Long-term follow up of patients with classical phenylketonuria after diet relaxation at 5 years of age. The Paris Study.

The age for discontinuing dietary treatment of phenylketonuria (PKU) has been a worldwide source of controversy for many years. It is the reason we report here the results of a prospective, controlled study in which the diet was relaxed at 5 years of age in 31 so far well-treated children with classical PKU. The increase of phenylalanine (Phe) plasma levels to about 1500 mumol/l (25 mg/dl) after relaxing the diet was not associated with any significant decline of intellectual performance as measured by the Wechsler scores. Paired comparisons at 7-8 years and 11-13 years of age (n = 12) have shown WISC scores of 102.6 +/- 16.2 and 104.8 +/- 16, respectively, which were not significantly different. Similarly, paired comparisons at 9-10 years and 14-16 years (n = 6) did not demonstrate a significant loss of IQ points (107.7 +/- 13 vs 104.8 +/- 18). Of course, it is possible to argue that we should have observed an increase in IQ with increasing age in our patients and that the absence of deterioration cannot be considered by itself as a good result. Nevertheless, it cannot be excluded that the subtle but global intellectual impairments that have been documented in early-treated subjects are, to a very substantial degree, determined in the pre-school years, long before there is any question of stopping or relaxing the diet.