According to Leahy1 the most common condition caused by entrapment at the pronator teres is the carpal tunnel syndrome and “the most common site of peripheral nerve entrapment is the pronator teres.”
The median nerve can be constricted by a fibrous band within the pronator teres, by hypertrophy of the pronator teres, or can be compressed as it passes deep to both heads of this muscle.2 Leahy1 finds that the pronator teres is much more frequently involved than the actual carpal tunnel as a causative factor; that it is always necessary to evaluate and treat multiple areas of adhesion of the particular nerve which often exist before the actual symptoms appear. Other common areas affecting the median nerve may be at the subscapularis, ligament of Struthers (originating from a spur located on the anteromedial surface of the humerus about 5cm above the medial epicondyle and attaching to the medial epicondyle), or distally past the pronator teres down the forearm.
In a Mayo Clinic series,3 seven of 35 patients were operated on for carpal tunnel who later were found to have a pronator teres syndrome. In this series 50 percent of the patients with definite pronator teres syndrome had a positive Phalen’s test. It was thought that compression of the median nerve at the pronator teres caused the nerve to be more susceptible to compression at the level of the carpal tunnel. A positive Tinel’s sign associated with firmness and tenderness over the pronator teres, compared to the opposite side and associated with carpal tunnel symptoms, points to involvement at the pronator teres level.
Of course an actual pronator syndrome can exist without creating any carpal tunnel symptoms. The median nerve after crossing the elbow must pass beneath the lacertus fibrosus which is a thick fascial band extending from the biceps tendon to the forearm fascia. A thickened lacertus fibrosus could compress the median nerve by indenting the flexor muscle mass2 and visibly depress the contour of the forearm. Besides the pronator teres as described above, the median nerve may be compressed by a tight fibrous arch of the flexor digitorum superficialis.2
The most frequent symptom of pronator symptom may be a mild to moderate aching pain in the proximal forearm described as “tiredness” or “aching,”2 especially with repetitive movements. Pain may radiate to the elbow and shoulder. Dawson et al.,2 states that carpal tunnel syndrome can be differentiated by its frequency of night symptoms and exacerbation by wrist movements which pronator syndrome would not express. Apparently he does not recognize the frequency of the pronator teres as a cause of the carpal tunnel syndrome. In my practice I have found the pronator syndrome to be causative of carpal tunnel syndrome in at least 50 percent of the cases which were effectively treated by Leahy’s method of “active release” as described in articles by Leahy and Mock.4,5 In severe cases of pronator syndrome there may be weakness in the intrinsic muscles of the hand and muscles of the forearm, but usually the weakness is not severe compared to compression at the level of the anterior interosseous nerve.
To determine the level of compression of the median nerve about the elbow, Spinner6 has developed several tests which, although often negative, are significant if found positive.
Pain at the level of the lacertus fibrosus is surmised by resisting pronation of a patient’s arm which is fully supinated and flexed at the elbow. This test contracts the biceps and tightens the lacertus fibrosus.
Compression at the level of the pronator teres is determined by instructing the patient with the elbow extended to place the forearm in full pronation with the wrist in flexion. The patient resists against the examiner attempting to supinate and extend the wrist. Pain in the proximal forearm indicates possible pronator teres involvement.
Evaluation of compression of the flexor digitorum superficialis is determined by having the patient flex the proximal interphalangeal joint of the middle finger against resistance. If this test creates forearm pain there may be compression of the median nerve at the level of the superficialis arch.