Mononeuropathies & Plexopathies
Mononeuropathies & Plexopathies
What Do You Need to Know?
- 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
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
Ulnar Nerve
Anatomy & Entrapment Sites
| Site | Location | Sensory Loss | Motor Deficit | Key Feature |
|---|---|---|---|---|
| Cubital tunnel | Elbow (between medial epicondyle and olecranon) | Digit 5 + medial digit 4 (dorsal and palmar) | All ulnar hand intrinsics + FDP 4/5 + FCU | Most common site; conduction velocity slowing across elbow |
| Guyon's canal | Wrist (between pisiform and hook of hamate) | Palmar digit 5 + medial digit 4 (dorsal hand spared) | Ulnar hand intrinsics (FDP/FCU spared) | Dorsal ulnar cutaneous branch spared (exits proximal to wrist) |
| Deep motor branch (pisohamate) | Deep within Guyon's canal | None (pure motor) | Interossei + adductor pollicis (hypothenar muscles may be spared) | Ganglion cyst; may mimic ALS (pure motor hand weakness) |
Clinical Signs
- Froment's sign: patient grips paper between thumb and index finger; weak adductor pollicis → compensatory FPL flexion (thumb IP flexion) — median nerve substitution
- Ulnar claw hand: MCP hyperextension + DIP/PIP flexion of digits 4–5; caused by loss of lumbricals 3–4 (unopposed FDP pull)
- Ulnar paradox: higher lesions → LESS clawing (FDP to digits 4/5 also denervated, so no unopposed flexion at DIP)
- Wartenberg sign: abducted small finger at rest (weak 3rd palmar interosseous)
- Tardy ulnar palsy: delayed-onset ulnar neuropathy at elbow from remote fracture/deformity (cubitus valgus)
Martin-Gruber Anastomosis
- Present in 20–25% of the population; median-to-ulnar nerve crossover in the forearm
- Motor fibers from median nerve cross to ulnar nerve in the forearm → innervate ulnar hand muscles via ulnar nerve distally
- NCS pitfall: stimulating median nerve at elbow gives a larger CMAP than at wrist (volume conduction artifact); initial positive deflection on ulnar recording
- Can cause diagnostic confusion in CTS and ulnar neuropathy — apparent conduction block or unusual CMAP drop
💎 Board Pearl
- Dorsal ulnar cutaneous nerve branches proximal to wrist — if dorsal hand sensation is spared, lesion is at Guyon's canal (not cubital tunnel)
- Ulnar paradox: higher lesions produce LESS clawing because FDP 4/5 is also weak
- Martin-Gruber anastomosis (20–25%) can cause median CMAP at elbow > wrist — do not mistake for conduction block
Radial Nerve
Radial Nerve Anatomy & Compression Sites
| Site | Syndrome | Motor Deficit | Sensory Loss | Key Features |
|---|---|---|---|---|
| Spiral groove (mid-humerus) | Saturday night palsy | Wrist drop + finger drop; triceps spared (branches above groove) | Dorsal hand/forearm (superficial radial nerve territory) | Compression during sleep, intoxication, improper crutch use |
| Axilla | Crutch palsy / prolonged arm abduction | Wrist drop + finger drop + triceps weakness | Posterior arm + dorsal forearm/hand | Triceps involvement distinguishes from spiral groove |
| Posterior interosseous nerve (PIN) | PIN syndrome | Finger drop (extensors of digits) + weak wrist ulnar deviation; no wrist drop (ECRL spared) | None (pure motor) | Wrist deviates radially with extension (ECRL intact, ECU weak); arcade of Frohse |
