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Pronator Syndrome

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Pronator Syndrome
Kevin E. Burroughs
Basics
Seyffarth 1st described compression of the median nerve as it passes through the pronator teres just distal to the elbow in 1951.
Description
  • Median nerve entrapment that occurs (1) within the ligament of Struthers, (2) within the lacertus fibrosis, (3) between the humeral and ulnar heads of the pronator teres muscle, or (4) at a tight fibrous arch proximally between the heads of the flexor digitorum superficialis, the sublimis arch.
  • Compression between the heads of the muscle is the most common etiology, with the sublimis arch the 2nd most common.
  • Synonym(s): Lacertus fibrosis syndrome; Pronator teres syndrome; Sublimis bridge syndrome
Epidemiology
  • Relatively uncommon but should be considered in any patient with carpal tunnel symptoms or volar hand numbness
  • Carpal tunnel syndrome is responsible for over 90% of cases of median nerve entrapment neuropathies.
Prevalence
  • Usually presents in the 5th decade, and 4× more common in women
  • Delay in diagnosis ranging from 9 mos to 2 yrs
Risk Factors
  • Repetitive occupational pronation/supination
  • Acute forceful pronation or wrist flexion
  • Weight lifters with hypertrophied pronator-flexor mass
Etiology
  • In over 80% of individuals, the pronator teres arises from 2 heads, superficial (humeral) and deep (ulnar), and the median nerve travels between as it enters the antecubital area.
  • Variants have been demonstrated where the nerve travels posterior to both heads, under a single head, or pierces the superficial head.
  • Compression can occur proximally in individuals with a supracondyloid process. This is an anomalous bone spur on the anteromedial distal 1/3 of the humerus and occurs in 1% of people of European ancestry.
Diagnosis
History
  • Typically patients complain of an aching or fatigue-like pain in proximal forearm. Other symptoms may include occasional hand pain, dysesthesias of the radial 3½ digits, cramping in the hand (writer's cramp).
  • Many report an increase in pain with exercise or an increase in activity.
  • Frank hand weakness occurs only in long-standing or severe cases.
  • Motor and sensory symptoms can be poorly defined.
  • Onset typically is insidious, but a specific event may cause a sudden increase in pain.
  • Typically, pain does not occur at night, as it would with carpal tunnel syndrome.
Physical Exam
  • Thickened or hypertrophied pronator muscle mass
  • Tenderness or positive Tinel sign in the proximal anterior forearm over the pronator muscle, with radiation to the palm and/or radial 3½ digits
  • May have weakness or atrophy of the hand intrinsics innervated by the median nerve (LOAF muscles): lumbricals (1st 2 on radial side of hand), opponens pollicis, abductor pollicis brevis, and flexor pollicis brevis
  • May have weakness of the extrinsic finger flexors, wrist flexors, and pronator quadratus
  • Tests of Spinner can determine location of median nerve entrapment.
    • Pain or paresthesia reproduced with resisted flexion of the forearm in a position of >120 degrees of elbow extension, maximal supination: Lacertus entrapment of median nerve
    • Reproduction of pain or paresthesia with resisted pronation (forearm in neutral) as the elbow is extended: Pronator entrapment of median nerve
    • Reproduction of symptoms with resisted proximal interphalangeal (PIP) joint flexion of the middle finger with forearm fully supinated and elbow extended: Flexor digitorum superficialis entrapment of median nerve

