Chest Wall



Chest Wall


Bernard F. Laya

Evan J. Zucker

Mark C. Liszewski

Ricardo Restrepo

Edward Y. Lee



INTRODUCTION

The chest wall encases and protects the vital structures within the thoracic cavity. The layers of the chest wall include the skin, subcutaneous fat, muscle, bone, cartilage, and pleura. Chest wall abnormalities are common in the pediatric population and may arise from any of these layers. Pathologic conditions include congenital anomalies, infections, benign and malignant tumors, traumatic lesions, and vascular malformations. Imaging plays a critical role in accurate detection and characterization of these lesions, which are essential to appropriate management.

This chapter discusses the embryologic development and normal radiologic anatomy of the chest wall. Various imaging techniques for evaluation of pediatric chest wall anomalies and abnormalities are discussed. In addition, myriad pediatric chest wall disorders are reviewed, with discussion of clinical and imaging manifestations as well as current management strategies.


ANATOMY


Embryology

The thoracic wall consists of skin, fascia, nerves, vessels, muscles, bones, cartilage, and pleura. Its four main functions include: (1) protection of the thoracic and upper abdominal viscera; (2) resistance to negative intrathoracic pressure during respiration; (3) provision of support for the upper limbs; and (4) provision of attachment sites for the muscles of respiration, as well as many muscles of the upper limbs, neck, abdomen, and back.1 The various components of the chest wall develop both sequentially and simultaneously. For practical purposes, the skeletal development of the vertebral column and thoracic wall is described in this section.

During the precartilaginous phase, mesenchymal cells from the sclerotomes are found around the notochord, surrounding the neural tube, and in the body wall. At the 4th week of gestation, some densely packed mesenchymal cells from each sclerotome move cranially to form the intervertebral disk, while some migrate around the spinal cord and notochord and merge with the cells from the opposing side to form the primordial vertebral body, called the centrum. Resegmentation occurs such that the caudal half of each sclerotome grows into and fuses with the cephalic half of the adjacent sclerotome. Thus, each vertebra is formed from a combination of the cranial half of one somite and the caudal half of the adjacent somite.2

The intersegmental arteries develop along each side of the vertebral bodies, while the nerves lie close to the intervertebral disks. The notochord eventually degenerates and becomes incorporated into the intervertebral disk as the nucleus pulposus. In the meantime, the mesenchymal cells that surround the neural tube organize and eventually develop into the vertebral arch. By the 6th week of gestation, chondrification centers appear on each centrum, paving the way for the development of the cartilaginous vertebral column (Fig. 5.1A). Extensions of chondrification from the neural arch also develop, forming the spinous and transverse processes. At the 7th week of gestation, both ventral and dorsal primary ossification centers of the primordial vertebra appear and later fuse (Fig. 5.1B). By the 8th week of gestation, the three primary ossification centers include the fused vertebral centrum and the paired neural arches. At birth, the typical vertebra consists of an ossified single vertebral body, and paired neural arches and transverse processes attached to one another by synchondroses. The processes of fusion and ossification continue through childhood and even beyond puberty.2,3

In the precartilaginous stage of gestation, mesenchymal cells from the sclerotomes are also found in the body wall and later form the costal process of the thoracic vertebrae from which the ribs develop (Fig. 5.1A). The costal processes are cartilaginous during the early embryonic phase and ossify during the fetal period. A synovial articulation also develops, connecting the vertebrae and the ribs posteriorly.3 The
sternum develops from paired sternal bars that are initially located at the ventrolateral aspects of the anterior body wall. These sternal bars chondrify, migrate medially, and eventually fuse in the midline. At 10 weeks of gestation, the cartilaginous manubrium, sternal body, and xiphoid process are formed. Ossification of the manubrium and sternal body occur during fetal maturation, but the xiphoid process may not be completely ossified even after birth.3,4 The sternum is subject to growth and morphologic changes, as well as segmental fusion that may continue until early adulthood.4






FIGURE 5.1. Embryologic development of the vertebral body and ribs. A: Vertebral foramen (VF), neural arch (NA), chondrification centers (CC), costal process (CP). B: Primary ossification centers (POC).


Normal Development and Anatomy

The chest wall is a symmetrical structure that broadens in the cranial to caudal direction, encasing many important vital organs of the upper body. It is divided into three distinct layers: (1) a superficial layer comprised of the skin and subcutaneous tissues; (2) an intermediate layer that includes the pectoralis region as well as the bones and muscles of the shoulder; and (3) a deep layer that includes the spinal column, ribs, and intercostal spaces; sternum; fascial layers; and parietal pleura (Fig. 5.2).5,6 The anteroposterior diameter of an infant’s chest is generally wider than that of an older child, and in infants, the ribs are oriented more horizontally.7

The superior border of the chest wall is the superior thoracic aperture or the anatomical thoracic inlet, which communicates with the neck and the upper limbs. The inferior thoracic aperture or thoracic outlet is the inferior border delineated by the diaphragm, which almost completely separates the thoracic and abdominal cavities.1,8 The anterior skeletal border is the sternum, a flat, vertically oriented bone situated subcutaneously in the mid anterior aspect of the chest wall.






FIGURE 5.2. Layers of the chest wall. Diagrammatic representation (A) and axial enhanced CT image (B) show the three distinct layers of the chest wall. The superficial (S) compartment includes the skin and subcutaneous tissue. The intermediate (I) compartment includes the shoulder girdle and pectoralis muscles. The deep (D) compartment includes the thoracic spine, ribs, intercostal space, sternum, fascia, and parietal pleura.

