There are no external deformities and children at birth can appear normal.  However, within a year developmental delays are evident in activities such as sitting alone.  Later, evidence of intellectual disability becomes evident although this is variable in severity.  Speech may also be delayed and is never normal with even some older children only speaking single words or abbreviated sentences.  Some slurring of words is common.  Walking is never normal and requires some assistance from the beginning.  Contractions of joints can lead to further difficulties in ambulation and by the second decade some individuals are confined to a wheelchair.

Eye movements are often abnormal.  Some individuals have constant to-and-fro movements while others a have rotatory component (nystagmus).  Still others have an intermittent or nearly constant upward gaze while still others have difficulty tracking moving targets, instead using head turns or jerky movements of the eyes.  The optic nerve that connects the eye to the brain is often damaged (optic atrophy) but the level of vision has not been measured.

The intellectual level of function does not seem to worsen.  However, MRI imaging of the brain shows progressive damage in the cerebellum.