The influence of blood phenylalanine levels on neurocognitive function in adult PKU patients

It is well known that hyperphenylalaninemia caused by phenylketonuria (PKU) negatively influences cognitive performance. Several tests have been used to study these functions. Until now, no universal, optimal tool has been developed for detecting PKU-caused brain dysfunctions. Using computerized neuropsychological tests during daily routine would be helpful for screening subclinical brain deficits in adult PKU patients. In a monocentric, cross-sectional study, adult patients with PKU (n = 46; median age = 29.5 years; female/male ratio = 21/25) were tested with the computerized Cambridge Cognition (CANTAB) test measuring neurocognitive functions. Patients were divided into two groups: The “on diet” group included patients whose blood Phe-level was under 600 μmol/l (n = 20), and the “loose diet” group included patients whose blood Phe-level was above 600 μmol/l (n = 26) at the examination time. The results of the PKU-affected individuals were compared with a healthy control group (n = 31; median age = 25 years; female/male ratio = 11/20). Compared with the control group, PKU patients had significantly worse test results in memory, problem-solving skills, and strategy. However, there were no significant differences in response speed or initial thinking time. There was no correlation between the blood Phe-level, tyrosine (Tyr)-level or Phe/Tyr ratio and the different cognitive test results. There were no significant differences in test results between the two PKU subgroups. Several cognitive functions measured by CANTAB are negatively influenced by hyperphenylalaninemia in adult PKU patients. However, response speed and initial thinking time were not impaired as seriously as other functions. Patients with lower Phe-levels failed to achieve better test results than patients whose Phe-levels were notably elevated.