Most children with SMS have developmental delay and moderate-to-severe learning difficulties. In infancy low-muscle tone, feeding difficulties, failure to thrive and frequent ear infections are common. Speech delay tends to be more pronounced than motor delay, and language comprehension is more impaired than expression.
The most distinctive features of SMS are the behavioural problems. These include self-injurious behaviours such as hand biting, self-pinching or scratching, and picking at sores. Other behavioural problems include aggression, frequent temper tantrums, hyperactivity, restlessness and distractibility, and severe sleep disturbance. In many cases, the severe behaviour difficulties in children with the syndrome persist into adulthood.
Autistic-type behaviours (see entry Autism Spectrum disorders, including Asperger syndrome), such as resistance to change, repetitive questioning and preoccupations with particular topics are also common.
In contrast, children with SMS are often described as loving and caring, eager to please and with a good sense of humour. They usually enjoy interacting with adults.
Facial features of SMS include a flat, broad head and prominent forehead, heavy brows, up-slanting eyes, depressed nasal bridge, and a wide mouth with an inverted central portion of the upper lip. Other features include a hoarse, deep voice, short stature, eye problems (squint and abnormalities of the iris), hearing loss and scoliosis. Congenital heart disease (see entry Heart Defects), epilepsy and kidney abnormalities are less consistent features.
Clinical signs of peripheral neuropathy are found in 75 per cent of individuals, and include decreased sensitivity to pain and temperature, gait disturbances (leading to walking abnormalities) and muscle weakness.