Pronator Syndrome

Pronator syndrome is the compression of the median nerve around the elbow and palmar side of the forearm. Because it is the same nerve that causes carpal tunnel syndrome, there is overlap in the symptoms making the diagnosis difficult, at times.

Pathology

Pronator syndrome is the compression of the median nerve around the elbow and palmar side of the forearm.

The median nerve is one of the five nerves arising from the brachial plexus and traveling down the arm to innervate the forearm and hand muscles.

Responsibilities of the Median Nerve

  • Most of the flexor mass muscles
  • The muscles on the palmar side of the thumb
  • Sensations of the thumb, index, long and thumb side of the ring finger

When the median nerve is compressed, the sensation of these 3 ½ fingers is progressively lost and the muscle girth of the forearm muscles decrease due to muscle atrophy.

Depending on where the compression occurs will depend upon whether the sensation and muscle strength is affected.

Four Sites of Median Nerve Compression

  1. A bony or cartilaginous spur called a supracondylar process. The spur creates a tunnel above and inside the elbow with a restricted diameter. This causes both sensory and motor deficits in the forearm and hand.
  2. The pronator muscle, for which this syndrome is named. The median nerve travels under this muscle as it leaves the elbow. Fascial bands in the pronator muscle can constrict the median nerve. This constriction will produce similar symptoms as in the compression produced by the supracondylar process.
  3. The bicipitalaponneurosis, a normal fascial band on the volar side of the elbow. The biceps tendon contributes some fibers to thisanother structure the median nerve travels under and on rare occasions can compress the median nerve producing similar sensory and motor symptoms.
  4. As the median nerve travels down the arm, it will divide into the anterior interosseous nerve (AIN) and the remainder of the median nerve travels down the arm to innervate the hand. As the AIN splits from the median nerve proper, it travels under a fibrous arch of the muscles called the flexor digitorum superficialis (FDS) arch which is the fourth area of median compression in pronator syndrome. Since the AIN is mostly a motor nerve to the forearm muscles, the forearm muscles atrophy and develop weakness resulting in difficulty bending the last joint of the thumb and index finger without affecting sensation to the hand.

The physical exam and EMG/NCS nerve study are the only two studies necessary to make a diagnosis. Many patients with pronator syndrome have a false negative nerve conduction study and the experience of the surgeon will dictate if surgery will be required.

The median nerve also causes carpal tunnel syndrome, resulting in an overlap of symptoms. This can make diagnosing Pronator syndrome difficult.

Pronator syndrome can also present as a dynamic condition that only occurs with repetitive forearm activities involving wrist flexion, extension and forearm rotation.

Treatment

Pronator syndrome is initially treated conservatively.

Initial Treatment

  • Elimination of the offending repetitive activity
  • Therapy
  • Anti-inflammatory medication
  • A resting wrist splint

If these measures fail, or if the forearm and the arm atrophy (loss of muscle girth in the forearm and thumb) is severe or progressive, surgery is the best option.

Surgery for pronator syndrome, like carpal tunnel surgery, is done to release the offending structures.

The presence of a supracondylar process (bony spur on the inside of the arm bone) is rare. When a supracondylar process is present, the incision must be above the elbow.

However, the majority of cases of pronator syndrome are the result of pronator muscle compression, FDS sheath edge effect or the bicipital aponeurosis. These anatomically tight areas are all released though a small single forearm incision.

After surgery, a soft dressing is applied and motion exercises are begun immediately. Full recovery is expected within one month after surgery if severe muscle atrophy is not present.