Noonan syndrome: partial factor XI deficiency.

Among the multiple extracardiac abnormalities seen in the Noonan syndrome, several authors have mentioned an unexplained bleeding tendency. In 1983 Kitchens and Alexander reported on a partial deficiency of coagulation factor XI in four patients with Noonan syndrome. We examined 9 patients with Noonan syndrome and 9 of their 18 parents (3 of them also had Noonan syndrome). Four Noonan patients were found to have a partial deficiency of factor XI (30-65%). The Noonan patients had a mean factor XI level of 67%, significantly lower than the normal parents (mean level of 109%). Our study suggests a relationship between Noonan syndrome and a partial deficiency of factor XI.