Diagnostic Criteria

Findings typically present in affected individuals

Normal prenatal and birth history, normal head circumference at birth, no major birth defects.
Normal metabolic, hematologic, and chemical laboratory profiles.
Structurally normal brain by magnetic resonance imaging (MRI) or computed tomography, although mild cortical atrophy or dysmyelination may be observed.
Delayed attainment of developmental milestones without loss of skills.
Evidence of developmental delay by age 6 to 12 months, eventually classified as severe.
Speech impairment, with minimal to no use of words; receptive language skills and nonverbal communication skills higher than expressive language skills.
Movement or balance disorder, usually ataxia of gait and/or tremulous movement of the limbs.
Behavioural uniqueness, including any combination of frequent laughter/smiling; apparent happy demeanour; excitability, often with hand-flapping movements; hypermotoric behaviour; short attention span.

Delayed or disproportionately slow growth in head circumference, usually resulting in absolute or relative microcephaly by age 2 years.
Seizures, usually starting before age 3 years.
Abnormal electroencephalogram (EEG), with a characteristic pattern of large-amplitude slow-spike waves.

Flat occiput.
Occipital groove.
Protruding tongue.
Tongue thrusting; suck/swallowing disorders.
Feeding problems and/or muscle hypotonia during infancy.
Prognathia.
Wide mouth, wide-spaced teeth.
Frequent drooling.
Excessive chewing/mouthing behaviours.
Strabismus.
Hypopigmented skin, light hair and eye colour (compared with family); seen only in those with the deletion genotype.

Hyperactive lower-extremity deep-tendon reflexes.
Uplifted, flexed arm position, especially during ambulation.
Wide-based gait with out-going (i.e. pronated or valgus-positioned) ankles.
Increased sensitivity to heat.
Abnormal sleep-wake cycles and diminished need for sleep.
Attraction to/fascination with water; fascination with crinkly items such as certain papers and plastics.
Abnormal food-related behaviours.
Obesity (in the older child; more common in those who do not have a deletion).
Scoliosis.
Constipation.