Description

• Neuropathic pain is defined as pain in association with primary injury or dysfunction of the nervous system, with or without ongoing tissue damage.
• Divided into 2 groups based on the location of the suspected lesion: Central vs. peripheral:
  • Newer data suggest a possibility that most neuropathic syndromes have a component of both central and peripheral mechanisms.
• An important consideration with a patient with neuropathic pain is the absence of any deficit or injury. Socially, the patient appears “normal,” yet suffers from a chronic painful condition. This can lead to psychosocial distress, as the patient is labeled “drug seeking.”
• May be triggered by numerous insults, including direct nerve injury, infection, metabolic dysfunction, autoimmune disease, neoplasm, drugs, radiation, and neurovascular disorders.
• May reflect the pathologic operation of a dysfunctional nervous system rather than a manifestation of any underlying pathology itself (i.e., phantom limb pain, complex regional pain syndrome [CRPS]):
  • Patients may paradoxically experience pain and hypersensitivity in an area of denervation.
• System(s) affected: Nervous; Musculoskeletal

Epidemiology

• >3 million Americans suffer from painful diabetic neuropathy (PDN).
• 1 million Americans suffer from postherpetic neuralgia (PHN).

Prevalence
Estimated at 1.5% of the population, although recent studies in England, Germany, and France place it around 6–8% of the population.

Risk Factors

• Radiculopathy
• Polyneuropathy (diabetes mellitus, alcohol-induced, postchemotherapy, heritable neuropathies)

• Trauma (nerve entrapment, postsurgical, nerve injury)
• Infection (HIV, herpes zoster)
• Central mechanisms (stroke, multiple sclerosis [MS], spinal cord injury, limb amputation)
• Nutritional deficiencies (B₁₂, folate)
• Medications (AIDS medications DDC and DDI, antibiotics metronidazole and isoniazid, some chemotherapeutics, amiodarone, hydralazine, phenytoin, nitrofurantoin)

Pathophysiology

• Positive symptoms due to changes in peripheral nerves, loss of inhibitory mechanisms in CNS, and central sensitization
• Negative symptoms likely due to axonal or neuronal loss

Etiology

• Associated with a predisposing factor
• In addition to possible etiologies listed as risk factors above, others include:
  • Demyelinating disorders (MS, Guillain-Barré)
  • Neoplasm (primary/metastatic)
  • Neurovascular (central post-stroke syndrome, trigeminal neuralgia)
  • Autoimmune disease (Sjögren syndrome, polyarteritis nodosa)
  • Structural disease (herniated disc disease)

Commonly Associated Conditions

• Depression
• Anxiety
• Sleep disturbance
• Fibromyalgia