Anomalies of the Skull and Meninges

Main Text

Preamble

Anomalies of the skull and meninges represent maldevelopment of the embryonic mesenchyme. These include cephaloceles, congenital calvarial defects, and other meningeal malformations, including lipomas.

Cephaloceles

Preamble

Cephaloceles are external protrusions of CNS contents through a calvarial defect, usually in the midline. Cephaloceles can occur in the occipital, parietal, frontal, petrous apex, or intrasphenoidal regions.

“Cephaloceles” or “encephaloceles” are generic terms for the protrusion of intracranial contents through a calvarial or skull base defect (46-1). Cephaloceles that contain herniations of brain tissue, meninges, and CSF are called meningoencephaloceles. If the meninges and accompanying CSF are herniated without brain tissue, the lesion is termed a meningocele. Glioceles are glial-lined, CSF-containing cysts. Atretic cephalocele (APC) is a small defect that contains just dura, fibrous tissue, and degenerated brain tissue.

Cephaloceles can be congenital or acquired and are defined by their contents and location. The most common congenital cephaloceles are occipital, frontoethmoidal, parietal, and skull base cephaloceles.

Cephalocele imaging has four goals: (1) Depict the osseous defect, (2) delineate the sac and define its contents, (3) map the course of adjacent arteries and determine the integrity of the dural venous sinuses, and (4) identify any coexisting anomalies.

Occipital Cephaloceles

Terminology and Classification

Three subtypes of occipital cephalocele (OC) are recognized and identified according to the involved bone(s). From most to least extensive, they are occipitocervical (involving the occipital bone, foramen magnum, and neural arches of the upper cervical spine), low occipital (involving the occipital bone and foramen magnum) (46-3), and high occipital (involving only the occipital bone).

High OCs and low OCs are the most common and are typically located in the midline of the occipital bone between the lambda and foramen magnum. They are sometimes divided into two subtypes: Supra- and infratorcular OCs.

Clinical Issues

OCs account for 75% of cephaloceles in European and North American White patients but are relatively rare in South and Southeast Asia. There is a 2.4:1 male predominance.

OCs are usually recognized at birth as an occipital or suboccipital soft tissue mass (46-1). Size varies from small to very large. The term “giant” OC is used when the size of the OC is equal to or greater than the size of the head. In such cases, the affected infant is often microcephalic with visible craniofacial disproportion.

Neurodevelopmental outcome in infants with OCs is related to cephalocele size and contents as well as the presence and type of associated abnormalities.

Imaging

Bone CT with 3D reconstruction delineates the osseous defect well, and multiplanar MR best depicts the sac and its contents. The herniated brain—which can derive from both supra- and infratentorial structures—is always abnormal, appearing dysmorphic, disorganized, and dysplastic. Depending on the size of the cephalocele, severe traction and distortion of the brainstem and supratentorial structures can be present (46-2).

Dura and CSF-filled structures (including the fourth ventricle and sometimes part of the lateral ventricles) are often contained within the sac. In addition to delineating the sac and its contents, identifying the course and integrity of the dural venous sinuses is essential for preoperative planning.

Hydrocephalus occurs in 60-90% of posterior cephaloceles. At least 1/2 of all patients with OCs have associated abnormalities, such as callosal dysgenesis, Chiari 2, Dandy-Walker spectrum disorders, and gray matter heterotopias.

Parietal Cephaloceles

Parietal cephaloceles compose just 5-10% of all cephaloceles. Most have underlying brain and vascular anomalies, such as a persistent falcine sinus or sinus pericranii (46-4).

MR best delineates cephalocele contents. Defining the position of the superior sagittal sinus and adjacent cortical draining veins with MRV, CTV, or DSA prior to surgery is essential.

Atretic Cephaloceles

APCs are occult or rudimentary cephaloceles that present clinically as midline scalp masses near the posterior vertex (46-5). APCs are congenital skull defects with herniation of rudimentary intracranial structures through the defect. Most are covered by skin and consist of meninges &/or neuronal or glial cells.

