Limb nerve lesions
Upper and lower limb nerve lesions, roots, pulses and veins (FCPS Part 1)
Carpal tunnel, ulnar claw, wrist drop, foot drop, brachial plexus injury, disc prolapse roots and limb pulses for FCPS Part 1. Matching motor, sensory and reflex findings to a nerve or root solves most clinical anatomy questions.
One of the most tested topics in past papers
Roots: reflexes, myotomes and dermatomes
- A root lesion gives weakness in a myotome, sensory loss in a dermatome and a lost reflex; a peripheral nerve lesion follows the territory of that nerve. Comparing the two is the commonest way FCPS separates them.
| Root | Key movement | Reflex | Dermatome landmark |
|---|---|---|---|
| C5 | Shoulder abduction, elbow flexion | Biceps (C5–C6) | Lateral upper arm |
| C6 | Wrist extension | Supinator (C5–C6) | Thumb and index finger |
| C7 | Elbow extension | Triceps | Middle finger |
| C8 | Finger flexion | Finger jerk | Little finger |
| T1 | Finger abduction (interossei) | None | Medial forearm |
| L2–L4 | Hip flexion, knee extension, hip adduction | Knee (L3–L4) | L3 medial knee, L4 medial leg and medial malleolus |
| L5 | Dorsiflexion, great toe extension, foot inversion | None reliable | Dorsum of foot, great toe |
| S1 | Plantarflexion, eversion | Ankle | Lateral foot, little toe, sole |
- Other landmarks: T4 nipple, T10 umbilicus, L1 groin, S2–S4 saddle area. Femoral and obturator nerves both arise from L2–L4 (femoral from posterior divisions, obturator from anterior).
Upper limb nerves
| Nerve and site | Typical cause | What is lost | Hallmark sign |
|---|---|---|---|
| Median at wrist (carpal tunnel) | Pregnancy, hypothyroidism, diabetes, rheumatoid arthritis, acromegaly, dialysis amyloid, lunate dislocation | Lateral two lumbricals, opponens pollicis, abductor and flexor pollicis brevis; lateral 3½ digits | Thenar wasting, night pain, Tinel and Phalen tests; palm sensation spared |
| Median at elbow | Supracondylar fracture | Plus long flexors (FDS, lateral FDP, FPL) | Index stays straight on making a fist (hand of benediction) |
| Anterior interosseous | Forearm compression | FPL, FDP to index, pronator quadratus | Cannot make an "OK" sign; no sensory loss |
| Ulnar at elbow | Medial epicondyle fracture, leaning on elbows, cubitus valgus | Interossei, medial lumbricals, hypothenar, adductor pollicis, FCU, medial FDP; medial 1½ digits front and back | Claw of ring and little fingers, Froment sign, wasted first dorsal interosseous |
| Ulnar at wrist (Guyon canal) | Cyclists, ganglion, hook of hamate fracture | Intrinsics only (FDP intact) | Worse claw (ulnar paradox); dorsal skin often spared |
| Radial at spiral groove | Humeral shaft fracture, arm over a chair | Wrist and finger extensors; triceps spared | Wrist drop; numb first dorsal web space |
| Radial in axilla | Crutches | Also triceps | Wrist drop plus weak elbow extension |
| Posterior interosseous | Radial head fracture, supinator arcade | Finger and thumb extensors; ECRL spared | Finger drop without wrist drop; no sensory loss |
| Axillary | Anterior shoulder dislocation, surgical neck fracture | Deltoid, teres minor | Numb regimental badge area |
| Musculocutaneous | Rare | Biceps, brachialis | Lateral forearm numb |
| Long thoracic | Axillary surgery, heavy loads | Serratus anterior | Winged scapula |
| Suprascapular | Ganglion, overhead athletes | Supraspinatus and infraspinatus | No skin loss |
- Suprascapular nerve passes the suprascapular notch (supplies both supraspinatus and infraspinatus) and then the spinoglenoid notch (infraspinatus only). Isolated infraspinatus wasting with a normal supraspinatus means compression at the spinoglenoid notch, often by a labral ganglion in volleyball players.
- Ulnar paradox: the higher lesion (elbow) also paralyses medial FDP, so the fingers cannot flex hard and the claw looks milder; the low lesion leaves FDP pulling unopposed.