The sternum is divided into three parts: (1) the manubrium, a trapezoid-shaped bone that forms the superior component at the T3-T4 level and has lateral articulations with the clavicle and costal cartilages of the first two ribs; (2) the body, a vertically oriented, elongated portion of sternum spanning the T5-T9 levels, which articulates with the costal cartilages of ribs 2-7; and (3) the xiphoid process, which is the smallest component, located inferior to the body of the sternum at the T10 level, and composed of unossified cartilage in most children.1,4,8

The thoracic vertebrae form the posterior skeletal border of the chest wall. There are 12 thoracic vertebral bodies with vertebral arches and processes for muscular and articular attachments. Characteristic features that distinguish the thoracic vertebrae include bilateral costal facets on the vertebral bodies and transverse processes that articulate with the heads and tubercles of the ribs, respectively. Intervertebral disks separate the thoracic vertebral segments.1,8 The lateral osseocartilaginous border of the chest wall, connecting the sternum anteriorly and the thoracic vertebral bodies posteriorly, are the 12 paired ribs. The ribs are curved, flat, remarkably lightweight, resilient bones that form most of the thoracic cage. Each rib has an anterior cartilaginous component that provides a flexible articulation with the sternum. Ribs 1-7 are designated as true vertebrocostal ribs, as their costal cartilages directly connect with the sternum anteriorly. Ribs 8-10 are designated as false vertebrocostal ribs since they are anteriorly attached to the sternum indirectly, by way of the cartilage of the adjacent superior rib. Ribs 11 and 12 are considered floating ribs because, although they are attached to the vertebrae posteriorly, there is no anterior attachment to the sternum (Fig. 5.3).1,3,8







FIGURE 5.3. Skeletal anatomy of the chest wall. Three-dimensional reformatted CT images of the bones in anterior (A) and posterior (B) views.

The intercostal spaces contain three layers of intercostal muscles that aid in the respiratory function of the chest wall: (1) external intercostal muscles in the superficial layer, (2) internal intercostal muscles in the middle layer, and (3) innermost intercostal muscles in the deepest layer.1,7 Other important muscle groups of the anterior chest wall are the pectoralis major, pectoralis minor, and serratus anterior, which helps expand the thoracic cage during deep and forceful inspiration. The latissimus dorsi is the predominant muscle of the posterior chest wall, and some anterolateral abdominal muscles, neck, and other back muscles also attach to the thoracic cage.1

The thoracic spine contains 12 pairs of thoracic spinal nerves that divide into anterior and posterior rami as they leave the intervertebral foramina. The posterior rami of the spinal nerves innervate the bones, joints, muscles, and skin of the back. The anterior rami comprise the intercostal nerves that run along the intercostal spaces within the costal grooves, just inferior to the intercostal arteries. With the exception of the 10th and 11th intercostal spaces, each intercostal space is supplied by a large posterior intercostal artery, its collateral branch, and paired anterior intercostal branches. The intercostal veins also pass through the costal grooves and lie superior to the intercostal arteries and nerves.1


Anatomic Variants

In pediatric patients, normal variants of chest wall anatomy, specifically those involving the bones and muscles, are a common source of concern. Although they are mostly asymptomatic, these findings can sometimes be palpable, leading to suspicion for true lesions. Radiologic imaging evaluation, though rarely needed, can elucidate the anatomic structures producing “lumps and bumps” and help to distinguish them from pathological conditions.


Sternalis

The sternalis muscle, also described in the literature as “parasternalis,” “rectus sterni,” “rectus sternalis,” and “biceps sternalis,” among other terms, is an uncommon, band-like muscle located superficial to the pectoral fascia that can be found unilaterally (4.5%) or bilaterally (1.7%).9,10,11 Its presence has been found to be variable among different ethnic groups,10,12 but there is no gender predilection.10 This developmental anatomic variant is believed to be an embryonic remnant of the rectus muscle or a derivative of the pectoralis muscle.9,11,13 The function of the sternalis muscle is still unknown, although its location and orientation suggest that it may help in elevation of the lower chest wall9,10 or shoulder joint movement.9 As an anatomical variant, it is not believed to cause symptoms.9 However, it has recently been reported to be a cause of pain and soft tissue swelling in the sternal region.12 Possible etiologies of pain relate to a compensatory increase in tension and inflammation at the muscle insertion site or compression of the anterior cutaneous branches of the lateral pectoral nerve or traversing intercostal nerve branches.12

Grossly and on imaging, the sternalis is a vertical strip of muscle that lies superficial and perpendicular to the pectoralis major and parallel to the sternum. Its fibers arise from the upper sternum and the infraclavicular region and can variably insert onto the pectoral fascia, lower ribs, costal cartilages, rectus abdominis muscle sheath, or the abdominal external oblique muscle aponeurosis (Fig. 5.4).9,10,11,12,13

It is important for clinicians and radiologists to be aware of this structure, as it can cause alterations in electrocardiogram tracings and breast or chest wall asymmetry, or mimic a benign or malignant mass on routine imaging.9,10 Various imaging techniques, particularly computed tomography (CT) or magnetic resonance imaging (MRI), may help to prove the presence of this variant and prevent diagnostic dilemmas.9,10







FIGURE 5.4. Diagrammatic representation of sternalis muscle. Sternalis muscle (SM) arises from the tendon of the sternal origin of sternocleidomastoid muscle (SCM) and the upper segment of the pectoralis major (PM), courses lateral to the sternum (ST), and inserts onto the 10th to 12th costal cartilages or costochondral junction on each side of the chest.


Sternal Foramen

A sternal foramen represents a congenital anatomic defect in the sternum that occurs as a result of impaired and incomplete fusion of the paired sternal bar ossification centers during early gestation.4,14,15 The bony defect can be single or multiple and is usually seen in lower sternal body.4,14 Its incidence is significantly variable among different studies in the literature, reported in 0.06% to 18.3% of individuals, with a male-to-female ratio of 4:1.4 Although it is a developmental variant, it may mimic a pathological defect of the sternum caused by infection, primary malignancy, or metastases. Notably, these acquired defects can be distinguished from normal variant sternal foramina by irregular margins, erosions, and associated soft tissue components. Due to its proximity to important thoracic structures, accurate identification of sternal foramina is important in preventing fatal complications (e.g., pneumothorax or pericardial tamponade) during sternal bone marrow aspiration, acupuncture, and other procedures involving the anterior chest wall.14,15