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Diagnostic Tests & Interpretation
Imaging
  • Plain radiographs of the elbow rule out bone involvement such as a supracondyloid process (exostosis that attaches to ligament of Struthers).
  • MRI:
    • Since the median nerve often is poorly depicted at the elbow because of the minimal amount of perifascial fat in this region, MRI is not indicated for the diagnosis of pronator syndrome but may be used for excluding other etiologies. The nerve can look normal at the site of entrapment.
    • The pronator teres and other muscles innervated by the median nerve distally to the site of the lesion may show abnormally high signal intensity on T2-weighted fat-suppressed, STIR (Short TI Inversion Recovery), or T1-weighted images in more advanced cases.
Diagnostic Procedures/Surgery
Electrodiagnostic testing:
  • Needle electromyography (EMG) is the most helpful to identify evidence of denervation.
  • Symptoms often must persist for minimum of 4–6 wks before positive findings on EMG.
  • Nerve conduction may be normal despite compressive pathology.
  • Some report that only 10% of patients show abnormal EMG findings that support the clinical diagnosis.
  • In the electrodiagnostic evaluation of carpal tunnel syndrome, EMG of muscles innervated by the median nerve proximal to the wrist should be performed to decrease unnecessary carpal tunnel surgeries.
Differential Diagnosis
  • Carpal tunnel syndrome (common): Double crush phenomena can occur.
  • C6 or C7 radiculopathy (common)
  • Anterior interosseous syndrome (Kiloh-Nevin syndrome) (uncommon)
  • Thoracic outlet syndrome (rare)
  • Neuralgic amyotrophy (brachial neuritis, Parsonage-Tuner syndrome) (rare)
Treatment
Rehabilitation is the initial treatment and can be summarized using the acronym APORIM:
  • Activity modification: Avoidance of provocative activities
  • Protection from external compression
  • Orthoses: Night splints to prevent excessive elbow flexion
  • Rehabilitation: Assess proximal and distal kinetic chain (shoulder girdle and wrist) for strength, flexibility, and movement.
  • Injections can be considered at point of compression, but exercise caution to avoid direct injection into nerve bundle.
  • Medication: Nonsteroidal or steroid anti-inflammatory medications
Surgery/Other Procedures
  • If conservative measures fail, or if muscle atrophy or denervation occurs, one should consider exploration.
  • For a proven supracondyloid process, excision of the bone prominence and release of the ligament of Struthers should provide resolution.
  • For evaluation of entrapment within the forearm, an incision begins in the distal arm about 5 cm above the elbow and along the medial aspect of the biceps muscle. The incision curves toward the lacertus fibrosus at the elbow crease and then continues distally over the flexor-pronator mass. Dissection distally should ensure visualization of the nerve to its course at the flexor superficialis origin.
Additional Reading
Bencardino JT, Rosenberg ZS. Entrapment neuropathies of the shoulder and elbow in the athlete. Clin Sports Med. 2006;25:465–87, vi–vii.
Dawson DM, Hallett M, Wilbourn AJ, eds. Median nerve entrapment in entrapment neuropathies. Philadelphia: Lippincott-Raven Publishers, 1999:99–111.
Dumitru D. Etectrodiagnostic medicine. Philadelphia: Hanley & Belfus, 1994:856–867.
Hartz CR, Linscheid RL, Gramse RR, et al. Pronator teres syndrome: compressive neuropathy of the median nerve. J Bone Joint Surg. 1981;63A:885–890.
Kopell HP, Thompson WA. Pronator syndrome: a confirmed case and its diagnosis. N Engl J Med. 1958;259:713–715.
Tetro AM, Pichora DR. High median nerve entrapments. An obscure cause of upper-extremity pain. Hand Clinics. 1996;12:691–703.
Werner CO, Rosen I, Thorngren KG. Clinical and neurophysiologic charateristics of the pronator syndrome. Clin Ortho Related Res. 1985;197:231–236.
Wiggins CE. Pronator syndrome. South Med J. 1982;75:240–241.
Codes
ICD9
354.1 Other lesion of median nerve
Clinical Pearls
  • Prognosis usually depends on the severity of median neuropathy. Worse prognosis with extensive axon loss and atrophy. Expect some reinnervation of distal musculature once the median nerve is free from compromising structures.
  • Symptoms of pronator syndrome typically do not occur at night. Also, wrist splints typically do not improve symptoms.


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