How is 22q13 deletion / Phelan-McDermid Syndrome diagnosed?

TL;DR: Phelan-McDermid syndrome (22q13 deletion) is primarily diagnosed through chromosomal microarray (CMA) testing, which identifies a deletion or mutation involving the SHANK3 gene on chromosome 22. Because clinical symptoms like global developmental delay and hypotonia are broad, a definitive diagnosis relies on genetic confirmation rather than physical examination alone.

How is Phelan-McDermid syndrome diagnosed?

The diagnostic process for Phelan-McDermid syndrome typically begins when a pediatrician or neurologist observes developmental delays, particularly in speech. Because the condition is rare, many families endure a "diagnostic odyssey," often waiting years for an accurate identification. A clinical geneticist is the primary specialist responsible for ordering the definitive test: a chromosomal microarray (CMA). This test is highly effective at detecting the terminal or interstitial deletions of the 22q13.3 region that characterize Phelan-McDermid syndrome. In cases where a microarray is negative but clinical suspicion remains high, whole-exome sequencing (WES) may be used to identify smaller mutations or disruptions of the SHANK3 gene.

What are the key tests and criteria for 22q13 deletion?

There are no specific "physical" diagnostic criteria, as the presentation of Phelan-McDermid syndrome is highly variable. Instead, diagnosis is based on genetic evidence. Physicians look for the following indicators, though these vary significantly between individuals:

• Global developmental delay, specifically characterized by absent or severely delayed speech.


• Neonatal hypotonia (low muscle tone).


• Characteristic physical features, such as large fleshy hands, dysplastic toenails, or a long, narrow face.


• Behavioral traits often associated with autism spectrum disorder.


• Genetic confirmation of a 22q13.3 deletion or a pathogenic SHANK3 variant.

Which conditions can be confused with Phelan-McDermid syndrome?

Because the clinical features of Phelan-McDermid syndrome overlap with many other neurodevelopmental disorders, it is frequently misdiagnosed initially. Differential diagnoses often include non-specific autism spectrum disorder, Angelman syndrome, Prader-Willi syndrome, or other chromosomal microdeletion syndromes. We understand how exhausting it is to navigate these misdiagnoses; the uncertainty is a heavy burden, but the genetic confirmation provided by a microarray is the "gold standard" that ends the search for answers.

Why is seeing a specialist essential for 22q13 deletion?

General practitioners may not be familiar with the nuances of Phelan-McDermid syndrome, which can lead to delayed testing. Geneticists and neurologists who specialize in rare genetic disorders are better equipped to interpret microarray results and distinguish Phelan-McDermid syndrome from other genetic conditions. At DiseaseMaps.org, we have seen 35 community members share their unique journeys, highlighting that specialized care is often the turning point in accessing appropriate therapies and support.

Next steps

• Consult a clinical geneticist to request a chromosomal microarray (CMA) if you suspect a diagnosis.


• Request a referral to a neurologist or developmental pediatrician for a comprehensive evaluation of symptoms.


• Join the DiseaseMaps.org community to connect with other families navigating the same diagnosis.


• Visit the Phelan-McDermid Syndrome Foundation website for specialized resources and clinical trial information.

Medical disclaimer: This content is for educational purposes only and should not replace professional medical advice, diagnosis, or treatment; always seek the advice of your physician with any questions regarding a medical condition.

References

• NIH Genetic and Rare Diseases Information Center (GARD): Phelan-McDermid syndrome.


• Orphanet: 22q13.3 deletion syndrome.


• OMIM (Online Mendelian Inheritance in Man): #606232 Phelan-McDermid Syndrome.


• Phelan-McDermid Syndrome Foundation: Clinical guidance and research updates.