Neuropathic Pain

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NP: Definition & Mechanisms - Nerve Zingers Defined

  • Definition (IASP): Pain caused by a lesion or disease of the somatosensory nervous system. Characterized by burning, shooting, or electric shock-like sensations.
  • Common Etiologies:
    • Metabolic: Diabetic Neuropathy (most frequent).
    • Infectious: Post-Herpetic Neuralgia (PHN), HIV neuropathy.
    • Compressive/Traumatic: Radiculopathy, Trigeminal Neuralgia (TN).
    • Post-Surgical Pain, Phantom Limb Pain, Chemotherapy-Induced Peripheral Neuropathy (CIPN). Mechanisms of Neuropathic Pain: Peripheral and Central
  • Key Pathophysiological Mechanisms:
    • Peripheral Sensitization: Lowered nociceptor activation threshold and increased responsiveness (e.g., altered ion channel expression like Nav1.7).
    • Central Sensitization: Enhanced excitability of CNS neurons (spinal cord, brain); involves NMDA receptors, wind-up, and Long-Term Potentiation (LTP).
    • Ectopic Discharges: Spontaneous, aberrant firing from damaged primary afferent neurons.
    • Disinhibition: Reduced function of inhibitory pathways (e.g., GABAergic, glycinergic).
    • Structural Reorganization: e.g., Aβ fibers sprouting into superficial dorsal horn laminae.

⭐ Allodynia (pain evoked by a stimulus that does not normally provoke pain) is a hallmark feature frequently tested. 📌 Nerve fibers: Aδ (fast, sharp "zingers") & C fibers (slow, burning/aching).

NP: Clinical Features & Diagnosis - Spotting the Signs

  • Characteristic Symptoms:

    • Spontaneous pain: Burning (common), shooting, electric shock-like, stabbing.
    • Evoked pain: Allodynia (pain from non-painful stimuli like light touch), hyperalgesia (↑ pain response to noxious stimuli).
    • Paresthesia (pins & needles, tingling), dysesthesia (unpleasant abnormal sensation, e.g., crawling).
  • Clinical Examination:

    • Focus: Identify lesion/disease affecting somatosensory system.
    • Sensory testing: Assess for positive signs (allodynia, hyperalgesia) & negative signs (hypoesthesia, hypoalgesia).
    • Look for autonomic signs (e.g., skin color/temperature changes).
  • Screening Tools:

    ToolScore for NPKey Features
    DN4≥4/107 symptom, 3 examination items
    LANSS≥12/245 symptom, 2 examination items
    PainDETECT≥19/38Patient-reported, 7 weighted sensory items

⭐ The 'Leeds Assessment of Neuropathic Symptoms and Signs' (LANSS) scale helps differentiate neuropathic from nociceptive pain.

  • Diagnostic Flow:
  • Key Investigations (when indicated):
    • Nerve Conduction Studies (NCS) & Electromyography (EMG).
    • Quantitative Sensory Testing (QST).

NP: Management Strategies - Taming the Tingles

Stepwise approach is key. Always consider non-pharmacological therapies adjunctively.

Pharmacological Agents:

ClassMOA HighlightsExamplesKey SEs / Notes (Doses are typical starting/max)
TCAsNE & 5-HT reuptake ↓Amitriptyline (10-25 mg hs, up to 150 mg)Anticholinergic (dry mouth, constipation), sedation, cardiotoxicity. Start low.
SNRIsNE & 5-HT reuptake ↓Duloxetine (30-60 mg/day, max 120 mg), VenlafaxineNausea, dizziness, insomnia, HTN. Good for comorbid depression/anxiety.
GabapentinoidsBind α2-δ Ca²⁺ channels, ↓ NT releaseGabapentin (start 300 mg, max 3600 mg/day), Pregabalin (start 75 mg BD, max 600 mg/day)Sedation, dizziness, peripheral edema, weight gain. Renal dose adjustment.
OpioidsWeak µ-agonist; NE/5-HT reuptake ↓ (Tramadol)Tramadol (50-100 mg q4-6h, max 400 mg/day), Morphine (refractory)Nausea, constipation, sedation, dependence risk. Tramadol: seizure risk.
Topical AgentsLocal Na⁺ channel blockadeLidocaine 5% patchLocal skin reactions (erythema, rash). For localized NP. Max 3 patches/12h.

Non-Pharmacological:

  • TENS (Transcutaneous Electrical Nerve Stimulation)
  • Physiotherapy, exercise
  • Psychological therapies (CBT, mindfulness)

Interventional & Other Third-Line Options (Refractory Pain):

  • Nerve blocks (local anesthetic +/- steroid)
  • Spinal Cord Stimulation (SCS)
  • Intrathecal drug delivery
  • Other Antiepileptics (e.g., Carbamazepine for Trigeminal Neuralgia; Lamotrigine)

High‑Yield Points - ⚡ Biggest Takeaways

  • Neuropathic pain results from lesion or disease of the somatosensory nervous system.
  • Key symptoms include allodynia (pain from non-painful stimuli) and hyperalgesia.
  • Examples: Diabetic neuropathy, postherpetic neuralgia (PHN), trigeminal neuralgia.
  • First-line pharmacotherapy: Gabapentinoids (e.g., pregabalin), TCAs (e.g., amitriptyline), SNRIs (e.g., duloxetine).
  • Carbamazepine is the drug of choice specifically for trigeminal neuralgia.
  • Opioids are generally considered second or third-line options.
  • Topical agents like lidocaine or capsaicin can be useful for localized neuropathic pain.

Practice Questions: Neuropathic Pain

Test your understanding with these related questions

Slow, burning pain is primarily carried by which type of nerve fibers?

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Flashcards: Neuropathic Pain

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_____ is an IV anesthetic that can be used to reduce postoperative pain

TAP TO REVEAL ANSWER

_____ is an IV anesthetic that can be used to reduce postoperative pain

Ketamine

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Neuropathic Pain - Free Indian Medical PG Review