Posterior Fossa Malformations

Main Text

Preamble

Neural structures in the midbrain and posterior fossa are derived from three sources: (1) The embryonic mesencephalon gives rise to midbrain structures. (2) The embryonic hindbrain (rhombencephalon) gives rise to the posterior fossa structures. (3) Mesodermal elements give rise to the meninges and bone that surround and protect these neural structures. Developmental errors can give rise to a spectrum of midbrain and hindbrain malformations.

Chiari Malformations

Introduction to Chiari Malformations

Chiari 1 and 2 are pathogenetically distinct disorders. Chiari 1is inferior dislocation of the cerebellar tonsils (41-1) in a typically otherwise normal patient. Chiari 2is herniation of the medulla and vermis and is caused by an associated myelodysplasia (spinal cord anomalies) (41-8), usually an open neural tube defect. Chiari 3is characterized by herniation of posterior fossa contents through a low occipitocervical bony defect.

Chiari 1

Terminology

Chiari 1 malformation (CM1) is defined as caudal cerebellar tonsillar ectopia. The precise distance of the tonsils below the foramen magnum (FM) required to diagnose CM1 is not agreed upon. Some investigators consider tonsillar ectopia measuring ≥ 5 mm as sufficient to establish the diagnosis of CM1. However, most insist additional abnormalities, such as tonsillar deformity (“pointing”), obliterated CSF spaces at the FM, or altered CSF flow dynamics, should also be present.

Etiology

Abnormal Posterior Fossa

The primary problem in CM1 is a size mismatch between the bony posterior fossa and the cerebellum. Many patients with CM1 demonstrate abnormal geometry of the bony posterior fossa (“normal-sized hindbrain housed in a too-small bony envelope“). Various combinations of congenitally reduced clival length, shortened basiocciput, and craniovertebral junction (CVJ) fusion anomalies may all result in diminished posterior cranial fossa depth &/or an abnormally small posterior fossa volume.

Altered CSF Dynamics

Descent of the cerebellar tonsils below the FM into the upper cervical spine cases leads to crowding of structures and effacement of CSF spaces. This effacement of CSF spaces leads to reduced CSF flow between the intracranial and spinal compartments, resulting in poor intracranial pressure control during the cardiac cycle. Syringomyelia is present in 40-80% of individuals with symptomatic CM1 who undergo surgical treatment. The exact mechanism for development of syrinx in CM1 is not well understood, but it is likely related to altered CSF flow dynamics at the craniocervical junction and upper cervical spine.

Pathology

Grossly, the herniated tonsils in CM1 are inferiorly displaced and grooved by impaction against the opisthion (41-3A). In severe cases, they may appear firm and sclerotic. Arachnoid thickening and adhesions around the CVJ are common.

Clinical Issues

Epidemiology and Demographics

CM1 is the most common of the Chiari malformations and can be identified in patients of all ages. When using tonsillar ectopia of > 5 mm as the imaging definition of CM1, the prevalence is quite high, somewhere between 0.24-3.6% of the population.

Presentation

While the incidence of an imaging diagnosis of CM1 is quite high, the incidence of symptomatic CM1 is relatively low (~ 0.01% of the population).

Clinical symptoms arise from 1) altered CSF flow through the FM, 2) direct mass effect on the brainstem, or 3) development of a syrinx. The most common symptom in the pediatric age group is an occipitocervical headache (27-70%). However, presentation of symptomatic CM1 differs with age. Children who are two years and younger most commonly present with oropharyngeal dysfunction. Older children more often present with headache and, less frequently, with sleep apnea and ataxia. Children with syrinx may present with scoliosis, motor/sensory deficits, spasticity, hyperreflexia, and poor coordination.

Natural History

The vast majority of patients with tonsillar ectopia > 5 mm below the plane of the FM remain asymptomatic. The imaging findings of CM1 may worsen or improve over time, especially in very young children who are undergoing significant head growth.

