PMSFUK - Phelan McDermid Syndrome Foundation UK

What Is PMS?

Phelan-McDermid Syndrome – A Scientific Explanation

Human cells have 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes), giving a total of 46 chromosomes per cell. Each chromosome has a short (p) arm and a long (q) arm. Phelan-McDermid Syndrome can be a 22q13 chromosome deletion, ring chromosome, mosaic or unbalanced rearrangement of the chromosomes.


SHANK3 is a type of protein that is missing in almost all cases of PMS. SHANK3 is the named genes for the neurological deficit in PMS and results in global development delay and absent speech.

Some of the major features of people with Phelan-McDermid Syndrome are:


  • Hypotonia (low tone) – 80%
  • Normal or accelerated growth – 90%
  • Absent or severely delayed speech – 95%
  • Global developmental delay – 95%
  • Minor dysmorphic features
  • Common Facial Features are wide brow, wide nasal bridge and flat midface
  • Full or puffy cheeks and or eyelids
  • Ptosis
  • Deep set eyes and long eyelashes
  • Large, fleshy hands
  • Some affected by lymphedema


  • Autistic or ‘autistic like’ behaviour
  • Persistent mouthing and chewing, sometimes teeth grinding or tongue thrusting
  • Decreased perception of pain
  • Aggressiveness such as hitting, biting or hair pulling


  • Seizures – 25%
  • Anachroid cysts – 15%
  • Sleep disturbance
  • Sacral dimples – 50%

Size of Chromosome Deletion – it matters!

Deletions are measured in units, kilobases or 1000 base pairs, and megabases or 1 million base pairs.

Individuals with Autism Spectrum Disorder (ASD) were found to have smaller deletion sizes (median size of 3.9MB) than those without ASD, Median deletion size 6.03MB.

Deletion size among 71 patients with PMS ranged from 0.2 to 9.2MB.