Peripheral nerve disorders

Intro to Neuropathies - Nerve Network Nonsense

• Peripheral Neuropathy: A result of damage to nerves outside the brain and spinal cord, leading to sensory, motor, or autonomic dysfunction.
• Key Patterns:
  • Mononeuropathy: Single nerve (e.g., Carpal Tunnel).
  • Mononeuritis Multiplex: Multiple, non-contiguous nerves (e.g., vasculitis).
  • Polyneuropathy: Symmetrical, distal "stocking-glove" pattern.
• Primary Pathologic Processes:
  • Axonal Degeneration: "Dying back" process from metabolic/toxic injury; reduces signal amplitude.
  • Segmental Demyelination: Schwann cell/myelin damage from immune/hereditary causes; reduces conduction velocity.

⭐ Wallerian Degeneration: Anterograde degeneration of the axon and myelin sheath distal to a point of injury, like transection.

Guillain-Barré Syndrome - Ascending Paralysis Panic

• Patho: Acute, immune-mediated demyelinating polyneuropathy. Triggered by infections (e.g., Campylobacter jejuni, CMV) via molecular mimicry.
• Clinical: Rapidly progressive, symmetric ascending weakness/paralysis. "Ground-to-Brain."
  • ↓ or absent deep tendon reflexes (LMN sign).
  • Autonomic dysfunction & respiratory failure are major risks.
• Diagnosis:
  • CSF: ↑ protein with normal cell count (albuminocytologic dissociation).
  • Nerve conduction studies confirm demyelination.
• Treatment: IVIG or plasmapheresis. Corticosteroids are not effective.

⭐ Exam Favorite: The classic CSF finding is albuminocytologic dissociation, with protein levels typically rising after the first week of symptoms.

📌 Mnemonic: Ground-to-Brain Syndrome.

Diabetic Neuropathy - Sweet & Sour Signals

• Pathogenesis: Hyperglycemia → ↑intracellular glucose in nerves/Schwann cells.

  • Polyol Pathway: Glucose → sorbitol (via aldose reductase) → ↑osmotic stress, ↓Na+/K+ ATPase activity.
  • Non-enzymatic Glycation: Forms advanced glycation end-products (AGEs) → inflammation & microvascular damage.

• Clinical Types:

  • Distal Symmetrical Polyneuropathy (DSPN): Most common. "Stocking-glove" sensory loss (numbness, paresthesias).
  • Autonomic Neuropathy: Gastroparesis, orthostatic hypotension, neurogenic bladder, erectile dysfunction.
  • Mononeuropathy: Focal nerve damage. 📌 Cranial Nerves III, IV, VI, VII are commonly affected.

⭐ Exam Favorite: Diabetic CN III palsy classically presents with ptosis and a "down and out" eye but spares the pupil. This is because the ischemic damage affects the central somatic fibers while sparing the peripheral parasympathetic fibers.

Hereditary & Traumatic - Genes, Jeans & Jams

• Charcot-Marie-Tooth (CMT): Most common inherited neuropathy.

  • CMT1A (demyelinating): PMP22 gene duplication.
  • CMT2 (axonal): Various gene mutations (e.g., MFN2).
  • Classic signs: Distal muscle atrophy (stork legs), pes cavus, foot drop, palpable nerves.

• Traumatic Injury:

  • Wallerian Degeneration: Anterograde degeneration of axon & myelin distal to injury.
  • Traumatic Neuroma: Painful, non-neoplastic mass from haphazard nerve regeneration after transection.

⭐ Repetitive demyelination & remyelination (as in CMT1) creates concentric Schwann cell layers visible as "onion bulbs" on histology.

High‑Yield Points - ⚡ Biggest Takeaways

• Guillain-Barré syndrome presents as an acute, ascending paralysis with albuminocytologic dissociation in the CSF, often post-Campylobacter infection.
• Diabetic neuropathy, the most common type, manifests as a "stocking-glove" sensory loss due to non-enzymatic glycosylation and osmotic damage.
• Charcot-Marie-Tooth disease is a hereditary motor-sensory neuropathy causing pes cavus, foot drop, and palpably thickened nerves.
• Wallerian degeneration is axonal breakdown distal to injury, while segmental demyelination from Schwann cell damage slows conduction velocity.