Neurological complications of acquired cobalamin deficiency: clinical aspects

Neuropsychiatric syndromes occur in about 40% of Cbl-deficient patients and are characterized by progressive and variable damage to the spinal cord, peripheral nerves and cerebrum. The first abnormality is usually sensory impairment, most often presenting as distal and symmetrical paraesthesiae of the lower limbs and frequently associated with ataxia. Almost all patients demonstrate loss of vibratory sensation, often in association with diminished proprioception and cutaneous sensation and a Romberg sign. Corticospinal tract involvement is common in more advanced cases, with abnormal reflexes, motor impairment and, ultimately, spastic paraparesis. A minority of patients exhibit mental or psychiatric disturbances or autonomic signs, but these rarely if ever occur in the absence of other neurological changes. Because N2O inactivates Cbl, abuse of the gas may lead to typical Cbl neuropathy. Haematological changes are minimal and serum Cbl levels and Schilling tests normal in most patients. The severity of neurological abnormalities prior to treatment correlates with the duration of symptoms and the haemoglobin level. Initial severity, symptom duration and initial haemoglobin also correlate with residual neurological damage after Cbl therapy. The inverse correlation between severity of anaemia and neurological damage is not understood. Diagnosis of Cbl neuropathy can usually be made in the presence of the typical neuropsychiatric abnormalities, a low serum Cbl level and evidence of megaloblastic haemopoiesis. In some patients serum MMA and HCYS determinations or a therapeutic trial may be required. A neurological response usually occurs within the first 3 months, although further improvement may occur with time. Patients with advanced disease may be left with major residual disability. Therefore early diagnosis is critical. Pharmacological doses of folic acid reverse the haematological abnormalities of Cbl deficiency. This may allow neuropathy to develop or progress and make recognition of deficiency more difficult. There is no clear evidence that folic acid therapy precipitates or exacerbates Cbl neuropathy. Haematological improvement may occur in a fraction of patients receiving small doses of folate, but the data are inadequate to predict the danger of low levels of folate supplementation in the general population.