Nerve Injuries and Clinical Anatomy

Peripheral nerve injuries are common in clinical practice, and the pattern of motor and sensory loss precisely reflects the anatomy of the affected nerve. Identifying which nerve is injured, and at what level, requires understanding the nerve's course, its relationship to adjacent structures, and the muscles and skin it supplies. This guide is for educational purposes only.

## Structure and Classification of Nerve Injuries

Peripheral nerves consist of bundles (fascicles) of axons surrounded by connective tissue sheaths: the endoneurium (around individual axons), the perineurium (around each fascicle), and the epineurium (around the whole nerve). The Seddon classification describes three grades: neuropraxia (focal conduction block without axonal disruption — full recovery expected), axonotmesis (axon interrupted but connective tissue sheaths intact — axonal regeneration at 1 mm/day), and neurotmesis (complete nerve transection, including sheaths — surgical repair required). Sunderland's five-grade system further subdivides the intermediate injuries.

The clinical hallmarks of a complete peripheral nerve injury are flaccid paralysis of muscles it supplies (lower motor neurone pattern — reduced tone, reduced reflexes, wasting), sensory loss in its autonomous zone, autonomic changes (loss of sweating, skin atrophy), and a positive Tinel's sign (tingling on percussion over the nerve — indicating regenerating axons) as recovery begins.

## Median Nerve: Carpal Tunnel Syndrome

The median nerve is formed from the medial and lateral cords of the brachial plexus (C6–T1). In the forearm it supplies all the flexors of the forearm except flexor carpi ulnaris and the medial half of flexor digitorum profundus (the "LOAF" muscles of the hand that it supplies in the palm are Lumbricals 1 and 2, Opponens pollicis, Abductor pollicis brevis, and Flexor pollicis brevis). Carpal tunnel syndrome (CTS) — compression of the median nerve at the wrist — is the most common entrapment neuropathy.

The carpal tunnel is an osseofibrous canal bounded posteriorly and laterally by the carpal bones (arranged in a C-shape) and anteriorly by the flexor retinaculum (transverse carpal ligament), which attaches from the scaphoid tubercle and trapezium tubercle radially to the hook of hamate and pisiform ulnarly. The tunnel is compartmentally inelastic; any increase in its contents (fluid in pregnancy, synovial thickening in rheumatoid arthritis, a space-occupying lesion, hypothyroidism increasing perineurial glycosaminoglycan deposition) raises the intracompartmental pressure and compresses the nerve. The median nerve is the most superficial structure in the tunnel, lying directly beneath the flexor retinaculum and therefore most susceptible to compression.

Clinically, CTS produces pain and paraesthesia in the radial three and a half digits and the radial palm (the autonomous zone of the median nerve's palmar cutaneous branch, which passes superficial to the flexor retinaculum, is actually spared in CTS, helping localise the lesion to within the tunnel). With progression, thenar wasting (abductor pollicis brevis, which is purely median-nerve-innervated, is the first to atrophy) and weakness of thumb opposition develop. Tinel's sign (percussion over the wrist flexion crease) and Phalen's test (sustained wrist flexion for 60 seconds reproducing symptoms) are the standard provocative tests.

## Radial Nerve: Saturday Night Palsy

The radial nerve (C5–T1) is the largest branch of the posterior cord of the brachial plexus and is unique in its spiral course around the posterior aspect of the humerus in the radial groove (between the medial and lateral heads of triceps). Here it is in direct contact with the humeral shaft — an anatomical vulnerability that makes it susceptible to injury from mid-shaft humeral fractures and from prolonged compression ("Saturday night palsy" — arm draped over a chair back during intoxication, compressing the nerve against the humerus). At this level, the triceps (which receive branches proximal to the groove) are spared, but all muscles distal to the groove are affected: the brachioradialis and supinator (elbow flexion in mid-pronation and supination), and all wrist and finger extensors supplied by the posterior interosseous nerve (the deep branch of the radial nerve after it pierces the supinator). The result is wrist drop (inability to extend the wrist) and finger drop, with sensory loss limited to the dorsum of the first web space (the autonomous zone of the superficial radial nerve). The brachioradialis reflex is reduced or absent.

## Common Peroneal Nerve: Foot Drop

The common peroneal (fibular) nerve (L4, L5, S1, S2) is the smaller terminal branch of the sciatic nerve, dividing behind the knee and winding around the neck of the fibula (just posterior to the biceps femoris tendon) before dividing into the superficial and deep peroneal nerves. The neck of the fibula is the single most common site of peroneal nerve injury, due to its subcutaneous, bony location where the nerve is exposed to compression from plaster casts, tight bandages, leg crossing, or prolonged squatting. Fractures of the fibular neck and knee dislocations are important traumatic causes.

Deep peroneal nerve injury produces foot drop — inability to dorsiflex the foot and extend the toes — because the nerve supplies tibialis anterior (dorsiflexion), extensor hallucis longus (great toe extension), extensor digitorum longus, and extensor digitorum brevis. The patient adopts a high-stepping gait to avoid tripping. Sensory loss from the deep peroneal nerve alone is confined to the first web space. The superficial peroneal nerve supplies the peronei (foot eversion) and the dorsum of the foot and toes (except the first web space). Complete common peroneal palsy produces both foot drop and loss of eversion, with sensory loss over the anterolateral lower leg and dorsum of the foot.

## Facial Nerve: Bell's Palsy

The facial nerve (CN VII) exits the brainstem at the pontomedullary junction, enters the internal acoustic meatus with the vestibulocochlear nerve (CN VIII), traverses the petrous temporal bone in the facial canal (the longest intraosseous course of any cranial nerve — approximately 3 cm), exits through the stylomastoid foramen, and divides within the parotid gland into its five terminal branches (temporal, zygomatic, buccal, marginal mandibular, cervical) to supply all muscles of facial expression. Bell's palsy — acute, idiopathic, unilateral lower motor neurone facial nerve palsy — is caused by inflammation and oedema of the facial nerve within the rigid bony facial canal, producing ischaemic compression. Most cases are associated with herpes simplex virus type 1 reactivation.

The key anatomical distinguishing feature of a lower motor neurone (peripheral) facial palsy is involvement of the forehead muscles (frontalis). Both hemispheres of the cerebral cortex send upper motor neurone fibres to the portion of the facial nucleus that controls the forehead bilaterally — therefore, a unilateral cortical (upper motor neurone) lesion (such as a stroke) spares the forehead (frontalis is still innervated by the intact contralateral corticobulbar fibres). In Bell's palsy (lower motor neurone, peripheral), the entire ipsilateral facial nerve is affected below the nucleus, including the branch to frontalis — so the patient cannot wrinkle the forehead or raise the eyebrow. Additional features reflect where in the facial canal the oedema is greatest: if proximal to the chorda tympani (which carries taste from the anterior two-thirds of the tongue and parasympathetics to the submandibular and sublingual glands), taste loss and dry mouth accompany the palsy; if proximal to the nerve to stapedius, hyperacusis (abnormal sensitivity to loud sounds) is present.

## General Principles of Anatomical Vulnerability

Nerve injuries are most common where nerves are superficial, cross bone prominences, pass through fibro-osseous tunnels, or lie adjacent to frequently fractured bones. The radial nerve at the radial groove, the ulnar nerve at the medial epicondyle (cubital tunnel), the common peroneal nerve at the fibular neck, and the median nerve in the carpal tunnel are the four most commonly entrapped peripheral nerves in clinical practice. The anatomical principles — course, depth, adjacent structures, bony relationships — explain every vulnerability.