Sustained attention in adult phenylketonuria: the influence of the concurrent phenylalanine-blood-level.

The effect of concurrent phenylalanine levels (Phe-level) on sustained attention was tested in a group of 19 early treated adult PKU patients. Mean age was 20.5 years; WAIS IQs were in the normal range (M = 109.3). Phe-levels were manipulated in a high-low-high design by reintroduction of a strict phenylalanine-reduced diet for 4 to 5 weeks between test time 1 and 2 and returning to usual diet between test time 2 and 3. A control group of 20 healthy subjects, mean age 20.7 years was tested twice. Results of a sustained attention task are presented. In adult PKU patients with high concurrent Phe-levels, sustained attention is significantly impaired and reaction times are prolonged. In the low Phe-level condition, performance improved significantly. Nevertheless, the PKU group did not reach the level of performance of the control group. Results are not influenced by IQ and suggest a sustained attention deficit in adult PKU patients that varies according to the concurrent Phe-level. The partial reversibility of the deficits provides support for the hypothesis that biochemical mechanisms rather than structural changes of the brain underlie the relationship between concurrent Phe-level and sustained attention.