Pronator Teres Syndrome

Etiology

Quick and repetitive grasping or pronation movements (prolonged hammering, ladling food, cleaning dishes, tennis) may cause PT muscle hypertrophy and entrapment of MN, especially in those individuals who have additional fibrous brands. [7] Pronator teres syndrome has been described after local trauma, compression with Schwanomma, and in patients undergoing anticoagulation therapy and renal dialysis. [7][8][9] Tight lacertus fibrosis (also known as the bicipital aponeurosis) may exacerbate the symptoms of pronator teres syndrome.

Epidemiology

Pronator teres syndrome is a rare condition that may be easily overlooked and mistaken for the more prevalent carpal tunnel syndrome (CTS). [10][11] No age group preference has been described, but one study reports a higher prevalence of pronator teres syndrome in men. [10]

History and Physical

Pronator teres syndrome may manifest with pain in the volar forearm region, aggravated by resisted pronation of the forearm and flexion of the elbow, and a positive Tinel sign over the proximal edge of PT. [7]  The patient may report significant weakness. Wasting of the median nerve innervated muscles is rare in pronator teres syndrome, but a mild weakness of flexor pollicis longus (FPL) and abductor pollicis brevis (APB) is not uncommon, with some involvement of flexor digitorum profundus (FDP) to digits 2 and 3 and opponens pollicis (OP). The PT itself is usually spared because it received innervation before the MN pierces it. Sensory loss is variable, involving the palm of the hand or mimic that of carpal tunnel syndrome, including the thenar eminence, thumb, index, middle, and ring fingers. A positive Phalen test over the PT muscle can be present in 50% of cases.[7]

Evaluation

It is important that nerve conduction studies (NCS) are done in pronator teres syndrome to rule out other neuropathies, but they seldom show abnormalities. [1] Sensory and motor amplitudes are reduced more than conduction velocities, mostly in patients with severe and axonal symptoms. [7][10] Electromyography (EMG) abnormalities occur in FPL and FDP to digits 2 and 3, less often in the FDS and APB, and only rarely in PT because compression at the site most often occurs distal to its innervation. If any of the median muscles are abnormal, other muscles innervated by the same myotomes as the proximal median muscles but supplied by a different nerve should be tested to exclude more proximal lesions within the brachial plexus or cervical roots. Some authors report abnormal electrodiagnostic findings in 10% of pronator teres syndrome only. [8][12]

Pronator teres syndrome, among other entrapments within the upper limb, also can be diagnosed by ultrasound and magnetic resonance imaging, but ultrasound is advantageous due to its dynamic character and lower cost. [3][10] Several studies with an ultrasound evaluation of median nerve between the humeral and ulnar heads of PT concluded that cross-sectional area of MN positively correlates with severity, duration of symptoms, and nerve conduction failure. [10][13]

Treatment / Management

Conservative treatment includes rest, modification of activities that exacerbate the symptoms, physical and occupational therapy, nonsteroidal anti-inflammatory medications, and local injections with corticosteroids or local anesthetic. [3][14] Patients usually may continue to work unless they present with prominent motor or sensory deficits that limit work activities. Conservative treatment should be conducted for at least 6 weeks. If conservative treatment fails, surgery is considered.

Surgical treatment is indicated if a patient is symptomatic, has one or more objective findings in a physical examination of weakness or motor atrophy, and has abnormal electrodiagnostic studies. [14] Surgery usually includes exploration of MN in the forearm and release of PT muscle as well as all other compressive structures – ligament of Struthers, lacertus fibrosis, and/or fascia of flexor digitorum superficialis. Although MN is sometimes decompressed at multiple sites, this is considered a single procedure. [14]

An endoscopic release has advantages over open surgery because it does not compromise the nerve's blood supply, reduces scar formation, and has been reported to have excellent results. [6] In the case of recurrent symptoms, open surgery has been indicated. [6](B2)

Differential Diagnosis

Due to similar clinical presentation, some authors combine pronator teres syndrome with the following proximal median nerve entrapment syndromes [8][12]:

