HIGH MEDIAN NERVE ENTRAPMENT

Course of median nerve

  • From medial and lateral cords of brachial plexus - (C5), C6-T1
  • Startsantero-lateral to artery, lies over it in midportion and then to its medial side at the elbow coursing between brachialis muscle and medial intermuscular septum
  • nerve consistently resides anterior to the brachialis muscle and deep to the lacertus fibrosus.
  • Enters the cubital fossa lateral to brachialis tendon, under the bicipital aponeurosis. Biceps tendon, brachial artery, and median nerve are arranged from lateral to medial respectively.
  • Continues most commonly between deep (ulnar) and superficial (humeral) heads of pronator teres(82%) 4cm below the humeral epicondylar line. May pass posterior to both heads(7%) or pierce the superficial head. (2%).) In 9% deep head is absent
  • Enters the forearm deep to the FDS arch (7cm below the humeral epicondylar line)
  • Gives off branches to Pronator, Palmaris longus, FCR and FDS, and rarely FDP.
  • first branch, off the medial aspect approximately 1 cm to 4 cm proximal to the elbow, penetrates and innervates the pronator teres muscle
  • AIN branches off immediately distal to the proximal border of the superficial head of pronator (5-8cm below epicondylar line) - arises from the posterolateral surface of the nerve.
  • AIN innervates radial half of FDP, FPL and pronator quadratus
  • branches to the FPL and FDP arise at the tendinous origin of the FDS (approximately 4 cm. distal to AIN origin
  • Surface marking – groove behind coracobrachialis to medial to brachial artery at cubital fossa

Sites of high entrapment

  1. Supracondylar process/ligament of Struthers

a) Suppressed head of coracobrachialis extending from supracondylar process to medial epicondyle

b) Present in 0.7-3%, more common on the left than the right and more common in females than males.

c) presence of pronator weakness distinguishes this syndrome from pronator and anterior interosseous syndromes

  1. lacertus fibrosus

a) at front of elbow, median nerve lies behind bicipital aponeurosis (lacertus fibrosus) and in front of the brachialis

b) often associated with brachialis hypertrophy

c) suspect the lacertus fibrosis with pain in the forearm with resisted elbow flexion beyond 120 in supination - can also be caused by pressure from the origin of the FDS

  1. Pronator Teres

a) Pain with forced flexion of middle finger, resisted pronation in full extension

  1. FDS arch
  2. Accessory muscles in forearm

a) Gantzer’s muscle – accessory head of FPL (present in 45%)

b) Palmaris profundus

c) Flexor carpi radialis brevis

Pronator Teres Syndrome

  • Due to compression of above structures(proximal to AIN)
  • most common areas of compression are at the level of the FDS arch or by the nerve passing through the pronator teres muscle
  • A great mimicker:
  • ". . . many patients with CTS have been found to have a nerve compression at a higher, more proximal level, such as the elbow." – Dellon
  • “The significance of the pronator syndrome is that it can easily be mistaken for carpal tunnel compression and the wrong treatment undertaken.” - Lister

Epidemiology

  • presents in the fifth decade and is four times more common in women

Clinical

  • insidious in onset, with a delay in diagnosis ranging from 9 months to 2 years
  • Clinical examination is often very characteristic, with tenderness and, frequently, firmness noted over the pronator teres muscle.
  • most commonly complains of aching pain in the proximal volar forearm and the distal arm on pronation/supination
  • often aggravated by use of the upper limb, especially with resisted forceful pronation or repeated pronation/supination movements
  • usually associated paresthesias and, possibly, altered sensibility in the radial three and a half digits, but night awakenings and nocturnal pain are uncommon- a key symptom to help differentiate from CTS
  • early fatigue of the forearm muscles is seen with repetitive stressful motion, especially pronation;
  • Atrophy of the forearm musculature is uncommon.
  • despite their anatomic proximity, pts with pronator teres syndrome do not have a higher incidence of AIN syndrome
  • Tinel's sign over the median nerve at the level of the pronator teres is frequently positive but often takes 4 to 5 months to develop

Provocation tests

  1. Pronator compression:Resisted forearm pronation in full extension (in elbow flexion, only deep(ulnar) head is tense)
  2. FDS arch compression: Resisted contraction of the FDS to the middle finger
  3. Lacertus Fibrosis:resisted flexion of the forearm in a position of greater than 120 degrees of elbow flexion, maximal supination

