Upper Limb Nerve Injuries for MRCEM Primary: Localise the Lesion

Recognising a familiar nerve injury is only the first step in an anatomy SBA. The harder task is deciding where the lesion lies: does wrist drop indicate a proximal radial injury, a spiral-groove lesion or a problem affecting a more distal branch?

The most useful sequence is weak movement → responsible muscle → nerve → lesion level. Sensory findings refine the answer, while preserved movements reveal which branches are still working. This guide applies that sequence to the major upper-limb nerves before testing it with five original questions.

Use it alongside your MRCEM Primary revision plan. Syllabus note, checked 30 October 2026: RCEM states that its updated Basic Sciences Syllabus applies from the 21 April 2027 examination onwards. Check the syllabus for your sitting; this article does not imply a fixed allocation of nerve-injury questions. RCEM syllabus announcement.

1. A reliable method for localising an upper-limb nerve injury

Identify the movement that is weak

Translate descriptions such as “cannot straighten the fingers” into a specific joint movement. Extension at the metacarpophalangeal (MCP) joints depends on the long finger extensors supplied through the radial nerve’s deep motor pathway. Interphalangeal extension also involves the lumbricals and interossei through the extensor apparatus. These are not interchangeable tests.

Start proximally and move distally: shoulder abduction, elbow flexion and extension, forearm rotation, wrist movement, then individual finger and thumb movements. A label such as “weak grip” is insufficiently specific because grip depends on several nerves, intact tendons and a stable wrist.

Add the sensory findings

A dermatome describes skin supplied predominantly by one spinal root. A peripheral nerve territory describes the distribution of a named nerve containing fibres from several roots. A numb little finger may support ulnar involvement, but it does not automatically establish an isolated ulnar lesion.

Useful representative testing sites include the lateral shoulder for the axillary nerve, lateral forearm for the musculocutaneous nerve’s cutaneous continuation, dorsal first web space for the radial nerve, index fingertip for the median nerve and little fingertip for the ulnar nerve. Sensory territories overlap and vary: interpret the pattern alongside motor findings.

Use preserved function to find the lesion level

Ask which unaffected branches must have left the nerve before the injury. For example, wrist and finger extension weakness with preserved elbow extension points away from a complete radial lesion above the triceps branches. Add dorsal first web-space sensory loss and an isolated posterior interosseous lesion becomes less likely.

Preserved function narrows the possibilities; it rarely proves an exact level by itself. Partial lesions, anatomical variation, pain and tendon injury can produce incomplete patterns. In an SBA, choose the option explaining the whole stem with the fewest contradictions.

2. Axillary and musculocutaneous nerve injuries

Axillary nerve: shoulder movement and lateral shoulder sensation

The axillary nerve arises from the posterior cord, predominantly C5–C6. It passes through the quadrangular space and around the surgical neck of the humerus, supplying deltoid and teres minor. This relationship explains its vulnerability in shoulder dislocation and proximal humeral injury.

Deltoid weakness impairs shoulder abduction, particularly through the middle part of the movement. Supraspinatus helps initiate abduction, but the familiar division into exact angular stages is a simplification of overlapping muscle activity. Reduced sensation over the lateral shoulder supports axillary involvement; normal sensation does not exclude a motor injury.

Pain can inhibit movement after trauma. Assess deltoid contraction as well as attempted abduction, and look for a consistent neurological pattern rather than treating every painful shoulder as an axillary palsy.

Musculocutaneous nerve: elbow flexion and lateral forearm sensation

The musculocutaneous nerve arises from the lateral cord, usually C5–C7, and supplies coracobrachialis, biceps and most of brachialis. It continues as the lateral cutaneous nerve of the forearm. Injury therefore links weakened elbow flexion and supination with altered lateral forearm sensation.

Elbow flexion need not disappear: brachioradialis remains available through the radial nerve, and brachialis may receive an additional radial contribution. Supination is also assisted by supinator. Think “weakened action” rather than assuming that loss of one nerve abolishes every movement it contributes to.

When one peripheral nerve does not explain the findings

Isolated deltoid weakness fits axillary injury. Deltoid weakness together with biceps weakness and lateral forearm sensory change spans axillary and musculocutaneous territories. Consider an upper plexus or root-level problem rather than forcing the findings into one terminal nerve.

