The main symptoms of the condition are severe learning disability, breathing abnormalities with intermittent periods of over breathing and daytime apnoea and characteristic facial features including a wide mouth, prominent lips and deep-set eyes. Individuals with Pitt-Hopkins syndrome are often short compared to other family members and have a small head size. Seizures may be present and brain scans often show specific abnormalities including agenesis of the corpus callosum. There are minor differences in the shape of the fingers and slim feet, often with shortening of the little toes. Constipation may be a problem. Speech is very limited in Pitt-Hopkins syndrome, often to a few words. It has been noted that many people with Pitt-Hopkins syndrome have a tendency to smile frequently and this characteristic, together with some unsteadiness on walking leads to the diagnosis often being confused with Angelman Syndrome. The breathing abnormalities, sometimes associated with colour change, are not present in every person and may not appear until mid-childhood. This diagnosis is therefore often made relatively late.
(the above answer is taken with kind permission from the Contact a Family website and from Prof Jill Clayton-Smith, Consultant Clinical Geneticist, St Mary’s Hospital Manchester Medical text written January 2009)