The major defect of B12deficiency is an impairment of growth, particularly of rapidly dividing cells such as immature RBC. Infants with severe deficiency present with anemia and neurological problems, such as flaccidity, poor muscular control, twitching, and abnormal electroencephalogram. In adults, it is characterized by megaloblastic anemia and later development of neuropsychiatric symptoms. Neurological symptoms include numbness of the hands and feet, parasthesias, decreased vibration sense, and ataxia. CNS symptoms may occur without anemia and are irreversible. Poor growth, sore, smooth tongue, spleenomegaly, thrombocytopenia, and leucopenia are also seen.
Vitamin B12 toxicity: There are no signs of vitamin B12 toxicity, per se. There are a few rarely reported side effects that might be attributable to the vitamin, but such side effects are not necessarily related to the dose. These possible side effects include: diarrhea, blood clots in the legs, feelings of swelling over the entire body. These are the signs of an allergic reaction: hives or a rash, itching, swelling of the lips, mouth, or throat, wheezing or other difficulty breathing.
Vitamin B12 is usually considered a non-toxic substance. Even taking it by injection at high doses does not seem to increase the risk for toxicity
Elevated levels of Vitamin B12 can occur in polycythemia vera. Polycythemia vera is a disease state in which the proportion of blood volume that is occupied by red blood cells increases. Diagnosis is characterized by an absolute increase in red blood cells and in the total blood volume, although it is not unusual to also have increases in white blood cells and platelets. A bone marrow examination may be done. However, it is not functional in determining a definitive diagnosis. Laboratory studies confirm the diagnosis by showing increased RBC mass and normal arterial oxygen saturation in association with splenomegaly (spleen enlargement) or two of the following: thrombocytosis, leukocytosis, elevated leukocyte alkaline phosphatase level, or elevated serum vitamin B12 or unbound B12-binding capacity.
