VITAMIN-B-12 DEFICIENCY IN ADOLESCENTS AND YOUNG-ADULTS WITH PHENYLKETONURIA

Following several years absence from clinical follow up, an 18-year-ol d female on diet therapy for phenylketonuria presented with spastic pa raparesis, tremor, disorientation, slurred speech, distractibility, de teriorating mental function and megaloblastic anaemia (Hb 64 g/l mean corpuscular volume 121). Plasma phenylalanine levels were 100-600 mu m ol/l for the first 18 months of life but thereafter, because of seriou s psyche-social factors, > 1200 mu mol/l. Her diet had strictly exclud ed all meats, eggs and dairy products but she had been ingesting her m edical food (Lofenalac) only irregularly. Further investigation reveal ed a vitamin B-12 level of 65.8 pmol/l (normal 150-670). Treatment wit h oral B-12 quickly corrected her anaemia and there was a gradual impr ovement in speech, gait, tremor, disorientation and mood but mild spas tic diplegia remained. This case prompted us to survey 37 adolescent a nd young adult phenylketonuria patients in our clinic - 28 were on die t therapy, 9 were off(age 11-35 years, mean 21.6 years, 17 males, 20 f emales). Those on diet were not under ideal metabolic control. Six (16 %) had subnormal serum B-12 levels (< 150 pmol/l) and another six had borderline low values (150-200 pmol/l). None had specific neurological signs of subacute combined degeneration. Serum methylmalonic acid and homocysteine were not measured. On the basis of this survey we recomm end that complete blood count, serum B-12, RBC folate, methylmalonic a cid and homocysteine be routinely measured in adolescents and young ad ults with phenylketonuria.