CT typically shows a round-to-oval-shaped defect in the midline of the sternal body, usually with a smooth, welldemarcated border (Fig. 5.5).4,14 Irregular margins are less common.4 Sternal foramina have an average diameter of 6 mm and can be solitary or multiple. In the axial plane, they can demonstrate a “bowtie” appearance.4 Although they are most commonly seen in the lower body of the sternum, they have also been reported in the xiphoid process.4


IMAGING TECHNIQUES

The chest wall is composed of various structures including skin, subcutaneous tissues, muscles, bones, nerves, arteries, and veins. Various lesions or pseudolesions can arise from any of these structures, including normal variants, developmental or congenital disorders, traumatic injuries, infections, and either benign or malignant tumors. Clinical history and careful physical examination often provide information that is useful for localizing the area of abnormality and guiding the generation of an appropriate differential diagnosis. Various imaging techniques play important roles in diagnosing chest wall abnormalities in children.6,7,16 Specific roles of medical imaging include (1) confirming the presence of an abnormality, (2) localizing and characterizing the lesion, (3) depicting the lesion’s origin and its relationship with adjacent structures, (4) differentiating normal variants from abnormalities, (5) aiding in the formulation of a differential diagnosis, (6) facilitating image-guided interventions, and (7) evaluating patients during or after medical or surgical interventions.






FIGURE 5.5. Sternal foramen. Coronal reformatted CT image of the sternum demonstrates an oval-shaped defect in the distal body of the sternum, compatible with a sternal foramen (arrow). Incomplete segmentation (arrowhead) is also noted in the superior aspect of the sternal body.


Radiography

The initial imaging test for chest wall lesions in children is usually a chest radiograph, because of its widespread availability, relatively low cost, and rapidity and ease of acquisition. Both frontal (anteroposterior or posteroanterior) and lateral views are usually obtained; however, oblique and other dedicated views of the area of concern may be added for more precise localization of the lesion (Fig. 5.6A).6,17,18 Low-kilovoltage technique used for bone radiography can more accurately define soft tissue planes, particularly in
fat-containing tumors such as lipomas. It can also more accurately delineate calcifications.19






FIGURE 5.6. Chest wall abnormalities seen on radiographs. A: Oblique chest radiograph of an 8-year-old boy with left-sided chest pain shows a sclerotic lesion (arrowhead) in the lateral aspect of the left rib, later proven to be an osteosarcoma. B: Frontal chest radiograph of a neonate girl with respiratory distress demonstrates bilateral short ribs giving a bell-shaped appearance to the chest, compatible with thoracic asphyxiating dystrophy.

To localize a lesion on a radiograph, it is helpful to place a radiopaque marker adjacent to but not directly on the lesion, so it does not become obscured. Some findings may have typical radiographic appearances for particular diagnoses. In other cases, radiographs can localize and help narrow the differential diagnosis or at least provide valuable information for directing selection of further diagnostic studies.18

Palpable but asymptomatic osseous abnormalities, including developmental variants and benign osseous lesions of the ribs and sternum, can sometimes be identified on chest radiographs (Fig. 5.6B). Radiographs are particularly useful in the setting of trauma, infection, and malignancy.17 For chest wall masses, a chest radiograph is usually obtained initially to confirm the presence of the lesion, localize and characterize the mass, and show bony erosion and intrathoracic involvement. 6,19,20 Although this technique is useful for detecting cortical destruction that indicates extracompartmental extension, it does not allow comprehensive evaluation of tumors.21


Ultrasound

Ultrasound (US) is increasingly utilized in the evaluation of palpable, superficial chest wall abnormalities. It is a noninvasive, widely available, relatively rapid test that is less expensive than CT and MRI. Importantly in the pediatric population, US is especially advantageous because it does not require sedation or impart a dose of ionizing radiation.6,22

US is useful in children with palpable anterior chest wall masses and negative chest radiographs. These findings are often relatable to costochondral cartilage deformities that are common in children.16,23 US can play a primary role in the evaluation of chest wall infection, differentiating cellulitis from abscess formation and osteomyelitis.23 Because superficial chest wall lesions can be visualized on US, they are often amenable to US-guided biopsy.17

US utilizing a high-frequency linear transducer focused on the area of concern can aid in determining whether a lesion is present and if it is solid, cystic, or part of a normal structure. Assessment of vascular flow within the lesion can be performed using Doppler US with spectral tracings. This is particularly important in the evaluation of vascular malformations and vascular masses.6 It can be difficult to evaluate the depth and extent of involvement of many congenital vascular malformations on US. Deeper involvement of chest wall lesions, such as extension into the spinal foramina or middle and posterior mediastinum, is also difficult to assess sonographically.6,22


Computed Tomography

Due to recent technological advances that enable improved spatial resolution, faster acquisition speeds that reduce the need for sedation, and multiplanar and 3D reconstruction capabilities that improve lesion localization, CT has emerged as a key modality for evaluation of chest wall pathology in children.6,17,18,24 CT may be indicated for further evaluation when radiographs and US are normal (with ongoing clinical suspicion for pathology), inconclusive, or nondiagnostic. In particular, CT is excellent at providing characterization of the lesion’s composition, defining its size and extent, assessing involvement of adjacent chest wall and intrathoracic structures (particularly the lungs and mediastinum), detecting metastatic spread, and identifying tumor vascularity. Ultimately, CT provides important information for determining the nature of the disorder, narrowing the differential considerations, and sometimes providing the diagnosis.6,18,20,21

For CT evaluation of chest wall abnormalities in children, the smallest possible field of view should be used in order to maximize spatial resolution. Intravenous administration of nonionic iodinated contrast media, with a suggested dose of 2.0 mL/kg, is particularly useful in neoplastic and infectious processes.6 Radiation-related risks remain an important
concern in pediatric CT; thus, CT in children should follow the “as low as reasonably achievable” (ALARA) principle and be performed with the lowest possible radiation dose that maintains diagnostic quality, which is largely determined by patient size. In addition, radiation dose optimization and dose reduction techniques should be utilized to their fullest capabilities in order to help diminish these radiation-related risks.6,18


Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is a cross-sectional imaging tool that offers excellent spatial resolution and soft tissue differentiation without the use of ionizing radiation or iodinated contrast.6,17,18 MRI enables accurate tissue characterization and assessment of vascular enhancement and therefore plays a key role in preoperative staging and assessment of involvement of neurovascular structures.17,21,25 MRI is complementary to other imaging tools when diagnoses have not been established with radiographs, US, or CT.18 It is also helpful for evaluating potential spinal involvement by posterior chest wall lesions.20 MRI is superior to the other imaging modalities in detecting bone marrow lesions. Contrast-enhanced MRI can reveal the most cellular or vascular portions within tumors and identify necrotic regions, which can improve diagnostic yield in biopsies and facilitate the planning of other imageguided interventional and surgical procedures. Additionally, it can be used for assessing response to therapy and detecting recurrence and/or metastasis in post-therapeutic followup.25 However, utilization remains limited in the evaluation of pediatric chest wall masses, particularly in younger children, due to relatively long examination durations that often necessitate sedation in order to minimize respiratory motion in pediatric patients who cannot follow instructions for breathhold acquisition.6

For infants and toddlers, a head or cardiac coil can be utilized, but for older children, a phased-array surface/torso coil may be used. To maximize spatial and temporal resolution, the smallest possible field of view should be selected. Intravenous administration of gadolinium-based contrast is performed, particularly for infectious and neoplastic processes and vascular malformations. The recommended dose is 0.1 mmol/kg body weight, injected by rapid bolus technique.6,18 To obtain high-quality MR images, respiratory triggering, flow compensation, and flow presaturation techniques may be helpful.18 The specific imaging protocol may vary depending on the nature of the mass (osseous or soft tissue lesion), its location, and the presence of vascularity.

Typical MRI sequences include multiplanar T1- and T2-weighted sequences, a fat-suppressed sequence such as short tau inversion recovery (STIR), and a gradient echo T1-weighted sequence with fat suppression. Diffusionweighted imaging (DWI) sequences can also be helpful for characterizing soft tissue masses. In particular, DWI is useful for defining the amount and location of cellular tissue within tumors based on representation of decreased Brownian motion of water molecules in various regions (i.e., lower diffusivity is present within more cellular tissues with higher nucleus-to-cytoplasm ratios). Fat-suppressed, T1-weighted multiplanar sequences are also obtained following intravenous contrast administration. In selected vascular malformations and vascular soft tissue tumors, MR angiography may be contributory.6


Nuclear Medicine

Nuclear medicine studies, particularly bone scintigraphy and fluorodeoxyglucose (FDG) positron emission tomography (PET), play an important role in the assessment of both primary and metastatic chest wall malignancies, as well as infectious processes. Bone scintigraphy with radionuclide-labeled diphosphonate analogs, such as 99mTc-methylene diphosphonate (MDP), is one of the most frequently used nuclear medicine imaging studies for detection of bone metastases and investigation of various benign bone disorders, including bone pain, trauma, infection, bone viability, and metabolic bone disease. Utilization of SPECT imaging provides 3D information, enabling assessment of depth and location of the radionuclide activity.26

Many primary bone tumors are FDG-avid and readily detected on 18F-FDG PET/CT. 18F-FDG uptake reflects the degree of metabolic activity within a mass, while the morphologic characteristics are evaluable on the CT portion of the examination that is also acquired for attenuation correction. Aside from diagnosis of certain primary bone malignancies, such as osteosarcoma and Ewing sarcoma, 18F-FDG PET/CT can also be used for whole-body staging and evaluation of therapeutic response.27 18F-FDG PET/CT is also useful for detecting unsuspected and unusual metastatic sites of childhood sarcomas.28 In the context of malignancy, 18F-FDG PET/CT is most useful in identifying sites of metastasis and in choosing the most metabolically active area for biopsy.17,20 It is important to remember that not all hypermetabolic bone lesions are malignant—some benign processes, such as inflammation, infection, and fibro-osseous lesions in children, can have elevated standard uptake values and mimic malignancy.29

Rapid technological development in nuclear imaging has generated interest in the use of the positron agent 18F-labeled sodium fluoride (18F-NaF), a bone-seeking agent that provides higher spatial resolution images than 99mTc-MDP. Most experience with 18F-NaF has been in imaging bone malignancies. However, 18F-NaF PET has also proven useful in the evaluation of benign bony abnormalities such as suspected bone trauma (including child abuse) and stress-related injury.30


SPECTRUM OF CHEST WALL DISORDERS


Congenital and Developmental Abnormalities


Nonvascular Abnormalities

Congenital and developmental abnormalities of the pediatric chest wall are common.5,7,24 There is asymmetry in the shape or size of the chest wall in up to one-third of children.5,31 Although chest wall asymmetry in otherwise asymptomatic children is usually due to underlying anatomical variation in the rib cage and sternum, it still frequently raises concern that prompts imaging evaluation.5







FIGURE 5.7. Asymmetric chest wall in two children. A: Axial enhanced CT image of a 2-year-old boy with a hard, painless, palpable “lesion” of the left anterior chest wall shows a prominent sternochondral junction (arrow) on the left. B: Axial bone window CT image of a 9-year-old boy with scoliosis and a protuberant right anterior chest demonstrates a prominent sternochondral junction (asterisk) on the right.


Rib Abnormalities

Congenital rib anomalies are common incidental findings in children, with an estimated frequency of about 1.4% to 2%.5,32 Rib anomalies can be classified as either numerical or structural. Numerical abnormalities are classified as either supernumerary (more than 12 pairs of ribs) or rib deficient (<12 pairs of ribs). Structural abnormalities include bifid, forked, fused, bridging, and hypoplastic ribs.32


Prominent Convexity of a Rib or Costal Cartilage

Developmental anatomical variations in chest wall configuration are very common in children. One of the most common variants is a prominent, convex rib or costal cartilage.24,31 Affected pediatric patients usually present with a palpable anterior chest wall “lump,” which is usually discovered by either a parent or an examining physician.33 It is important to recognize this pseudolesion and not mistake it for a true abnormality.

