Median Mononeuropathies

Disease/Disorder

Definition

Median mononeuropathy is defined as a lesion affecting the median nerve in isolation at any point along the course of the nerve, from the brachial plexus into the hand. The most common median nerve mononeuropathy from chronic compression at the level of the wrist¹ is carpal tunnel syndrome. For more information on carpal tunnel syndrome, please see the section titled “Carpal Tunnel Syndrome.” Median mononeuropathy proximal to the carpal tunnel can occur in the proximal arm, near the elbow (as in Pronator Syndrome), and in the forearm (as in Anterior Interosseous Syndrome).²

Etiology

• Compression
  • Under the ligament of Struthers, an anomalous structure between the supracondylar process of the humerus and the medial epicondyle
  • Along the tendinous edge of the deep head (most common) or between the two heads (ulnar and humeral) of the pronator teres (PT) muscle
  • Under the lacertus fibrosus (biceps aponeurosis) in the antebrachial fossa
  • Under the proximal arch (sublimis arch) of the flexor digitorum superficialis (FDS)
  • Gantzer’s muscle (accessory head of the flexor pollicis longus FPL muscle)
• Trauma
  • Humerus/supracondylar fractures (proximal arm)
  • Radius, ulna fractures (anterior interosseus AIN neuropathy)
  • Gunshot wounds (proximal arm) and lacerations
  • Iatrogenic: excessive traction or use of retractors²
  • Tourniquet injury
• Compartment syndrome²
• Tumors/schwanommas²
• Inflammatory/infectious conditions:
  • Idiopathic brachial neuritis resulting in AIN
  • CMV, Zoster, Hepatitis E infections
  • Diabetes, hypothyroidism (pronator syndrome)

Epidemiology

Pronator Syndrome

• Less than 1% of all median nerve entrapments³
• Second most common entrapment site of the median nerve after the carpal tunnel⁴
• Most common in ages 40 to 50 years⁴
• Risk factors include the following:
  • Female sex
  • Repetitive use of elbow, wrist, and hand
  • Volar forearm muscle hypertrophy, specifically the PT

Anterior Interosseous Syndrome

• Incidence is rare
• Risk factors include the following:
  • Anomalous fibrous bands arising from the PT or FDS muscles
  • Trauma
  • Inflammatory process (e.g., idiopathic brachial neuritis)

Patho-anatomy/physiology

• As the median nerve travels down the medial arm, it may pass beneath the ligament of Struthers (rare anomalous structure), which spans from a supracondylar spur to the medial epicondyle
• Then provides branches to the PT and flexor carpi radialis (FCR) just proximal to the elbow
• At the elbow, it passes deep to the lacertus fibrosus (biceps aponeurosis) and continues between the two heads of the PT
• Approximately 4-6 cm distal to the medial epicondyle, the anterior interosseous nerve (AIN) branches off as a pure motor nerve, supplying flexor pollicis longus (FPL), flexor digitorum profundus (FDP) to the index/middle fingers, and pronator quadratus
• The median nerve then courses deep to the FDS and its aponeurotic edge (the sublimis bridge)
• Motor innervation: PT, FCR, FDS, and in the hand to abductor pollicis brevis (APB), opponens pollicis, superficial head of flexor pollicis brevis (FPB), and the first and second lumbricals
• Sensory innervation: palmar cutaneous nerve to the palm (proximal to the wrist) and terminal digital branches to the palmar thumb, index, middle, and radial half of the ring finger

Essentials of Assessment

History

Pronator syndrome

• Insidious onset, more common in women, 4^th-5^th decade of life⁴
• Symptoms are not typically worse at night and paresthesias are not as well localized as in carpal tunnel syndrome⁶
• Aching discomfort in the forearm can be described as tiredness or heaviness
• Mild weakness
• Exacerbation by repeated or resisted forearm pronation and/or resisted or repeated proximal interphalangeal flexion of digits 2 and 3⁶

Anterior Interosseous Syndrome

• Compressive¹
  • Slow onset of symptoms
  • Pain is often not severe and localized to the forearm
  • Deficits are strictly confined to the AIN: no sensory changes
• Inflammatory: focal neuritis or part of acute brachial plexus neuropathy¹
  • Acute onset of severe pain in the forearm (if part of brachial plexopathy, pain can involve the shoulder and/or elbow)
  • Pain typically precedes weakness; weakness is most noticeable with thumb IP and index DIP flexion⁷
  • May have sensory symptoms if more than the AIN is involved