Treatment Options

Asymptomatic CM1 in the absence of an associated syrinx or scoliosis is usually not treated. Surgical treatment of symptomatic CM1 with suboccipital decompression attempts to restore normal CSF fluid dynamics at the FM. Depending on the severity of the tonsillar herniation, a simple bony decompression may be sufficient, whereas a duroplasty and tonsillar reduction may be necessary in more severe cases. When patient symptoms are clearly arising from the CM1, surgery usually results in improved or resolved symptoms. There is usually a decrease or resolution of associated spinal cord syrinx following surgery, although this is not universal.

Imaging

General Features

The basion-opisthion line (BOL) is a line drawn from the tip of the clivus to the posterior rim of the FM (41-1). Measuring the distance from this line to the inferior margin of the cerebellar tonsils on sagittal MR defines tonsillar position.

Midline tonsillar descent 5 mm or more below the BOL—often considered diagnostic of CM1—is, by itself, a poor criterion for definitive diagnosis. Tonsils 6 mm below the FM are common during the first decade of life. Almost 15% of normal patients have tonsils that lie 1-4 mm below the FM, and 0.5-1.0% have tonsils that project 5 mm into the upper cervical canal.

Great caution should be exercised in establishing a diagnosis of CM1, especially on the basis of borderline tonsillar ectopia alone. Unless (1) the tonsils appear compressed and pointed (peg-like) instead of gently rounded (41-2), (2) the tonsillar folia are angled obliquely or inferiorly (instead of horizontally), and (3) the retrocerebellar CSF spaces at the FM/C1 level are effaced (41-3B), the diagnosis may not be warranted. Low-lying tonsils that retain their rounded shapes and are surrounded by normal-appearing CSF spaces are usually asymptomatic and of no diagnostic significance.

CT Findings

NECT scans may reveal a “crowded” FM and effaced retrotonsillar CSF space. However, beware of streak artifact at the skull base mimicking tonsillar ectopia. Bone CT often demonstrates a combination of undersized, shallow posterior cranial fossa, short clivus, and CVJ assimilation anomalies.

MR Findings

Sagittal T1 and T2 scans show “pointed” tonsils with more vertically oriented folia, obliterated FM subarachnoid spaces, and a “crowded” FM (41-4) (41-5). The fourth ventricle usually appears normal. In severe cases, the dorsal cervicomedullary junction may be “kinked” or may demonstrate a dorsal “bump,” which is typically the result of longstanding pistoning of the tonsils causing remodeling of the dorsal cervicomedullary junction (41-6).

Sagittal phase-contrast CSF flow studies show diminished or absent alternating bright (systolic) and dark (diastolic) signals behind the cervicomedullary junction. Any change in signal intensity of the cerebellar tonsils on phase contrast sequences suggests tonsillar pulsations (41-7), sometimes referred to as “pistoning”. Cine MR may directly show this abnormal tonsillar motion/pistoning.

The proximal cervical spinal cord should be carefully examined for the presence of a syrinx. T2/FLAIR parenchymal hyperintensity without frank cyst formation may indicate a “presyrinx” state.

Differential Diagnosis

Congenital tonsillar descent (CM1) must be distinguished from normal variants (mild uncomplicated tonsillar ectopia). The most important pathologic differential diagnosis is acquired tonsillar herniation caused by increased intracranial pressure or intracranial hypotension.

Increased intracranial pressure due to supratentorial mass effect with transmission of the pressure cone through the tentorial incisura can be easily distinguished from CM1. Signs of descending transtentorial herniation are present along with downward midbrain displacement. Tonsillar herniation in such cases is a secondary effect and should not be termed “acquired Chiari 1.”

Intracranial hypotension shows a constellation of other findings besides inferiorly displaced tonsils. “Slumping” midbrain, enlarged pituitary gland, draping of the optic chiasm and hypothalamus over the dorsum sellae, engorged venous sinuses, and dura-arachnoid thickening and enhancement are typical abnormalities. Mistaking intracranial hypotension for CM1 can have disastrous consequences due to the very different treatments and management strategies for each.

