Gastrectomy is another cause of intrinsic factor deficiency, although it seldom causes symptomatic deficiency in isolation. Various disorders of the small intestine, such as surgical resection of the terminal ileum and blind loop syndrome, may result in malabsorption of vitamin B12. Rare cases of dietary insufficiency can be seen in strict vegetarians without evidence of malabsorption. High-risk groups for the deficiency syndrome include the elderly, patients taking ulcer medications over long periods, patients with acquired immunodeficiency syndrome, vegetarians, patients who have undergone stomach resection or small bowel resection, or both, and patients with dementia. Interestingly, after heavy abuse of nitrous oxide, patients can develop a clinical syndrome identical to that of vitamin B12 deficiency.
Subjects improve when nitrous oxide is stopped.
Serum vitamin B12 and the Schilling test are almost always normal. Subjects who are already deficient in vitamin B12 seem to be more susceptible to the neurologic effects of prolonged nitrous oxide exposure. With proper treatment, at least partial improvement can be expected in most instances. Most of the improvement occurs during the first 6 months of therapy. Remission of symptoms correlates inversely with the time lapse between onset of symptoms and initiation of therapy.