When imaging is necessary, a chest radiograph can be the initial imaging tool. A radiopaque marker can be placed adjacent to the area of concern as a localizer. Although variants of bone can be readily identified radiographically, variants of cartilaginous structures are radiolucent and, thus, radiographically occult. US is, therefore, more useful for evaluating cartilaginous variants. If CT is warranted, multiplanar and 3D reformatted images are helpful in demonstrating causes of asymmetry (Fig. 5.7).24 Costal cartilage is considered to be prominent when its anteroposterior diameter is ≥3 mm larger than the contralateral cartilage at the same level.24,33


Bifid Ribs

A bifid rib exhibits bifurcation or cleavage of the anterior aspect of its bony or cartilaginous portion. It has a reported incidence ranging from 0.15% to 6.76% and equally affects right- and left-sided ribs.32,34,35 Bifid ribs may be unilateral or bilateral. The 3rd and 4th ribs are most commonly affected, while the 2nd, 5th, and 6th ribs are less common sites. They are frequently an isolated finding in asymptomatic pediatric patients that are incidentally detected on imaging. However, bifid ribs can be associated with genetic disorders such as congenital scoliosis, Gorlin-Goltz (basal cell nevus) syndrome, Kindler syndrome, and Job syndrome.32,34,35 Complete clinical evaluation is recommended for any pediatric patient who is found to have a bifid rib, in order to exclude underlying congenital syndromes.

It is necessary for radiologists to be familiar with the radiographic appearance of bifid ribs so that they are not mistaken for benign or aggressive rib lesions. Chest radiographs can readily demonstrate a forked appearance of the anterior aspect of a bifid rib (Fig. 5.8). However, radiographs alone are limited in showing the relationship between the bifid rib and the thoracic cavity. They cannot demonstrate cartilaginous articulations or anomalous fibrous attachments to adjacent ribs. Furthermore, radiographs provide limited assessment
of the ribs’ associated neurovascular bundles.35 Multidetector computed tomography (MDCT) with 3D reconstruction is a useful tool for precisely characterizing rib abnormalities initially identified on radiographs.35






FIGURE 5.8. A 2-year-old boy with a bifid rib who underwent chest radiography for fever. Frontal chest radiograph shows a bifid anterior portion (arrowheads) of the right 5th rib.


Cervical Rib

Cervical ribs are supernumerary accessory ribs that develop from the 7th cervical vertebrae. They have a reported incidence of 0.2% to 3.4%, are twice as common in girls, and are bilateral in more than 50% of cases. An overwhelming majority (˜90%) of individuals with a cervical rib are asymptomatic and do not require surgical resection. In 10% of affected individuals, thoracic outlet syndrome (TOS) results from compression of the brachial plexus and vascular sheath by the cervical rib or a fibrous band that connects the cervical rib with the first thoracic rib. TOS may manifest with neurogenic signs such as pain, paresthesia, and weakness, or it may present with arterial or even venous compression symptoms.5,32,36 Cervical ribs can be associated with Klippel-Feil deformity.

Radiographs ably demonstrate the course of cervical ribs and their anatomic relationships with other structures. A cervical rib may extend beyond the C7 transverse process, extend adjacent to the 1st thoracic rib, or be fused to the 1st thoracic rib (Fig. 5.9).32,36 CT can demonstrate associated cartilaginous and fibrous abnormalities and diagnose compression of the adjacent neurovascular structures and other associated complications.


Intrathoracic Rib

Intrathoracic ribs are rare congenital anomalies in which supra- or normonumerary ribs follow an abnormal course within the thorax. It has been proposed that these anomalies result from incomplete fusion between adjacent sclerotomes that leads to the formation of two lateral processes: one connects with the transverse process, while the other connects to the vertebral body. It has also been suggested that changes in intrathoracic or extrathoracic pressure interfere with rib development.37 Congenital intrathoracic ribs usually have the exact shape and structure of a typical rib but are found in an intrathoracic position. They usually arise from a vertebral body or from the posterior portion of a rib. Intrathoracic ribs are usually solitary and unilateral, frequently arising from the 3rd to 8th thoracic vertebral bodies, and are more common on the right. There may be fibrous attachments to the diaphragmatic pleura and intrathoracic fat.37,38






FIGURE 5.9. Cervical ribs in two children. A: Frontal chest radiograph of a 16-year-old girl who presented with right upper extremity pain shows a right cervical rib (arrow). B: Frontal chest radiograph of an asymptomatic 6-year-old boy reveals bilateral cervical ribs (arrowheads).

Intrathoracic ribs are subdivided into four categories: (1) type I-a is a typical supernumerary intrathoracic rib, arising from the anterolateral portion of a vertebral body and extending laterally and downward; (2) type I-b is also a supernumerary intrathoracic rib, arising from the posterior portion of a rib close to a vertebral body (proximal rib) and extending laterally and downward; (3) type II intrathoracic rib is a rare variant bifid intrathoracic rib; and (4) type III intrathoracic rib is caused by local rib depression, where one or more ribs are depressed into the thoracic cavity.38

Radiography alone can be used to diagnose a typical intrathoracic rib with a high degree of confidence. However, it may be challenging to definitively diagnose an atypical intrathoracic rib on radiography, and other imaging tests, such as MDCT with 3D reconstructions, are often necessary.37 Most individuals with an intrathoracic rib are asymptomatic and treatment is often unnecessary.


