Causes of numbness in the thumb include carpal tunnel syndrome and de Quervain's tendinitis, according to MedicineNet. Numbness in the thumb generally occurs due to a condition that affects the nerves or blood vessels that supply the hand.
Carpal tunnel syndrome is caused by a pinched nerve in the wrist that results in numbness, tingling and other symptoms in the hand or wrist, explains Mayo Clinic. The carpal tunnel is a passageway located on the palm side in wrist that protects the main nerve and tendons of the hand that control bending of the fingers. Compression of the nerve produces the numbness, tingling and hand weakness that usually starts in the thumb, index and middle fingers and may extend up the wrist and arm.
First dorsal compartment tendinitis, or de Quervain’s tendinitis, is characterized by irritation or inflammation of the wrist tendons at the base of the thumb that causes the tunnel or sheath around the tendon to swell and enlarge, notes Bethesda-Chevy Chase Orthopaedic Associates. The swelling and enlargement cause irritation of the nerve lying on top of the tendon sheath that results in numbness on the back of the thumb and index finger. It tends to occur as the result of new and repetitive activity, and is more common in women and usually starts after age 40. Although the cause is generally unknown, it is believed that past injuries to the joint increase the chances of developing this condition.