Brachial plexus injuries
| Injury | Roots | Mechanism | Picture |
|---|---|---|---|
| Erb palsy | C5–C6 | Head and shoulder forced apart (shoulder dystocia, motorcycle fall) | Arm adducted, internally rotated, elbow extended ("waiter's tip") |
| Klumpke palsy | C8–T1 | Arm pulled upward, Pancoast tumour, cervical rib | Total claw hand, medial forearm numb, ± Horner syndrome |
| Complete root avulsion | C5–T1 | High-energy traction (motorcycle crash) | Flail anaesthetic arm, Horner syndrome, severe burning (deafferentation) pain |
- Horner syndrome with a plexus injury means the T1 root has been torn from the cord (preganglionic), so the injury is proximal to the sympathetic outflow; such avulsions carry a poor prognosis because they cannot be repaired directly. Check the subclavian artery and cervical spine.
- Mononeuritis multiplex (several separate nerves): diabetes, vasculitis such as polyarteritis nodosa, leprosy, rheumatoid arthritis, SLE, sarcoidosis.
Lower limb nerves
| Nerve | Typical cause | What is lost | Key point |
|---|---|---|---|
| Common peroneal (fibular) | Fibular neck fracture, tight cast, leg crossing, squatting | Dorsiflexion and eversion; dorsum of foot and lateral leg | Foot drop, high-stepping gait; inversion and ankle reflex preserved |
| Deep peroneal | Anterior compartment injury | Dorsiflexion, toe extension | Numb first web space only; runs with the anterior tibial artery |
| Superficial peroneal | Fibular shaft fracture, lateral compartment injury | Eversion (peroneus longus and brevis) | Numb most of the dorsum of the foot, first web space and dorsiflexion spared |
| Tibial | Popliteal injury, tarsal tunnel | Plantarflexion, toe flexion, inversion | Numb sole |
| Sciatic | Posterior hip dislocation, misplaced gluteal injection | Hamstrings and everything below the knee | Gluteal muscles are spared (they have their own gluteal nerves) |
| Femoral | Iliopsoas haematoma, femoral catheterisation | Quadriceps | Knee jerk lost; numb anterior thigh and medial leg (saphenous) |
| Obturator | Pelvic surgery, obturator hernia | Hip adduction | Numb medial thigh |
| Lateral cutaneous nerve of thigh | Obesity, pregnancy, tight belts | Nothing motor | Meralgia paraesthetica: burning anterolateral thigh |
| Superior gluteal | Hip surgery, injection | Gluteus medius and minimus | Trendelenburg sign: pelvis drops on the opposite side |
| Inferior gluteal | Hip surgery | Gluteus maximus | Difficulty rising from a chair or climbing stairs |
| Sural | Small saphenous vein surgery | Nothing motor | Runs with the small saphenous vein behind the lateral malleolus; used for nerve biopsy and grafting |
- Common peroneal palsy versus L5 root: both cause foot drop, but an L5 lesion also weakens inversion (tibialis posterior, tibial nerve, L5) and hip abduction.
Abdominal wall nerves and the groin
- Iliohypogastric (L1) runs between internal oblique and transversus near the anterior superior iliac spine, the line of a gridiron (McBurney) incision. With the ilioinguinal nerve it supplies the lowest fibres of internal oblique and transversus that form the conjoint tendon; damage weakens the posterior wall of the inguinal canal and predisposes to a direct inguinal hernia.
- Ilioinguinal (L1) passes through the inguinal canal outside the spermatic cord, emerging through the superficial ring to supply the upper medial thigh and anterior scrotum or labium. It is at risk in hernia repair.
- Genitofemoral nerve (L1–L2): its genital branch lies inside the cord and supplies cremaster (cremasteric reflex L1–L2).
Disc prolapse and cauda equina
- A posterolateral disc compresses the traversing (lower) root: L4/L5 disc compresses L5, L5/S1 disc compresses S1. Straight leg raise is positive for L5 and S1; femoral stretch test for L3–L4.
- Cauda equina syndrome: saddle anaesthesia, painless urinary retention with overflow, faecal incontinence, reduced anal tone, bilateral leg symptoms; needs urgent MRI and decompression.
Exam trap Common peroneal palsy spares inversion and the ankle reflex; an L5 root lesion weakens inversion.
