Congenital Abnormalities of the Posterior Fossa: Comprehensive Neuroimaging and Clinical Guide

Fundamental Definitions and Classification of Posterior Fossa Abnormalities

• Conceptual Framework of Anomalies: Congenital posterior fossa anomalies are categorized into two primary groups based on their underlying cause:

  • Malformation: Defined verbatim as a congenital morphologic anomaly of a single organ or body part due to an alteration of the primary developmental program caused by a genetic defect. These may arise from de novo mutations (new in the child) or inherited mutations.

  • Disruption: Defined verbatim as a congenital morphologic anomaly caused by the breakdown of an anatomic structure that had a normal developmental potential. Common disruptive causes include prenatal infection, hemorrhage, and ischemia. These are generally acquired lesions without a risk of recurrence, though genetic predispositions to disruptions (e.g., $COL4A1$ mutations) may exist.

• Importance of Precise Diagnosis: Accurate identification is critical for three reasons:

  1. Determining the inheritance pattern and risk of recurrence (crucial for family planning).

  2. Identifying involvement of other systems, such as the kidneys or liver.

  3. Establishing prognostic implications for the child and family.

• Inheritance Patterns of Malformations:

  • Autosomal Recessive: Transmitted from an unaffected mother or father to the child.

  • Autosomal Dominant: Transmitted from an affected mother or father to the child.

  • X-linked: Transmitted from an affected mother to her son.

  • Mitochondrial: Transmitted from an affected mother to her child.

• General Classification Scheme: Based on the neuroimaging-based scheme proposed by Doherty et al., abnormalities are divided into four patterns:

  1. Predominantly cerebellar.

  2. Cerebellar and brainstem.

  3. Predominantly brainstem.

  4. Predominantly midbrain.

Normal Posterior Fossa Anatomy

• Neuroimaging Standards: Conventional MR imaging, specifically midline sagittal $T1$ or $T2$-weighted sequences, is vital for evaluating anatomy.

• Anatomic Landmarking and Ratios:

  • Vermis: Divided into three parts by the primary and prepyramidal fissures. It consists of $10$ lobules grouped into three major lobes: Anterior (lobules $I-V$), Posterior (lobules $VI-IX$), and Flocculonodular (lobule $X$).

  • Brainstem Proportion: The rostrocaudal length of the ventral pons should be approximately $2$ times that of the midbrain (measured from the isthmus to the third ventricle).

  • Midbrain-Medulla Proportion: The midbrain length should be roughly equal to that of the medulla (from the obex to the ventral pontomedullary junction).

  • Brainstem Contour: The posterior margin of the brainstem from the caudal aqueduct to the obex should form a straight line.

  • Fastigium: The summit of the fourth ventricle should lie just below the midpoint of the ventral pons on sagittal images.

  • Cerebellar Folia: These run parallel to the calvaria in an "onion-like" configuration.

• Cerebellar Peduncles: Superior, middle, and inferior peduncles are assessed for size, symmetry, contour, and location. Axial color-coded fractional anisotropic maps allow for identification of:

  • Middle Cerebellar Peduncles: Anteroposterior orientation (green).

  • Corticospinal Tracts: Superoinferior orientation (blue).

  • Medial Lemnisci: Posteriorly situated (arrowheads).

  • Transverse Pontine Fibers: Transverse orientation (red).

Neuroimaging Modalities and Sequences

• Key Modalities: Ultrasonography (US), computed tomography (CT), and magnetic resonance (MR) imaging.

• MR Sequences and Roles:

  • 3D T1W (Axial, Coronal, Sagittal): Excellent differentiation between gray and white matter; high-resolution anatomic information.

  • T2W (Axial, Coronal): Delineation of the cortex, white matter, and gray matter nuclei; detailed evaluation of folia and interfolial spaces.

  • Diffusion Tensor Imaging (DTI): Evaluation of white matter microstructural integrity and identification of tracts.

  • Constructive Interference in the Steady State (CISS): Evaluation of cerebellar folia, cranial nerves, ventricles, and foramina.

  • Susceptibility-Weighted Imaging (SWI): Identification of hemorrhage, blood products, calcification, and iron accumulation; supports disruptive pathomechanisms.

• Prenatal Imaging Constraints: Development of the cerebellar vermis is typically complete at around $18$ weeks gestation. Imaging before $18-20$ weeks may lead to false-positive results.

Predominantly Cerebellar Malformations

• Terminology: A malformed cerebellum is described as hypoplastic (reduced volume), dysplastic (abnormal foliation/architecture), or hypodysplastic (combination).

• Dandy-Walker Malformation (DWM):

  • Prevalence: Most common posterior fossa malformation; typically sporadic (recurrence risk $1\%-5\%$).

  • Genetics: Rare mutations in $ZIC1$, $ZIC4$, $FOXC1$, $FGF17$, $LAMC1$, and $NID1$.

  • Diagnostic Criteria: Requires (a) hypoplasia or agenesis of the cerebellar vermis, which is elevated and upwardly rotated, and (b) cystic-appearing dilatation of the fourth ventricle. The posterior fossa is usually enlarged with an elevated tentorium and torcular.

  • Symptoms: Increased intracranial pressure; macrocephaly affects $90\%-100\%$ of children. Hydrocephalus is present in approximately $90\%$.

• Blake Pouch Cyst (BPC):

  • Pathogenesis: Lack of fenestration of the Blake pouch ($18$ weeks gestation), preventing communication between the fourth ventricle and subarachnoid space.

  • Imaging: Enlarged fourth ventricle communicating with an infravermian cyst; normal vermis; consistent presence of tetraventricular hydrocephalus. Normal posterior fossa size.