| Superficial radial nerve | Wartenberg syndrome / cheiralgia paresthetica | None | Dorsal radial hand (1st web space) | Pure sensory; tight wristbands, handcuffs; positive Finkelstein-like maneuver |
Key Clinical Points
- Saturday night palsy: wrist drop + finger drop but triceps and brachioradialis usually spared (branches exit above spiral groove)
- Finger extensors extend at MCP joints — interossei extend at IP joints; patient with radial palsy can still extend IP joints via interossei
- PIN vs. radial nerve: PIN spares ECRL → wrist can extend but deviates radially; no sensory loss (pure motor)
- PIN compression at arcade of Frohse; causes include lipoma, synovitis (RA), fracture
💎 Board Pearl
- Saturday night palsy: triceps SPARED (innervated proximal to spiral groove) — if triceps is weak, lesion is at the axilla
- PIN = pure motor finger drop without wrist drop and without sensory loss — wrist deviates radially
- Wartenberg syndrome (superficial radial) = pure sensory — numbness over dorsal 1st web space
Peroneal Nerve & Footdrop
Common Peroneal Neuropathy
- Most common compressive neuropathy in the lower extremity; at fibular head (neck of fibula)
- Causes: leg crossing, prolonged bed rest, casts, weight loss, fibular head fracture
- Footdrop (weak ankle dorsiflexion) + weak eversion + weak toe extension
- Sensory loss: dorsal foot + lateral leg (superficial peroneal territory)
Deep vs. Superficial Peroneal
| Branch | Motor | Sensory |
|---|---|---|
| Deep peroneal | Tibialis anterior (dorsiflexion), EHL, EDL, peroneus tertius | 1st web space only |
| Superficial peroneal | Peroneus longus & brevis (eversion) | Dorsal foot + lateral lower leg |
Differential Diagnosis of Footdrop
| Feature | Common Peroneal | L5 Radiculopathy | Sciatic (Peroneal Division) | Lumbosacral Plexus | Parasagittal Lesion (UMN) |
|---|---|---|---|---|---|
| Foot inversion | Normal (tibialis posterior = tibial nerve) | Weak (tibialis posterior = L5) | Normal or weak (depending on extent) | Weak | Normal |
| Foot eversion | Weak | Weak | Weak | Weak | Normal |
| Hip abduction | Normal | May be weak (gluteus medius = L5) | Normal | Weak | Weak |
| Ankle reflex | Normal | Normal (S1) | May be reduced | May be reduced | May be brisk |
| Sensory loss | Dorsal foot + lateral leg | Lateral leg, dorsal foot, and lateral foot | Variable | Variable, broader | None or leg predominant |
| Paraspinal EMG | Normal | Abnormal | Normal | Normal | Normal |
| Tone/reflexes | LMN (flaccid) | LMN | LMN | LMN | UMN (spastic, Babinski+) |
🎯 Clinical Pearl
- KEY test: foot inversion (tibialis posterior, L5, tibial nerve) — NORMAL in peroneal neuropathy, WEAK in L5 radiculopathy. This is the single most important maneuver to differentiate the two.
- Paraspinal fibrillations on EMG → radiculopathy (not peripheral nerve lesion)
💎 Board Pearl
- Common peroneal at fibular head is the #1 lower extremity entrapment neuropathy
- Sciatic nerve: the peroneal division is more susceptible to injury than the tibial division (lateral position in the nerve, less connective tissue)
- Parasagittal meningioma (falx) → bilateral lower extremity UMN weakness mimicking bilateral footdrop — do not miss this!
