Neuromuscular — Last Minute Review
A last-minute review of high-yield facts — dense tables and one-liners for RITE/Board prep. Not a substitute for the full notes.
Polyneuropathy Classification
Axonal vs Demyelinating NCS
| Feature | Axonal | Demyelinating |
|---|
| Amplitudes (CMAP/SNAP) | ↓↓ (primary finding) | Normal or ↓ (secondary to CB) |
| Conduction velocity | Normal or mildly ↓ (>70% LLN) | Markedly ↓ (<70% LLN) |
| Distal latencies | Normal | Prolonged (>130% ULN) |
| F-waves | Normal | Prolonged or absent |
| Conduction block | Absent | Present (acquired > hereditary) |
| Temporal dispersion | Absent | Present (acquired only) |
GBS Subtypes
| Subtype | Antibody | NCS Pattern | Key Feature | Prognosis |
|---|
| AIDP | None specific | Demyelinating | Most common in Western countries; sural sparing pattern | Good (80% walk at 6 mo) |
| AMAN | GM1, GD1a | Axonal (motor only) | Post-Campylobacter (C. jejuni is the most commonly identified GBS trigger, ~25–30% of cases); common in Asia/Mexico | Variable; can recover fast |
| AMSAN | GM1, GD1a | Axonal (motor + sensory) | Severe axonal; worst prognosis | Poor |
| MFS | GQ1b | Often normal | Ophthalmoplegia + ataxia + areflexia | Excellent |
| Pharyngeal-cervical-brachial | GT1a, GQ1b | Axonal (upper limb) | Bulbar + neck + arm weakness; legs spared | Good |
GBS vs CIDP
| Feature | GBS | CIDP |
|---|
| Onset | Acute (<4 weeks to nadir) | Chronic (≥8 weeks progressive) |
| Course | Monophasic | Relapsing or progressive |
| Weakness pattern | Ascending, distal > proximal | Proximal + distal, symmetric |
| CSF | Albuminocytologic dissociation | Elevated protein (may have mild pleocytosis) |
| NCS | Demyelinating (sural sparing) | Demyelinating (no sural sparing) |
| Steroids | NOT effective | Effective |
| IVIg / PLEX | Yes | Yes |
- Sural sparing pattern (absent sensory SNAPs in arms but preserved sural) is the electrodiagnostic hallmark of AIDP
- GQ1b antibody → think MFS (ophthalmoplegia + ataxia + areflexia) or Bickerstaff brainstem encephalitis
- If GBS relapses ≥3 times or progresses >8 weeks → reclassify as CIDP
Key Neuropathy Syndromes
CMT Comparison
| Type | Gene | Inheritance | NCS | Unique Feature |
|---|
| CMT1A | PMP22 duplication | AD | Uniform slowing (<38 m/s) | Most common CMT; onion bulbs on biopsy |
| CMT1B | MPZ (P0) | AD | Very slow (<20 m/s in severe) | More severe than 1A; early onset |
| CMT2A | MFN2 | AD | Axonal (normal CV, low amplitudes) | Optic atrophy possible; most common axonal CMT |
| CMTX1 | GJB1 (Connexin 32) | X-linked | Intermediate CV (25–40 m/s) | Males > females; may have transient CNS symptoms |
| HNPP | PMP22 deletion | AD | Prolonged distal latencies; CB at entrapment sites | Recurrent pressure palsies; “tomaculous” (sausage-shaped) myelin |
CMT (HMSN) — onion-bulb formations of chronic demyelinating neuropathy.Jensflorian · CC BY-SA 3.0 · Wikimedia Commons
Hereditary vs acquired demyelinating neuropathy: Uniform slowing + no conduction block = hereditary (CMT1). Non-uniform slowing + conduction block + temporal dispersion = acquired (CIDP).