Vessels, veins and lymphatics
- Pulses: radial lateral to flexor carpi radialis (Allen test before arterial puncture); brachial medial to the biceps tendon; femoral at the mid-inguinal point (nerve, artery, vein from lateral to medial); popliteal deep in the fossa with the knee flexed; posterior tibial behind the medial malleolus; dorsalis pedis lateral to extensor hallucis longus (absent in about 10%).
- Popliteal fossa: bounded by biceps femoris (superolateral), semimembranosus and semitendinosus (superomedial) and the two heads of gastrocnemius (below). From superficial to deep: tibial nerve, popliteal vein, popliteal artery (deepest), so a supracondylar fracture can tear the artery.
- Veins: great saphenous lies anterior to the medial malleolus (cutdown site) and joins the femoral vein at the saphenous opening; small saphenous passes behind the lateral malleolus with the sural nerve to the popliteal vein. Varicose veins in an otherwise healthy adult are due to incompetent valves (often saphenofemoral junction), which let blood reflux from deep to superficial veins.
- Lymph: the lateral foot and back of the calf drain with the small saphenous vein to the popliteal nodes; the rest of the limb drains to the superficial inguinal nodes.
- DVT embolus from leg veins passes through the right heart and lodges in the pulmonary arteries.
- Intermittent claudication: calf pain after walking a fixed distance, relieved by rest, from superficial femoral artery atherosclerosis. Immobilisation in a cast causes disuse atrophy of the calf.
- Compartment syndrome: pain out of proportion and pain on passive stretch (earliest sign); pulses persist until late; treat with urgent fasciotomy.
Remember Foot drop with numb dorsum but normal inversion is the common peroneal nerve at the fibular neck.
How FCPS asks about this
- Wasting of infraspinatus only: suprascapular nerve compressed at the spinoglenoid notch.
- Flail anaesthetic arm with Horner syndrome after a crash: C5–T1 root avulsion (preganglionic).
- Fibular fracture, weak eversion, numb dorsum, first web space normal: superficial peroneal nerve.
- Direct hernia after gridiron incision: iliohypogastric nerve; nerve in the canal outside the cord is the ilioinguinal.
- Deepest structure in the popliteal fossa: popliteal artery; nerve beside the small saphenous vein used for grafting is the sural.
- Carpal tunnel: thenar wasting with palm sensation spared; ulnar lesion at the wrist gives the worse claw.
- Varicose veins in a healthy man: incompetent venous valves; DVT emboli lodge in the pulmonary arteries.
Test yourself on this topic
5 of the 119 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
Which nerve roots form the femoral nerve?
Answer: L2, L3, L4
The femoral nerve arises from the posterior divisions of the ventral rami of L2–L4 in the lumbar plexus. It passes lateral to the femoral artery under the inguinal ligament, supplies the anterior thigh muscles (quadriceps, sartorius, pectineus), the knee jerk (L3–L4) and, via the saphenous nerve, the skin of the medial leg. The obturator nerve comes from the anterior divisions of the same roots.
Femoral nerve
- L2–L4, posterior divisions
- Lateral to femoral artery
- Quadriceps; knee jerk
- Saphenous: medial leg skin
Remember Femoral and obturator share L2–L4: back and front.
Exam trap The saphenous nerve is the only branch of the femoral nerve reaching the foot.
Why the other options are wrong
- L1, L2 Ilioinguinal and genitofemoral region.
- L4, L5, S1 Part of the sciatic nerve.
- S1, S2, S3 Sacral plexus.
- L5, S1, S2 Part of the sciatic nerve.
After a stab wound in the upper arm, a man has weak elbow flexion and forearm supination and numbness along the lateral forearm. Which nerve is injured?
Answer: Musculocutaneous
The musculocutaneous nerve (C5–C7) pierces coracobrachialis and supplies the anterior arm muscles: biceps, brachialis and coracobrachialis. It continues as the lateral cutaneous nerve of the forearm. Injury weakens elbow flexion and supination (brachioradialis, supplied by the radial nerve, still allows some flexion) and causes numbness along the lateral forearm.
Musculocutaneous nerve
- Pierces coracobrachialis
- Supplies biceps, brachialis
- Sensation: lateral forearm
Remember Biceps and lateral forearm.
