Phelan-McDermid syndrome

What Phelan-McDermid syndrome is

Phelan-McDermid syndrome is a rare genetic condition that affects how a child develops, communicates, and moves. Every one of us carries chromosomes, the tightly packed strands that hold our genes. In Phelan-McDermid syndrome, there is a change at the very end of one chromosome, chromosome 22, in a spot doctors label 22q13.3. Most often a small piece of that region is missing, which is why the condition is sometimes called 22q13 deletion syndrome. In some children, the change is not a missing piece but an alteration inside a single gene in that region called SHANK3.

SHANK3 matters because it helps brain cells build and use the tiny connection points where they pass signals to one another. When that gene is missing or altered, those connections do not work in quite the usual way, and that shows up as delays in development, in language, and in coordination. In many families the change happened for the first time in the child rather than being passed down, but only genetic testing and a conversation with a geneticist can tell you what happened in your child's case.

Here is the most important thing to hold onto: a diagnosis names something that was already true about your child. It does not change who they are. It gives you and your care team a shared word, a map of what to watch for, and a doorway into a community of families who understand.

A syndrome is a description, not a destiny. Your child is a whole person first, and the label is simply a tool that helps the right people show up with the right support.

How it can affect a child day to day

Phelan-McDermid syndrome looks different in every child, and the range is wide. Some children are more affected than others, and features can change as a child grows. These are the areas families most often notice, described in general terms rather than as a checklist your child must match.

• Low muscle tone. Many babies with the condition feel floppy or have low tone (hypotonia), which can affect early milestones like holding up the head, sitting, and later walking.
• Developmental delay. Reaching milestones often takes longer, and many children have intellectual disability that ranges from mild to significant.
• Speech and communication. Spoken language is frequently delayed or absent. This does not mean a child has nothing to say. It means they may need other tools to say it.
• Autism and social differences. Many, though not all, children also have autism or autistic traits, which shape how they play, connect, and handle sensory input.
• Everyday health and behavior patterns. Families often mention things like a high pain tolerance, a tendency to run warm or sweat less, chewing on non-food objects, disrupted sleep, and digestive issues. Seizures and kidney differences can also occur in some children.

Alongside the challenges, parents describe real strengths and a great deal of joy: affection, humor, a love of music or water, and a way of connecting that is entirely their own. Both things are true at once, and a good day is still a good day.

Therapies and supports families often explore

There is no medication that treats Phelan-McDermid syndrome itself. Instead, care is about supporting your child's development and health across time, usually with a team of specialists who each help with a different piece. Families commonly explore some mix of the following.

• Physical therapy (PT) to build strength, balance, and motor skills, and to support goals like sitting, standing, and walking.
• Occupational therapy (OT) to work on everyday skills, fine motor control, sensory needs, and growing independence.
• Speech-language therapy for both communication and, when needed, feeding and swallowing. This is often where augmentative and alternative communication (AAC) enters, from picture boards to speech-generating devices that give a child a reliable voice.
• Early intervention and special education, including an Individualized Family Service Plan (IFSP) for the youngest children and, later, an Individualized Education Program (IEP) at school.
• Medical follow-up for specific issues as they arise, such as seizures, sleep, or digestive concerns, guided by your pediatrician and relevant specialists.

No single plan fits every child, and the right combination shifts over the years. Decisions about therapies, medications, and equipment should always be made with your child's physician and care team, who know your child directly. Think of this guide as a starting vocabulary for those conversations, not a substitute for them.

Getting an evaluation and building a care team

Phelan-McDermid syndrome is confirmed through genetic testing. A chromosomal microarray can find a missing piece at 22q13, and gene sequencing can catch a change inside SHANK3 that a microarray might not see. Families often arrive at testing after a doctor notices low muscle tone, delayed milestones, or absent speech and refers them onward.

If you are early in this process, a few steps tend to help:

1. Start with your pediatrician. Share your observations plainly and ask directly whether a developmental evaluation and genetic testing make sense.
2. Ask for a referral to genetics. A geneticist or genetic counselor can order the right tests, interpret results, and explain what they mean for your child and for future family planning.
3. Connect with early intervention. In the United States, every state has a free early intervention program for children under three, and you can usually refer your own child without waiting for a diagnosis.
4. Build your team over time. That team may grow to include a developmental pediatrician, neurologist, therapists, and your child's school, with you as the constant thread who ties it all together.

You do not have to assemble everything at once. Take the next single step, keep copies of reports and evaluations in one place, and let the team fill in around you.

Finding your community

A rare diagnosis can feel isolating, especially in the early days. One of the most steadying things families do is find other families who live this. The Phelan-McDermid Syndrome Foundation connects parents worldwide, shares up-to-date information, and points families toward research and registries. Condition-specific groups, local disability organizations, and family support networks can turn a lonely search into a shared road.

Community is not a luxury on top of care. It is part of care. It is where you learn the practical shortcuts, hear how someone else handled the same school meeting, and remember that your child's crown is seen by people who truly get it. You are not walking this alone.