Vitamin B12 Deficiency and Unilateral Wrist Drop
Vitamin B12 deficiency is an extremely unlikely cause of unilateral wrist drop and should prompt immediate investigation for alternative etiologies, particularly radial nerve compression or mononeuropathy.
Why B12 Deficiency Does Not Cause Unilateral Wrist Drop
B12 deficiency characteristically causes distal symmetric polyneuropathy, not focal mononeuropathies. The neurological pattern of B12 deficiency involves:
- Symmetric involvement affecting both sides of the body equally, typically starting distally in the lower extremities 1
- Posterior and posterolateral spinal cord involvement (subacute combined degeneration), which manifests as sensory ataxia, proprioceptive loss, and upper motor neuron signs—not isolated motor weakness in a single nerve distribution 2
- Sensory symptoms predominating over motor symptoms, with paresthesias, numbness, and gait ataxia being the hallmark features 3, 4
The Clinical Pattern of B12 Neuropathy
When B12 deficiency does cause neurological manifestations, the presentation includes:
- Bilateral and symmetric distal sensory loss in a stocking-glove distribution 1
- Posterior column dysfunction causing loss of vibration sense and proprioception, leading to sensory ataxia 2
- Cognitive difficulties, memory problems, and neuropsychiatric symptoms often preceding or accompanying peripheral symptoms 5, 3
- Megaloblastic anemia may be present but is absent in one-third of cases 5
What Actually Causes Unilateral Wrist Drop
Unilateral wrist drop indicates radial nerve pathology, which requires urgent evaluation for:
- Radial nerve compression at the spiral groove (Saturday night palsy), axilla, or forearm
- Mononeuropathy from trauma, compression, or entrapment
- Lead toxicity (classically bilateral but can be asymmetric)
- Focal nerve infarction in diabetic mononeuropathy
- Structural lesions affecting the radial nerve
Critical Diagnostic Approach
If B12 deficiency is suspected in a patient with wrist drop:
- Check serum B12 level as part of polyneuropathy screening, but recognize this will not explain unilateral findings 1
- Measure methylmalonic acid and homocysteine if B12 is low-normal (200-500 pg/dL), as these metabolites are more sensitive markers of functional deficiency 1, 5
- Perform electrodiagnostic studies to localize the lesion—B12 neuropathy shows symmetric distal axonal polyneuropathy, while radial nerve pathology shows focal demyelination or axonal loss in radial nerve distribution 1
- Obtain complete blood count looking for macrocytic anemia, though its absence does not exclude B12 deficiency 5
Common Pitfall to Avoid
Do not attribute focal, unilateral motor deficits to B12 deficiency. While B12 can rarely present acutely with posterior or posterolateral cord syndromes causing "useless hand syndrome" bilaterally due to cervical posterior column involvement 2, isolated unilateral wrist drop from radial nerve dysfunction is not a recognized manifestation. Missing the true cause (such as compressive radial neuropathy requiring urgent decompression) could lead to permanent nerve damage and functional impairment.
When to Test for B12 in Neuropathy Patients
B12 screening is appropriate when evaluating symmetric distal polyneuropathy in patients with risk factors 1, 3:
- Age >50-75 years (25% of elderly have metabolic B12 deficiency) 5
- Metformin use >4 months or proton pump inhibitor use >12 months 5, 3
- Vegetarian/vegan diet 5, 3
- Gastric or small bowel surgery, inflammatory bowel disease 3
- Atrophic gastritis or pernicious anemia 5
In these populations, B12 deficiency accounts for approximately 2.2-8% of polyneuropathy cases 1, making it a reasonable screening test but not an explanation for focal mononeuropathy.