Other Lower Extremity Mononeuropathies
Femoral Neuropathy
- Motor: quadriceps weakness (knee extension) + iliopsoas may be weak if lesion is proximal
- Sensory: anterior thigh + medial leg (saphenous nerve)
- Reflex: absent/reduced patellar (knee jerk)
- Causes: psoas/iliacus hematoma (anticoagulation!), diabetes, pelvic surgery, hip hyperextension (lithotomy position)
- Hip hyperextension worsens symptoms (stretches femoral nerve over inguinal ligament)
- CT abdomen/pelvis to evaluate for retroperitoneal hematoma
Meralgia Paresthetica
- Lateral femoral cutaneous nerve (L2/L3) — pure sensory
- Compressed under inguinal ligament near ASIS
- Symptoms: burning pain/numbness of anterolateral thigh; no motor deficit, no reflex change
- Risk factors: obesity, tight belts/clothing, pregnancy, diabetes
- Treatment: remove offending compression, weight loss; local nerve block; usually self-limited
Tarsal Tunnel Syndrome
- Tibial nerve compressed behind the medial malleolus under the flexor retinaculum
- Symptoms: burning pain/numbness of the sole of the foot (medial and lateral plantar nerves)
- Tinel's sign at medial malleolus; worsened by prolonged standing/walking
- NCS: prolonged distal latency or reduced amplitude of medial/lateral plantar nerves
Sciatic Neuropathy
- Two divisions within one sheath: peroneal (lateral) and tibial (medial)
- Peroneal division more vulnerable (lateral position, tethered at fibular head, less connective tissue protection)
- Short head of biceps femoris: only muscle above the knee innervated by the peroneal division of the sciatic nerve — if denervated on EMG, localizes to sciatic (not peroneal at fibular head)
- Causes: hip arthroplasty, posterior hip dislocation, deep IM injection (gluteal), piriformis syndrome
💎 Board Pearl
- Psoas hematoma (anticoagulated patient) is the classic cause of femoral neuropathy — order CT abdomen/pelvis
- Meralgia paresthetica = pure sensory, anterolateral thigh — no motor deficit, no reflex change
- Short head of biceps femoris on EMG differentiates sciatic neuropathy from peroneal neuropathy at fibular head
Cervical Radiculopathy Localization
| Root | Pain / Radiation | Sensory Loss | Motor Weakness | Reflex |
|---|---|---|---|---|
| C5 | Lateral shoulder, upper arm | Lateral arm (axillary nerve territory) | Deltoid, biceps, supraspinatus, infraspinatus | Biceps (↓) |
| C6 | Lateral forearm to thumb | Lateral forearm, thumb, index finger | Biceps, brachioradialis, wrist extensors (ECRL) | Brachioradialis (↓) |
| C7 | Posterior arm/forearm to middle finger | Middle finger | Triceps, wrist flexors, finger extensors (EDC), pronator teres | Triceps (↓) |
| C8 | Medial forearm to ring/small finger | Medial forearm, ring and small finger | Finger flexors (FDP), hand intrinsics (interossei), FPL | Finger flexor (↓) |
🎯 Clinical Pearl
- C5/C6 disc → C6 radiculopathy (nerve exits above pedicle in cervical spine)
- C6/C7 disc → C7 radiculopathy — most common cervical radiculopathy
- Inverted brachioradialis reflex (tap brachioradialis → finger flexion instead of elbow flexion) suggests C5/C6 myelopathy with C6 radiculopathy
💎 Board Pearl
- C7 is the most commonly affected cervical root — remember “C7 = lucky 7 = longest nerve = most vulnerable”
- Cervical roots exit ABOVE their corresponding vertebra (C6 root exits above C6 pedicle) — opposite of lumbar spine
- Spurling's test (neck extension + lateral flexion + axial compression) reproduces radicular pain — high specificity
Lumbosacral Radiculopathy Localization
| Root | Pain / Radiation | Sensory Loss | Motor Weakness | Reflex |
|---|---|---|---|---|
| L2/L3 | Anterior thigh | Anterior thigh | Hip flexion (iliopsoas), hip adduction | None reliably (↓ patellar sometimes) |
| L4 | Anterior thigh to medial leg | Medial leg (saphenous territory) | Knee extension (quadriceps), hip adduction | Patellar (knee jerk) (↓) |