Typical finding is a midline osseus defect within the sagittal suture (46-6A). Venous anomalies are common. A recent review identified fenestration (“splitting”) of the superior sagittal sinus in almost 1/2 of all cases (46-7). Other vascular anomalies include a persistent falcine sinus (47%) (46-6B), vertical positioning of the straight sinus (44%), absent straight sinus (40%), and vein of Galen anomalies (27%). Prognosis depends on the coexistent intracranial abnormalities.

Frontoethmoidal Cephaloceles

Frontoethmoidal cephaloceles are the most common type of cephalocele seen in Southeast Asia. Brain tissue, CSF, and leptomeninges herniate into the midface, typically the forehead or dorsum of the nose (46-13).

Frontonasal cephaloceles represent 40-60% of frontoethmoidal cephaloceles. Brain herniates into the forehead between the frontal bones above and the nasal bones below (46-8). In nasoethmoidal cephaloceles (30%), the sac herniates through a midline foramen cecum defect into the prenasal space. The cribriform plate is deficient or absent; the crista galli may be absent or bifid. Naso-orbital and combined nasoethmoidal and naso-orbital are less common location-based subtypes of frontonasal cephaloceles (46-14).

The management of frontoethmoidal cephaloceles is surgical. Careful preoperative, imaging-based planning minimizes the risk of intra- and postoperative complications. NECT scans show a well-demarcated, heterogeneous, mixed-density mass that extends extracranially through a bony defect. MR shows a soft tissue mass in direct contiguity with the intracranial parenchyma.

Skull Base Cephaloceles

Skull base cephaloceles account for 10% of all cephaloceles (46-11). MR of skull base cephaloceles is essential to delineate the sac contents (46-10). The pituitary gland, optic nerves and chiasm, hypothalamus, and third ventricle can all be displaced inferiorly into the cephalocele (46-12). Associated anomalies, such as corpus callosum dysgenesis and an azygous anterior cerebral artery, are common.

Craniosynostoses

Craniosynostosis Overview

Craniosynostosis (from Greek meaning skull + together + bone interlocking and fastening) refers to early fusion of one or several cranial sutures resulting in craniofacial abnormalities.

The craniosynostoses are a heterogeneous group of disorders characterized by abnormal head shape resulting from premature fusion of the skull sutures. Any of the skull sutures can be prematurely fused. Craniosynostosis can be nonsyndromic (70-75% of cases) or syndromic and may affect a single suture or multiple sutures.

The cranial sutures form relatively late (at around 16 weeks of gestation). As long as the brain grows rapidly, the calvarium expands. As brain growth slows, the sutures close.

The normal order of closure is metopic first, followed by the coronal and then the lambdoid sutures. The sagittal suture normally closes last. Craniosynostosis occurs when osseous obliteration of one or more sutures occurs prematurely. Premature closure means bones with persisting open sutures will develop in a compensatory way, usually without limiting the growth of brain volume.

In 85% of cases, craniosynostosis is a sporadic (i.e., nonsyndromic) abnormality with no clear genetic association. In the other 15%, craniosynostosis occurs as part of an identifiable syndrome (i.e., syndromic craniosynostosis). One or multiple sutures can be affected.

True synostosis must be distinguished from positional deformities. In contrast to craniosynostosis (rare), nonsynostotic head shape problems are benign entities caused by external mechanical forces that mold the infant skull (common). The sutures themselves are normal. This is often referred to as “deformational” or “positional” plagiocephaly or positional brachycephaly (plagiocephaly—”skewed”—simply refers to an asymmetry of the head). Here, the altered head shape is due to positioning of the head with respect to its surroundings (in utero, at birth, or postnatally) and is not caused by sutural stenosis. Treatment is nonsurgical.

Nonsyndromic Craniosynostosis

Nonsyndromic craniosynostoses (NCSs) occur in the absence of a recognizable syndrome, although a likely pathogenic germline variant has been reported in nearly 10% of children with NCSs.

Craniosynostosis results in restricted growth around the fused suture. Clinically, this growth pattern leads to abnormal bulging or bossing of bone plates that have patent sutures, causing characteristic dysmorphic craniofacial features.

