The most common cause is entrapment of the median nerve between the two heads of the pronator teres muscle. Other causes are compression of the nerve from the fibrous arch of the flexor superficialis, or the thickening of the bicipital aponeurosis.
Anatomy
The median nerve passes through the cubital fossa and passes between the two heads of pronator teres muscle into the forearm. It then runs between flexor digitorum superficialis and flexor digitorum profundus muscles and enters the hand through the carpal tunnel. It innervates most of the flexor muscles in the forearm and hand. Its sensory component supplies the skin of the palm, thumb, index and middle finger as well as half the ring finger, and the bones of the wrist. In the proximal forearm it gives rise to the anterior interosseous nerve which innervates the flexor of the thumb, the flexor digitorum profundus of the index finger, and the pronator quadratus, and terminates in a sensory branch to the bones of the wrist, i.e., the carpal tunnel. Compression of the proximal median nerve results in weakness of these three muscles, and can cause aching pain in the wrist on the basis of the sensory nerve to the carpal bones.
Diagnosis
The characteristic physical finding is tenderness over the proximal median nerve, which is aggravated by resisted pronation of the forearm. The flexor pollicis longus and FDP of the index finger are weak, leading to impairment of the pincer movement. This reflects involvement of the anterior interosseous nerve. Sensory changes may be found in the first three fingers as well as in the palm, indicating impairment of the median nerve proximal to the flexor retinaculum. The clinical and electrophysiological features of pronator teres syndrome are quite different from patients with carpal tunnel syndrome or pure anterior interosseous syndrome. Proper localisation is crucial to treatment options. Conduction velocity of the median nerve in the proximal forearm may be slow but the distal latency and sensory nerve action potential at the wrist are normal. Although MRI may show denervation atrophy of the affected muscles, its role in the evaluation of pronator teres syndrome is unclear. If the EMG or the MRI are abnormal for the pronator teres muscle and the flexor carpi radialis, this implies that the problem is at or proximal to the elbow, as the takeoff of the nerves to these muscles occurs proximal to the elbow.
Treatment
Injection of corticosteroids into the pronator teres muscle may produce relief of symptoms. Massage therapy can also provide relief for individuals experiencing this condition. Surgical decompression can provide benefit in selected cases.
