Neuropathic Pain

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).
• 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:

  Tool	Score for NP	Key Features
  DN4	≥4/10	7 symptom, 3 examination items
  LANSS	≥12/24	5 symptom, 2 examination items
  PainDETECT	≥19/38	Patient-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:

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.