Methylmalonic acid in serum from patients with neurological symptoms consistent with cobalamin deficiency

Patients may have neurological symptoms due to cobalamin deficiency in spite of a normal plasma cobalamin level. The aim of the present study was to test if serum-methylmalonic acid (S-MMA) can identify these cobalamin deficient patients. Patients with neurological symptoms consistent with cobalamin deficiency and normal plasma cobalamin had an estimation of S-MMA. If S-MMA was elevated, treatment with by droxycobalamin was offered. The patients were followed for one year in a prospective study with blood tests, clinical assessment and neurophysiological tests. Serum-MMA was above 0.37 μmol/l (+2 SD of normal mean) in 15 out of 134 patients (11.2%) tested. Twelve of these patients were treated with hydroxycobalamin injections. A significant decrease in S-MMA below 0.37 μmol/l was found in 11 patients treated, indicating a functional cobalamin deficiency. Five patients were followed for one year. Documented improvement of neurological symptoms and nerve conduction velocity was shown in three of these patients. Of the remaining two patients treated one did not change and one deteriorated. S-MMA can identify patients with neurological symptoms due to cobalamin deficiency in spite of a normal plasma cobalamin level.