3. Radial nerve lesions: axilla, spiral groove or posterior interosseous nerve?

Compare elbow, wrist and finger extension

The radial nerve carries fibres from C5–T1 and arises from the posterior cord. Its branches leave at different levels, making the pattern of spared muscles particularly useful.

These are typical localisation patterns rather than guarantees for every partial injury. The radial nerve’s relationship with the humeral shaft and its division into superficial and deep branches explain these differences. AO: humeral shaft anatomy; AO: forearm nerve anatomy.

Why preserved triceps function matters

After a humeral shaft injury, a patient cannot extend the wrist or finger MCP joints, but elbow extension remains strong. The triceps branches are functioning, supporting a lesion distal to their origins. In the appropriate context, the spiral groove is a better answer than a high axillary-level radial injury.

However, triceps is also preserved in PIN palsy. The distinction requires the remaining findings: substantial wrist drop and cutaneous sensory loss favour a radial lesion proximal to its terminal division over isolated PIN involvement.

Motor and sensory branches behave differently

In PIN palsy, extensor carpi radialis longus remains supplied proximal to the lesion. Wrist extension can therefore persist, often with radial deviation because extensor carpi ulnaris is weak. Extensor carpi radialis brevis innervation varies, so the degree of wrist weakness is not identical in every case.

The PIN has no cutaneous sensory territory, although it carries deep sensory fibres. Thus, “no skin numbness” is more accurate than saying it contains no sensory fibres at all. Conversely, an isolated superficial radial lesion produces sensory symptoms without motor weakness. Clinical PIN cases and localisation.

4. Median nerve lesions: proximal injury, carpal tunnel or anterior interosseous nerve?

Separate thenar function from forearm function

The median nerve receives contributions from the lateral and medial cords. In the hand, test thumb palmar abduction and opposition. Palmar abduction lifts the thumb perpendicular to the palm; opposition brings its pulp towards the little finger with rotation. Neither is the same as thumb adduction.

A proximal median lesion can also weaken pronation, wrist flexion and selected finger flexion. Carpal tunnel compression occurs after the forearm motor branches have left, so those proximal functions remain intact. Thenar weakness is more characteristic of advanced motor involvement than of every mild carpal tunnel presentation.

The anterior interosseous nerve and the precision pinch

The anterior interosseous nerve (AIN) supplies flexor pollicis longus, the radial portion of flexor digitorum profundus—most consistently the index component—and pronator quadratus. Weak thumb interphalangeal and index distal interphalangeal flexion produces a flattened pulp-to-pulp pinch rather than a normal tip-to-tip “OK” circle.

An isolated AIN lesion produces no cutaneous sensory deficit and spares thenar muscles. Pronator teres may preserve useful pronation despite pronator quadratus weakness. An abnormal pinch alone is not diagnostic: pain and flexor tendon injury can mimic weakness, so use the complete examination pattern.

Use sensory sparing to distinguish lesion sites

The median palmar cutaneous branch arises proximal to the wrist and travels superficial to the flexor retinaculum. Sensation over the thenar eminence is therefore usually preserved in carpal tunnel syndrome, despite symptoms in the median-innervated digits. A more proximal lesion can affect both territories. Anatomical study of the palmar cutaneous branch.

5. Ulnar nerve lesions: elbow versus wrist

Interossei, thumb adduction and Froment’s sign

The ulnar nerve arises from the medial cord, predominantly C8–T1. Dorsal interossei abduct the fingers; palmar interossei adduct them. Test these actions against resistance rather than relying only on grip strength.

Adductor pollicis supports key pinch. In Froment’s sign, weakness prompts compensatory thumb interphalangeal flexion through flexor pollicis longus, supplied by the median nerve via the AIN. The visible flexion is compensation by a functioning muscle, not evidence that the ulnar nerve supplies that movement. RCEMLearning hand nerve examination.

Compare forearm involvement and hand sensation

A proximal ulnar lesion can affect flexor carpi ulnaris and the ulnar portion of flexor digitorum profundus as well as intrinsic hand muscles. Weak ring- and little-finger distal interphalangeal flexion therefore supports involvement proximal to the relevant forearm branches. Wrist lesions spare those branches.

The dorsal cutaneous branch leaves the ulnar nerve proximal to the wrist. Dorsoulnar hand sensation is usually spared in Guyon’s canal lesions, whereas a sufficiently proximal lesion may affect it. Within Guyon’s canal, the precise site matters: involvement before division can produce mixed deficits, while individual branch lesions may be predominantly motor or sensory.

