The most advanced neural changes occur in the peripheral nerves, particularly of the legs. The distal segments are characteristically affected earliest and most severely. Degeneration of the medullary sheath can occur in all tracts of the spinal cord, especially in the posterior columns and in the anterior and posterior nerve roots. Changes also occur in the anterior horn and posterior ganglion cells. Lesions of hemorrhagic polioencephalitis occur in the brain when deficiency is severe.
Vasodilation occurs and can result in some edema before frank high-output heart failure occurs.
Early deficiency produces fatigue, irritation, poor memory, sleep disturbances, precordial pain, anorexia, abdominal discomfort, and constipation. The syndrome of peripheral neurologic changes due to thiamine deficiency is called dry beriberi. Calf muscle tenderness, difficulty in rising from a squatting position, a decrease in the vibratory sensation in the toes, and plantar dysesthesia are early signs. A diagnosis of mild peripheral neuropathy can be made when ankle jerks are absent. Continued deficiency causes loss of knee jerk, loss of vibratory and position sensation in the toes, atrophy of the calf and thigh muscles, and finally footdrop and toedrop.