Vitamin B12 deficiency

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By Patrick J Bosque MD

Vitamin B12 deficiency is also known as or subsumes Cobalamin deficiency. -ed.

B12 deficiency may cause an extraordinary variety of progressive neurologic syndromes. In this clinical summary, Patrick Bosque, Associate Professor of Neurology at Denver Health Medical Center and the University of Colorado, Denver, describes the well-established manifestations of B12 deficiency as well as more controversial associations with macular degeneration, cerebrovascular disease, and dementia. In addition, while severe B12 deficiency has long been known to cause severe developmental delay, this update includes emerging information suggesting that a relative B12 deficiency may cause a common metabolic syndrome in infancy that is potentially linked to subtle developmental problems.

Key Points

  • B12 deficiency should be suspected in any patient with otherwise unexplained peripheral neuropathy, myelopathy, optic neuropathy, dementia, ataxia, movement disorder, or psychiatric disturbance.
  • Serum B12 level should be determined in any patient with suspected B12 deficiency; abnormal blood cell indices are neither sensitive nor specific for B12 deficiency.
  • In cases of borderline low B12 levels, or when B12 deficiency is strongly suspected despite reported normal levels from an automated assay, elevated serum methylmalonic acid and homocysteine levels may confirm a physiological deficiency.
  • Daily, high-dose oral B12 supplementation appears as effective as parenteral therapy, and is substantially less costly. A brief parenteral course of therapy may still be needed for patients with significant neurologic signs of B12 deficiency.
  • Modest elevations in serum methylmalonic acid and homocysteine levels are common in breastfed infants and may represent a physiologically significant B12 deficiency.

In This Article

Historical note and nomenclature
Clinical manifestations
Clinical vignette
Pathogenesis and pathophysiology
Differential diagnosis
Diagnostic workup
Prognosis and complications
References cited