Infants are usually born with a normal head circumference but fall behind (2 SD or more) in the first year.  They have neonatal and infantile central hypotonia with brisk peripheral tendon reflexes during early childhood.  They are sometimes described as drowsy or lethargic.  Facial and limb edema can be extensive but transient sometimes and can disappear later in childhood.  The fingers are tapered.  The cheeks are full, the mouth is usually open and the upper lip appears ‘tented’.  Global developmental delay is common and normal milestones are seldom attained.  Some patients have been described as severely retarded mentally.  Infantile spasms and myoclonic jerkingcan be seen within the first months of life while frank seizures with hypsarrhythmia are common in the first year of life.  Status epilepticus is a common occurrence.  General drowsiness and poor feeding are often features.  Death usually occurs in infancy or early childhood.  Midface hypoplasia and micrognathia are often present.

Brain imaging (MRI) and histology show severe alterations in myelination and cellular organization.  Neuronal loss is seen in the inner granular layer of the cerebellum but there is relative preservation of Purkinje cells.  General and progressive atrophy of the cerebellum and brain stem have been described.