Benefits of normalizing plasma phenylalanine: impact on behaviour and health. A case report.

An elderly man with mental retardation who had never received dietary treatment for his phenylketonuria was placed on a phenylalanine-restricted diet. Social skills and walking gait improved and a new interest in the objects in his environment developed spontaneously. A 2-year analysis of diet, blood plasma phenylalanine levels and behavioural state indicated that small differences in phenylalanine intake impacted his well-being. Of significant note, leg tremor and spasm that precipitated severe self-injury were only reversible when plasma blood phenylalanine concentrations were titrated to near normal ranges and daily phenylalanine intake was strictly controlled. This case may offer a potential explanation for some of the late treatment failures that have been reported and suggest new avenues to explore in the late treatment of PKU.