Because megaloblastosis caused by folate or cobalamin deficiency leads to a functional folate coenzyme deficiency, the morphologic manifestations of both deficiencies are understandably indistinguishable. However, only cobalamin deficiency results in a patchy demyelination process, which is expressed clinically as cerebral abnormalities and subacute combined degeneration of the spinal cord.16 The precise role of cobalamin in maintaining the integrity of the central nervous system has not been completely defined (see box on Clues for Distinguishing Cobalamin and Folate Deficiencies).
The demyelinating process involves patchy swelling of the myelin sheath followed by its breakdown (demyelination), leading to axonal degeneration. Microscopic foci coalesce with one another, giving the surface of the spinal cord (on cross section) a spongy appearance; later there is secondary Wallerian degeneration of long tracts. Patchy demyelination usually begins in the dorsal columns in the thoracic segments of the spinal cord (Fig. 1) and then spreads contiguously to involve corticospinal tracts. These lesions spread throughout the length of the cord and ultimately involve spinothalamic and spinocerebellar tracts. There is also degeneration of the dorsal root ganglia, celiac ganglia, the Meissner plexus, and the Auerbach plexus.
Fig1. SPINAL CORD IN COBALAMIN DEFICIENCY. The cross section of the spinal cord stained with Luxol blue shows demyelination of the dorsal columns (a) and early demyelination of the lateral columns (b).
Although demyelination may also extend to the white matter of the brain, it is unclear whether the peripheral neuropathy is caused by a distinct lesion or results from spinal cord disease; the clinical manifestations may be extremely varied.
Vegetarians with cobalamin neuropathy in India had cognitive impairment in nearly one-half of 36 patients; it was mostly global with impaired recall and “serial sevens” (which are useful bedside tests of attention); impaired naming was found among one-quarter of the patients. Patients can present with anxiety, apathy, aggression, irritability, hallucinations, delusions, and disinhibition. Nearly one-half had abnormal evoked potential (using the oddball auditory paradigm), which revealed P300 latency that was reversible in 3 months of cobalamin replacement; in one-fifth P300 was unrecordable. Objective tests to document cobalamin neuropathy include nerve conduction studies and motor- and sensory-evoked potentials, visual pathway abnormalities, and magnetic resonance imaging (MRI) that shows T2 hyperintensity and atrophy. Myelopathy is present in nearly all patients. In 70% of patients, there is neuropathy which is mainly axonal with some demyelinating features; but there may also be reversible frontal-subcortical neurobehavioral and cognitive abnormalities ascribed to cortical and subcortical dysfunction.
Among another cohort of patients with cobalamin neuropathy from the United States, 65% had mild, about 25% had moderate, and about 10% had severe neurologic deficits. Paresthesias or ataxia were most commonly the first symptoms, and diminished vibratory sensation and proprioception in the lower extremities were the most common objective early signs. Although multiple neurologic syndromes were often seen in the same patient, the spectrum of objective signs could include loss of fine or coarse touch, decreased or increased deep tendon reflexes with spasticity or muscle weakness, urinary or fecal incontinence, orthostatic hypotension, amaurosis, dementia, psycho sis, or mood disturbances. Overall, although the neurologic deficits were mild in most cases, the severity was judged related to the duration of symptoms before diagnosis; not unexpectedly, those with the shorter duration of symptoms responded most to appropriate replacement.
