Peripheral Neuropathies

PN Basics - Nerve Nuisance Intro

• Peripheral Neuropathy (PN): Disorder of peripheral nerves (motor, sensory, autonomic).
• Classification (Distribution):
  • Mononeuropathy: Single nerve (e.g., Carpal Tunnel).
  • Mononeuritis Multiplex: ≥2 separate nerves (e.g., Vasculitis).
  • Polyneuropathy: Symmetrical, distal (e.g., Diabetic).
• Classification (Pathology):
  • Axonal:
    • Primary axon damage.
    • ↓CMAP/SNAP amplitude.
    • NCV normal/mildly ↓.
  • Demyelinating:
    • Primary myelin damage.
    • ↓NCV, ↑distal latencies.
    • Conduction block common.
• Pathophysiology:
  • Wallerian Degeneration: Axon breakdown distal to injury.
  • Segmental Demyelination: Focal myelin loss.
  • Neuronopathy: Nerve cell body (soma) damage.

⭐ Demyelinating neuropathies primarily slow conduction velocity (CV) & prolong distal latencies. Axonal neuropathies reduce SNAP/CMAP amplitude.

Causes & Clues - The Why & How

• Metabolic:
  • Diabetes Mellitus (DM): Most common; length-dependent.
  • Uremia (CKD).
• Immune-Mediated:
  • Guillain-Barré Syndrome (GBS): Acute, ascending.
  • Vasculitis: Mononeuritis multiplex.
  • Paraneoplastic syndromes.
• Infectious:
  • Leprosy: Affects cool areas.
  • HIV: Distal symmetric.
  • Lyme disease: Cranial nerves, radiculopathy.
  • Diphtheria: Exotoxin.
• Hereditary:
  • Charcot-Marie-Tooth (CMT): Distal weakness/atrophy.
• Toxic:
  • Alcohol.
  • Lead: Wrist/foot drop.
  • Drugs: Vincristine, Isoniazid (INH).
• Nutritional:
  • Deficiency: Vit B12, B1, B6, E.
  • Excess: Vit B6 (sensory).
• Other:
  • Amyloidosis: Protein infiltration.

⭐ Diabetes mellitus is the leading cause of peripheral neuropathy in developed countries, typically presenting as a length-dependent sensorimotor polyneuropathy.

Signs & Sleuthing - Spotting the Damage

• Clinical Features:
  • Sensory:
    • Negative: Numbness, ↓proprioception, ataxia.
    • Positive: Paraesthesia, dysesthesia, allodynia, neuropathic pain.
  • Motor: Weakness (distal > proximal), atrophy, fasciculations, cramps, ↓/absent reflexes.
  • Autonomic: Orthostatic hypotension, gastroparesis, bowel/bladder/sweating issues, erectile dysfunction.
• Patterns:
  • Stocking-glove, length-dependent.
  • Mononeuropathy, Mononeuritis multiplex, Radiculopathy.
• Diagnosis:
  • History & Neuro Exam: Sensory, motor, reflexes, gait.
  • Electrodiagnosis:
    • NCS: ↓Velocity, ↓amplitude, ↑latency, conduction block.
    • EMG: Fibrillations, positive sharp waves, MUP changes.
  • Nerve Biopsy Indications: Vasculitis, amyloidosis, CIDP, leprosy.

⭐ NCS differentiates axonal (↓amplitude) vs. demyelinating (↓velocity, ↑latency, conduction block) neuropathies, guiding investigation.

Key Syndromes - Neuropathy Notables

• Guillain-Barré Syndrome (GBS): Acute inflammatory demyelinating polyradiculoneuropathy (AIDP). Ascending paralysis, areflexia. 📌 GBS: Ground to Brain Syndrome. CSF: Albuminocytologic dissociation. Rx: IVIG/Plasmapheresis.
• Diabetic Neuropathy: Distal Symmetric Polyneuropathy (DSPN) "stocking-glove"; autonomic (gastroparesis); focal (CN III palsy). Rx: Glycemic control, pain relief.
• Charcot-Marie-Tooth (CMT): Hereditary Motor & Sensory Neuropathy (HMSN). Pes cavus, distal muscle wasting ("stork legs"). Biopsy (CMT1A): "Onion bulb".
• Bell's Palsy: Idiopathic CN VII palsy. Acute unilateral facial weakness (upper/lower face). Rx: Corticosteroids +/- antivirals.
• Carpal Tunnel Syndrome (CTS): Median nerve entrapment at wrist. Pain/paresthesia (thumb, index, middle fingers). Tinel's/Phalen's signs. Rx: Splinting, steroids, surgery.

⭐ Albuminocytologic dissociation (elevated CSF protein with normal or minimally increased white blood cell count) is a hallmark finding in Guillain-Barré Syndrome, typically seen after the first week.

High‑Yield Points - ⚡ Biggest Takeaways

• Guillain-Barré Syndrome (GBS): Ascending paralysis, areflexia, albuminocytologic dissociation in CSF.
• Charcot-Marie-Tooth (CMT): Most common hereditary neuropathy; pes cavus, distal muscle atrophy.
• Diabetic Neuropathy: Most common cause; stocking-glove sensory loss, autonomic dysfunction.
• Bell's Palsy: Unilateral LMN facial nerve (CN VII) palsy; sudden onset, good prognosis.
• Carpal Tunnel Syndrome: Median nerve compression at wrist; Phalen's/Tinel's positive, nocturnal symptoms.
• Vitamin B12 Deficiency: Subacute combined degeneration; dorsal columns, corticospinal tracts, peripheral neuropathy.
• Leprosy (Hansen's Disease): Mononeuritis multiplex, anesthetic skin patches, thickened nerves.