Cubital tunnel syndrome is ulnar nerve compression or irritation at the elbow. It can produce little- and ring-finger sensory symptoms and hand weakness, but a partial entrapment need not affect every ulnar-supplied muscle. BSSH: cubital tunnel syndrome.

Understand the ulnar paradox

Ulnar clawing combines MCP hyperextension with interphalangeal flexion, especially in the ring and little fingers, following loss of intrinsic muscle balance. With a distal lesion, the long finger flexors remain active and can accentuate the flexed posture.

A more proximal lesion may also weaken the ulnar portion of flexor digitorum profundus, reducing distal interphalangeal flexion and making the claw less conspicuous. Thus, less clawing can accompany a more extensive deficit. The appearance alone cannot reliably establish severity, timing or lesion level.

6. Brachial plexus versus individual nerve injury

Recognise a pattern spanning several terminal nerves

An upper-trunk pattern, typically involving C5–C6 fibres, can combine weakness of shoulder abduction, external rotation and elbow flexion. These movements involve different terminal nerves. A lower-trunk pattern, involving C8–T1 fibres, may affect intrinsic hand function across both median and ulnar territories.

Medial forearm sensory loss is particularly useful when considering a lower plexus lesion: this territory belongs to the medial cutaneous nerve of the forearm, not the ulnar nerve. Do not attribute every sensory change on the medial side of the limb to ulnar neuropathy.

Keep roots, trunks and peripheral nerves separate

The brachial plexus is organised as roots, trunks, divisions, cords and terminal branches. Its roots are the anterior rami, usually C5–T1; the upper trunk combines C5–C6, the middle continues C7, and the lower combines C8–T1. Cords are named by their relationship to the axillary artery.

A root lesion and a trunk lesion are different anatomical diagnoses even when their motor patterns overlap. Neck symptoms, reflexes, additional muscle testing and investigations may be needed to distinguish them clinically. In questions, first establish whether one peripheral nerve can explain all the abnormalities. Further reading: clinical anatomy of upper-limb nerve supply.

Avoid diagnosing from the mechanism alone

A shoulder dislocation suggests axillary injury, but associated deficits may indicate a broader injury. Similarly, a humeral fracture suggests radial involvement without proving a complete lesion. Use mechanism to generate a hypothesis, then test it against the motor and sensory pattern.

7. Five original MRCEM Primary anatomy SBAs

Attempt the questions before reading the answers

These are original educational questions, not recalled or official RCEM examination material. Choose the single best answer from the findings provided.

Question 1: shoulder trauma

Following reduction of an anterior shoulder dislocation and adequate analgesia, a patient has weak deltoid contraction and reduced sensation over the lateral shoulder. Elbow flexion, supination and lateral forearm sensation are normal. Which nerve is most likely injured?

A. Musculocutaneous nerve
B. Suprascapular nerve
C. Axillary nerve
D. Radial nerve
E. Long thoracic nerve

Question 2: wrist drop

A patient with a midshaft humeral fracture cannot extend the wrist or finger MCP joints. Elbow extension is strong, but sensation in the dorsal first web space is reduced. Which lesion best explains the findings?

A. Posterior interosseous nerve lesion
B. Radial nerve lesion at the spiral groove
C. Superficial radial nerve lesion
D. High radial lesion proximal to the triceps branches
E. Median nerve lesion at the elbow

Question 3: finger extension weakness

A patient has marked weakness of finger MCP and thumb extension. Wrist extension is retained but deviates radially. Elbow extension and cutaneous sensation throughout the hand are normal. Which nerve is most likely affected?

A. Superficial radial nerve
B. Ulnar nerve at the wrist
C. Median nerve at the wrist
D. Posterior interosseous nerve
E. Radial nerve in the axilla

Question 4: an abnormal precision pinch

A patient cannot flex the thumb interphalangeal joint or index distal interphalangeal joint effectively and makes a flattened pulp-to-pulp pinch. Tendon continuity is confirmed. Thumb opposition and palmar abduction are normal, with no cutaneous sensory loss. Which lesion is most likely?

A. Recurrent motor branch of the median nerve
B. Median nerve within the carpal tunnel
C. Ulnar nerve at the elbow
D. Superficial radial nerve
E. Anterior interosseous nerve

Question 5: ulnar localisation

A patient has weak finger abduction and a positive Froment’s sign. Little-finger pulp sensation is reduced, but dorsoulnar hand sensation is normal. Flexor carpi ulnaris and ring- and little-finger distal interphalangeal flexion are strong. Which site best fits this combined motor and sensory deficit?

