We reviewed 153 episodes of cobalamin deficiency involving the nervous system 
that occurred in 143 patients seen over a recent 17-year period at 2 New York 
City hospitals. Pernicious anemia was the most common underlying cause of the 
deficiency. Neurologic complaints, most commonly paresthesias or ataxia, were 
the first symptoms of Cbl deficiency in most episodes. The median duration of 
symptoms before diagnosis and treatment with vitamin B12 was 4 months, although 
long delays in diagnosis occurred in some patients. Diminished vibratory 
sensation and proprioception in the lower extremities were the most common 
objective findings. A wide variety of neurologic symptoms and signs were 
encountered, however, including ataxia, loss of cutaneous sensation, muscle 
weakness, diminished or hyperactive reflexes, spasticity, urinary or fecal 
incontinence, orthostatic hypotension, loss of vision, dementia, psychoses, and 
disturbances of mood. Multiple neurologic syndromes were often seen in a single 
patient. In 42 (27.4%) of the 153 episodes, the hematocrit was normal, and in 31 
(23.0%), the mean corpuscular volume was normal. Neutropenia and 
thrombocytopenia were unusual even in anemic patients. In nonanemic patients in 
whom diagnosis was delayed, neurologic progression frequently occurred although 
the hematocrit remained normal. In 27 episodes, the serum cobalamin 
concentration was only moderately decreased (in the range of 100-200 pg/ml) and 
in 2 the serum level was normal. Neurologic impairment, as assessed by a 
quantitative severity score, was judged to be mild in 99 episodes, moderate in 
39 and severe in 15. Severity of neurologic dysfunction before treatment was 
clearly related to the duration of symptoms prior to diagnosis. In addition, the 
hematocrit correlated significantly with severity, independent of the longer 
duration of symptoms in nonanemic patients. Four patients experienced transient 
neurologic exacerbations soon after beginning treatment with cyanocobalamin, 
with subsequent recovery. Followup evaluation was adequate to assess the 
neurologic response to vitamin B12 therapy in 121 episodes. All patients 
responded, and in 57 (47.1%), recovery was complete, with no remaining symptoms 
or findings on examination. The severity score was reduced by 50% or greater 
after treatment in 91% of the episodes. Residual long-term moderate or severe 
neurologic disability was noted following only 7 (6.3%) episodes. The extent of 
neurologic involvement after treatment was strongly related to that before 
therapy as well as to the duration of symptoms. The percent improvement over 
baseline neurologic status after treatment was inversely related to duration of 
symptoms and hematocrit. Some evidence of response was always seen during the 
first 3 months of treatment.(ABSTRACT TRUNCATED AT 400 WORDS)