NeurologyTier 1Disease (DEADMAN)

Mononeuropathies, common

Description

Focal lesion of one named nerve -> LMN pattern in that nerve's territory only
- Wasting, flaccid weakness, dec/absent reflex, sensory loss to nerve (not dermatomal) distribution
Level is set by the most proximal branch involved - examine for what is spared, not only what is weak

Median

Forearm: all flexors except FCU and ulnar half of FDP; pronators
Hand: LOAF - Lateral 2 lumbricals, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis
Sensory: radial 3.5 digits, palmar surface + dorsal fingertips
- Palmar cutaneous branch (thenar skin) arises proximal to the tunnel -> thenar sensation spared in CTS**
AIN branch: FPL, FDP to index, pronator quadratus
- Pure motor - no sensory loss; cannot make an "OK" sign, pinches flat

Ulnar

Forearm: FCU + ulnar half FDP (ring/little)
Hand: all intrinsics except LOAF
Sensory: little finger + medial half ring finger, palmar and dorsal
- Dorsal cutaneous branch leaves ~5 cm above wrist -> dorsal sensation spared in Guyon's canal lesions

Radial

Triceps, brachioradialis, supinator, all wrist/finger extensors
Sensory: dorsum of hand - test the anatomical snuffbox
PIN branch: finger drop + wrist extension with radial deviation (ECRL spared), no sensory loss

Common peroneal

Deep branch: dorsiflexion (TA, EHL, EDB); sensory = 1st web space only
Superficial branch: eversion (peronei); sensory = lateral calf + dorsum of foot
Inversion (tibialis posterior, tibial nerve) preserved - the single discriminator from L5

Others worth knowing

Long thoracic (C5-7) -> serratus anterior -> commonest cause of winged scapula
Lateral femoral cutaneous -> meralgia paraesthetica; pure sensory, anterolateral thigh
Axillary -> deltoid + regimental badge patch; shoulder dislocation, humeral neck fracture

Epidemiology

CTS = commonest mononeuropathy (~3-5% adults; F>M ~3:1)
Ulnar at elbow = 2nd commonest upper limb
Common peroneal = commonest lower limb mononeuropathy
Bimodal: entrapment peaks middle age; traumatic peaks young men

Aetiopathogenesis

Mechanism

Compression/entrapment - at anatomical tunnels and where the nerve is superficial over bone
- Demyelination first (conduction block, fully reversible) -> axonal loss if prolonged
Traction/trauma, ischaemia (vasa nervorum), infiltration (leprosy, amyloid, lymphoma)

By nerve - causes follow the anatomical course

Nerve	Site	Causes
Radial	Axilla (posterior cord)	Crutch palsy - triceps involved
	Spiral groove	"Saturday night palsy", humeral shaft fracture - triceps spared
	Forearm (PIN)	Radial fracture, lipoma, repetitive supination
Ulnar	Cubital tunnel	Commonest; elbow OA, valgus deformity from old fracture (tardy palsy), leaning
	Guyon's canal (wrist)	Ganglion, hook of hamate fracture, cyclists
Median	Carpal tunnel	See CTS
	Forearm (AIN)	Fracture, forearm trauma, Parsonage-Turner
Peroneal	Fibular neck	Plaster/tourniquet, prolonged squatting or leg crossing, rapid weight loss ("slimmer's palsy"), ganglion of superior tibiofibular joint, tendinous edge of peroneus longus

Diabetes predisposes all of the above - nerves are more susceptible to compression
Leprosy: commonest infective mononeuropathy worldwide (ulnar, peroneal, greater auricular - thickened palpable nerves)

Mononeuritis multiplex

Two or more named nerves, asymmetric, stepwise. Almost always a systemic disease.

Acute / vascular - the important group
- Polyarteritis nodosa, EGPA (Churg-Strauss), GPA, rheumatoid vasculitis, cryoglobulinaemia
- Diabetes (commonest single cause overall)
Chronic
- Sarcoidosis, amyloid, acromegaly, HIV, leprosy, hereditary neuropathy with liability to pressure palsy (HNPP), lymphoma/carcinoma infiltration
- Multiple compressive neuropathies at separate sites

Diagnosis

Nerve vs root vs plexus - the exam question

	Nerve	Root	Plexus
Sensory loss	Nerve territory, sharp edge	Dermatome	Multiple dermatomes
Weakness	Nerve's muscles	Myotome - muscles from >1 nerve	Widespread, one limb
Extends above wrist?	No	Yes	Yes
Pain	Local +/- distal	Radicular, worse on cough/strain	Deep, aching
Extras	-	-	Horner's, apical mass -> lower trunk

Ulnar lesion

Wasting: 1st dorsal interosseous + abductor digiti minimi (dorsal guttering)
Claw hand - MCP hyperextension + IP flexion of ring and little fingers
Froment's sign - thumb IP flexes on key pinch (adductor pollicis weak, FPL substitutes)
Weak finger abduction/adduction
Ulnar paradox - the higher the lesion, the less the clawing
- Elbow lesion also paralyses ulnar FDP -> no unopposed flexor pull -> less claw
- Wrist lesion spares FDP -> more claw
Level: FCU weak = at/above elbow. FCU spared + adductor pollicis weak = wrist
*A true claw of all fingers is not isolated ulnar* - think combined median+ulnar, C8-T1/lower trunk, syringomyelia, polio, Volkmann's ischaemic contracture, advanced RA

