Long term studies of untreated phenylketonuria I: intelligence or mental ability.

A group of 90 patients over the age of 2 years with untreated phenylketonuria was studied by 3 independent observers in 3 different ways for evidence of mental deterioration. In a small subset (N = 10), psychometric tests showed a fall in Intelligence or Developmental Quotients between the ages of 4 and 18 years; the same was found in a matched group of nonphenylketonuric patients from the same institution. In a second subset (N = 83), a Habit Score derived from the observations of the patients' caretakers showed a small overall improvement in adaptive behaviour over an average span of 19 years. In a third subset (N = 81), clinical assessment by one observer detected no significant change in mental ability over an average span of 11 years. No evidence of progressive mental deterioration was found in untreated phenylketonuria from middle childhood to middle age.