| L5 | Lateral leg, dorsal foot to great toe | Lateral leg, dorsal foot, 1st web space | Ankle dorsiflexion (tibialis anterior), toe extension (EHL), foot eversion, foot inversion (tibialis posterior), hip abduction (gluteus medius) | None (medial hamstring sometimes) |
| S1 | Posterior leg, lateral and plantar foot | Lateral foot, sole, posterior calf | Ankle plantarflexion (gastrocnemius), foot eversion (peroneus longus), hip extension (gluteus maximus) | Achilles (ankle jerk) (↓) |
🎯 Clinical Pearl
- L4/L5 disc → L5 radiculopathy; L5/S1 disc → S1 radiculopathy (lumbar roots exit below pedicle)
- L5 is the most common lumbosacral radiculopathy
- Far-lateral disc herniation can hit the exiting root (e.g., far-lateral L4/L5 disc → L4 root, not L5)
💎 Board Pearl
- L5 radiculopathy has NO reliable reflex — if footdrop + absent ankle jerk, think S1 involvement or sciatic lesion
- L5 affects tibialis posterior (foot inversion) — this distinguishes L5 from peroneal neuropathy on exam
- Lumbar roots exit BELOW their corresponding vertebra (opposite of cervical) — L4/L5 disc hits L5 root
Brachial Plexopathy
Brachial Plexus Anatomy
| Level | Components | Details |
|---|---|---|
| Roots | C5, C6, C7, C8, T1 | Ventral rami; exit intervertebral foramina |
| Trunks | Upper (C5/C6), Middle (C7), Lower (C8/T1) | Form in posterior triangle of neck; supraclavicular |
| Divisions | Anterior & Posterior from each trunk (6 total) | Pass behind clavicle; no named clinical syndromes |
| Cords | Lateral (ant. upper + ant. middle), Posterior (all 3 posterior), Medial (ant. lower) | Named by position relative to axillary artery; infraclavicular |
| Terminal branches | Musculocutaneous, axillary, radial, median, ulnar | Lateral cord → musculocutaneous + lateral contribution to median; Posterior cord → axillary + radial; Medial cord → ulnar + medial contribution to median |
Classic Brachial Plexus Syndromes
| Syndrome | Roots / Trunk | Mechanism | Motor Deficit | Posture / Sign | Other Features |
|---|---|---|---|---|---|
| Erb-Duchenne | C5/C6 (upper trunk) | Birth injury (shoulder dystocia), motorcycle accident, downward traction on shoulder | Deltoid, biceps, brachialis, supraspinatus, infraspinatus, brachioradialis | Waiter's tip: arm adducted, internally rotated, forearm pronated, wrist flexed | Sensory loss: lateral arm/forearm |
| Klumpke | C8/T1 (lower trunk) | Upward arm traction (grabbing overhead), birth injury (breech delivery) | Hand intrinsics (interossei, lumbricals, thenar, hypothenar), finger flexors | Claw hand | Horner syndrome if T1 sympathetic fibers involved (ptosis, miosis, anhidrosis) |
| Posterior cord | C5–T1 (all posterior divisions) | Shoulder dislocation, humeral fracture | Deltoid (axillary) + wrist/finger extensors (radial) | Wrist drop + shoulder abduction weakness | Sensory: lateral shoulder (axillary) + dorsal hand (radial) |
Parsonage-Turner Syndrome (Neuralgic Amyotrophy)
- Acute onset severe shoulder/arm pain → followed by flaccid weakness days to weeks later
- Upper plexus/trunk most commonly affected (C5/C6); patchy, multifocal distribution possible
- Antecedents: viral illness, vaccination, surgery, postpartum
- Long thoracic nerve (serratus anterior → winged scapula) is classically involved
- EMG: multifocal denervation of upper trunk muscles; may see involvement outside single trunk/cord
- Prognosis: self-limited; ~80–90% recover good function over 1–3 years
- Treatment: pain management (acute phase); PT for range of motion
Neoplastic vs. Radiation Plexopathy
| Feature | Carcinomatous (Tumor Invasion) | Radiation Plexopathy |
|---|---|---|
| Pain | Painful (early and prominent) | Painless (or minimal pain) |
| Trunk affected | Lower trunk (C8/T1) most common | Upper trunk more common |
| EMG | Denervation; no myokymic discharges | Denervation + myokymic discharges |
| Horner syndrome | Common (Pancoast tumor) | Uncommon |
| Classic tumor | Pancoast (lung apex) → lower plexus + Horner | Post-radiation (breast, lung, lymphoma) |
