Subacute Combined Degeneration

In chronic B₁₂ deficiency, both ascending proprioceptive (dorsal column), and descending corticospinal axons are affected, causing subacute (slowly developing) combined (motor and sensory) degeneration, accompanied by loss of proprioception, vibration sensitivity and two-point discrimination, and the appearance of Romberg's Sign (sensory) of dorsal column lesion: inability to stand without marked swaying or even falling, with the eyes closed, because of the removal of visually guided postural stabilisation, in the absence of proprioceptive afferent information. These signs are also prominent in neurosyphilis. Temperature and pain sensitivity remain intact. The cord shown in the illustration was from a patient with pernicious anaemia. The lesion of the cord affects mainly the dorsal and dorso-lateral columns, and the symptoms were consistent with gross dorsal column and corticospinal tract degeneration. It is unclear why the myelinated fibres appear to be preferentially damaged in this condition. It may be that impaired metabolic energy supply especially affects the large, fast conducting fibres which have a higher energy demand. [Return to Main Document]