Chapter 03
Recognizing the pattern
No single sign is decisive. The combination is what points at SMS.
SMS has a recognizable behavioral phenotype, but the individual features are not diagnostic on their own. It is their co-occurrence in one child — especially with severe sleep disturbance and the physical/developmental pattern — that should prompt genetic testing.
| Feature | What the sources support |
|---|---|
| Self-injurious and physically aggressive behavior | >50% |
| Inverted melatonin rhythm | over 90% of studied profiles |
| Nail-yanking, skin-picking, and foreign-object insertion | 25%-90%, depending on age and group studied |
| Self-hugging / upper-body squeeze | Highly associated with SMS; no percentage is given by GeneReviews |
Alongside these: developmental delay, speech and language impairment, infantile hypotonia, sensory processing differences, and — appearing more clearly with age — a broad square face, midface retrusion, short broad hands, hearing problems, and dental anomalies.
The self-hugging is worth knowing by name. GeneReviews describes it as a spasmodic upper-body squeeze, often when the child is pleased, and as highly associated with SMS.
This is educational, not a diagnostic tool. A matching pattern is a reason to ask a clinician about genetic testing — it is not a diagnosis, and nothing on this page can assess an individual child.