There is diffuse damage to the nerves. The mechanism by which lack of B12 damages nerves is not clear. It is thought that lack of this vitamin causes an abnormal formation of fatty acids, which are used by the body to build cell membranes and the nerve myelin sheath. The disorder affects approximately 1 out of 10,000 people. An examination usually shows symmetrical sensorimotor polyneuropathy , with muscle weakness and sensation disturbance.
Reflexes are often abnormal.
Muscles may develop spasticity . Mental changes are variable, from mild forgetfulness to complete dementia or psychosis . Severe dementia is uncommon, but in some cases, it is the initial symptom of the disorder. A CBC may show anemia if pernicious anemia is also present. If folic acid has been used to treat anemia, neurologic symptoms may develop even if the anemia is no longer evident.