Diabetic Neuropathy Types
- Distal symmetric polyneuropathy — most common; length-dependent, small fiber → large fiber
- Diabetic amyotrophy (lumbosacral radiculoplexopathy) — acute proximal thigh pain + weakness; weight loss; often self-limited
- Autonomic neuropathy — orthostatic hypotension, gastroparesis, erectile dysfunction
- Cranial mononeuropathy — CN III palsy with pupil sparing (vs. aneurysm = pupil involved)
- Mononeuropathy/entrapment — median (CTS), ulnar
- Truncal radiculopathy — band-like thoracoabdominal pain; often misdiagnosed
Other Key Neuropathies
| Syndrome | Key Facts |
|---|
| Small fiber neuropathy | Burning pain, autonomic symptoms, NCS normal, diagnose with skin punch biopsy (↓ IENFD); causes: DM, Fabry, amyloid, sarcoid, Sjögren |
| Anti-MAG neuropathy | IgM kappa paraprotein; distal acquired demyelinating; wide-spaced myelin; slowly progressive; poor response to IVIg; rituximab preferred |
| POEMS syndrome | Polyneuropathy, Organomegaly, Endocrinopathy, M-protein (IgA or IgG, overwhelmingly lambda-restricted; kappa cases rare but reported), Skin changes; sclerotic bone lesions; ↑ VEGF; demyelinating NCS |
| Vasculitic neuropathy | Mononeuritis multiplex; sural nerve biopsy → necrotizing vasculitis; treat with steroids + cyclophosphamide |
| MMN | Pure motor; asymmetric distal UE; anti-GM1 IgM; conduction block on NCS; IVIg works; steroids worsen |
| Amyloid neuropathy | Painful small fiber + autonomic; CTS common; TTR (hATTR) or AL; Congo red → apple-green birefringence |
TTR Amyloidosis Treatments
| Class | Drugs | Mechanism | U.S. indication |
|---|
| RNAi (siRNA) | Patisiran, vutrisiran | Knock down hepatic TTR mRNA | hATTR polyneuropathy |
| Antisense oligonucleotides | Inotersen, eplontersen | Knock down hepatic TTR mRNA | hATTR polyneuropathy |
| TTR stabilizers | Tafamidis, acoramidis | Stabilize tetramer → prevent dissociation/misfolding | ATTR cardiomyopathy (ATTR-CM) in the U.S. — NOT approved for polyneuropathy in the U.S. (tafamidis has neuropathy approval in some non-U.S. regions) |
Bottom line: For U.S. hATTR polyneuropathy, disease-modifying therapy = TTR knockdown (patisiran, vutrisiran, inotersen, eplontersen). Stabilizers (tafamidis, acoramidis) are U.S. cardiomyopathy drugs.
- POEMS is overwhelmingly IgA or IgG lambda-restricted; kappa cases are rare but have been reported. Anti-MAG is typically IgM kappa
- MMN = IVIg responsive; steroids and PLEX can worsen MMN
- Absent SNAPs in the hands but preserved sural SNAP → think GBS or CIDP, not axonal neuropathy
Mononeuropathies & Entrapment
Upper Extremity Mononeuropathies
| Nerve | Syndrome | Site | Motor Deficit | Sensory Deficit |
|---|
| Median | Carpal tunnel | Wrist | APB weakness (thenar atrophy late) | Digits 1–3 + radial half of 4 |
| AIN (anterior interosseous) | Proximal forearm | FPL, FDP (index), PQ — can’t make OK sign | None (pure motor) |
| Pronator syndrome | Proximal forearm | Similar to CTS but includes forearm pronation | Palmar cutaneous branch involved (vs. CTS) |
| Ulnar | Cubital tunnel | Elbow | FDI, ADM, lumbricals 4–5; claw hand (more severe distally) | Digit 5 + ulnar half of 4 |
| Guyon canal | Wrist | Hypothenar + interossei (variable by zone) | Variable; may spare dorsal hand |
| Radial | Saturday night palsy | Spiral groove | Wrist drop + finger drop; spares triceps | Dorsal hand (first web space) |
| PIN (posterior interosseous) | Proximal forearm | Finger drop (no wrist drop — ECRL spared) | None (pure motor) |
Lower Extremity Mononeuropathies
| Nerve | Site | Motor Deficit | Sensory Deficit |
|---|
| Common peroneal | Fibular head | Footdrop (dorsiflexion + eversion) | Lateral leg + dorsal foot |
| Femoral | Pelvis/inguinal | Quad weakness; ↓ knee jerk | Anterior thigh + medial leg (saphenous) |
