Median nerve mononeuropathies

Author(s): Christie M. Lehman, MD

Originally published:09/20/2014

Last updated:09/20/2014

1. DISEASE/DISORDER:

Definition

The median nerve is most commonly damaged at the level of the wrist1resulting in carpal tunnel syndrome, but more proximal lesions can occur. Pronator syndrome and anterior interosseous syndrome, or Kiloh-Nevin syndrome,2 can develop at the level of the proximal forearm and elbow region.

Pronator syndrome may mimic carpal tunnel syndrome symptoms with additional weakness in median forearm muscles and sensory changes in the thenar eminence. Anterior interosseous syndrome affects select muscles in the forearm and does not cause cutaneous sensory changes.

Etiology

Pronator syndrome

Possible compression sites include the following:

  1. Supracondylar region at the ligament of Struthers3
  2. Between the 2 heads of the pronator teres muscle
  3. Under the lacertus fibrosus (biceps aponeurosis)
  4. Beneath the flexor digitorum superficialis sublimis arch4

Anterior interosseous neuropathy

  1. Compression of the largest branch of the median nerve in similar locations to pronator syndrome, trauma, or inflammatory conditions1

Epidemiology including risk factors and primary prevention

Pronator syndrome

  1. Less than 1% of all median nerve entrapment5
  2. Second most common entrapment site of the median nerve after the carpal tunnel6
  3. Risk factors include the following:
    • Being a woman
    • Repetitive use of elbow, wrist, and hand movements
    • Volar forearm muscle hypertrophy, specifically the pronator teres
    • Anomalous fibro-osseous tunnel
      • Biceps aponeurosis (lacertus fibrosis)
      • Ligament of Struthers
      • Only found in about 1% of the population
      • Flexor digitorus sublimus arch
    • Most common in ages 40 to 50 years6

Anterior interosseous syndrome

  1. Incidence is rare
  2. Risk factors include the following:
    • Anomolous fibrous bands arising from the pronator teres or flexor digitorum superficialis muscles
    • Trauma-supracondylar fractures7
    • Inflammatory process (eg, focal neuritis, acute brachial plexus neuropathy)
    • Multifocal motor neuropathy

Patho-anatomy/physiology

Above elbow

  1. As the median nerve descends through the medial arm it may become compressed by the *ligament of Struthers. This tendinous band may cross over the nerve from a supracondylar spur just cephalad to the medial epicondyle.8

Elbow

  1. The median nerve runs beneath the *lacertus fibrosus or biceps aponeurosis before entering the forearm. The nerve typically continues between the 2 heads of the *pronator teres muscle. 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 itself.1
  2. The anterior interosseous nerve divides off approximately 4 to 6 cm distal to the level of the elbow at the superior border of the pronator teres muscle.4
  3. The anterior interosseous branch contains no superficial sensory branches. It dives posteriorly to provide innervation to the flexor pollicis longus, flexor digitorum profundus (index/middle), and pronator quadratus muscles.

Forearm

  1. The median nerve then passes deep to the flexor digitorus superficialis muscle and its aponeurotic tendinous edge, the *sublimus bridge.8
  2. It innervates the flexor carpi radialis and flexor carpi sublimis and continues through the forearm to the hand to innervate the abductor pollicis longus, opponens pollicis, and first and second lumbricals.
  3. Proximal to the wrist, the palmar cutaneous branch divides and provides sensation to the thenar eminence. Terminal fibers of the median nerve provide sensation to the palmar surface of the thumb and second, third, and radial half of the fourth digit.

*Areas of possible compression.

Disease progression including natural history, disease phases or stages, disease trajectory (clinical features and presentation over time)

Disease progression appears to depend on whether there is a compressive lesion or an inflammatory cause.

  1. Compressive
    • Onset may be insidious
    • Pain is less severe and worsened by repetitive activity
    • Slowly progressive weakness, loss of dexterity, clumsiness, and fatigue
    • Surgical decompression may provide relief9,10
  2. Inflammatory
    • Onset is typically acute
    • Pain can be severe but short lasting
    • Muscle weakness may be quite severe in the acute phase

Some advocate that conservative treatment be considered for a year or more because late spontaneous recovery can occur.11,12

2. ESSENTIALS OF ASSESSMENT

History

Pronator syndrome

  1. Symptoms are not typically worse at night like carpal tunnel syndrome8
  2. Aching discomfort in the forearm
  3. Weakness in flexor pollicis longus, median sensory loss, and positive tinels over pronator13
  4. Exacerbation by repeated or resisted forearm pronation and/or resisted or repeated proximal interphalangeal flexion of digits 2 and 39

