What is PMS?

The science, in plain English

Our cells contain 23 pairs of chromosomes: 22 numbered pairs plus one pair of sex chromosomes, making 46 in total. Each chromosome has a short arm (called p) and a long arm (called q).

In Phelan-McDermid Syndrome, part of the long arm of chromosome 22 is affected, at a place called 22q13. This can happen in several ways: a deletion (a missing piece), a ring chromosome, mosaicism, or an unbalanced rearrangement of the chromosomes.

A gene called SHANK3 is missing in almost all cases of PMS. SHANK3 is the gene linked to the neurological effects of PMS, which lead to global developmental delay and absent speech.

The size of the deletion matters

Deletions are measured in kilobases (1,000 base pairs) and megabases (1 million base pairs, written mb).

In one study of 71 people with PMS, deletions ranged from 0.2mb to 9.2mb. People who were also autistic tended to have smaller deletions (a median of 3.9mb) than those who were not (a median of 6.03mb).

Common features of PMS

Not everyone with PMS has all of these. Where a percentage is given, it shows roughly how many people with PMS have that feature.

Physical

• Low muscle tone (hypotonia): 80%
• Normal or faster than usual growth: 90%
• No speech, or severely delayed speech: 95%
• Global developmental delay: 95%
• Minor differences in appearance, such as a wide brow, a wide bridge of the nose and a flat midface
• Full or puffy cheeks or eyelids
• Drooping eyelids (ptosis)
• Deep-set eyes and long eyelashes
• Large, fleshy hands
• For some people, swelling caused by lymphoedema

Behaviour

• Autistic or autistic-like behaviour
• Frequent mouthing and chewing
• Sometimes teeth grinding or tongue thrusting
• A reduced sense of pain
• Aggressive behaviour, such as hitting, biting or hair pulling

Neurological

• Seizures: 25%
• Arachnoid cysts: 15%
• Disturbed sleep
• Sacral dimples (a small dip at the bottom of the spine): 50%