Slipping Rib Syndrome

Slipping rib syndrome is a condition in which the rib intermittently slips out of place. This causes stretching of the
ligaments supporting the ribs, because ribs have no muscular scaffold. Affected individuals present with chronic lower chest or upper abdominal pain with superimposed episodes of severe pain. Affected patients sometimes describe the slipping movement of the ribs as a “popping sensation.” The loose ribs may also pinch intercostal nerves, which causes radiation of excruciating pain around the chest into the back. Slipping rib syndrome is caused by weakness of the costosternal, costochondral, and/or costovertebral/costotransverse ligaments, which allows rib hypermobility. False ribs (ribs 8 to 10) are more susceptible to hypermobility and trauma than true ribs. This is because they are attached to one other anteriorly only by a relatively weak cartilaginous or fibrous cap (Fig. 5.10).39,40,41,42,43 There is no difference in the prevalence of the syndrome between men and women, and the condition may be unilateral or bilateral. Although this syndrome may occur at any age, it is reportedly more common in middle-aged adults39,40,41 and less common in children because of the more flexible nature of their chest wall tissues.40

Slipping rib syndrome is relatively uncommon and often underdiagnosed because it is difficult to detect.40,41 The diagnosis rests on clinical suspicion and a positive “hooking maneuver,” a physical examination technique in which the examiner curls his or her fingers beneath the costal margin and lifts straight up. A positive test reproduces the patient’s pain and causes a click. The role of imaging in the evaluation of patients with slipping rib syndrome is largely to exclude other potential causes of chest wall pain, such as rib fractures, bone metastases, muscle tears, and pleural and abdominal diseases39,40,41; however, subluxation of cartilaginous ribs has been successfully demonstrated by US.41,44






FIGURE 5.10. False ribs. Three-dimensional reformatted oblique CT image of the bones shows that the cartilages of ribs 8 to 10 are attached to the sternum via the cartilage of rib 7 (arrow), making them more prone to slipping rib syndrome.

Medical pain management or placement of a nerve block should be the first step in the management of slipping rib syndrome. If pain is not alleviated, partial resection of the rib and costal cartilage may be performed.39,40,41


Chest Wall Abnormalities


Pectus Excavatum

Pectus excavatum or “funnel chest” is a chest wall deformity characterized by inward depression and tilting of the lower portion of the sternum, resulting in anterior chest wall concavity with relative protrusion of the attached costal cartilages on each side. The degree of concavity varies from mild to severe.5,6,18,33,45,46,47 The incidence of pectus excavatum in the general population is between 1:400 and 1:1,000.33 The deformity is five to nine times more common in males than in females.33,47 The pathophysiology is poorly understood, but the deformity is generally thought to result from abnormal cartilage growth.48

Pectus excavatum is the most common chest wall deformity (90%) and is usually an isolated abnormality that occurs sporadically. In some cases, however, it is inherited in an autosomal dominant fashion. Certain disorders are associated with pectus excavatum, including Turner syndrome, osteogenesis imperfecta, muscular dystrophy, and connective tissue disorders such as Marfan and Ehlers-Danlos syndromes.5,46,49 Pediatric patients with severe pectus excavatum may present with chest pain and cardiorespiratory symptoms, but most patients are asymptomatic.5,6,18,47,48

On frontal chest radiography, the anterior rib ends have a steep downward course, while the posterior ribs are more horizontally oriented. The heart is shifted to the left and slightly rotated due to the thoracic deformity. The right parasternal soft tissues and hilar vessels produce a paracardial density that partially obscures the right heart border, which should not be mistaken for a middle lobe consolidation.5,6,33,45 On the lateral view, the anteroposterior dimension of the chest is narrowed due to sternal depression (Fig. 5.11A and B). Cross-sectional imaging with either CT (Fig. 5.11C) or MRI is useful to determine and quantify the degree of deformity, evaluate the degree to which the heart and tracheobronchial structures are compressed, plan surgery, and predict the potential for clinical improvement following surgery.5,18,33,46

The Haller index is a quantitative measure of the chest wall in patients with pectus excavatum. It is the ratio of the internal transverse chest diameter to the narrowest anteroposterior diameter of the chest, usually obtained from a single axial CT image (Fig. 5.11D).5,6,18,33,46,50 A normal Haller index value ranges from 1.9 to 2.7. The normal index in children under the age of 2 is significantly lower than that in older children. A Haller index of 3.2 or greater usually requires surgical correction.5,33,51 Haller index measurements can also be obtained using MRI, which also allows dynamic assessment of the chest wall and diaphragm.5 A significant correlation between Haller indices estimated with CT and radiographs has been reported.45,46

Pediatric patients with severe pectus excavatum typically undergo surgical correction, commonly with placement of
a convex metal bar behind the sternum (Nuss procedure) or a Nuss procedure combined with surgical resection of costal cartilage and sternal manipulation (modified Ravitch procedure).33,46,47,48,52






FIGURE 5.11. A 10-year-old boy with pectus excavatum. A: Frontal chest radiograph shows a steep downward course of the anterior ribs (arrowheads) and more horizontally oriented posterior ribs (arrows). A right paracardiac density (asterisk) represents summation of parasternal soft tissue and hilar vessel shadows. B: Lateral chest radiograph shows a narrow anteroposterior diameter of the chest caused by the sternal depression (arrowheads). C: Three-dimensional oblique reformatted CT image of the bones shows rightward tilt and depression of the lower portion of the sternum (arrowheads), resulting in a narrowed anteroposterior chest diameter. D: Axial bone window CT image shows the measurements used to calculate the Haller index, which is the widest transverse thoracic diameter (TRV) divided by the narrowest anteroposterior thoracic diameter (AP).


Pectus Carinatum

Pectus carinatum or “pigeon breast” is defined as anterior protrusion of the sternum with flattening of the lateral chest wall.5,6,33,47,49,53,54 Pectus carinatum is less common than pectus excavatum, with an incidence of 1:1,500 live births, and a male-to-female ratio of 4- 9:1.33,47,54 The congenital deformity is postulated to be due to growth disturbance of both the sternum and costal cartilages, with premature sternal fusion.5,49 Two variants of pectus carinatum have been described: (1) the more common chondrogladiolar variant, defined by protrusion of the middle and lower sternum; and (2) the less common chondromanubrial variant, in which the manubrium and upper sternum protrude anteriorly. Distinguishing between the two variants helps guide surgical management.5,53,54

Nonsyndromic patients are mostly asymptomatic5 but typically present with cosmetic complaints during puberty, as the deformity becomes more prominent during the adolescent growth spurt.48,53 In a minority of affected pediatric patients, pectus carinatum can present with shortness of breath, tachypnea, exercise intolerance, and musculoskeletal symptoms.33,53 Approximately one-third of individuals with
pectus carinatum have scoliosis, and underlying congenital heart disease is present in a small percentage of patients.18,33,53 There is a higher incidence in various disorders including Marfan, Ehlers-Danlos, Noonan, Morquio, and prune belly syndromes, and osteogenesis imperfecta, among others.5,33,49






FIGURE 5.12. A 9-year-old boy with pectus carinatum. Frontal (A) and lateral (B) chest radiographs show anterior protrusion of the sternum (arrowheads), resulting in an increased anteroposterior thoracic diameter.