Physical examination

• Sensory findings
  • Pronator syndrome/entrapment at ligament of Struthers
    • Sensory deficits may be detected in the entire median territory, including the palm, which is spared in carpal tunnel syndrome (palmar cutaneous nerve branches proximal to carpal tunnel)
  • Anterior interosseous syndrome
    • No sensory deficits
• Motor findings
  • Pronator syndrome/entrapment at ligament of Struthers
    • Weakness may be detected in entire median territory
    • PT muscle weakness is usually spared in pronator syndrome (There is variable innervation to the pronator teres with the main branch of the median nerve providing innervation either proximal to or within the muscle itself¹)
    • Significant weakness rare, mild weakness of FPL and APB most common⁶
    • Muscle bulk of PT may be greater in symptomatic volar forearm
  • Anterior interosseous syndrome
    • Weakness may be detected in the FPL, pronator quadratus, and FDP (index and middle finger)
    • OK sign: inability to flex first and second DIP joint indicating FPL and FDP weakness
    • AIN deficits will result in flattening at the DIP joint
    • Resisted pronation when the elbow is fully flexed for pronator quadratus weakness in AIN²¹
• Special tests
  • PT compression test (or Tinel’s sign) with manually applied pressure proximal and lateral to the PT muscle belly volar to the forearm with positive test producing pain within 30 seconds.²¹

Functional assessment

• Review type of work performed, including tools used, forceful gripping, or twisting forearm movements
• Assess recreational activities that may involve repetitive use including, but not limited, to the following
  • Racquet sports
  • Rowing
  • Weightlifting
  • Chopping wood
  • Throwing
  • Arm wrestling

Imaging

Magnetic Resonance Imaging (MRI)

• MRI allows visualization of mass lesions and nerve enlargement or inflammation, and is superior in detecting intrinsic mass lesions such as neurofibromas or schwanomas¹⁰
• Signal changes or atrophy of the muscle innervated by the nerve in question are also seen on MRI¹⁰

Ultrasound

• Less expensive than MRI
• Can be more easily accessed when done in the office
• Can detect mass lesions and nerve enlargement (increased cross-sectional area) and loss of fascicular arrangement²
• Can provide dynamic imaging
• Can differentiate whether the nerve is in-continuity or not after trauma

Supplemental assessment tools

Electrodiagnostic Testing

Nerve Conduction Studies/Needle Electromyography (EMG)

• Nerve conduction studies
  • Sensory nerve conduction studies (sensory nerve action potentials or SNAP) to the digits and at least one median vs ulnar comparison at the wrist to exclude carpal tunnel; consider radial sensory and lateral antebrachial cutaneous nerve conduction studies to rule out upper trunk/lateral cord lesions
  • Motor nerve conduction studies (compound motor action potential CMAP) recording at APB wrist and antecubital fossa to the APB.
• EMG: Perform needle EMG of the APB, pronator teres, and one of the following: FPL, FCR, or FDS⁵
  • If the APB is abnormal, rule out lower trunk brachial plexopathy, polyneuropathy, or C8-T1 radiculopathy by testing other nonmedian lower trunk/C8-T1 muscles
  • If the proximal median muscles are abnormal, rule out proximal brachial plexopathy or C6-C8 radiculopathy by testing nonmedian C6-C7 and C7-C8 muscles

Pronator syndrome

• Reduced median compound muscle action potentials (CMAPs) and/or sensory nerve action potentials (SNAPs) amplitudes with relatively normal distal latencies⁵
• Conduction block/temporal dispersion or marked conduction velocity slowing between the wrist and elbow or between the elbow and axilla, with relatively normal CMAP distal latency
• Prolonged median F waves despite a relatively normal distal CMAP and distal latency
• Muscles distal to the lesion should demonstrate abnormalities (fibrillations and positive waves) on needle EMG if axon loss is present

Anterior interosseous syndrome

• Routine median SNAPs and CMAPs are normal
• Median motor study to the pronator quadratus may demonstrate evidence of demyelination and/or axon loss¹
• Needle EMG abnormalities such as fibrillations and positive sharp waves seen in the pronator quadratus, FPL, and FDP of digits 2 and 3 if axon loss is present

