The median nerve is formed by the anterior divisions of the
lateral and medial cords. It travels down the arm adjacent to the brachial
artery just anterior to the medial intermuscular septum and posterior to the
biceps. As it progresses distally, it wraps anteriorly over the artery to go
from lateral to medial by the time it reaches the biceps tendon. It passes
beneath the lacertus fibrosa (bicipital aponeurosis) to enter into the forearm.
It passes between the two heads of the pronator teres muscle, where it crosses
medial to the ulnar artery. The median nerve and ulnar artery are separated
from each other by the deep (ulnar) head of the pronator teres. The median
nerve then passes beneath the two heads of the flexor digitorum superficialis
arch and runs between the flexor digitorum superficialis and the flexor
digitorum profundus.
The first motor branch, the nerve to the pronator teres, comes
off just 1 cm to 4 cm proximal to the elbow. Soon after, comes the nerve to the
flexor carpi radialis, the nerve to palmaris longus and the nerve to flexor
digitorum superficialis arch. Approximately 5 cm to 8 cm distal to medial
epicondyle, just distal to the proximal border of the superficial head of the
pronator teres, the anterior interosseous nerve takes off from the
posterolateral surface and travels with the median nerve underneath the flexor
digitorum superficialis arch. The anterior interosseous nerve then comes to lie
on the interosseous membrane to give off its branches to the flexor pollicis
longus, the flexor digitorum profundus to the index and long and pronator
quadratus. The anterior interosseous nerve terminates with sensory branches to
the carpus.
The remaining median nerve continues between the flexor digitorum
superficial arch and flexor digitorum profundus, where it gives off the palmar
cutaneous nerve 5 cm to 7 cm proximal to the proximal wrist crease, then passes
into the carpal tunnel and terminates into common digital nerves to the first,
second and third web spaces in hand.
There are four potential sites of compression of the proximal
median nerve:
- Supracondyloid process/ligament of Struthers
The supracondyloid process is an anteromedial bony prominence of
the distal humerus where the humeral head of the pronator teres originates. It
is estimated to occur in 3% of the general population. It may be an ontologic
remnant, as it serves as a protector of the neurovascular bundle in climbing
animals.
- Lacertus fibrosis (bicipital
aponeurosis)
- Pronator teres fibrous bands
This is the most common cause of proximal median nerve
compression. It can be due to pronator hypertrophy or fibrous bands on the
humeral head (50%) or on the ulnar head (40%). In 9%, the nerve passes deep to
both heads.
- Sublimis arch (proximal edge of the
FDS)
The FDS usually has two heads of origin: the humeral and the
interosseous membrane. But occasionally, it will have a third head of origin
from the radius. The sublimis arch is the second most common site of
compression
Pronator syndrome (PS) is characterised by pain more than motor
loss, while Anterior Interosseous Nerve Syndrome (AINS) is a pure motor palsy.
They represent the two extremes of a clinical continuum, and have similar
pathologic etiologies.
PS can occur from compression proximal to the branching of the
AIN, but because the two run together through the four sites of compression it
can occur at any of the four sites. Similarly, partial compression of the
median nerve at any of these four sites can result in AINS.
The causes of PS can be broken down into five basic
categories:
- Acute trauma
- Supracondylar humerus fractures/elbow dislocations
- Penetrating injuries
- Repetitive (cumulative) trauma
- Four sites of compression
- Accessory muscles
- Exertional compartment syndrome
- Inflammation
- Neuralgia amyotrophy
- CMV infection
- Polyarteritis nodosa
PS is much less common than carpal tunnel syndrome. It usually
presents in the fifth decade and is four times more common in women. Patients
usually complain of the insidious onset of aching pain in the proximal volar
forearm, which may radiate proximally and is aggravated by use. There is
associated paresthesias, but night awakenings are uncommon (a key to
differentiation from carpal tunnel syndrome).
On physical exam, altered sensibility may be noted. Atrophy is
uncommon. Weakness is variable. Tinel’s sign is usually positive.