| Imaging | Mass on MRI/CT | Diffuse enhancement, no mass |
💎 Board Pearl
- Carcinomatous plexopathy = painful; radiation plexopathy = painless + myokymic discharges on EMG — this is a classic board question
- Pancoast tumor (lung apex) → lower trunk (C8/T1) + ipsilateral Horner syndrome
- Parsonage-Turner: do NOT confuse with C5/C6 radiculopathy — EMG shows multifocal pattern beyond one root/trunk; no paraspinal denervation
- Long thoracic nerve palsy → winged scapula — think Parsonage-Turner, especially if preceded by acute shoulder pain
Lumbosacral Plexopathy
Anatomy
| Plexus Component | Roots | Major Nerves | Key Muscles |
|---|---|---|---|
| Lumbar plexus | L1–L4 | Femoral (L2–L4), obturator (L2–L4), lateral femoral cutaneous (L2/L3) | Iliopsoas, quadriceps, hip adductors |
| Lumbosacral trunk | L4–L5 | Connects lumbar to sacral plexus | Critical link; vulnerable during pelvic surgery/delivery |
| Sacral plexus | L5–S4 | Sciatic (L4–S3), superior gluteal (L4–S1), inferior gluteal (L5–S2), pudendal (S2–S4) | Glutei, hamstrings, all below-knee muscles |
Diabetic Amyotrophy (Diabetic Lumbosacral Radiculoplexus Neuropathy)
- Acute/subacute severe unilateral thigh pain → proximal leg weakness + atrophy; may spread contralaterally
- Predominantly lumbar plexus (L2–L4): quadriceps weakness, absent knee jerk, anterior thigh sensory loss
- Pathology: ischemic injury from microvasculitis of vasa nervorum (not metabolic)
- Can occur with mild or newly diagnosed diabetes; HbA1c may be only mildly elevated
- Often associated with weight loss
- Self-limited over 6–18 months; some advocate IV methylprednisolone or IVIG (limited evidence)
Other Causes of LS Plexopathy
- Pelvic masses/tumors: colorectal, cervical, prostate, lymphoma — insidious onset, progressive
- Retroperitoneal hemorrhage: anticoagulation, hemophilia — acute lumbar plexus involvement, femoral neuropathy overlap
- Radiation: typically 1–5 years after pelvic radiation; painless; myokymic discharges on EMG (same principle as brachial)
- Surgical injury: lithotomy position (lumbosacral trunk compression), cesarean section, hip replacement
- Idiopathic lumbosacral plexitis: LS equivalent of Parsonage-Turner syndrome
💎 Board Pearl
- Diabetic amyotrophy: ischemic microvasculitis (NOT metabolic neuropathy) — can occur with mild diabetes; severe pain + proximal weakness + weight loss
- Lumbosacral trunk (L5) is vulnerable during labor/pelvic surgery → postpartum footdrop
- Radiation LS plexopathy: painless + myokymic discharges — same rule as brachial plexus
Cauda Equina vs. Conus Medullaris
| Feature | Conus Medullaris (L1–L2 level) | Cauda Equina (below L2) |
|---|---|---|
| Onset | Sudden and bilateral | Gradual and unilateral → bilateral |
| Pain | Less prominent; bilateral perineal | Severe; radicular, asymmetric |
| Sensory | Saddle anesthesia (bilateral, symmetric, perianal) | Saddle anesthesia (may be asymmetric); radicular pattern in legs |
| Motor (type) | UMN + LMN mixed (may have Babinski) | LMN only (flaccid, areflexic) |
| Motor (distribution) | Symmetric; mild | Asymmetric; may be severe (multisegmental) |
| Reflexes | Knee jerk preserved; ankle jerk absent; Babinski may be present | Variable loss depending on roots affected |
| Bladder/bowel | Early and prominent (detrusor areflexia) | Late (unless severe) |
| Sexual dysfunction | Frequent and early | Less frequent; later onset |
| Symmetry | Symmetric | Asymmetric |
💎 Board Pearl
- Conus = early bladder, symmetric, UMN+LMN; Cauda equina = severe pain, asymmetric, pure LMN, late bladder
- Conus medullaris ends at L1–L2 in adults — lesions at this level can produce mixed UMN/LMN findings
- Cauda equina syndrome (large central disc, tumor, abscess) is a surgical emergency — bladder retention + saddle anesthesia = urgent MRI
- Many lesions affect both conus and cauda equina simultaneously — look for the dominant pattern