Approximately 60% of all single-suture craniosynostosis cases involve premature fusion of the sagittal suture followed in frequency by those that involve the coronal (22%) and metopic (15%) sutures. Lambdoid craniosynostosis is very rare, causing just 2% of all cases.

Craniosynostosis is generally classified by head shape. There are four major abnormal head shapes: Scaphocephaly or dolichocephaly (long and narrow, associated with sagittal synostosis), brachycephaly (broad and flattened, bicoronal synostosis), trigonocephaly (triangular at the front, from metopic synostosis) (46-15), or plagiocephaly (“skewed,” from unilateral coronal or lambdoid synostosis). Focal synostosis or diffuse bony ridging or “beaking” along the affected suture are typical findings (46-16). Compensatory spreading of the noninvolved sutures is common (46-17).

CT is required to determine whether part or all of the affected suture(s) is fused. Thin-section CT scans with multiplanar reconstruction and 3D shaded surface display (SSD) are invaluable for detailed evaluation and preoperative planning. MR is helpful to rule out coexisting anomalies, such as hydrocephalus, corpus callosum dysgenesis, and gray matter abnormalities.

Syndromic Craniosynostoses

A minority of craniosynostosis cases are related to craniofacial syndromes, such as Apert or Crouzon syndrome. Syndromic craniosynostoses account for just 25-30% of all cranial stenoses but are much more likely to be associated with additional craniofacial or skeletal anomalies. These include limb abnormalities, dysmorphic facial features, and skull deformity. In addition, brain malformations are common, and developmental delay is more frequent. In contrast to NSCs (in which the sagittal suture is most often affected), bilateral coronal synostosis is the most common pattern in these patients.

Examples of syndromic craniosynostoses include Apert syndrome (a.k.a. acrocephalosyndactyly type 1). Bilateral coronal synostosis is the most common calvarial anomaly (46-17). Hypertelorism, midface hypoplasia, and cervical spine anomalies are common.

Of all the syndromic craniosynostoses, patients with Apert syndrome are most severely affected in terms of intellectual disability, developmental delay, CNS malformations, hearing loss, and limb anomalies. Intracranial anomalies occur in > 1/2 of all cases and include hydrocephalus, callosal dysgenesis, and abnormalities of the septi pellucidi (25-30% each).

Meningeal Anomalies

Preamble

Anomalies of the cranial meninges commonly accompany other congenital malformations, such as Chiari 2 malformation. Lipomas and arachnoid cysts are two important intracranial abnormalities with meningeal origin. Arachnoid cysts were considered in detail in Chapter 32. We therefore conclude our discussion of congenital anomalies by focusing on lipomas.

Lipomas

The 2021 WHO classification of CNS tumors no longer includes mention of intracranial lipomas as potentially neoplastic. Because lipomas do not demonstrate neoplastic behavior, it is more appropriate to consider them on the spectrum of malformative overgrowths that arise during brain development. We include lipomas in this chapter because of their frequent association with other congenital malformations.

Fat—adipose tissue—is not normally found inside the arachnoid. Therefore, any fatty tissue inside the skull or spine is abnormal. Because fat deposits commonly accompany congenital malformations, such as callosal dysgenesis or tethered spinal cord, imaging studies should be closely scrutinized for the presence of additional abnormalities.

Terminology

So-called ordinary lipoma is the most common of all soft tissue tumors and is composed of mature adipose tissue. “Complex lipomatous lesions” may contain other mesenchymal tissues, such as striated muscle, and have sometimes been referred to as choristomas.

Etiology

Lipomas were once thought to be congenital malformations of the embryonic meninx primitiva (the undifferentiated mesenchyme). The primitive meninx normally differentiates into the cranial meninges, invaginating along the choroid fissure of the lateral ventricle. Maldifferentiation and persistence of the meninx was thought to result in deposits of mature adipose tissue, i.e., fat, along the subpial surface of the brain and spinal cord and within the lateral ventricles.

Recent fluorescence in situ hybridization (FISH) and comparative genomic hybridization (CGH) studies have identified clonal cytogenetic aberrations in nearly 60% of ordinary systemic lipomas.