Approximately 20% of patients with idiopathic intracranial hypertension exhibit cerebellar tonsillar ectopia ≥ 5 mm. 50% have a peg-like tonsil configuration, and many have a low-lying obex. Looking for other signs of idiopathic intracranial hypertension (e.g., optic nerve head protrusion into the globe) is essential to avoid misdiagnosis as CM1.

CHIARI 1 IMAGING

General Features

Differential Diagnosis

Chiari 2

Terminology and Definition

Chiari 2 malformation (CM2) is a complex hindbrain malformation that is caused by an associated myelodysplasia (neural tube defect), almost always an open neural tube defect.

Etiology

General Concepts

CM2 is postulated to arise due to fetal brain development in the setting of a CSF leak from the open neural tube defect and resultant intracranial hypotension. The CSF leak leads to collapse of the developing ventricular system and caudal displacement of the posterior fossa structures, resulting in a small posterior fossa and hindbrain structures. The etiology of other associated anomalies, such as callosal dysplasia and falcine deficiency, are not as well understood but are also likely secondary to the neural tube defect.

Genetics

Nearly 1/2 of all neural tube closure anomalies have mutations on the methylene-tetra-hydrofolate reductase gene (MTHFR). Maternal folate deficiency and teratogens, such as anticonvulsants, have been linked to increased risk of CM2.

Pathology

Grossly, a broad spectrum of findings can be present in CM2. Myelomeningocele and a small posterior fossa with concave clivus and petrous pyramids are virtually always present (41-8). The cerebellar vermis (typically the nodulus) is displaced inferiorly along the dorsal aspect of the cervical spinal cord. The fourth ventricle, pons, and medulla are elongated and partially dislocated into the cervical spinal canal. The lower medulla may be kinked.

Unlike CM1, supratentorial abnormalities are the rule in CM2, not the exception. Hydrocephalus is present in the majority of cases. Corpus callosum dysgenesis and gray matter anomalies, such as subependymal gray matter heterotopia, are common.

Clinical Issues

Epidemiology and Demographics

The overall prevalence of CM2 is 0.44 in 1,000 live births but has been decreasing with prophylactic maternal folate therapy. A dose of 4 mg per day reduces the risk of CM2 by at least 70%.

Presentation

CM2 is identified in utero with ultrasound or fetal screening for elevated α-fetoprotein. At birth, coexistent myelomeningocele and hydrocephalus are dominant clinical features in > 90% of cases. Lower cranial nerve deficits, apneic spells, and bulbar signs may be present. Lower extremity paralysis, sphincter dysfunction, and spasticity often develop later.

Treatment Options

Fetal repair of myelomeningocele is increasingly common and may reduce subsequent symptoms. Surgical repair within 72 hours following delivery reduces mortality and morbidity from the open dysraphism.

Imaging

CM2 affects many regions of the skull, brain, and spine, so a variety of imaging abnormalities may be seen.

Skull and Dura

The calvarial vault forms from membranous bone. With failure of neural tube closure and absence of fetal brain distention, normal induction of the calvarial membranous plates does not occur. Disorganized collections of collagen fibers and deficient radial growth of the developing calvarium ensue, resulting in lacunar skull (i.e., Lückenschädel) (41-10).

Focal calvarial thinning and a scooped-out appearance are typical imaging findings of lacunar skull. The calvarium appears thinned with numerous circular or oval lucent defects and shallow depressions. The craniofenestra diminish with age and typically resolve by six months, although some scalloping of the inner table often persists into adulthood.

A small, shallow, bony posterior fossa with low-lying transverse sinuses is almost always present in CM2. A large, “gaping” FM is common. Concave petrous temporal bones and a short concave clivus are often present (41-12).