The following limitations exist

• If nerve conduction studies show a nonlocalized median neuropathy, needle EMG can only localize a lesion if motor axon damage is involved
• Variability of pronator teres innervation complicates lesion localization
• Axonal injury may result in distal conduction slowing via loss of fastest axons
• Concomitant median neuropathy at the wrist challenges localization of proximal lesion.
• Median neuropathy in the arm is difficult to evaluate due to lack of muscles innervated in the arm by the median nerve
• Electrodiagnostics cannot differentiate between neurotmesis and complete axonotmesis²

Environmental

• Controversy remains regarding the role of muscle hypertrophy and overuse in developing pronator syndrome
• Controversy remains regarding true entrapment of the AIN versus inflammatory etiology (focal neuritis, acute brachial plexus neuropathy) given the lack of abnormalities intraoperatively and variable response with surgery¹

Rehabilitation Management and Treatments

At different disease stages

Pronator Syndrome

• New onset/acute
  • Conservative management initially (trial of 3-6 months) has a high incidence of improvement^6,15
  • Avoidance of provocative activity
  • Splinting to neutralize forearm pronation/supination and elbow flexion
  • Physical therapy: modalities for inflammation and swelling, soft tissue mobilization, gentle range of motion, and graduated strengthening
  • Patient education
• Subacute
  • Corticosteroid injections into the PT muscle and activity modifications have been reported to give relief¹⁵
• No clinical improvement after conservative treatment with median nerve dysfunction
  • Extensive surgical exploration and decompression after failing conservative treatment at intervals of 9-12 months¹
  • Surgery may be expedited if blunt trauma or space-occupying lesions are derivatives.

Anterior Interosseous Syndrome

• New onset/acute
  • Due to self-limiting nature of AINS, conservative management (same as above) should be explored for up to 6mo as first line therapy. No significant difference in recovery time when compared to immediate surgical management^18-21
• No clinical improvement after conservative treatment
  • Surgical exploration and decompression
    • Timing for surgical intervention varies from after 8 weeks to up to 12 months^7,8,9

Patient & family education

• Ergonomics education
• Modification of recreational and work-related activities

Emerging/unique interventions

• Impairment-based measurement
  • American Medical Association’s Guides to the Evaluation of Permanent Impairment
• Measurement of patient outcomes
  • Disabilities of the Arm, Shoulder, and Hand score¹⁵

Translation into practice: Practice “pearls”/performance improvement in practice (PIPs)/changes in clinical practice behaviors and skills

Although routine history, physical examination, and electrodiagnostic studies can localize most cases of median neuropathy, the incorporation of neuromuscular ultrasound along the course of the nerve can aid in localization as well as provide structural information about the cause of median neuropathy; therefore, integration of neuromuscular ultrasound with electrodiagnostic studies should be considered

Cutting Edge/Emerging and Unique Concepts and Practice

• High-frequency ultrasound is becoming the imaging modality of choice for peripheral nerves and may be an additional tool to aid in diagnosis^2,19
• Ultrasound has been able to identify stenotic lesions of the AIN. Nagano¹³ reported good recovery in patients who underwent surgical neurolysis of these lesions. Closer attention to identifying these lesions may lead to better patient outcomes
• Endoscopically assisted release,²⁰ lacertus fibrosus release,¹¹ and deep fascial release¹⁷ of the superficial PT are novel surgical techniques being explored

Gaps in the Evidence-Based Knowledge

More outcome studies are needed to guide management. One area which may be particularly useful to the clinicians are longitudinal studies outlining residual nerve and muscular defects in those managed conservatively versus surgically. Ultrasound may also play a role in diagnosing proximal median nerve entrapments in individuals with normal electrodiagnostic studies (analogous to EMG-negative carpal tunnel syndrome).