Chest radiography, particularly the lateral view, demonstrates an increased anteroposterior diameter and anterior protrusion of the sternum (Fig. 5.12).33,53 A Haller index (transverse thoracic diameter divided by the anteroposterior diameter) and assessment of the severity of the chest wall deformity can be obtained with radiographs, CT, or MRI. A Haller index of <1.98 is indicative of pectus carinatum.33 Cross-sectional imaging is obtained for children with associated disorders in order to exclude alternative pathology and to monitor the results of corrective treatment.53,54

Correction of pectus carinatum is indicated in children with significant deformity that causes pain or a disturbed body image.53 Orthotic bracing is currently the preferred initial management technique in the majority of children with pectus carinatum that requires correction.47,53,54


Syndromic Disorders


Poland Syndrome

Poland syndrome is a rare congenital malformation of the chest wall characterized by unilateral hypoplasia or aplasia of the pectoralis muscles and adjacent cartilaginous, osseous, breast, and soft tissue structures.5,6,18,55,56 It is hypothesized to occur as a result of temporary reduction of circulation in the ipsilateral subclavian artery and vertebral artery. It can be associated with ipsilateral brachysyndactyly of the upper extremity.6,18,55,56 The incidence is 1:30,000, and the syndrome is more common in males, with a male-to-female ratio of 2-3:1. The right side is involved in ˜60% to 75% of cases.6,56 The majority of affected individuals are asymptomatic; however, affected pediatric patients may present with chest pain, respiratory distress, or limited shoulder range of motion depending on the degree of cardiopulmonary disease or functional impairment of the shoulder.55

On chest radiography, hypoplasia or aplasia of the chest wall soft tissues results in characteristic hyperlucency of the affected hemithorax.5,6 Differential considerations include congenital lobar emphysema, obstruction from a foreign body or mucus plug, or Swyer-James syndrome.6 Multiplanar cross-sectional imaging with CT or MRI with multiplanar reformations can show the degree of pectoralis major aplasia/hypoplasia, as well as deficiency of the adjacent bone and soft tissues (Fig. 5.13). Detailed characterization of these findings is important for reconstructive surgical planning.5,6,18,55

In pediatric patients who require surgical correction for significant deformities of the chest wall and overlying soft tissue deficiency, surgery should be deferred until the patient is finished growing.56


Cleidocranial Dysostosis

Cleidocranial dysostosis or dysplasia is a rare autosomal dominant inherited skeletal disorder affecting intramembranous and enchondral ossification.7,57,58 Approximately 60% to 70% of affected individuals have mutations in RUNX2, a gene that encodes a transcription factor that is responsible for bone differentiation.57 The disorder is characterized by short stature, hypoplasia or absence of one or both clavicles resulting in droopy, hypermobile shoulders, and other cranial and pelvic skeletal abnormalities.5,7,59 Suspected skull mineralization defects require prompt identification so that the cranium can
be protected. While hypoplastic clavicles do not necessarily cause significant disability, associated thoracic anomalies have been reported to cause respiratory distress in early infancy. Long-term follow-up is necessary because additional abnormalities may become evident as the patient grows.57






FIGURE 5.13. A 3-year-old boy with an asymmetrical appearance of the chest wall due to Poland syndrome. Axial enhanced CT image shows absence of the pectoralis muscles (white arrowheads), deficient shoulder musculature (black arrowhead), and hypoplastic ribs (arrows) in the right chest wall.

Cleidocranial dysostosis is diagnosed primarily through its typical radiographic findings in the cranium, chest, and pelvis and confirmed by genetic analysis.57 Radiographic manifestations in the chest include absent or small clavicles, small scapulae, deficient sternal ossification, posterior wedging of thoracic vertebrae, scoliosis, kyphosis, and short ribs with prominent downward sloping (Fig. 5.14).5,7,59 Cranial features include Wormian bones, delayed or failed closure of the sutures, and delayed or failed eruption of permanent teeth. There may also be narrowing of the iliac wings, absence or hypoplasia of the pubic bone, and tapering of the distal phalanges.5






FIGURE 5.14. A 1-month-old boy with cleidocranial dysostosis. A: Frontal skull radiograph shows prominent sutures and a substantially widened anterior fontanelle (arrowheads). B: Frontal chest radiograph demonstrates small clavicles (arrows) and deficient scapulae (arrowheads).


Vascular Abnormalities


Malformations


Venous Malformations

Venous malformations (VMs) are the most common type of vascular malformation, accounting for 50% to 75% of all cases.60,61 They represent a spectrum of congenital abnormalities of the superficial or deep venous system and range from isolated venous dilatation (varix) to multiple, dilated, tortuous venous structures. The most common sites for “low-flow” venous and lymphatic malformations (LMs) are the neck and face, followed by the limbs, trunk, internal viscera, bones, and skeletal muscle.61,62 VMs can range from focal abnormalities within the subcutaneous soft tissues to diffuse lesions that extensively involve the deeper soft tissues, bones, body wall, extremities, and other organs.6 Affected pediatric patients often present with pain affecting the involved body part,61 but they typically do not become symptomatic until later in childhood or even adulthood.6

