Hindbrain Malformations (eg, Chiari Malformation)

The major hindbrain malformations

Chiari malformation

A syndrome of cerebellar herniation through the foramen magnum, which subdivides into three types: 1 through 3, from most mild to most severe.

Dandy Walker malformation

There is gross enlargement of the fourth ventricle and numerous associated posterior fossa anomalies.

Typical radiographic imaging of cerebellar herniation in chiari malformation.

An axial radiograph of the cervico-medullary junction.

• The cerebellar tonsils squash the surrounding CSF space. In a normal radiograph, CSF (the white space) flows freely around the medulla.

Sagittal, mid-sagittal view

• Chiari malformation involves herniation of the cerebellar tonsils below the foramen magnum (we consider a dip of up to 3 mm to be normal with a dip of > 5 mm being undoubtedly abnormal).

Chiari malformation

Type 1 Chiari Malformation

• Chiari malformation always involves downward displacement of the cerebellar tonsils through the foramen magnum but what determines its morbidity is the degree of cerebellar displacement, the degree of displacement of additional brainstem structures, and the associated pathologic involvement of other areas of the central nervous system.
• There is typically a normal ventricular system: 3rd ventricle and cerebral aqueduct, and 4th ventricle.
• There is an associated syringomyelia (a central cavitation of the spinal cord).

Syringomyelia (Cervical)

• The spinothalamic tract fibers cross at the ventral commissure.
• A fluid-filled cavity (a syrinx) expands outward from the central canal and disrupts the crossing fibers and ascending spinothalamic tracts.
• The clinical exam findings of a cervical syringomyelia include a suspended-sensory level wherein small fiber sensation is lost in a cape-like distribution in the arms and upper trunk but preserved in the legs, upper neck, and face.
• The location of the syrinx in the at the cervical level affects the cervical crossing fibers; hence the cape-like distribution.
• This upper limb distribution of deficit also occurs because the inner (arm) spinothalamic fibers lie more central than the outer white matter leg fibers (and, thus, are affected first).

Although the literature suggests that the association between syringomyelia and Chiari Type 1 malformation is quite common, frequent MRI imaging has taught us about the gross underestimation of the prevalence of more mild, asymptomatic Chiari Type 1 malformation cases.

Type 2 Chiari Malformation (aka Arnold-Chiari malformation)

• There is cerebellar tonsillar herniation and kinking below the foramen magnum.
• This kinking can block the output of CSF, which can cause hydrocephalus, which manifests with a dilated 3rd ventricle, cerebral aqueductal stenosis (narrowing) or atresia, and downward displacement of the 4th ventricle.
• The effect is to cause callosal dysgenesis (thinning) and enlargement of the diencephalon with often an absent septum pellucidum.
• Type 2 malformation is most often associated with a lumbar meningomyelocele (a protrusion of the spinal cord and meninges through a defect in the posterior vertebral column).

We can imagine that this tethering of the lumbar cord could serve as the nidus of the downward displacement of the posterior fossa structures by the tethering effect of the meningomyelocele but however logical this theory, it hasn't been substantiated in the scientific literature.

Type 3 Chiari Malformation

The severest, rarest form.

• There is a high cervical and occipital encephalocele, which involves:
  -Occipital lobe herniation
  -Cerebellar herniation
  -Spinal cord herniation
  -Cystic dilatation of the 4th ventricle
  -And squashing of the corpus callosum and diencephalon from the caudal dragging of the occipital and cerebellar herniations.

*Dandy-Walker malformation*

Cerebellar agenesis and a grossly dilated 4th ventricle.

• Hydrocephalic 3rd ventricle
• Enlarged diencephalon
• Dysgenic corpus callosum (much like in the Chiari Type 2).
• Cerebellar agenesis and a grossly dilated 4th ventricle.

• Dandy Walker malformation involves cystic dilatation of the fourth ventricle, agenesis of the cerebellar vermis, and hydrocephalus with thinning or absence of the corpus callosum may be absent.
• Dandy Walker malformation may cause impaired motor development, macrocephaly (enlargement of the skull), and indicate that it can be associated with other syndromes such as Walker Warburg syndrome, which combines a Type 2 Lissencephaly with Dandy Walker malformation, or other syndromes involving cardiac, neural, and limb developmental anomalies.