Dural abnormalities are common. A widened, open, heart-shaped tentorial incisura and thinned, hypoplastic, or fenestrated falx are frequent findings. The fenestrated falx allows gyri to cross the midline. Interdigitating gyri and the deficient falx result in the appearance of an irregular interhemispheric fissure on imaging studies (41-11C)(41-15).

Midbrain, Hindbrain, and Cerebellum

Hindbrain and cerebellum anomalies are a constant in CM2. The medulla and inferior cerebellum are displaced downward into the upper cervical canal for a variable distance. A cervicomedullary “kink”with a medullary “spur”is common in the upper cervical canal but may lie as low as T1-T4 in severe cases (41-9).

On sagittal T1 and T2 scans, the inferiorly displaced vermis, medulla, and choroid plexus form a “cascade” of tissue that protrudes downward through the gaping FM to lie behind the spinal cord. The superiorly herniated cerebellum may compress and deform the quadrigeminal plate, giving the appearance of a beaked tectum (41-13).

In addition to the cephalocaudal displacement of posterior fossa contents, the cerebellar hemispheres often curve anteromedially around the brainstem. In severe cases, the pons and medulla appear nearly engulfed by the “creeping” cerebellum on axial imaging studies.

The cerebellar hemispheres are pushed upward through the incisura, giving the appearance of a towering cerebellum on coronal T1 and T2 scans.

Ventricles

Abnormalities of the ventricles are present in > 90% of CM2 patients. The fourth ventricle is caudally displaced, typically lacks a fastigium (dorsal point), and appears thin and elongated (“soda straw” fourth ventricle) (41-13) (41-14). The third ventricle is often large and has a very prominent massa intermedia (41-13).

The lateral ventricles vary in size and configuration. Hydrocephalus is almost always present at birth, but sometimes develops later, and usually requires shunting. The atria and occipital horns are often disproportionately enlarged (“colpocephaly”), suggesting the presence of callosal and forceps major dysgenesis.

Following shunting, the lateral ventricles frequently retain a serrated or scalloped appearance. A large CSF space between the occipital lobes often persists.

Cerebral Hemispheres

Malformations of cortical development, such as subependymal gray matter heterotopia, are common, especially in cases with hydrocephalus. In those patients with severe hydrocephalus treated with shunting, there is development of numerous and narrow gyri (“stenogyria”) (41-15).

Callosal dysgenesis is found in nearly 2/3 of all cases, and abnormalities of the fornices are also common.

Spine and Spinal Cord

Open spinal dysraphism with myelomeningocele is present in almost all cases of CM2. Hydrosyringomyelia is seen in ~ 50%.

Differential Diagnosis

The major differential diagnosis of CM2 is other Chiari malformations.

In Chiari 1, myelomeningocele is absent, and, other than being somewhat small, the posterior fossa and its contents appear relatively normal.

If findings of CM2 + a low occipital or high cervical cephalocele are present, the diagnosis is Chiari 3.

CHIARI 2 MALFORMATION

Pathoetiology

Clinical Issues

Imaging Findings

Chiari 3

Chiari 3 malformation (CM3) is the rarest of the Chiari malformations. CM3 consists of a small posterior fossa with a caudally displaced brainstem and variable herniation of meninges/posterior fossa contents through a low occipital or upper cervical bony defect.

The cephalocele contains meninges together with variable amounts of brain tissue, vessels, and CSF spaces. The brain is often featureless, dysplastic-appearing, and disorganized with extensive gliosis and gray matter heterotopias.

NECT scans show bony features similar to those seen in CM2, i.e., a small posterior cranial fossa, short, scalloped clivus, lacunar skull, a defect in the ventral chondral portion of the supraoccipital bone, and low cranium bifidum that may extend inferiorly to involve much of the cervical spine (41-17).

MR best delineates sac contents, which often include dysplastic-appearing cerebellum &/or brainstem, as well as distorted CSF spaces and vessels. A deformed fourth and, sometimes, third ventricle can be partially found within the mass of herniated brain and meninges. Vascular structures, both venous and arterial, are sometimes “pulled” into the defect (41-16).

