13-06: Vitamin B₁₂ Deficiency

Vitamin B₁₂ belongs to the family of cobalamins and serves as a cofactor for two important reactions in humans (eFigure 13–7). As methylcobalamin, it is a cofactor for methionine synthetase in the conversion of homocysteine to methionine, and as adenosylcobalamin for the conversion of methylmalonyl-coenzyme A (CoA) to succinyl-CoA. These enzymatic steps are critical for annealing Okazaki fragments during DNA synthesis, particularly in erythroid progenitor cells. Vitamin B₁₂ comes from the diet and is present in all foods of animal origin. The daily absorption of vitamin B₁₂ is 5 mcg.

After being ingested, vitamin B₁₂ is bound to intrinsic factor, a protein secreted by gastric parietal cells in an acid environment. Other cobalamin-binding proteins (called R factors) compete with intrinsic factor for vitamin B₁₂. Vitamin B₁₂ bound to R factors cannot be absorbed. The vitamin B₁₂–intrinsic factor complex travels through the intestine and is absorbed in the terminal ileum by cells with specific receptors for the complex. It is then transported through plasma and stored in the liver. Three plasma transport proteins have been identified. Transcobalamins I and III (differing only in carbohydrate structure) are secreted by white blood cells. Although approximately 90% of plasma vitamin B₁₂ circulates bound to transcobalamins I and III, only transcobalamin II is capable of transporting vitamin B₁₂ into cells.

The liver contains 2–5 mg of stored vitamin B₁₂. Since daily utilization is 3–5 mcg, the body usually has sufficient stores of vitamin B₁₂ so that it takes more than 3 years for vitamin B₁₂ deficiency to occur if all intake or absorption immediately ceases.

Since vitamin B₁₂ is present in foods of animal origin, dietary vitamin B₁₂ deficiency is extremely rare but is seen in vegans—strict vegetarians who avoid all dairy products, meat, and fish (Table 13–6). Pernicious anemia is an autoimmune illness whereby autoantibodies destroy gastric parietal cells (that produce intrinsic factor) and cause atrophic gastritis or bind to and neutralize intrinsic factor, or both. Abdominal surgery may lead to vitamin B₁₂ deficiency in several ways. Gastrectomy will eliminate the site of intrinsic factor production; blind loop syndrome will cause competition for vitamin B₁₂ by bacterial overgrowth in the lumen of the intestine; and surgical resection of the ileum will eliminate the site of vitamin B₁₂ absorption. Rare causes of vitamin B₁₂ deficiency include fish tapeworm (Diphyllobothrium latum) infection, in which the parasite uses luminal vitamin B₁₂; pancreatic insufficiency (with failure to inactivate competing cobalamin-binding proteins [R-factors]); severe Crohn disease, causing sufficient destruction of the ileum to impair vitamin B₁₂ absorption; and perhaps prolonged use of proton pump inhibitors.

A. Symptoms and Signs

Vitamin B₁₂ deficiency causes a moderate to severe anemia of slow onset; patients may have few symptoms relative to the degree of anemia. In advanced cases, the anemia may be severe, with hematocrits as low as 10–15%, and may be accompanied by leukopenia and thrombocytopenia. The deficiency also produces changes in mucosal cells, leading to glossitis, as well as other vague gastrointestinal disturbances such as anorexia and diarrhea. Vitamin B₁₂ deficiency also leads to a complex neurologic syndrome. Peripheral nerves are usually affected first, and patients complain initially of paresthesias. As the posterior columns of the spinal cord become impaired, patients complain of difficulty with balance or proprioception, or both. In more advanced cases, cerebral function may be altered as well, and on occasion dementia and other neuropsychiatric abnormalities may be present. It is critical to recognize that the nonhematologic manifestations of vitamin B₁₂ deficiency can be manifest despite a completely normal complete blood count.

Patients are usually pale and may be mildly icteric or sallow. Typically, later in the disease course, neurologic examination may reveal decreased vibration and position sense or memory disturbance (or both).