A. Ulnar nerve in Guyon’s canal before its motor–sensory division
B. Complete ulnar lesion proximal to the elbow
C. Isolated deep motor branch of the ulnar nerve
D. Isolated dorsal cutaneous branch of the ulnar nerve
E. Median nerve within the carpal tunnel

Answers and worked explanations

1. C — Axillary nerve. Deltoid weakness and lateral shoulder sensory loss form a coherent axillary pattern. Preserved elbow flexion, supination and lateral forearm sensation argue against musculocutaneous involvement.

A would weaken biceps/brachialis and alter lateral forearm sensation. B affects supraspinatus and infraspinatus rather than deltoid or lateral shoulder skin. D would produce a radial motor/sensory pattern. E affects serratus anterior and may cause scapular winging, without this sensory deficit. Learning point: confirm the nerve from both movement and sensation.

2. B — Radial nerve at the spiral groove. The combination of wrist drop, finger extension weakness and dorsal first web-space sensory loss supports a radial lesion before its terminal division. Strong elbow extension indicates preserved triceps supply.

A usually spares cutaneous sensation and retains some wrist extension. C cannot explain motor weakness. D would be expected to weaken triceps in a complete lesion at the specified level. E affects flexion, pronation and median hand functions rather than this extensor pattern. Learning point: triceps sparing narrows the level; sensation helps separate radial trunk from PIN involvement.

3. D — Posterior interosseous nerve. The decisive combination is finger/thumb extension weakness, retained radially deviating wrist extension and normal skin sensation. Preserved extensor carpi radialis longus explains the remaining wrist movement.

A is sensory and cannot cause finger drop. B weakens intrinsic hand muscles but does not explain loss of long extensor function. C affects median hand functions. E would cause a broader deficit, including more proximal radial muscles, and may impair sensation. Learning point: a radial motor-branch lesion does not necessarily produce complete wrist drop.

4. E — Anterior interosseous nerve. Weak flexor pollicis longus and index flexor digitorum profundus explain the pinch abnormality. Normal thenar function and skin sensation support selective AIN involvement.

A impairs thenar muscles rather than these forearm flexors. B lies distal to the AIN origin and therefore spares them. C affects ulnar intrinsic muscles and potentially the ulnar FDP component, not the thumb/index pair. D has no motor supply to these muscles. Learning point: a motor-only median branch lesion differs from carpal tunnel compression.

5. A — Guyon’s canal before division. Intrinsic weakness and little-finger sensory loss require both motor and sensory involvement. Preserved forearm muscles and dorsoulnar sensation favour a distal site beyond those branches.

B should affect additional proximal functions and the dorsal sensory territory if complete. C cannot explain little-finger pulp sensory loss. D cannot explain intrinsic weakness or palmar digital sensory loss. E affects the wrong motor and sensory distribution. Learning point: specify both the nerve and which branches the lesion includes.

8. Consolidate the patterns

One comparison table for rapid revision

Common mistakes

Confusing thumb abduction with adduction: palmar abduction tests predominantly median-innervated abductor pollicis brevis; adduction tests ulnar-innervated adductor pollicis. Radial abduction in the plane of the palm involves abductor pollicis longus through the radial/PIN pathway. Specify the movement before naming the nerve.

Treating all finger extension as one action: distinguish MCP extension by the long extensors from the intrinsic contribution to interphalangeal extension. Mixing dermatomes with peripheral territories: ask whether the sensory map fits a root or named nerve. Ignoring preserved function: an intact movement may be the strongest reason to reject an otherwise tempting option.

Turn recognition into recall

Cover the diagnosis column and identify each lesion from its findings. Then reverse the exercise: name a lesion and predict one lost movement, one sensory finding and one preserved function. Revisit errors after a delay and mix similar cases so that the distinguishing feature has to be actively retrieved.

For related clinical topics, read our broader emergency neurology revision guide.

Practise applying anatomy to unfamiliar SBA stems

Build on this guide with the StudyMedical MRCEM Primary question bank. It offers over 7,800 questions, explanations with teaching diagrams, custom topic-based quizzes, timed quizzes and the option to revisit previously incorrect answers.

Try free MRCEM Primary sample questions without signing up, then practise explaining why each alternative fits less well—not just recognising the correct nerve.