Subacute combined degeneration of the cord
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Subacute combined degeneration of the cord is caused by vitamin B12 deficiency.
Most common in patients older than 40 years and especially older than 60 years 7.
The clinical presentation of subacute combined degeneration is usually with loss of vibration and proprioception in the hands and feet, with eventual progression to sensory loss of all modalities, sensory gait ataxia, and distal muscle weakness, especially of the legs. Features of dementia may also become apparent.
Subacute combined degeneration can be a sequela of any cause of vitamin B12 deficiency. In the developed world where nutrition is generally adequate, it tends to result most commonly from pernicious anemia, but has other causes:
use of proton pump inhibitors (acid is required to release vitamin B12 from meat)
gastrointestinal surgery (especially if affecting the terminal ileum)
abuse of nitrous oxide (usually from recreational abuse) 3
Additionally, although uncommon, strict vegetarians or those who are undernourished may develop sufficient deficiencies to develop SACD.
Vitamin B12 levels may be normal in up to 30% of patients, and thus looking at levels of other more sensitive and specific biomarkers may be utilized instead:
holotranscobalamin or active vitamin B12 (low)
methylmalonic acid (high)
MRI spine may be normal in 60-85% of cases 14. When abnormal, most commonly there is symmetric bilateral high signal within the dorsal columns. This appearance has been described as the inverted "V" sign 4,14. The signal changes typically begin in the upper thoracic region, with ascending or descending progression 5.
The lateral corticospinal tracts, and sometimes lateral spinothalamic tract may also be involved. Although very unusual, there has been at least one described case of anterior cord involvement 6. Usually these areas have no contrast enhancement, although very mild enhancement has been rarely reported 1,11.
Often there is also accompanying cerebral white matter change. Both spinal and cerebral changes resolve after correction of vitamin B12 deficiency.
Treatment and prognosis
Management depends on the cause, but generally, patients should be provided with intramuscular hydroxocobalamin injections, followed by oral supplementation. Additionally, patients should avoid using nitrous oxide.
Approximately half of all affected patients will completely recover 7.
On imaging the differential includes:
other nutritional or metabolic deficiencies/toxicities
leukoencephalopathy with brainstem and spinal cord involvement and lactate elevation (LBSL): characteristically also involves the cerebral white matter and the brainstem 10
Friedreich ataxia: also causes cervical cord atrophy ('thinning') 10
hypomyelination with brainstem and spinal cord involvement and leg spasticity (HBSL): characteristically in an infantile demographic and also involves the cerebral white matter and brainstem 15