On radiographs, VMs appear as soft tissue masses, which sometimes contain phleboliths that result from slow blood flow and thrombosis. CT is more sensitive for detection of phleboliths (Fig. 5.15A), but with higher associated cost and ionizing radiation dose.60,61 On US, VMs are compressible, thin-walled, multicystic structures that may be hypoechoic, isoechoic, or hyperechoic relative to the adjacent soft tissues. 6,60 Calcified phleboliths appear as echogenic intravascular foci with posterior shadowing.6,60 Color Doppler US demonstrates either low-flow spectral tracings or absence of flow.6,60 MRI is the imaging modality of choice for evaluating anatomic extent and characterizing flow.6 VMs present as multiloculated structures with lobular contours. They
are typically hypo- to isointense to muscle on T1-weighted MR images and high in signal on T2-weighted MR images. Phleboliths can be seen as signal voids on T2-weighted and gradient echo sequences (Fig. 5.15B).6,60 VMs demonstrate diffuse central enhancement during the venous phase following intravenous contrast administration (Fig. 5.15C).6






FIGURE 5.15. A 10-year-old boy with a venous malformation who presented with a palpable mass in the anterior chest wall. A: Axial enhanced CT image at the level of the midclavicles shows an anterior, lobulated, serpiginous soft tissue mass (arrow) with focal calcifications known as phleboliths (arrowheads). B: Axial T2-weighted MRI demonstrates these phleboliths (arrowheads) in better detail. C: Axial enhanced T1-weighted MRI with fat suppression shows central enhancement of the tubular mass (arrow), characterizing it as a venous malformation. A vitamin E capsule on the skin surface (arrowhead) was used as a marker.

Treatment of VMs is highly variable, depending on the size of the mass and the symptoms of the patient. Treatment options include medical management, laser treatment, embolization, sclerotherapy, and surgery. Combined treatments are used in many instances.61


Lymphatic Malformations

Lymphatic malformations (LMs) are low-flow malformations on a spectrum with other congenital abnormalities of the lymphatic system. LMs are composed of abnormally numerous, dilated lymphatic channels lined by endothelium. They have been classified as microcystic, macrocystic, or combined.6,22,60,61,63,64,65,66 LMs can occur anywhere in the body but are most common in the axilla, chest, and cervicofacial regions.6,22,60 In the chest, LMs can be focal or diffuse cystic masses that are confined to the subcutaneous tissues or extend to involve the spine or mediastinum.6 Approximately 50% to 65% are diagnosed at birth, and 80% to 90% are diagnosed by 5 years of age.22,66

On US, microcystic LMs tend to be echogenic, while macrocystic LMs are predominantly anechoic, multiloculated lesions containing septations of variable thickness (Fig. 5.16). Mixed lesions containing both micro- and macrocysts can occur. Color Doppler US reveals vascular channels only in the septa, characterized as both veins and arteries by spectral analysis.6,61,64,66 US is limited in defining the extent of LMs, particularly if they involve the deeper layers of the chest wall or exhibit intrathoracic extension. In these cases, cross-sectional imaging (preferably MRI) is usually indicated.6

On MRI, macrocystic LMs are composed of discrete cystic structures with intervening septations. The cysts have homogenously high signal intensity on T2-weighted sequences and low signal intensity on T1-weighted sequences. The septations are low in signal on both T1- and T2-weighted sequences and may demonstrate contrast enhancement. Sometimes, blood products and proteinaceous content within the cysts can result in fluid-fluid levels. The individual cysts in microcystic LMs are often too small to discern on MRI. Microcystic LMs tend to have diffusely low signal intensity on T1-weighted sequences and high signal intensity on T2-weighted sequences. After administration of intravenous contrast, microcystic LMs may demonstrate mild diffuse enhancement of the closely applied cyst walls that can mimic a solid soft tissue mass.6,60,61,64,66

Treatment options for LMs include surgery, sclerotherapy, laser therapy, and even radiation therapy. Therapeutic selection depends on the patient’s symptoms, size of the cysts, and location and extent of the lesion.61,63,65







FIGURE 5.16. Chest wall lymphatic malformations in two neonates. A: Frontal chest radiograph of a 2-week-old boy shows a nonspecific soft tissue mass (white asterisks) in the left neck and chest wall. B: Transverse grayscale ultrasound image of the same child demonstrates a large cystic structure containing layering proteinaceous sediment (arrows). C: Coronal enhanced CT image of a 3-week-old girl shows a fluid attenuating mass (black asterisks) in the right lateral chest wall.


Tumors


Infantile Hemangioma

Infantile hemangiomas (IHs) are high-flow vascular neoplasms caused by endothelial proliferation. IHs are the most common vascular tumors of infancy, affecting 2% to 10% of the general population.33,67,68 Immunohistochemical staining of resected tissue is characteristically positive for the glucose transporter GLUT-1.68 IH is usually solitary, but multiple lesions are found in 15% to 30% of patients.67,68 They are commonly found in the cervicofacial region but can also be seen in other parts of the body. The thorax is involved in 25% of affected individuals.33

IHs are usually not seen at birth but exhibit rapid growth during early childhood, typically manifesting as palpable, raised, bluish-red masses. After the proliferative phase, these lesions enter an involutional phase with near complete resolution by age 10.33 The final stage is the fibrotic stage, defined by persistence of a relatively small focus of residual, loose fibrofatty scar tissue.68 The diagnosis of chest wall IH is often strongly suggested by the clinical history and physical examination, but radiologic imaging may be performed to define the extent of the lesion and its relationship with adjacent structures.33

On radiographs, large IHs appear as nonspecific soft tissue opacities. On US, they may appear as heterogeneous hyperechoic or hypoechoic masses. Doppler US imaging shows multiple arterial vessels with low-resistance waveforms and one or more draining veins. Vessel density >5 vessels/cm2 combined with a maximum systolic Doppler shift of >2 kHz has a specificity of 98%.23,33,64,68 On CT and MRI, IHs can have either well-demarcated or ill-defined margins, with prominent arterial supply and contrast enhancement. On MRI, they show intermediate signal intensity on T1-weighted sequences and hyperintensity on T2-weighted sequences (Fig. 5.17). Prominent flow voids indicate fast arterial flow, and contrast enhancement tends to be uniform and avid.62,64,68 With increasing age, IHs become more heterogeneous and hyperintense on T1-weighted MRI due to fibrofatty replacement.22,33,64,68

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May 8, 2019 | Posted by in PSYCHOLOGY | Comments Off on Chest Wall
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