large, horizontal superior cerebellar peduncles extending

posteriorly toward the cerebellum and a longitudinal cleft in

the superior vermis. Make sure that the components of the

brainstem are of normal size; in a child, the height of the pons

should be double that of the midbrain on the midline sagittal

image. An important clue can be provided by looking at the

size of the pons compared with that of the cerebellar vermis.

Because much of the anterior pons is composed of the

decussation of the middle cerebellar peduncles, development

hypoplasia of the cerebellum is nearly always associated with

hypoplasia of the ventral pons. If the pons is normal in the

setting of a small cerebellum, it is most likely that the

cerebellum lost volume near the end of gestation or after

birth. Remember that a small posterior fossa, intracranial

hypotension, or intracranial hypertension can result in

descent of the cerebellum below the foramen magnum. Look

for causes of a small posterior fossa (clival anomaly, anomaly

of the craniovertebral junction), intracranial hypertension

(space-occupying mass, hydrocephalus), or evidence of

intracranial hypotension (large dural venous sinuses, large

pituitary gland, "slumping" brainstem) before making a

diagnosis of Chiari 1 malformation.

Brain: Pathology-Based Diagnoses: Malformations,

Trauma, and Stroke

12