White matter disorder due to vitamin B12 deficiency

**Definition:** Neurological features occur in 40% of patients with B12 deficiency. Subacute combined degeneration is a potentially reversible neurological complication of a vitamin B12 deficiency. Symptoms develop insidiously and neuropathic manifestations include progressive paraesthesia distally, numbness, gait ataxia and diminished proprioception in the lower limbs, while the myelopathic component leads to variable motor impairment due to pyramidal tract dysfunction. Incontinence of bowel and bladder with impotence and postural hypotension occur as part of the myelopathy. Central manifestations include confusion, depression, progressive hallucination and mental slowing. There may also be optic neuropathy present.

**Long definition:** MRI scan may show extensive white matter change which can become confluent with disease progression culminating in leucoencephalopathy. Symmetric T2-hyperintensity within the dorsal column of spinal cord is commonly seen in subacute combined degeneration of the spinal cord patients with a linear pattern in the cervical spine and a nodular pattern in the thoracic spine. The pathological changes of subacute combined degeneration of the spinal cord include spongy change with focal loss of myelin and axonal destruction in the white matter of the spinal cord particularly affecting posterior and lateral columns of the cervical and upper thoracic spinal cord.