We present a patient with findings of subacute combined degeneration (SCD) in the setting of a mild B12 deficiency, with a normal methylmalonic acid level. 

A healthy 45-year-old female presented to the clinic with loss of balance, and paresthesia in her extremities. On physical examination, the patient had length-dependent loss of temperature with brisk reflexes and positive Romberg sign. B12 levels were measured to be mildly decreased at 258 (normal >300), with a methylmalonic acid level of 92 (normal 0-378). MRI imaging showed patchy dorsal cord T2 hyperintensity. Given the low B12 level, the patient was diagnosed with likely SCD possibly due to intrinsic factor deficiency or malabsorption. 

An elevated MMA level continues to remain a gold standard test to diagnose and confirm a low B12 level. The patient we present had significant symptoms of neuropathy without an elevated methylmalonic acid level and only a slightly decreased B12 level. These findings suggest that providers should not solely rely on MMA levels to diagnose B12 deficiency and subsequent SCD. Given that in aging patients, a B12 deficiency has been associated with a significantly increased risk of peripheral neuropathy and eventual total disability, aggressive treatment with B12 should be considered even if MMA levels are normal, as this may reflect an opportunity for providers to intervene early in the disease course.