Electrodiagnostics

  • rarely diagnostic for pronator syndrome.
  • Even with good history and examination findings, <50% can be confirmed with electrodiagnostics.
  • Slowing of motor and sensory conduction between the wrist and elbow is neither specific nor diagnostic for proximal median nerve entrapment.
  • Patients with CTS may have slowing of median nerve conduction for a variable distance proximal to the wrist.
  • most valuable tool in the electrophysiologic studies is needle electromyography. The diagnosis of pronator syndrome is supported in some cases in which denervation potentials are noted in median nerve-innervated- muscles.
  • value of electrophysiologic studies is primarily to help rule out other conditions that have similar clinical presentations, such as CTS, and to determine coexistent sites of compression (double crush syndrome or multiple nerve entrapment)

Criteria for diagnosis of pronator syndrome (Johnson and Spinner)

  1. Absence of night symptoms
  2. Reproduction of symptoms with use of blood pressure cuff
  3. Variable weakness of median nerve innervated muscles, muscles of AIN (fpl, fdp, quad) are relatively spared;
  4. Muscle spasm / cramping with repetitive pinch-type exercise
  5. Pronator teres tenderness / firmness to palpation
  6. increased paresthesias of radial three and a half digits with weak compression of pronator
  7. Positive Tinel's sign over pronator teres
  8. Reproduction of symptoms with provocative test
  9. Abnormal electrophysiological study

Differential diagnosis

  1. thoracic outlet syndrome, proximal brachial plexus neuropathies, cervical radiculopathy, and polyneuropathy.
  2. If principally involves C6,7 - cause numbness of thumb, index, and long fingers, and the median nerve innervated muscles of the forearm; The correct diagnosis is made by establishing the function of the muscles innervated by the C6-C7 portions of the radial nerve (ie thefunction of the wrist extensors and the triceps);
  3. carpal tunnel syndrome
  4. note that when pronator syndrome is present, CTS is frequently present (Phalen test positive in up to 50%)

Similarities and differences between pronator syndrome and carpal tunnel syndrome

  1. Similarities
  2. Pain in wrist and forearmregions
  3. Weakness of thenar muscles
  4. Numbness/paresthesias of radial three and a half digits
  5. Differences
  6. No nocturnal pain in pronator syndrome
  7. Negative Tinel's sign at wrist in pronator syndrome
  8. Nerve conduction study-no delay at wrist in pronator syndrome
  9. Dysesthesia in palmar cutaneous distribution in pronator syndrome

Management

Non-Operative

  • Indicated for mild cases (majority)
  • Avoidance of aggravating activities, rest of the extremity and nonsteroidal anti-inflammatories.
  • Splint – long arm removable splint in 90 elbow flexion; elbow in slight pronation and wrist in slight flexion
  • corticosteroid injection in the region of the pronator teres has been reported
  • 50% success rate with conservative treatment.
  • generally regarded that 8 to 12 weeks of conservative treatment are warranted before recommending surgery

Operative

  • pathologic features seen at the time of surgery are often minimal seems to correlate with the vagueness of the symptoms, lack of specific clinical findings, and, often, nonspecific electrophysiologic studies.
  • Exploration of the entire proximal forearm region, examining all possible sites, therefore should be done to rule out multiple or unusual compression sites.
  • Incision: Lazy S at least 5cm proximal to crease (10cm if supracondylar spur present), curving laterally at the crease and then back medially 5cm in forearm
  • Identify medial cutaneous nerve of the forearm and the terminal fibers of the lateral cutaneous nerve of forearm
  • deep fascia of the arm is divided to expose the median nerve and brachial artery at the medial edge of the biceps.
  • Lacertus fibrosus is completely divided along its length to continue the distal exposure of the median nerve
  • If a supracondylar process or Struther's ligament is present, it is excised
  • "safe side" of the nerve is the radial/lateral side because the proximal motor branches to the flexor/pronator mass arise ulnarly.
  • infrequently, a Z-lengthening of the superficial head is required in severe cases of scarring (such as in Volkmann's contracture). If so, the superficial head is rerouted deep to the nerve.
  • More distal dissection is most often carried out by opening the interval between the pronator teres and FCR. The FDS tendinous arch, if present, is readily visible and divided.
  • Dellon advocates epineural and, possibly, intraneural neurolysis.

Outcome

  • most series report approximately 85% to 90% good to excellent outcomes.
  • Mayo Clinic series noted the best results occurring when a definite anatomic constriction was found.