Exam trap Elbow flexion is not lost completely because brachioradialis helps.
Why the other options are wrong
- Radial Wrist drop; posterior arm sensation.
- Median Pronation and thenar weakness.
- Axillary Deltoid; regimental badge.
- Ulnar and median Hand weakness; elbow flexion spared.
Compartment pressure is measured in a swollen leg. Which value most supports urgent fasciotomy?
Answer: Diastolic BP minus compartment pressure under 30 mmHg
Absolute compartment pressure alone is less useful than the perfusion gradient. A delta pressure (diastolic blood pressure minus compartment pressure) below 30 mmHg indicates inadequate perfusion and the need for fasciotomy. Normal compartment pressure is under about 10 mmHg. Hypotension lowers the threshold at which compartment syndrome occurs.
Delta pressure
- Delta P = diastolic BP − compartment P
- Fasciotomy if < 30 mmHg
- Normal compartment < 10 mmHg
- Hypotension lowers tolerance
Remember Less than thirty between diastolic and compartment: cut.
Exam trap Clinical diagnosis should not wait for pressure measurement in an awake patient.
Why the other options are wrong
- Compartment pressure of 10 mmHg Near normal.
- Systolic BP minus compartment pressure of 90 mmHg Shows good perfusion.
- Compartment pressure equal to CVP Not a criterion.
- Any reading above 5 mmHg Normal values are under about 10 mmHg.
A 32-year-old woman who lost 25 kg after bariatric surgery often sits with her legs crossed. Over two weeks she has developed right foot drop, tripping on stairs, with numbness over the dorsum of the foot and no back pain. Which of the following findings would favour a common fibular nerve palsy over an L5 root lesion?
Answer: Normal strength of ankle inversion
Weight loss removes the fat that cushions the common fibular nerve at the fibular neck, and leg crossing compresses it there. The nerve supplies the dorsiflexors and evertors, but inversion is mainly by tibialis posterior, supplied by the tibial nerve. An L5 root lesion weakens tibialis posterior and gluteus medius as well, so weak inversion or hip abduction points to the root. Dorsiflexion, big toe extension and dorsal foot numbness are affected in both.
Fibular nerve vs L5
- Both: weak dorsiflexion, EHL
- Nerve: inversion normal
- L5: weak inversion and hip abduction
- Weight loss, leg crossing compress nerve
Remember Test inversion to separate nerve from root.
Exam trap Back pain may be absent in L5 radiculopathy, so test inversion in every foot drop.
Why the other options are wrong
- Weak ankle dorsiflexion Weak in both lesions.
- Numbness over the dorsum of the foot Present in both lesions.
- Weak extension of the big toe EHL is weak in both lesions.
- A normal ankle jerk The ankle jerk is S1 and normal in both.
A diabetic man who rests on his elbows at his desk has tingling of the little finger and weak finger spreading. Which finding places the lesion at the elbow rather than the wrist?
Answer: Numbness over the dorsal ulnar hand
The dorsal cutaneous branch of the ulnar nerve leaves the main trunk in the distal forearm, about 5 cm above the wrist, and supplies the dorsal ulnar side of the hand. A lesion in Guyon's canal at the wrist spares this branch, while compression at the cubital tunnel involves it. Weakness of flexor carpi ulnaris and the ulnar half of flexor digitorum profundus also points to the elbow. The intrinsic hand muscles are weak in both.
Ulnar nerve: elbow or wrist
- Dorsal branch leaves above the wrist
- Elbow: dorsal hand numb
- Elbow: weak FDP to ring, little
- Wrist (Guyon): dorsum spared
Remember Check the back of the hand to localise an ulnar lesion.
Exam trap Clawing is worse in distal lesions, the ulnar paradox.
Why the other options are wrong
- Weak first dorsal interosseous Weak in both elbow and wrist lesions.
- Positive Froment sign Adductor pollicis weakness occurs in both.
- Wasting of the hypothenar muscles Hypothenar muscles are supplied distal to both sites.
- Clawing of the ring and little fingers Clawing is actually milder at the elbow because FDP is weak too.
Where this sits in the CPSP syllabus
- Medicine & Allied, Paper 2: Regional anatomy for physicians, Upper and lower limb: nerves, vessels and common lesions