• Mega Cisterna Magna (MCM):

  • Characterization: Enlarged cisterna magna ($\ge 10\,mm$ on midsagittal images) with an intact vermis and normal fourth ventricle. No hydrocephalus is present.

• Posterior Fossa Arachnoid Cysts (PFAC):

  • Imaging: Fluid-filled collection isointense to CSF on all sequences; does not communicate with the fourth ventricle. May produce mass effect and occipital bone scalloping.

• Rhombencephalosynapsis:

  • Imaging: Absence of the vermis with fusion/continuity of the cerebellar hemispheres, dentate nuclei, and superior cerebellar peduncles. Creates a "keyhole-shaped" fourth ventricle and horizontal folial pattern.

  • Associations: Key feature of G3mez-L3pez-Hern&ndez syndrome (parietal alopecia, trigeminal anesthesia).

• Macrocerebellum (Cerebellar Hyperplasia):

  • Definition: Disproportionately large cerebellum with preserved architecture. Associated with Costello syndrome, Sotos syndrome, and neurometabolic diseases like fucosidosis.

• Cerebellar Dysplasia:

  • Imaging: Abnormal foliation, abnormal white matter arborization, and obscured gray-white junction. Global findings suggest malformations (e.g., Chudley-McCullough syndrome, caused by $GPSM2$ mutations).

Cerebellar and Brainstem Malformations

• Pontocerebellar Hypoplasia (PCH): A group of autosomal recessive neurodegenerative disorders.

  • PCH Type 2: Caused by $TSEN54$ mutations. Imaging shows a "dragonfly" appearance on coronal views (flattened hemispheres/wings with a preserved vermis/body).

  • PCH Subtypes (Table 4):

    • Type 1: Involvement of anterior horn cells ($VRK1$, $TSEN54$).

    • Type 3: Optic atrophy, nonprogressive course.

    • Type 4: Severe neonatal course; hypertonia; absent inferior olivary prominence.

    • Type 6: Lactic acidemia ($RARS2$).

    • Type 9: Dysgenesis of the corpus callosum ($AMPD2$).

• Joubert Syndrome:

  • Diagnostic Criterion: The "molar tooth sign" ($MTS$), consisting of elongated, thickened, horizontally oriented superior cerebellar peduncles, a deep interpeduncular fossa, and vermian hypoplasia.

  • Pathogenesis: Ciliopathy involving proteins of nonmotile primary cilia ($26$ associated genes).

  • Systemic Involvement: Includes nephronophthisis (renal), colobomas (ocular), and congenital hepatic fibrosis.

• $\alpha$-dystroglycanopathies: Congenital muscular dystrophies (e.g., Walker-Warburg syndrome, Muscle-eye-brain disease). Imaging shows cerebellar cysts, pontine hypoplasia, and cobblestone lissencephaly.

Predominantly Brainstem and Midbrain Malformations

• Pontine Tegmental Cap Dysplasia:

  • Imaging: Flattened ventral pons with a "cap" covering the dorsal pons protruding into the fourth ventricle. DTI shows an absence of transverse pontine fibers and a dorsal ectopic axonal band.

• Horizontal Gaze Palsy with Progressive Scoliosis (HGPPS):

  • Pathogenesis: Mutations in $ROBO3$, required for axonal guidance.

  • Imaging: "Butterfly-shaped" medulla due to missing gracile/cuneate prominence. DTI shows an absence of decussation in the corticospinal tracts and superior cerebellar peduncles.

• Midbrain Malformations: Diencephalic-mesencephalic junction dysplasia, characterized by a butterfly-like contour of the midbrain on axial images.

Cerebellar Disruptions

• Cerebellar Agenesis: Near-complete absence of cerebellar tissue. If associated with diabetes mellitus, it suggests a mutation in $PTF1A$.

• Unilateral Cerebellar Hypoplasia: Range from mild asymmetry to complete aplasia of one hemisphere. Often caused by prenatal hemorrhage. Posterior fossa size remains normal.

• Cerebellar Cleft: A cleft extending into the parenchyma, often from fetal hemorrhage. Unlike DWM, it involves the cerebellar hemisphere while sparing the vermis.

• Vanishing Cerebellum in Myelomeningocele: Secondary to prolonged prenatal CSF leak and hindbrain herniation leading to ischemic damage.

• Cerebellar Injury in Prematurity:

  • Vulnerability Phase: Gestational weeks $24-32$.

  • Incidence: Occurs in up to $20\%$ of preterm infants before $32$ weeks.

  • Imaging Appearance: "Skeletonized" appearance of cerebellar hemispheres, flattened ventral pons, and a balloon-shaped fourth ventricle.

  • Causes: Primary destructive injuries (hemorrhage/ischemia) or secondary lesions (glucocorticoids, undernutrition, or impaired transsynaptic trophic effects).

Congenital posterior fossa anomalies are classified into malformations (arising from genetic defects) and disruptions (due to breakdown of normal structures, often acquired). Accurate diagnosis is crucial for assessing inheritance patterns, involvement of other organs, and prognosis. Malformations can be autosomal recessive, autosomal dominant, X-linked, or mitochondrial. They can be categorized into four neuroimaging patterns: predominantly cerebellar, cerebellar and brainstem, predominantly brainstem, and predominantly midbrain. Normal anatomy evaluation relies on MRI standards, with key landmarks such as the vermis, brainstem proportions, and cerebellar folia. Key imaging modalities include US, CT, and MR, each with specific sequences for detailed assessment, such as T1W, T2W, and DTI. Several malformations, including Dandy-Walker Malformation (most common), Blake Pouch Cyst, and rhombencephalosynapsis, reflect varying degrees of cerebellar tissue development, while disruptions can lead to conditions like cerebellar agenesis and cerebellar injury in prematurity.