Hindbrain Malformations

Cystic Posterior Fossa Anomalies

Terminology

Cystic posterior fossa anomalies include a spectrum or “continuum” of cystic abnormalities, including Classic Dandy-Walker malformation (DWM), vermian hypoplasia (VH), Blake pouch cyst (BPC), and mega cisterna magna (MCM). Arachnoid cysts are also considered in the posterior fossa cystic malformation differential diagnosis. Due to overlapping features and inconsistency in definitions, the Dandy-Walker continuum terminology has been a source of confusion for many radiologists. In past iterations, there has been an emphasis on the cystic portion of the malformation and less emphasis on the anatomic changes in the cerebellum, the vermis in particular. A most recent attempt to refine the “Dandy-Walker phenotype” by Whitehead et al has resulted in five distinct diagnoses: DWM, inferior VH (IVH), VH, BPC, and VH + BPC. This newest refinement of neuroimaging criteria is based upon five anatomic features of the posterior fossa: 1) Vermis, 2) tegmentovermian angle (TVA), 3) fastigial recess, 4) tail sign, and 5) taenia/tela choroidea complex/choroid plexus location (TCC/CPL) (Table 41-1). The main goal of the neuroradiologist should be to accurately describe the posterior fossa anatomy and identify associated supratentorial abnormalities (e.g., hydrocephalus).

A key component of this system is assessing the vermis and whether it is hypoplastic based upon the major vermian landmarks (i.e., primary fissure).

The traditional definition of the classic DWM has focused on cystic enlargement of the posterior fossa with elevation of the torcular Herophili. Whitehead et al have suggested that posterior fossa size and torcular location should no longer be considered diagnostic criteria for this diagnosis. Rather, the diagnosis of DWM is defined as having IVH + increased TVA + obtuse fastigial recess ± tail sign ± down and out location of the TCC/CPL.

VH (old term = “Dandy-Walker variant”) is divided into 1) IVH and 2) VH. The major anatomic landmark of the vermis in the sagittal view is the primary fissure. Approximately 1/3 of the vermis should be above the primary fissure and ~ 2/3 of the vermis should be below the primary fissure. The diagnosis of IVH is made when < 1/2 the vermian tissue is below the primary fissure (41-21). In contrast, VH is defined as a diffusely small vermis with normal proportions of vermian tissue above and below the primary fissure (41-22).

BPC is defined as a normal vermis + increased TVA + normal fastigial recess (41-23). A common pitfall is the misdiagnosis of IVH whenever the TVA is increased. If the vermis is normal, but simply rotated up and away, the diagnosis is BPC.

Etiology

Etiology and Genetics

Three main genes have been associated with DWM: FOXC1 and the linked ZIC1 and ZIC4 genes.

DWM is accompanied by > 18 types of chromosomal abnormality and cooccurs with > 40 genetic syndromes. In addition, DWM can arise as a result of maternal diabetes or fetal infection (e.g., cytomegalovirus or Zika virus).

Pathology

Gross Pathology

The most common findings in DWM are (1) an enlarged posterior fossa with (2) upward displacement of the tentorium and accompanying venous sinuses and (3) cystic dilatation of the fourth ventricle. Vermian abnormalities range from complete absence to varying degrees of hypoplasia. The posterior fossa cyst in DWM is typically lined by an outer layer of pia-arachnoid and an inner layer of ependyma.

DWM is frequently associated with other CNS anomalies. Almost 2/3 of patients have gyral abnormalities (e.g., pachy- or polymicrogyria and heterotopic gray matter). Callosal dysgenesis and hydrocephalus are common.

Clinical Issues

DWM occurs in ~ 1:30,000. The most common presentation is increased intracranial pressure secondary to hydrocephalus.

Imaging

The spectrum of imaging abnormalities in classic DWM is broad, affecting—to varying degrees—the skull and dura, ventricles and CSF spaces, and brain.

