What is Phelan-McDermid Syndrome?

Phelan-McDermid syndrome (PMS) is a rare genetic condition that causes developmental and speech delays, behavioral problems and a weakened or no ability to feel pain or sweat. Phelan-McDermid syndrome is a congenital condition (condition that is present at birth) that can affect people of all genders.

What Causes PMS?

PMS is caused by a mutation (change) or deletion of theSHANK3 gene (piece of genetic material). In most cases, PMS happens at random. It is not anyone’s fault.

How Can a Person Develop PMS?

There are 4 different ways that a person can develop PMS:
  • Simple deletion. This is the most common type of PMS. It is when a piece of chromosome 22 (piece of genetic material) on the SHANK3 gene gets lost. This usually happens at random.
  • Ring chromosome. This is when chromosome 22 on the SHANK3 gene forms a circle instead of the typical X-shape.
  • Unbalanced translocation. This can happen when parents do not know that one or both parents carry the chromosome for PMS.
  • Mutation in the DNA sequence (order) of the SHANK3 gene. This usually happens at random.

What Are the Signs of PMS?

Most children with PMS show signs in early childhood. Other children show signs as early as their first 6 months of life. Signs can be different in every person with PMS.

PMS signs can affect development, behavior and physical appearance.


  • Developmental delays, such as delayed sitting up, rolling over, walking or talking
  • Delayed or absent speech
  • Autism spectrum disorder (ASD, or a condition that causes problems with communication, behavior and social skills)


  • Repetitive behaviors, such as hand flapping
  • Anxiety or nervousness
  • Sleep and/or seizure disorders
  • Some lower ability to sweat, which can lead to overheating
  • Lower sensation or expression of pain

Physical appearance

  • Hypotonia (low muscle tone)
  • Long head shape with full or puffy cheeks
  • Puffy or droopy eyelids with long eyelashes
  • Wide, flat nose or face with a pointed chin
  • Large hands
  • Underdeveloped toenails

Rev. 5/2018. This handout is intended to provide health information so that you can be better informed. It is not a substitute for medical advice and should not be used to treatment of any medical conditions.