Anterior Interosseous Syndrome

Epidemiogy

  • accounts for fewer than 1% of all upper extremity neuropathies

Aetiology

  1. "Pseudo-anterior interosseous nerve syndrome"-a partial lesion of the median nerve proximally, involving primarily the fascicles of the AIN
  2. Trauma
  3. Fractures (supracondylar humeral fractures, forearm fractures)
  4. Dislocations-of elbow Penetrating injuries-missile, stab wounds Crush injuries
  5. Repetitive trauma with muscle hypertrophy
  6. latrogenic
  7. Arterial or venous access (cutdowns, catheterization, venepuncture)
  8. Muscle release
  9. Open reduction/internal fixation of fractures
  10. Anatomic
  11. musculotendinous bands-pronator teres, FDS arch
  12. Anomalous course deep to pronator teres
  13. Accessory muscles: Ganzter's muscle (aberrant head of FPQ, FCR, palmaris profundus, forearm mass
  14. Accessory bicipital aponeurosis
  15. Vascular
  16. Aberrant radial artery
  17. Thrombosed ulnar collateral artery
  18. Inflammation
  19. Neuralgic amyotrophy (Parsonage Turner syndrome)
  20. Multifocal motor neuropathy
  21. Infection-cytomegalovirus
  22. Arteritis-polyarteritis nodosa

Clinical

  • Weakness is usually preceded by pain, with the pain often subsiding partially or completely over weeks to months
  • pain in the proximal volar forearm in the region of the pronator teres muscle and volar wrist. Symptoms tend to increase with activity, especially repetitive forearm motion. (purely motor nerves can still cause pain ?due to afferent fibers to joints)
  • weakness or paralysis of the FPL, FDP to index and sometimes middle fingers, and Pronator quadratus
  • patient often complains of difficulty with writing or picking up small objects.
  • May be partial or complete
  • can mimic single tendon lesions - isolated paralysis of the FPL or the index FDP muscles
  • EMG useful to differentiate this
  • Characteristic pinch deformity

  • Variations:
  1. The ulnar nerve may innervate the middle finger (partial or complete in 50%),therefore preserving the middle finger FDP strength. The index FDP is always supplied by the AIN.
  2. The Martin-Gruber anastomosis(17%). One of the common variants has a connection from the anterior interosseous nerve to the ulnar nerve (i.e., connection supplies some intrinsic muscles of hand), therefore resulting in AIN syndrome with the addition of intrinsic weakness.
  3. The AIN may innervate the complete FDP muscle, therefore leaving weakness of all the fingers
  4. The AIN may also supply part of the FDS (30% of people)
  • Discriminated from pronator syndrome by the lack of paresthesias and altered sensibility.
  • still much debate as to the causes of the AIN syndrome, particularly the relative contribution of compression versus inflammation of the nerve
  • surgical literature emphasizes the theory of compression, whereas the neurologic literature favors the inflammatory 'neuritis' etiology

Diagnosis

  • most commonly misdiagnosed problem is that of a tendon rupture, usually of the FPL and also of the FDP.
  • look for the tenodesis effect produced by intact flexor tendons.
  • Exclude proximal lesions: acute brachial plexus neuropathy neuritis, partial proximal median nerve injury (pseudoanterior interosseous syndrome), thoracic outlet syndrome, and cervical radiculopathy.
  • Consider Parsonage-Turner syndrome or symmetrical polyneuropathy in bilateral cases
  • Electrophysiologic studies are much more useful in anterior interosseous nerve syndrome than in pronator syndrome.
  • Electromyography has a high success rate in localizing the muscles affected and therefore confirming the diagnosis of AIN syndrome

Management

Nonoperative

  • rest, avoidance of aggravating factors, splinting, and nonsteroidal anti-inflammatory medication
  • 8-12 week trial of conservative management
  • neurologic literature indicates that inflammation is the primary cause, and has questioned the need for surgical intervention. There are reports of improvement beyond 18 monthsand even 2.5 yearsafter conservative treatment.
  • if inflammation is suspected, conservative treatment should be carried out for 6 months.
  • Severe pain, extending beyond the elbow into the shoulder region, precedes neurologic deficit by a few days, associated ipsi- or contralateral brachial plexopathy

Operative

  • Same as for pronator syndrome
  • Interfascicular neurolysis of AIN within median nerve 2-8cm above elbow should be done if no obvious compression point identified.