Subacute combined degeneration; spinal cord, myelin stain: Subacute combined degeneration is due to vitamin B-12 deficiency. This leads to a spongy or "vacuolar" degeneration of the spinal cord white matter that begins in the posterior and lateral columns. Eventually, in untreated cases, involvement becomes more widespread with marked destruction of peripheral areas of the cord. A pathologically similar process is occasionally a complication of AIDS, but its pathogenesis is as yet undetermined.