Pathology

Location

Nearly 80% of intracranial lipomas are supratentorial, and most occur in or near the midline. The interhemispheric fissure is the most common overall site (40-50%). Lipomas curve over the dorsal corpus callosum, often extending through the choroidal fissures into the lateral ventricles or choroid plexus (46-20).

Between 15-25% are located in the quadrigeminal region, usually attached to the inferior colliculi or superior vermis (46-18). Approximately 15% are suprasellar, attached to the undersurface of the hypothalamus or infundibular stalk (46-22) (46-23). About 5% of lipomas are found in the sylvian fissure.

Approximately 20% of lipomas are infratentorial. The cerebellopontine angle cistern is the most common posterior fossa site (10%).

Lipomas are generally solitary lesions that vary from tiny, barely perceptible fatty collections to huge, bulky masses. Most are < 5 cm in diameter.

Gross Pathology

Lipomas appear as bright yellow, lobulated soft masses (46-18). They usually adhere to the pia and underlying parenchyma. At least 1/3 encase adjacent vessels &/or cranial nerves. Lipomas are composed of mature, nonneoplastic-appearing adipose tissue with relatively uniform fat cells.

Clinical Issues

Lipomas are relatively rare, accounting for < 0.5% of intracranial masses. They can be found in patients of all ages.

Lipomas are rarely symptomatic and are usually incidental findings on imaging studies. Headache, seizure, hypothalamic disturbances, and cranial nerve deficits have been reported in a few cases. Lipomas are benign lesions. They may grow during periods of significant body growth early in life or with increases in overall body fat. Some may expand with corticosteroid use.

Lipomas encase vessels and nerves, so they are generally considered “leave me alone” lesions (46-26). Surgery has high associated morbidity and mortality.

Imaging

General Features

Lipomas are seen as well-delineated, somewhat lobulated extraaxial masses that exhibit fat density/signal intensity.

Two morphologic configurations of interhemispheric fissure lipomas are recognized on imaging studies: A curvilinear type (a thin, pencil-like mass that curves around the corpus callosum body and splenium) (46-25A)and a tubulonodular type (a large, bulky interhemispheric fatty mass) (46-21). Dystrophic calcification occurs in both types but is more common in tubulonodular lesions.

CT Findings

NECT scans show a hypodense mass that measures -50 to -100 HU (46-24A). Calcification varies from extensive—nearly 2/3 of bulky tubulonodular interhemispheric lipomas are partially calcified (46-28A)—to none, generally seen in small lesions in other locations. Lipomas do not enhance on CECT scans.

MR Findings

Lipomas follow fat signal on all imaging sequences. They appear homogeneously hyperintense on T1WI (46-19)and become hypointense with fat suppression (46-24D). Lipomas exhibit chemical-shift artifact in the frequency-encoding direction.

Signal on T2WI varies. Fat becomes hypointense on standard T2WI but remains moderately hyperintense on fast spin-echo studies because of J-coupling (46-24B). Fat is hypointense on STIR and appears hyperintense on FLAIR (46-24C). No enhancement is seen following contrast administration.

On SWI, lipomas show hyperintensity surrounded by a low signal intensity band along the fat-water interface that is more prominent than seen on T2* GRE sequences (46-24C).

Other CNS malformations are common. The most frequent are corpus callosum anomalies. These range from mild dysgenesis (usually with curvilinear lipomas) to agenesis (with bulky tubulonodular lipomas) (46-27) (46-28).

Differential Diagnosis

Although fat does not appear inside the normal CNS, it can be found within the dura and cavernous sinus. Metaplastic falx ossification is a normal variant that can resemble an interhemispheric lipoma. Dense cortical bone surrounding T1-hyperintense, fatty marrow is the typical finding.

The major differential diagnosis of intracranial lipoma is unruptured dermoid cyst. Dermoids generally measure 20-40 HU, often calcify, and demonstrate more heterogeneous signal intensity on MR.

INTRACRANIAL LIPOMAS

Clinical Issues

• < 0.5% of intracranial masses

• Usually found incidentally; “leave me alone” lesions

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