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10 notes available
1 Last Minute Review 2 Polyneuropathies 3 Mononeuropathies & Plexopathies 4 Motor Neuron Disease 5 Myasthenia Gravis & NMJ Disorders 6 Inflammatory Myopathies 7 Muscular Dystrophies 8 Channelopathies & Metabolic Myopathies 9 Neuromuscular Emergencies 10 EMG/NCS Patterns
Clinical › Neuromuscular

Mononeuropathies & Plexopathies

Mononeuropathies & Plexopathies

What You'll Learn

  • Carpal tunnel syndrome: most common entrapment neuropathy; sensory latencies are more sensitive than motor; Phalen's, Tinel's, shake sign; focal slowing at wrist on NCS
  • Ulnar neuropathy: cubital tunnel is the most common site; Froment's sign (FPL substitution for weak adductor pollicis); Martin-Gruber anastomosis can confuse NCS
  • Footdrop DDx: peroneal neuropathy vs. L5 radiculopathy vs. sciatic — check foot inversion (tibialis posterior, tibial nerve) — normal in peroneal, weak in L5
  • Brachial plexopathy: Erb-Duchenne (C5/C6, upper trunk, waiter's tip) vs. Klumpke (C8/T1, lower trunk, claw hand + Horner syndrome)
  • Parsonage-Turner: acute severe shoulder pain → flaccid weakness (upper plexus); antecedent infection/vaccination; self-limited but recovery takes months
  • Cancer vs. radiation plexopathy: carcinomatous = painful; radiation = painless + myokymic discharges on EMG
  • Radiculopathy tables: cervical (C5–C8) and lumbosacral (L2–S1) localization by pain, sensory, motor, and reflex are extremely high-yield
HighYield Pearls
  • Foot drop localization: peroneal at fibular head spares inversion (tibialis posterior, tibial nerve) — weak inversion + weak EHL points to L5 radiculopathy, NOT peroneal.
  • Ulnar at elbow vs Guyon canal: dorsal ulnar cutaneous branch exits proximal to wrist — dorsal hand sensation SPARED → lesion is at Guyon canal (not cubital tunnel).
  • Radial spiral groove vs PIN: spiral groove = wrist drop + finger drop + BR weakness with PRESERVED triceps; PIN = finger drop only, NO wrist drop (ECRL spared), no sensory loss.
  • Parsonage-Turner (neuralgic amyotrophy): acute severe shoulder/scapular pain → patchy flaccid weakness after pain subsides; often post-vaccine/infection/surgery; long thoracic + suprascapular + axillary + AIN most commonly involved.
  • Radiation vs neoplastic plexopathy: radiation = painless, slowly progressive, myokymic discharges on EMG; neoplastic = painful, faster, mass on MRI — do not miss the myokymia.
  • Diabetic amyotrophy (Bruns-Garland): older T2DM (often new diagnosis) → severe asymmetric proximal LE pain + weakness + atrophy + weight loss; immune-mediated; may respond to steroids/IVIG.
  • CTS treatment ladder: nocturnal wrist splints → steroid injection → carpal tunnel release; release is the only durable fix for thenar atrophy / denervation on EMG.
  • Meralgia paresthetica: lateral femoral cutaneous nerve at inguinal ligament — pure sensory anterolateral thigh burning/dysesthesia; obesity, pregnancy, tight belts; NO motor or reflex changes.
  • Erb (C5–C6) vs Klumpke (C8–T1): Erb = waiter’s tip (upper trunk, breech/shoulder dystocia); Klumpke = claw hand + Horner syndrome (lower trunk, T1 sympathetics).
  • Hereditary neuralgic amyotrophy: recurrent Parsonage-Turner-like attacks + dysmorphic features → SEPT9 mutation (autosomal dominant).
🔍 Quick ReferenceClinical · EMG/NCS localization · Etiology / treatment
Clinical phenotype
  • Nocturnal hand paresthesias relieved by shaking → carpal tunnel syndrome (flick sign)
  • Thumb IP flexion when pinching paper → Froment sign — ulnar neuropathy (FPL substitutes for weak adductor pollicis)
  • Abducted small finger at rest → Wartenberg sign — ulnar neuropathy
  • Wrist drop + finger drop, triceps preserved, after arm draped over chair → Saturday-night palsy (radial at spiral groove)
  • Finger drop without wrist drop, no sensory loss → posterior interosseous neuropathy (PIN)
  • Foot drop with eversion weakness, inversion preserved → peroneal at fibular head
  • Burning anterolateral thigh in obese/pregnant patient, no weakness → meralgia paresthetica (LFC)
  • Medial heel pain + plantar paresthesias → tarsal tunnel syndrome (tibial)
  • Scapular winging after carrying a backpack → long thoracic neuropathy (serratus anterior)
  • Acute lancinating shoulder pain → patchy arm weakness/atrophy days later → Parsonage-Turner (neuralgic amyotrophy)
  • Newborn with arm adducted, internally rotated, pronated forearm → Erb palsy (C5–C6 — waiter’s tip)
  • Newborn with claw hand + ipsilateral Horner → Klumpke palsy (C8–T1)
EMG/NCS localization clues
  • Myokymic discharges in plexus muscles → radiation-induced plexopathy (pathognomonic)
  • Focal slowing on short-segment (inching) study at the wrist → CTS
  • Conduction velocity drop across the elbow >10 m/s on inching → ulnar neuropathy at the elbow
  • Ring-finger median vs ulnar sensory latency difference ≥0.5 ms → earliest NCS finding in CTS
  • Dorsal ulnar cutaneous SNAP preserved → ulnar lesion at Guyon canal (not elbow)
  • Ulnar CMAP at elbow > wrist (false conduction block) → Martin-Gruber anastomosis
  • Denervation in tibialis posterior (tibial-innervated L5) → L5 radiculopathy, NOT peroneal
  • Denervation in short head of biceps femoris → peroneal-division sciatic lesion above knee
  • Patchy denervation across multiple individual nerves (long thoracic, suprascapular, axillary, AIN) with normal cervical paraspinals → Parsonage-Turner
  • Denervation in lumbosacral paraspinals + proximal LE muscles, asymmetric → diabetic amyotrophy (radiculoplexus neuropathy)
  • Absent median SNAP + thenar fibs/PSWs → severe CTS with axon loss
Etiology / treatment pearls
  • Pregnancy, hypothyroidism, acromegaly, amyloidosis, RA → CTS risk factors
  • Nocturnal wrist splint → steroid injection → carpal tunnel release → CTS treatment ladder
  • Cubitus valgus from old supracondylar fracture → tardy ulnar palsy
  • Ganglion cyst at wrist → deep palmar (motor) branch ulnar neuropathy — mimics ALS
  • Crutch use / prolonged tourniquet → radial neuropathy at axilla (triceps weak, unlike spiral groove)
  • Prolonged squatting (“strawberry-picker’s palsy”) or rapid weight loss → peroneal at fibular head
  • Tight belts, obesity, pregnancy → meralgia paresthetica
  • Post-viral / post-vaccination / post-surgical acute shoulder pain → Parsonage-Turner
  • Recurrent brachial neuritis with dysmorphic features (hypotelorism, short stature) → hereditary neuralgic amyotrophy, SEPT9 mutation
  • Older T2DM with severe proximal LE pain + weight loss + asymmetric weakness → diabetic amyotrophy (Bruns-Garland); consider IVIG/steroids
  • Painless, late-onset, slowly progressive upper plexopathy after breast cancer XRT → radiation plexopathy
  • Painful, faster-progressing plexopathy with MRI mass / Pancoast tumor → neoplastic plexus infiltration
  • Hip arthroplasty / lithotomy positioning → sciatic (peroneal division) or femoral neuropathy
  • Retroperitoneal hematoma in anticoagulated patient → femoral neuropathy (weak quadriceps, absent patellar reflex)
Median Nerve

