Typically, apparently normal or near-normal early development is followed by partial or complete loss of acquired hand skills and of speech, together with deceleration in head growth, usually with an onset between seven and 24 months of age. Loss of purposive hand movements, hand wringing stereotypies and hyperventilation are particularly characteristic. Social and play development are arrested in the first two or three years but social interest tends to be maintained. During middle childhood, trunk ataxia and apraxia, associated with scoliosis or kyphoscoliosis tend to develop and sometimes there are choreoathetoid movements. Severe mental handicap invariably results.
Fits frequently develop during early or middle childhood.