Blacking out is a rare neurological symptom of pernicious anemia, according to Healthline. Additional symptoms include peripheral neuropathy, weight loss, headaches, and muscle stiffness and tightness.
Pernicious anemia, or Vitamin B-12 deficiency anemia, is an autoimmune condition characterized by the inability of the body to produce an adequate supply of healthy red blood cells, notes Healthline. The deficiency occurs due to a poor diet lacking in appropriate amounts of B-12, or if the small intestine cannot properly absorb the vitamin. Treatment depends on the severity of the deficiency, but mild cases are often treated with dietary changes, supplements and nasal sprays, explains Mayo Clinic. For more severe cases, vitamin B-12 injections are sometimes necessary.