What are the causes and treatments of allodynia?

Causes and Treatments of Allodynia

Allodynia should be treated with first-line medications including tricyclic antidepressants (nortriptyline or desipramine), SNRIs (duloxetine or venlafaxine), or calcium channel α2-δ ligands (gabapentin or pregabalin), with topical lidocaine 5% added for localized peripheral allodynia 1.

Understanding Allodynia

Allodynia is pain triggered by normally non-painful stimuli such as light touch, gentle pressure, or temperature changes 2. This reflects dysfunction in pain processing originating from either peripheral or central nervous system pathology 3. It affects 15-50% of patients with neuropathic pain and appears in conditions including:

• Postherpetic neuralgia
• Diabetic neuropathy
• Fibromyalgia
• Chemotherapy-induced neuropathy
• Complex regional pain syndrome
• Post-surgical pain states 2, 3

The pathophysiology involves peripheral sensitization combined with maladaptive central nervous system changes, including cortical circuit rewiring mediated by astrocyte reactivation in the somatosensory cortex 4. Importantly, large myelinated Aβ-fibers (not just C-fibers) play a critical role in mechanical allodynia 5.

Treatment Algorithm

Step 1: Initial Assessment and First-Line Therapy

Start with one of these first-line options 1:

Tricyclic Antidepressants (TCAs):

• Nortriptyline or desipramine preferred (fewer anticholinergic effects than tertiary amines)
• Start low, administer at bedtime, titrate slowly
• Screen ECG if patient >40 years old
• Keep dosage <100 mg/day when possible in cardiac disease
• Allow 6-8 weeks for adequate trial (including 2 weeks at maximum tolerated dose) 1

SNRIs:

• Duloxetine or venlafaxine
• Better tolerated than TCAs in many patients
• Effective for peripheral neuropathic pain 1

Calcium Channel α2-δ Ligands:

• Pregabalin: Start 150 mg/day in 2-3 divided doses, titrate to 300 mg/day after 1-2 weeks (maximum 600 mg/day, though higher doses show more adverse effects without consistent additional benefit)
• Gabapentin: More complex titration, may require 2+ months for adequate trial, titrate up to 3600 mg/day in 3 divided doses 1

Topical Lidocaine 5%:

• Essential addition for localized peripheral allodynia
• Patch or gel formulation
• Excellent tolerability with minimal systemic absorption
• Particularly advantageous in elderly or medically complex patients
• Most appropriate for well-localized peripheral neuropathic pain; unlikely to benefit central neuropathic pain 1

Step 2: Combination Therapy if Partial Response

If pain remains ≥4/10 after adequate trial of single agent, add a second first-line medication from a different class 1. For example, combine a TCA with pregabalin, or add topical lidocaine to systemic therapy.

Step 3: Second-Line Options

Tramadol or Opioid Analgesics:

• Reserve for patients failing first-line medications except in these specific circumstances where they become first-line 1:
  • Acute neuropathic pain
  • Cancer-related neuropathic pain
  • Episodic severe pain exacerbations
  • When rapid relief needed during titration of other first-line agents

Tramadol dosing:

• Start 50 mg once or twice daily
• Increase by 50-100 mg/day every 3-7 days as tolerated
• Maximum 400 mg/day (300 mg/day if >75 years old)
• Allow 4 weeks for adequate trial 1

Critical caveat: Tramadol lowers seizure threshold and can cause serotonin syndrome when combined with SSRIs/SNRIs 1.

Step 4: Refractory Cases

If first-line medications fail alone and in combination, refer to pain specialist or multidisciplinary pain center 1. Advanced options may include spinal cord stimulation, which has demonstrated efficacy in reducing allodynia in complex regional pain syndrome 6.

Common Pitfalls to Avoid

• Don't abandon TCAs or SNRIs prematurely – adequate trials require 6-8 weeks 1
• Don't use topical lidocaine for central pain – it requires localized peripheral pathology 1
• Don't combine tramadol with SNRIs/SSRIs without monitoring for serotonin syndrome 1
• Don't assume allodynia is a contraindication to spinal cord stimulation – recent evidence contradicts older retrospective data showing SCS effectively reduces allodynia 6
• Don't overlook the functional and emotional impact – comprehensive assessment beyond pain intensity is essential 2

Monitoring and Follow-up

Reassess pain and quality of life frequently. Target goal: average pain ≤3/10 with tolerable adverse effects 1. If achieving <30% pain reduction at target dosage after adequate trial, switch to alternative first-line medication rather than continuing ineffective therapy.