B. Laboratory Findings

The diagnosis of vitamin B₁₂ deficiency is made by finding a low serum vitamin B₁₂ (cobalamin) level. Whereas the normal vitamin B₁₂ level is greater than 210 pg/mL (155 pmol/L), most patients with overt vitamin B₁₂ deficiency have serum levels less than 170 pg/mL (126 pmol/L), with symptomatic patients usually having levels less than 100 pg/mL (74 pmol/L). The diagnosis of vitamin B₁₂ deficiency in low or low-normal values (level of 170–210 pg/mL [126–155 pmol/L]) is best confirmed by finding an elevated level of serum methylmalonic acid (greater than 1000 nmol/L) or homocysteine. Of note, elevated levels of serum methylmalonic acid can be due to kidney disease.

The anemia of vitamin B₁₂ deficiency is typically moderate to severe with the MCV quite elevated (110–140 fL). However, it is possible to have vitamin B₁₂ deficiency with a normal MCV from coexistent thalassemia or iron deficiency; in other cases, the reason is obscure. Patients with neurologic symptoms and signs that suggest possible vitamin B₁₂ deficiency should be evaluated for that deficiency despite a normal MCV or the absence of anemia. In typical cases, the peripheral blood smear is megaloblastic, defined as red blood cells that appear as macro-ovalocytes, (although other shape changes are usually present) and neutrophils that are hypersegmented (six [or greater]-lobed neutrophils or mean neutrophil lobe counts greater than four) (eFigures 13–8 and 13–9). The reticulocyte count is reduced. Because vitamin B₁₂ deficiency can affect all hematopoietic cell lines, the white blood cell count and the platelet count are reduced in severe cases.

Other laboratory abnormalities include elevated serum lactate dehydrogenase (LD) and a modest increase in indirect bilirubin. These two findings reflect the intramedullary destruction of developing abnormal erythroid cells.

Bone marrow morphology is characteristically abnormal (eFigure 13–10). Marked erythroid hyperplasia is present as a response to defective red blood cell production (ineffective erythropoiesis). Megaloblastic changes in the erythroid series include abnormally large cell size and asynchronous maturation of the nucleus and cytoplasm—ie, cytoplasmic maturation continues while impaired DNA synthesis causes retarded nuclear development. In the myeloid series, giant bands and meta-myelocytes are characteristically seen.

Vitamin B₁₂ deficiency should be differentiated from folic acid deficiency, the other common cause of megaloblastic anemia, in which red blood cell folic acid is low while vitamin B₁₂ levels are normal. The bone marrow findings of vitamin B₁₂ deficiency are sometimes mistaken for a myelodysplastic syndrome (MDS) or even acute erythrocytic leukemia. The distinction between vitamin B₁₂ deficiency and myelodysplasia is based on the characteristic morphology and the low vitamin B₁₂ and elevated methylmalonic acid levels.

Initially, patients with vitamin B₁₂ deficiency are usually treated with parenteral therapy. Intramuscular or subcutaneous injections of 100–1000 mcg of vitamin B₁₂ are adequate for each dose (with the higher dose recommended initially). Replacement is usually given daily for the first week, weekly for the next month, and then monthly for life. The vitamin deficiency will recur if patients discontinue their therapy. Oral or sublingual methylcobalamin (1 mg/day) may be used instead of parenteral therapy once initial correction of the deficiency has occurred. Oral or sublingual replacement is effective, even in pernicious anemia, since approximately 1% of the dose is absorbed in the intestine via passive diffusion in the absence of active transport. It must be continued indefinitely and serum vitamin B₁₂ levels must be monitored to ensure adequate replacement. For patients with neurologic symptoms caused by vitamin B₁₂ deficiency, long-term parenteral vitamin B₁₂ therapy is recommended, though its superiority over oral vitamin B₁₂ therapy has not proven conclusively. Because some patients are concurrently folic acid deficient from intestinal mucosal atrophy, simultaneous folic acid replacement (1 mg daily) is advised for the first several months of vitamin B₁₂ replacement.

Patients respond to therapy with an immediate improvement in their sense of well-being. Hypokalemia may complicate the first several days of therapy, particularly if the anemia is severe. A brisk reticulocytosis occurs in 5–7 days, and the hematologic picture normalizes in 2 months. Central nervous system symptoms and signs are potentially reversible if they have been present for less than 6 months. Red blood cell transfusions are rarely needed despite the severity of anemia, but when given, diuretics are also recommended to avoid heart failure because this anemia develops slowly and the plasma volume is increased at the time of diagnosis.

Referral to a hematologist is not usually necessary.