Provocative testing can help to localize the lesions or reproduce dynamic
compression.
Pronator teres: Resisted pronation of the forearm from the
neutral position, with symptoms reproduced as the elbow is gradually extended.
Flexor digitorum superficialis (FDS) arch: Symptoms reproduced
with resisted contraction of the long finger FDS.
Lacertus fibrosus: Symptoms reproduced with resisted elbow
flexion (> 120°) with the forearm in maximal supination.
The differential diagnosis includes atypical carpal tunnel
syndrome, including the possibility of double crush syndrome (in which a more
proximal compression leaves the distal area more susceptible to compression);
thoracic outlet syndrome; brachial plexus neuropathy; cervical radiculopathy;
and polyneuropathy.
Electrophysiologic studies are usually not diagnostic, but can be
helpful in ruling out other conditions. Plain x-rays of the humerus can reveal
the supracondyloid process, but because this can be present in 1% to 2% of the
normal adult population, its significance is questionable.
Nonoperative treatment includes avoidance of aggravating
activities, rest, splinting, nonsteroidal anti-inflammatories (NSAIDs). About
50% of patients will recover within 4 months. If there is no resolution of
symptoms within 3 to 4 months, then surgical exploration, which consists of
release of all four possible sites of compression, is indicated. An extendible
curvilinear incision is made over the median nerve approximately centered at
the elbow flexion crease and the lacertus fibrosus is divided. If a
supracondylar process is present, it is excised along with the ligament of
Struther’s. The superficial head of the pronator is taken down medially
and all fibrous bands are released. Finally, the FDS arch is identified and
divided (Slide 4). Satisfactory outcome with surgical decompression can be
expected in 90%.
Anterior interosseous nerve syndrome (AINS) has an etiology
similar to that of PS, although inflammatory causes are more likely. AINS is
characterized by weakness more than pain.
Although patients present with pain in the proximal volar forearm
and volar wrist weakness, finger flexion soon follows and pain gradually
subsides.
On physical examination, there is weakness of the FPL and FDP of
the index and long fingers. Spinner’s sign (Slide 5) occurs late: the
patient is unable to make the “O” with thumb tip to index tip
pretension. Being as the AIN has no cutaneous sensation, there should be no
sensory deficits.
Tendon rupture, particularly FPL, is the most common
differential, and can be diagnosed by observing tenodesis. Other diagnoses are
less common: acute brachial plexus neuritis; partial proximal median nerve
injury (pseudoanterior interosseous syndrome); thoracic outlet syndrome; and
cervical radiculopathy.
Electrophysiologic studies are usually diagnostic (unlike PS) and
helpful in localizing the lesion in AINS.
Nonoperative treatment includes rest and splinting, NSAIDs and
avoidance of aggravating factors. The duration of nonoperative treatment is
controversial. If the etiology is inflammatory, then surgical intervention is
probably never necessary, while if it is compressive, the exploration can be
carried out after about 12 weeks. Inflammatory etiology is suggested by severe,
rapid onset of pain beyond the elbow into the shoulder region with maximum
rapid neurological deficit and other signs of plexopathy. Entrapment is
suggested by insidious, mild, primarily forearm pain with neurological deficits
isolated to the AIN.
Surgical treatment is identical for that of PS with the complete
release of lacertus fibrosis, excision of the ligament of Struther’s,
decompression of the median nerve in the pronator teres and FDS arch, and
decompression of the anterior interosseous nerve.
- Eversmann WW. Proximal median nerve compression. Hand Clin.
1992;8(2):307-315.
- Haussmann P, Mukund RP. Intraepineural constriction of nerve
fascicles in pronator syndrome and anterior interosseous nerve syndrome. Orthop
Clin North Am. 1996;27(2):339-344.
- Tetro AM, Pichora DR. High median nerve entrapments: An obscure
cause of upper-extremity pain. Hand Clin. 1996;12(4):691-703.