1. Ligament of Struthers entrapment - MN compression by a ligament running from medial epicondyle to a bony spur on a distal medial humerus, which is present only in 1-2 % of the population. The pain in the forearm and paresthesias in the median-innervated digits is exacerbated by supination of the forearm and extension of the elbow (versus pronation in PTS). The radial pulse may also be decreased as the brachial artery runs next to the median nerve.
2. MN entrapment by hypertrophied lacertus fibrosis  (bicipital aponeurosis) - This is fascia attaching the biceps to the ulna and overlies the MN in the proximal forearm. The pain in the forearm and paresthesias in the median-innervated digits is exacerbated by resisted flexion of the elbow with the forearm in supination.
3. MN compression by the sublimus bridge of the flexor digitorum superficialis muscle - The pain in a forearm and paresthesias in the median-innervated digits is exacerbated by resisted flexion of the proximal interphalangeal joint of the middle finger while other fingers are held in extension.

Pronator teres syndrome should also be distinguished from carpal tunnel syndrome (CTS), MN entrapment at the wrist, which is the most common MN entrapment syndrome, as well as from AIN syndrome, brachial plexus injury, or cervical radiculopathy.

Pronator teres syndrome has a sensory loss in the entire median nerve distribution versus CTS, where sensation is spared over the thenar eminence because palmar cutaneous branch leaves the median nerve proximal to carpal tunnel but distal to PT muscle. In addition, pronation is spared in CTS, and nocturnal paresthesia symptoms are usually absent in pronator teres syndrome. [5] NCS and amplitude of the MN may decrease in the forearm, but the distal motor and sensory latencies are normal in pronator teres syndrome except when there is associated CTS. [1][10][15][16]

Because of overlapping symptoms, CTS is sometimes diagnosed, and more proximal pronator teres syndrome is missed when both are present in the same limb. [16] Thus in patients with CTS, pronator teres syndrome should be ruled out, especially when the patient is a candidate for surgery. [10][15]

To distinguish AIN from pronator teres syndrome, the pronation should be demonstrated with the elbow flexed (PQ function) to avoid the contribution of the PT, which is the primary muscle to pronate with the arm extension. [17][18] Also, there is no sensory loss in AIN syndrome (as it carries only deep sensory fibers to the wrist), and in some cases, pronator teres syndrome may have only mild paresthesias in MN distribution. In both cases, FPL and FDP to digits 2 and 3 may be affected; but in AIN, the pronator quadratus (PQ) does not weaken the PT as AIN leaves MN distal to PT muscle. Clinically, in both cases, the patient presents with an inability to flex the distal phalanx of the thumb, index, and middle fingers and weakness of pronation.

In the case of brachial plexus injury or cervical radiculopathy, the examination may reveal a weakness in other muscles outside the median nerve territory. Neck pain that radiates to the arm is a common complaint, suggesting cervical radiculopathy.

One group of authors describe a neuralgic amyotrophy as a potential competing condition with pronator teres syndrome due to similar symptoms, but it usually involves multiple nerves, and pain is more commonly localized in the shoulder rather than in the forearm. Neuralgic amyotrophy is an idiopathic condition that usually improves spontaneously in 6 to 12 months. [14]

Prognosis

Pronator teres syndrome only requiring PT muscle release has a good recovery in most of the cases, and patients return to light duty in approximately 3 weeks and regular duty in 6 weeks. [14]. Occupation therapy fastens recovery and is particularly important for patients who underwent tendon transfers or have a residual weakness. In these cases, patients may return to light-duty work in approximately 6 to 8 weeks and regular duty in 10 to 12 weeks. [14]

Complications

Complications of surgical treatment are rare. In one study of 72 patients treated with pronator teres syndrome, no complications were recorded during the operations with an overall postoperative satisfaction rate (very satisfied and satisfied) of 59%. [19] Theoretical complications from surgery include:

• Infection
• Seroma/hematoma formation
• Nerve injury
• Scar formation