A systematic review 1 including 7 studies with a total of 336 subjects was abstracted in DARE. Only 2 studies were judged to be of fairly good quality (Jadad score 3 out of 5). The other 5 studies were of a poor quality (Jadad score less than or equal to 2 out of 5). None of the studies involved an intention-to-treat analysis. Pain or somatosensory symptoms (6 studies): all studies showed a statistically significant benefit compared with baseline or placebo. Vibration perception threshold (4 studies): 3 studies showed a beneficial outcome with vitamin B12 compared with control or baseline, and one showed no improvement with methylcobalamin. Autonomic symptoms (3 studies): all 3 studies found improvements with methylcobalamin. Electophysiological measures (5 studies): in one trial that used a neuromotor assessment process to measure the current perception threshold, a beneficial treatment effect for vitamin B complex was observed. Of the 4 trials that included nerve conduction velocity testing, the only trial of vitamin B combination therapy and 2 of the 3 trials of methylcobalamin showed beneficial outcomes compared with placebo. The other study of methylcobalamin found no change.
Comment: The quality of evidence is downgraded by severe limitations in the methdology of the included studies and a high probability of publication bias. The authors classified some studies as showing positive results when there was an improvement from baseline, rather than an improvement in comparison with the control treatment; this can exaggerate the strength of the evidence.
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