Chapter 04

Getting a diagnosis

Which test finds it, which test misses it, and why it takes years.

Two tests matter, and they find different things.

• Chromosomal microarray (CMA) detects the typical 17p11.2 deletion — around 90% of cases.
• Gene sequencing is needed for the remaining ~10%, where RAI1 carries a point variant and the chromosome looks normal. A negative CMA does not rule SMS out.

That distinction is a practical failure point: the microarray can come back normal, SMS can be considered excluded, and sequencing may never be ordered.

Why it takes so long

SMS features can overlap with autism, ADHD, or non-specific intellectual disability. Each of those can be a plausible reading of the same child, and each can stop the search too early if the genetic question is not revisited. Because the condition is usually de novo there may be no family history to prompt genetic testing, and because it is rare it may be unfamiliar outside genetics and developmental clinics.