Peripheral nerve compression represents one of the most common reasons for presentation to hand surgery clinic. The most common peripheral nerve entrapment syndromes involve the median nerve, most frequently at the carpal tunnel, and the ulnar nerve, most commonly at the cubital tunnel. Although peripheral nerve anatomy generally lends to predictable patterns of symptomatology, some anomalies can exist. Thorough history and comprehensive, targeted physical examination are paramount to achieving an appropriate diagnosis. Given the time-dependent nature of nerve regeneration potential after mechanical decompression, it is important to implement appropriate treatment promptly.
18 Compression Neuropathies
I. Peripheral Nerve Anatomy
Neuron: Working unit of nervous system, composed of cell body along with axons and dendrites.
Axons: Larger processes with the function of conducting signals via action potentials:
Reside in the anterior horn in motor neurons, in the dorsal root ganglion in sensory neurons.
May be myelinated or unmyelinated.
Dendrites: Thinner processes with the function of receiving input from other nerves.
Vasa nervorum are small arteries which supply peripheral nerves.
Compression neuropathy, or nerve compression syndrome, is a condition caused by direct pressure on a peripheral nerve:
Direct compression induces local ischemia by reducing flow through the vasa nervorum.
Results in slowed action potentials, focal demyelination, axonal damage, and fibrosis.
II. Median Nerve Compression
A. Anatomic Course
Arises from the medial and lateral cords of the brachial plexus, with input from C5 to T1.
Gives off the anterior interosseous nerve (AIN) distal to the arch of the flexor digitorum superficialis (FDS).
AIN descends volar to the interosseous membrane between the radius and ulna, typically radial to the anterior interosseous artery.
After innervating and passing dorsal to the pronator quadratus, the AIN terminates as sensory branches to the volar wrist capsule.
Palmar cutaneous branch of the median nerve branches from the main nerve 5 to 7 cm proximal to the volar wrist crease:
Supplies sensation to the volar palm.
Runs between palmaris longus and flexor carpi radialis (FCR), passing volar to the transverse carpal ligament.
May run within the sheath of the FCR.
Median nerve proper passes between FCR and FDS just proximal to the wrist, prior to entering the carpal tunnel.
Recurrent motor branch travels radial to the median nerve through the carpal tunnel
Supplies thenar muscles.
Distal to the carpal tunnel, the median nerve branches into radial and ulnar divisions
Radial division supplies sensation to the thumb and radial index finger via digital nerves.
Ulnar division supplies the ulnar index finger, middle finger, and radial ring finger via digital nerves.
B. Motor Innervation
Flexor carpi radialis.
Flexor digitorum superficialis.
Flexor digitorum profundus to index and long fingers (via anterior interosseous nerve).
Flexor pollicis longus (via anterior interosseous nerve).
Pronator quadratus (via anterior interosseous nerve).
Flexor pollicis brevis (superficial head via recurrent motor branch).
Abductor pollicis brevis (via recurrent motor branch).
Opponens pollicis (via recurrent motor branch).
First and second lumbricals (via digital nerves).
C. Sensory Innervation
Articular branch to ulnohumeral joint.
Volar palmar skin (via palmar cutaneous nerve).
Volar thumb, index finger, middle finger, and radial half of ring finger.
D. Potential Compression Points
Anomalous axillary vascular structures.
Projects anteromedially from the anterior humeral shaft, approximately 3 to 5 cm proximal to the medial epicondyle, best seen on oblique radiographs of the humerus.
Only present in about 1% of individuals.
Connected to the medial epicondyle via ligament of Struthers:
The median nerve may be compressed as it passes underneath the ligament of Struthers, between the supracondylar process and medial epicondyle.
Anomalous muscle, present in 45% of limbs.
Arises from the medial condyle of the humerus, inserts into the flexor pollicis longus.
Arch of the flexor digitorum superficialis.
Anomalous muscle that originates proximal to the flexor pollicis longus from the radius, inserts onto the transverse carpal ligament.
Flexor capri radialis brevis
Anomalous muscle that originates near the proximal origin of the flexor pollicis longus from the radius, inserts onto the flexor carpi radialis.
Ulnar collateral artery.
Aberrant branches of radial artery to anterior interosseous nerve.
Fibro-osseous tunnel in the volar wrist; finger and thumb flexors travel with the median nerve on their way to the hand.
Borders include the transverse carpal ligament volarly, the carpal bones dorsally, the hook of the hamate and triquetrum ulnarly, and the scaphoid tubercle and trapezium radially.
Anatomic variants such as a transligamentous recurrent motor branch may put the nerve at risk during surgical release (► Fig. 18.1).