Anterior interosseous syndrome

  1. Compressive1
    • Slow onset of symptoms
    • Pain is often not severe
    • Pain is localized to the forearm
    • Deficits are strictly confined to the anterior interosseous nerve (AIN): no sensory changes
  2. Inflammatory: focal neuritis or part of acute brachial plexus neuropathy1
    • Pain is often severe, involving the shoulder, elbow, and/or foreram
    • Pain typically precedes neurologic symptoms
    • Motor deficits reach maximum rapidly within several days
    • May have sensory symptoms if more than the AIN is involved
    • Involvement of other upper limb muscles

Physical examination

  1. Cervical spine, shoulder, elbow, and wrist evaluation
  2. Findings may be normative in mild cases
  3. Sensory findings
    • Pronator syndrome/ligament of Struthers
      • Sensory deficits may be detected in the thenar eminence and median innervated fingers
    • Anterior interosseous syndrome
      • No sensory deficits
  4. Motor findings
    • Pronator syndrome/ligament of Struthers
      • Weakness may be detected in median and AIN innervated muscles ◦Anterior interosseous syndrome
      • Weakness may be detected in the flexor pollicis longus, pronator quadratus, and flexor digitorum profundus (index and middle)
      • OK sign: inability to flex first and second distal interphalangeal joints
      • Provocative tests
      • Tinel sign over the pronator or proximal to the medial epicondyle
  5. Muscle bulk may be greater in symptomatic volar forearm

Functional assessment

  1. Review type of work performed, technique, tools used, forceful gripping, or twisting
  2. Assess recreational activities that may involve repetitive use including, but not limited, to the following:
    • Racquet sports
    • Rowing
    • Weight lifting
    • Chopping wood
    • Throwing

Laboratory studies

Laboratory studies may be used to evaluate for conditions that may predispose to compressive/inflammatory neuropathies.

Imaging

Magnetic resonance imaging (MRI)

  1. MRI allows visualization of mass lesions and nerve enlargement or inflammation14
  2. Signal changes or atrophy of the muscle innervated by the nerve in question is also seen on MRI15

Ultrasound

  1. Less expensive than MRI
  2. Can be more easily accessed when done in the office
  3. Can detect mass lesions and nerve enlargement
  4. Can provide dynamic imaging

Ultrasound imaging demonstrated the anomalous ligament of Struthers and was deemed a useful alternative to MRI.16

Supplemental assessment tools

Electrodiagnostic Testing

Anterior interosseous syndrome

  1. Median motor study to the pronator quadratus may demonstrate evidence of demyelination and/or axon loss.1
  2. Demonstrate electromyographic abnormalities of the pronator quadratus, flexor pollicis longus, and flexor digitorum profundus of digits 2 and 3.
  3. Electromyography (EMG) of other median muscles is normative.
  4. Median sensory nerve action potentials (SNAPs) and compound muscle action potentials (CMAPs) to the abductor pollicis brevis are normative.

Pronator syndrome

  1. Reduced median CMAP and/or SNAP amplitudes with relatively normative distal latencies.8
  2. Conduction block/temporal dispersion or marked conduction velocity slowing between the wrist and elbow or between the elbow and axilla, with relatively normative CMAP distal latency.
  3. Prolonged median F waves despite a relatively normative distal CMAP and distal latency.

Electromyographic abnormalities may not be present in pronator syndrome.1If motor axon damage is present, muscles distal to the lesion should demonstrate abnormalities.

  1. Perform needle EMG of the abductor pollicis brevis, pronator teres, and one of the following: FPS, flexor carpi radialis, or FDS.8
  2. If the abductor pollicis brevis is abnormal, rule out plexopathy, peripheral neuropathy, or C8-T1 radiculopathy by testing other nonmedian lower trunk/C8/T1 muscles.
  3. If the proximal median muscles are abnormal, rule out plexopathy or C6-8 radiculopathy by testing nonmedian C6-7 and C7-8 muscles.

The following limitations exist:

  1. If nerve conduction studies show a nonlocalized median neuropathy, EMG can only localize a lesion if motor axon damage is involved.
  2. Variability of pronator teres innervation complicates lesion localization by EMG if axon loss is present.
  3. Axonal injury may result in distal conduction slowing via loss of fastest axons.
  4. Concomitant median neuropathy at the wrist challenges localization of proximal lesion.

Early predictions of outcomes

Not reported.

Environmental

  1. Controversy remains regarding the role of muscle hypertrophy and overuse in the development of pronator syndrome.
  2. 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 surgery.1

3. REHABILITATION MANAGEMENT AND TREATMENTS

Available or current treatment guidelines

No official guidelines.

