Subacute combined degeneration
Summary
Subacute combined degeneration (SCD) is a neurological disorder caused by vitamin B12 (cobalamin) deficiency, resulting in demyelination of the dorsal columns and lateral corticospinal tracts of the spinal cord. It presents with a combination of sensory ataxia (impaired proprioception/vibration sense) and upper motor neuron signs (spasticity, hyperreflexia, Babinski sign). It is a key board favorite linking hematology (megaloblastic anemia) with neurology.
Detail
Subacute combined degeneration results from vitamin B12 deficiency, which is required as a cofactor for methylmalonyl-CoA mutase (converting methylmalonyl-CoA to succinyl-CoA) and methionine synthase (converting homocysteine to methionine, using methylcobalamin). B12 deficiency leads to accumulation of methylmalonyl-CoA, which disrupts normal fatty acid synthesis and leads to abnormal myelin production, causing degeneration of myelin in the dorsal columns (fasciculus gracilis and cuneatus) and lateral corticospinal tracts, and sometimes spinocerebellar tracts.
Clinical features include: - Dorsal column dysfunction: loss of vibration and proprioception, sensory ataxia, positive Romberg sign - Lateral corticospinal tract dysfunction: spasticity, hyperreflexia, Babinski sign, weakness - Peripheral neuropathy: paresthesias, decreased reflexes (can sometimes mask the UMN hyperreflexia) - Cognitive changes: irritability, memory loss, dementia in severe cases
Causes of B12 deficiency include pernicious anemia (autoimmune destruction of parietal cells or anti-intrinsic factor antibodies), strict vegan diets, malabsorption syndromes (Crohn disease, ileal resection), Diphyllobothrium latum infection, and nitrous oxide abuse (inactivates B12).
Laboratory findings: macrocytic megaloblastic anemia, hypersegmented neutrophils, elevated homocysteine and methylmalonic acid (MMA) levels (MMA is specifically elevated in B12 deficiency, not folate deficiency, helping distinguish the two).
Important board pearl: Folate deficiency can cause megaloblastic anemia but does NOT cause subacute combined degeneration because folate is not involved in the methylmalonyl-CoA mutase pathway. Giving folate to a B12-deficient patient can mask the anemia while allowing neurological damage to progress—hence always check B12 levels before treating megaloblastic anemia with folate.
Treatment is parenteral vitamin B12 replacement, which can halt progression and may partially reverse neurological damage if caught early. MRI may show hyperintense signal in the dorsal columns on T2-weighted images ("inverted V" sign).
Sources
- First Aid for the USMLE Step 1
- Pathoma - Fundamentals of Pathology (Husain)
- Robbins and Cotran Pathologic Basis of Disease
- Kaplan USMLE Step 1 Lecture Notes - Pathology
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