| Lateral femoral cutaneous | Inguinal ligament | None (pure sensory) | Lateral thigh — meralgia paresthetica |
| Sciatic (peroneal division) | Buttock/posterior thigh | Footdrop + hamstring weakness | Below knee (peroneal + tibial) |
| Tibial | Tarsal tunnel | Toe flexion weakness; intrinsic foot muscles | Sole of foot |
Footdrop DDx
| Feature | Common Peroneal | L5 Radiculopathy | Sciatic (peroneal division) |
|---|
| Dorsiflexion | Weak | Weak | Weak |
| Eversion | Weak | Weak | Weak |
| Inversion | Normal (tibial nerve) | Weak (tibialis posterior = L5) | Normal or weak |
| Hip abduction | Normal | Weak (gluteus medius = L5) | Normal |
| Knee flexion | Normal | Normal | Weak (hamstrings) |
| Sensory | Dorsal foot + lateral leg | Dorsal foot + lateral leg + medial foot | Entire below-knee |
| EMG clue | NCS abnormal at fibular head | Paraspinal fibs; tibialis posterior involved | Short head biceps femoris involved |
- Footdrop with weak inversion (tibialis posterior) = L5, not peroneal nerve
- AIN = pure motor; can’t pinch (no OK sign); no sensory loss
- Ulnar claw hand paradox: higher lesion = less clawing (FDP paralyzed so MCP not hyperextended)
Radiculopathy & Plexopathy Quick Reference
Cervical Radiculopathy
| Root | Disc | Pain/Sensory | Motor | Reflex |
|---|
| C5 | C4–C5 | Lateral shoulder/arm | Deltoid, biceps | ↓ Biceps |
| C6 | C5–C6 | Lateral forearm, thumb, index | Biceps, wrist extensors (brachioradialis) | ↓ Brachioradialis |
| C7 | C6–C7 | Middle finger | Triceps, wrist flexors, finger extensors | ↓ Triceps |
| C8 | C7–T1 | Ring + little finger, medial forearm | Finger flexors, hand intrinsics | ↓ Finger flexor |
| T1 | T1–T2 | Medial arm/forearm | Hand intrinsics (interossei) | None reliable |
Lumbosacral Radiculopathy
| Root | Disc | Pain/Sensory | Motor | Reflex |
|---|
| L2–L3 | L2–3/L3–4 | Anterior thigh | Hip flexion (iliopsoas), quad | ↓ (none reliable / patellar) |
| L4 | L3–L4 | Medial leg (saphenous) | Knee extension (quad), tibialis anterior | ↓ Patellar (knee jerk) |
| L5 | L4–L5 | Lateral leg, dorsal foot, great toe | Ankle dorsiflexion, hip abduction, toe extension, inversion | ↓ Medial hamstring (inconsistent) |
| S1 | L5–S1 | Lateral foot, sole, calf | Ankle plantarflexion, eversion, hip extension | ↓ Achilles (ankle jerk) |
Brachial Plexopathy
Brachial plexus — roots → trunks → divisions → cords → branches: trace Erb vs Klumpke and each mononeuropathy.Modernized from Gray's Anatomy (public domain) · HighYieldNeuro
- Erb-Duchenne (upper trunk, C5–C6) — “waiter’s tip”; shoulder abduction + elbow flexion lost; traction/birth injury
- Klumpke (lower trunk, C8–T1) — hand intrinsic weakness + claw hand; may have Horner syndrome (T1 sympathetic); Pancoast tumor
- Parsonage-Turner (neuralgic amyotrophy) — acute shoulder pain → patchy UE weakness; often post-viral; long thoracic nerve (winged scapula) commonly affected
Cauda Equina vs Conus Medullaris
| Feature | Conus Medullaris (S3–S5) | Cauda Equina (L2–S5 roots) |
|---|
| Onset | Sudden, bilateral, symmetric | Gradual, unilateral → bilateral |
| Pain | Less prominent | Severe, radicular |
| Weakness | Symmetric; mild; distal LE | Asymmetric; can be severe; multisegmental |
| Sensory | Perianal saddle anesthesia (symmetric) | Asymmetric; radicular distribution |
| Bladder/bowel | Early; prominent | Late (unless severe) |
| Reflexes | Bulbocavernosus absent; may have UMN signs | Absent (LMN); areflexia |
Conus vs cauda — where the cord ends and the roots begin: the whole DDx in one image.Modernized from Gray's Anatomy (public domain) · HighYieldNeuro
🔒
Continue reading — sign in
The full note has more clinical pearls, tables, and board-focused tips. Free account, no fee.
