De Quervain's tenosynovitis
The muscles and bones of the hand are connected by thick flexible tissue called tendons. Tendons are covered by a thin soft sheath of tissue known as synovium. Extensor pollicis brevis and abductor pollicis longus are two tendons located on the thumb side of the wrist. Inflammation and swelling of the tendon sheaths puts pressure on the adjacent nerves and leads to pain and numbness in the thumb side of the wrist.
Strain on these tendons can cause swelling and irritation and lead to a condition called De Quervain's tenosynovitis, which is characterized by inflammation. The condition is also referred to as De Quervain’s tendinitis, De Quervain’s tendinosis, De Quervain syndrome, or De Quervain’s disease.