References

1. Stewart JD. Focal Peripheral Neuropathies. Philadelphia, PA: JBJ Publishing; 2010.
2. Shields LB et al. Proximal median neuropathy: electrodiagnostic and ultrasound findings in 62 patients. Front Neuro 2024; 5, 15. doi: 10.3389/fneur.2024.1468813
3. Mercier LR. Practical Orthopedics. St Louis, MO: Mosby; 2005.
4. Lee, MJ, LaStayo, PC. Compressions that mimic carpal tunnel syndrome. J Orthop Sports Phys Ther. 2004; 34:601-609.
5. Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders. Philadelphia, PA: Elsevier; 2005.
6. Hartz CR, Linscheid RL, Gramse RR, Daube JR. The pronator teres syndrome: compressive neuropathy of the median nerve. J Bone Joint Surg Am. 1981; 63:885-890.
7. Nigst H, Dick W. Syndromes of compression of the median nerve in the proximal forearm (pronator teres syndrome; anterior interosseous nerve syndrome). Arch Orthop Trauma Surg.1979; 93:307-312.
8. Seror P. Anterior interosseous nerve lesion: clinical and electrophysiological features. J Bone Joint Surg Br. 1996; 78:238-241.
9. Futami T, Kobayashi A, Itoman M, Shimajiri I, Fujita T. Clinical investigation on the anterior interosseous nerve syndrome. J Jpn Soc Surg Hand. 1993;10:338-341.
10. Spratt JD, Stanley AJ, Grainger AJ, Hide IG, Campbell RS. The role of diagnostic radiology in compressive and entrapment neuropathies. Eur Radiol. 2002;12:2352-2364.
11. Hagert E.  Clinical diagnosis and wide-awake surgical treatment of proximal median nerve entrapment at the elbow: a prospective study. Hand (N Y). 2013;8(1):41–6.
12. Morris HH, Peters BH. Pronator syndrome; clinical and electrophysiological features in seven cases. J Neurol Neurosurg Psychiatry. 1976;39:461-464
13. Nagano A. SPONTANEOUS ANTERIOR INTEROSSEOUS NERVE PALSY. J Bone Jt Surg [Br]. 2003;85-B(3):313-321. doi:10.1302/0301-620x.85b3.14147
14. Miller-Breslow A, Terrono A, Millender LH. Nonoperative treatment of anterior interosseous nerve paralysis. J Hand Surg Am. 1990;15(3):493-496. doi:10.1016/0363-5023(90)90069-4
15. SOOD MK, BURKE FD. Anterior Interosseous Nerve Palsy: A review of 16 cases. Journal of Hand Surgery. 1997;22(1):64-68. doi:10.1016/s0266-7681(97)80020-4
16. Ulrich D, Piatkowski A, Pallua N. Anterior interosseous nerve syndrome: retrospective analysis of 14 patients. Arch Orthop Trauma Surg. 2011;131(11):1561-1565. doi:10.1007/s00402-011-1322-5
17. Svernlov B, Nylander G, Adolfsson L. Patient-reported outcome of surgical treatment of nerve entrapments in the proximal forearm. Adv Orthop. 2011;2011:727689.
18. Sotereanos, Dean G., and Loukia K. Papatheodorou. Compressive Neuropathies of the Upper Extremity A Comprehensive Guide to Treatment. Edited by Dean G. Sotereanos and Loukia K. Papatheodorou. 1st ed. 2020. Cham: Springer International Publishing, 2020.
19. Kodama A, Sunagawa T, Ochi M. Early treatment of anterior interosseous nerve palsy with hourglass-like fascicular constrictions by interfascicular neurolysis due to early diagnosis using ultrasonography: a case report. J Hand Surg Eur Vol. In press.
20. Lee AK, Khorsandi M, Nurbhai N, Dang J, Fitzmaurice M, Herron KA. Endoscopically assisted decompression for pronator syndrome. J Hand Surg Am. 2012;37(6):1173–9.
21. Zancolli ER III, Zancolli EP IV, Perrotto CJ.  New mini-invasive decompression for pronator teres syndrome. J Hand Surg Am. 2012;37(8):1706–10.

Original Version of the Topic

Christie M. Lehman, MD. Median nerve mononeuropathies. Original Publication Date 9/20/2014

Previous Revision(s) of the Topic

Nassim Rad, MD. Median nerve mononeuropathies. Original Publication Date 11/5/2019

Thomas Chai, MD, Christian Vangeison, DO, Colton Reeh, MD, Alex Wilkinson. Median Nerve Mononeuropathies. 12/29/2022

Author Disclosure

Chrissa McClellan, MD
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Weston Rogers, MD
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Ramya Palaniappan, DO
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Drake Laughlin MD
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