Skull and Dura, Venous Sinuses

In contrast to CM2 in which the posterior fossa is abnormally small, the posterior fossa in DWM is usually enlarged, sometimes dramatically so. The straight sinus, sinus confluence, and tentorial apex are usually elevated, sometimes above the lambdoid suture (“lambdoid-torcular inversion”). The transverse sinuses descend at a steep angle from the torcular Herophili toward the sigmoid sinuses (41-18). While posterior fossa size and elevation of the torcular Herophili are not considered diagnostic criteria in the most recent Whitehead et al classification system, they are seen in the vast majority of DWM.

The occipital bone may appear scalloped, focally thinned, and remodeled. Retrocerebellar CSF cysts often demonstrate partially infolded dura-arachnoid (falx cerebelli) on axial T2 scans. The falx cerebelli is often absent in DWM.

Ventricles and Cisterns

In DWM and BPC, the fourth ventricle opens dorsally to a variably sized CSF-containing cyst that balloons posteriorly behind and between the cerebellar hemisphere remnants. This causes an increase in the TVA in both DWM and BPC. In DWM, the normal fastigium is absent or significantly attenuated to form an obtuse angle. In all other posterior fossa malformations, the relatively acute angle of the fastigial recess is maintained. Sometimes visible in DWM is a raised and dysplastic fourth ventricle roof, referred to as the tail sign (41-19). In DWM, the choroid plexus is displaces inferiorly and laterally (“down and out”).

Generalized obstructive hydrocephalus is present in > 80% of neonates with DWM at birth. If callosal dysgenesis is present, the lateral ventricles are widely separated and may have unusually prominent occipital horns (colpocephaly).

Axial images in DWM often demonstrate marked enlargement of the fourth ventricular vallecula and lateral displacement of the cerebellar hemispheres in a winged appearance (41-20).

Brainstem, Cerebellum, and Vermis

The brainstem appears normal in mild forms of DWM but often appears somewhat small in moderate to more severe DWM.

Varying degrees of VH are seen in DWM. In DWM, the vermian remnant is rotated and elevated above the large posterior fossa cyst (41-18).

The cerebellar hemispheres are often hypoplastic in DWM, and, in severe cases, the cerebellar remnants appear “winged” outward and displaced anterolaterally.

A normal vermis should have ~ 2/3 of the vermis inferior/posterior to the primary fissure. The diagnosis of IVH is made when < 1/2 the vermian tissue is below the primary fissure. In contrast, VH is defined as a diffusely small vermis with normal proportions of vermian tissue above and below the primary fissure.

Associated Abnormalities

Other CNS abnormalities are present in 70% of DWM. The most common finding is callosal agenesis or dysgenesis. A dorsal interhemispheric cyst may be present. Gray matter abnormalities (e.g., heterotopias, clefts, and pachy- and polymicrogyria) are common associated abnormalities. Hydrocephalus is seen in the majority of DWM cases. Careful attention and description of associated anomalies is important because they often have an equal or greater influence on the patient’s prognosis.

Differential Diagnosis

A retrocerebellar arachnoid cyst is an important differential diagnosis and, when small, can be difficult to distinguish from a MCM. It is an arachnoid-lined cyst located behind the vermis and fourth ventricle that does not communicate with the latter. When large, an arachnoid cyst may cause mass effect on adjacent structures. The cerebellum otherwise appears normal. Veins and a falx cerebelli do not traverse the CSF collection.

An MCM is the mildest cystic posterior fossa anomaly with an enlarged retrocerebellar CSF collection (> 10 mm). There is no mass effect on the cerebellar hemispheres or vermis. The vermis is well formed and normal. Cerebellar veins and elements of the falx cerebelli can be seen crossing through the MCM, and the adjacent bone can appear scalloped by pulsatile CSF in the MCM (41-24).

DANDY-WALKER CONTINUUM: DIFFERENTIAL DIAGNOSIS

Dandy-Walker Malformation

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