Radial lesion

Wrist drop + finger drop, wrist flexion preserved
Weak elbow flexion in mid-pronation (brachioradialis)
Passively extend the wrist -> fingers extend at IPJs (lumbricals/interossei) and apparent interosseous weakness resolves - do not call it an ulnar lesion
Triceps + triceps jerk involved = proximal to the spiral groove
No sensory loss = PIN

Median lesion

Wrist (CTS): wasting/weakness confined to APB only
Deep finger flexors + FPL weak = above the carpal tunnel

Common peroneal lesion

High-stepping gait; foot plantarflexed and inverted at rest
Weak dorsiflexion + eversion, inversion normal, tone and reflexes normal
Wasting of tibialis anterior (anterolateral compartment)
Sensory: lateral calf + dorsum of foot; deep branch alone -> 1st web space only

### Peroneal palsy vs L5 radiculopathy

	Peroneal	L5 root
Inversion	Normal	Weak
Eversion	Weak	Weak
Hip abduction / knee flexion	Normal	Weak
Sensory	Lateral calf, dorsum foot	L5 dermatome, spares sole + little toe
Back pain / positive SLR	No	Often

### Foot drop - full differential

Common peroneal palsy | sciatic palsy | lumbosacral plexopathy | L4/L5 radiculopathy
Peripheral motor neuropathy (CMT - bilateral, pes cavus) | distal myopathy
MND (painless, fasciculations, brisk reflexes elsewhere)
Contralateral parasagittal/precentral cortical lesion - foot drop with UMN signs

Investigation

Nerve conduction studies + EMG - localise, and separate conduction block (demyelinating, good prognosis) from axonal loss (denervation on EMG, slower recovery)
- Wallerian degeneration takes 7-10 days - NCS too early can be falsely reassuring; EMG denervation takes 2-3 weeks
Nerve ultrasound / MR neurography - structural cause (ganglion, nerve tumour, focal enlargement)
If mononeuritis multiplex: FBE + eosinophils, ESR/CRP, ANCA, ANA/ENA, RF, cryoglobulins, HBV/HCV/HIV, ACE, glucose/HbA1c, SPEP
- Nerve biopsy (sural) + muscle if vasculitis suspected and diagnosis not otherwise made

Management

Compressive mononeuropathy - by severity

Conservative first where deficit is sensory/mild and NCS show demyelination only
- Remove the cause: splinting in neutral (night splints), activity modification, avoid leaning/crossing legs, elbow padding
- Peroneal: ankle-foot orthosis, night splint to prevent equinus contracture
- Physio for range of motion and to maintain bulk
Corticosteroid injection - CTS only; symptomatic benefit is real but not durable
- ~40% still come to surgery; use to bridge, in pregnancy, or if not a surgical candidate
Surgical decompression if:
- Persistent symptoms despite conservative measures, OR
- Motor deficit or wasting, OR
- Axonal loss on NCS, OR
- A structural lesion (ganglion, tumour, fracture fragment)
Nerve repair/graft or tendon transfer for a transected nerve or failed reinnervation

Mononeuritis multiplex

Treat the systemic disease, urgently - vasculitic neuropathy is an organ-threatening manifestation
- High-dose corticosteroid + cyclophosphamide or rituximab for ANCA-associated/PAN
- Delay costs axons; recovery after axonal loss is slow and often incomplete
Diabetic: glycaemic control, exclude a coexistent vasculitis if stepwise and painful

Symptom control

Neuropathic pain: amitriptyline, duloxetine, pregabalin or gabapentin
Avoid oral corticosteroids in isolated compressive neuropathy - no evidence, real harm

Associations

Diabetes mellitus - both isolated entrapments and mononeuritis multiplex
Vasculitis - PAN, EGPA, GPA, rheumatoid vasculitis, cryoglobulinaemia
Connective tissue disease - RA, SLE, Sjogren's
HNPP (PMP22 deletion) - recurrent painless palsies after trivial compression, AD, mirror image of CMT1A duplication
Hypothyroidism, acromegaly, amyloidosis, pregnancy - CTS
Sarcoidosis, HIV, leprosy, Lyme disease
Alcohol, malnutrition, rapid weight loss - peroneal
Haematological malignancy / neurolymphomatosis

Natural history & complications

Neurapraxia (conduction block) - recovery in weeks, complete
Axonotmesis - axon regrows at ~1 mm/day from the lesion; endoneurium intact so recovery usually good
- Calculate expected time: an axillary radial lesion to the hand is >12 months
Neurotmesis - complete transection, no recovery without surgical repair
Bell's-type advancing Tinel's sign along the nerve tracks regeneration

Complications

Fixed contracture if unsplinted (equinus in peroneal palsy, claw in ulnar)
Persistent neuropathic pain, cold intolerance
Complex regional pain syndrome
Recurrent injury from an insensate territory
Chronic denervation >18-24 months -> motor end plates lost -> reinnervation fails even if the nerve is repaired

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