Classification systems for malformations of the cerebellum are varied and are constantly being revised as greater understanding of the underlying genetics and embryology of the disorders is uncovered. A classification proposed by Patel S in 2002 divides cerebellar malformations in two broad groups; those with cerebellar hypoplasia and; those with cerebellar dysplasia.

- I. Cerebellar hypoplasia

- A. Focal hypoplasia

- 1. Isolated vermis

- 2. One hemisphere hypoplasia

- B. Generalized hypoplasia

- 1. With enlarged fourth ventricle (“cyst,”), Dandy-Walker continuum

- 2. Normal fourth ventricle (no “cyst”)

- a. With normal pons

- b. With small pons i. Normal foliation

- a) Pontocerebellar hypoplasias of Barth, types I and II

- b) Cerebellar hypoplasias, not otherwise specified

Non-progressive early onset ataxia and poor motor learning are the commonest presentation.

Many of the signs and symptoms of aqueductal stenosis are similar to those of hydrocephalus. These typical symptoms include: headache, nausea and vomiting, cognitive difficulty, sleepiness, seizures, balance and gait disturbances, visual abnormalities, and incontinence.

- Headache may be a result of the raised intracranial pressure from the disrupted flow of CSF, and sometimes this symptom may come on suddenly as a “thunderclap headache”.

- In children, cognitive difficulty and developmental delay have been seen in a range of severities. Mild developmental delay is characterized by motor and neurological development that is no greater than 2 standard deviations below average for the age of the child, and moderate delay is characterized by greater than 2 standard deviations below. A child with severe delay may be unable to use spoken language or control movement or interact with others, and can behave abusively towards themselves.

- A patient's level of consciousness may also deteriorate with time, and this can lead to coma or death.

- The visual abnormalities previously mentioned include “upward gaze palsy”, where a person has difficulty looking up.

- Tremors have also been reported as a symptom, but are not as common as these previously mentioned.

Signs of aqueductal stenosis other than those mentioned in “Causes of stenosis” include detection of an enlarged lateral and third ventricle in conjunction with a smaller fourth ventricle. This variation in ventricle size is indicative of a blockage in the aqueduct because it lies between the third and fourth ventricles. Another sign of stenosis is deformation of the midbrain, which can be severe. This is caused by the pressure gradient formed from a blockage in the aqueduct.

Aqueductal stenosis is a narrowing of the aqueduct of Sylvius which blocks the flow of cerebrospinal fluid (CSF) in the ventricular system. Blockage of the aqueduct can lead to hydrocephalus, specifically as a common cause of congenital and/or obstructive hydrocephalus.

The aqueduct of Sylvius is the channel which connects the third ventricle to the fourth ventricle and is the narrowest part of the CSF pathway with a mean cross-sectional area of 0.5 mm in children and 0.8 mm in adults. Because of its small size, the aqueduct is the most likely place for a blockage of CSF in the ventricular system. This blockage causes ventricle volume to increase because the CSF cannot flow out of the ventricles and cannot be effectively absorbed by the surrounding tissue of the ventricles. Increased volume of the ventricles will result in higher pressure within the ventricles, and cause higher pressure in the cortex from it being pushed into the skull. A person may have aqueductal stenosis for years without any symptoms, and a head trauma, hemorrhage, or infection could suddenly invoke those symptoms and worsen the blockage.