Carpal Tunnel Syndrome (CTS)

  • Most common entrapment neuropathy; median nerve compressed under transverse carpal ligament (flexor retinaculum)
  • Symptoms: numbness/tingling in digits 1–3 and radial half of digit 4; nocturnal pain; shake sign (flicking hands for relief)
  • Provocative tests: Phalen's (wrist flexion 60 s), Tinel's (tapping at carpal tunnel), Durkan's (direct compression)
  • Thenar weakness: APB (abductor pollicis brevis) — late finding; opponens pollicis affected
  • Spared: palmar cutaneous branch exits proximal to carpal tunnel → thenar palm sensation preserved in CTS

NCS/EMG in CTS

  • Sensory latencies are more sensitive than motor latencies for early CTS
  • Focal slowing of median sensory and/or motor conduction at the wrist
  • Compare median to ulnar or radial sensory latencies (ring finger comparison study is most sensitive)
  • Severe CTS → absent median SNAP → thenar denervation (fibs/PSWs in APB)

Pronator Teres Syndrome

  • Median nerve compressed at pronator teres or fibrous arch of FDS
  • Key distinguisher from CTS: sensory loss includes thenar palm (palmar cutaneous branch involved)
  • Aching forearm pain worsened by pronation; no nocturnal symptoms (unlike CTS)
  • NCS: normal at wrist; EMG may show denervation in pronator teres, FCR, FDS

Anterior Interosseous Nerve (AIN) Syndrome

  • Pure motor branch of the median nerve — no sensory loss
  • Innervates: FPL (flexor pollicis longus), FDP to digits 2–3, pronator quadratus
  • Cannot make OK sign: unable to flex DIP of thumb and index finger → pinch is pad-to-pad instead of tip-to-tip
  • Causes: Parsonage-Turner syndrome (most common), fibrous bands, accessory muscles
💎 Board Pearl
  • Thenar palm sensation preserved in CTS (palmar cutaneous branch exits proximal to tunnel) but lost in pronator teres syndrome
  • AIN syndrome = pure motor → no numbness; failed OK sign is the exam hallmark
  • Ring finger comparison study (median vs. ulnar digit 4) is the most sensitive NCS for CTS
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