At different disease stages

Pronator syndrome

  1. New onset/acute
    • 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
  2. Subacute
    • Corticosteroid injections have been reported to give relief13
  3. No clinical improvement after conservative treatment with median nerve dysfunction
    • Extensive surgical exploration and decompression1

Anterior interosseous neuropathy

  1. New onset/acute
    • Same as previously listed
  2. No clinical improvement after conservative treatment
    • Surgical exploration and decompression
      • Timing for surgical intervention varies from after 8 weeks to up to 12 months10-12
      • Compressive or inflammatory?
      • Consider early decompression if felt to be compressive and worsening1

Patient & family education

  1. Ergonomics education
  2. Modification for recreational and work-related activities

Emerging/unique Interventions

  1. Impairment-based measurement
    • American Medical Association’s Guides to the Evaluation of Permanent Impairment
  2. Measurement of patient outcomes
    • Disabilities of the Arm, Shoulder, and Hand score17

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

  1. Thenar eminence sensory changes should warrant proximal investigation.
  2. Check the OK sign.
  3. In anterior interosseous syndrome, when a Martin-Gruber anastomosis is present, there may be associated weakness of the second and fifth digit abduction and adduction of the first digit because of the ulnar fiber cross-over.
  4. A normative physical exam does not not rule out pronator syndrome, anterior interosseous syndrome, or ligament of Struthers entrapment.

4. CUTTING EDGE/EMERGING AND UNIQUE CONCEPTS AND PRACTICE

Cutting edge concepts and practice

  1. High-frequency ultrasound is becoming the imaging modality of choice for peripheral nerves.18Ultrasound imaging may be an additional tool to aid in diagnosis.
  2. Ultrasound imaging was able to identify stenotic lesions of the AIN representing sites of internal fascicular restriction in a patient with sudden onset of severe pain in the volar proximal forearm with progressive severe weakness over several weeks. Surgical neurolysis of these lesions was performed, and the patient had good recovery.19
  3. Internal fascicular restriction
    • Nagano20reported good recovery in patients who underwent surgical neurolysis of these lesions in the AIN. Closer attention to identifying these lesions may lead to better patient outcomes.

5. GAPS IN THE EVIDENCE-BASED KNOWLEDGE

Gaps in the evidence-based knowledge

More outcome studies are needed to guide management.

REFERENCES

1. Stewart JD.Focal Peripheral Neuropathies. Philadelphia, PA: JBJ Publishing; 2010.

2. Kiloh L, Nevin S. Isolated neuritis of the anterior interosseous nerve.Br Med J. 1952;1:850-851.

3. Bilge T, Yalaman O, Bilge S, Cokneseli B, Barut S. Entrapment neuropathy of the median nerve at the level of the ligament of Struthers.Neurosurgery. 1990;27:787-789.

4. Johnson RK, Spinner M, Shrewsbury MM. Median nerve entrapment syndrome in the proximal forearm.J Hand Surg Am.1979;4:48-51.

5. Mercier LR.Practical Orthopedics. St Louis, MO: Mosby; 2005.

6. Lee, MJ, LaStayo, PC. Compressions that mimic carpal tunnel syndrome.J Orthop Sports Phys Ther. 2004;34:601-609.

7. Lipscomb PR, Burleson RJ. Vascular and neural complications in supracondylar fractures in children.J Bone Joint Surg Am. 1955;37:487-492.

8. Preston DC, Shapiro BE.Electromyography and Neuromuscular Disorders. Philadelphia, PA: Elsevier; 2005.

9. 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.

10. 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.

11. Seror P. Anterior interosseous nerve lesion: clinical and electrophysiological features.J Bone Joint Surg Br. 1996;78:238-241.

12. 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.

13. Morris HH, Peters BH. Pronator syndrome; clinical and electrophysiological features in seven cases.J Neurol Neurosurg Psychiatry. 1976;39:461-464.

14. 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.

15. Sallomi D, Janzen DL, Munk PL, Connell DG, Tirman PF. Muscle denervation patterns in upper limb nerve injuries: MR imaging findings and anatomic basis.AJR Am J Roentgenol. 1998;171:779-784.

16. Camerlinck M, Vanhoenacker FM, Klekens G. Ultrasound demonstration of Struther’s ligament.J Clin Ultrasound. 2010;38:499-502.

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. Strakowski JA.Ultrasound Evaluation of Focal Neuropathies: Correlation with Electrodiagnosis. New York, NY: Demos Medical Publishing; 2014.

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. Nagano A. Spontaneous anterior interosseous nerve palsy.Br J Bone Joint Surg.2003;85:313-318.

Author Disclosure

Christie M. Lehman, MD
Nothing to Disclose

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