What alternative treatment options are available for a patient with Small Fiber Neuropathy and anxiety who is currently taking Pregabalin (75mg twice a day) with inadequate pain relief?

Treatment Approach for Small Fiber Neuropathy with Inadequate Pregabalin Response

Direct Recommendation

Before switching to mirtazapine, optimize pregabalin dosing to 300-600 mg/day (the current 150 mg/day is subtherapeutic), or add duloxetine 60 mg/day to the existing pregabalin regimen rather than switching entirely. 1, 2

Current Dosing Problem

• The patient is receiving only 150 mg/day of pregabalin, which is below the minimum effective dose for neuropathic pain 2, 3
• The FDA-approved therapeutic range for neuropathic pain is 150-600 mg/day, with most patients requiring 300-600 mg/day for adequate pain relief 2
• In clinical trials, 300 mg/day achieved 50% pain reduction in 31-32% of patients versus 13-24% with placebo, while 600 mg/day achieved 41% response rates 3
• Declaring pregabalin a failure at 150 mg/day is premature—this dose represents only 25% of the maximum approved dose 2

Optimal Treatment Strategy

Option 1: Optimize Pregabalin Monotherapy (Preferred Initial Approach)

• Increase pregabalin to 75 mg three times daily (225 mg/day) immediately, then to 100 mg three times daily (300 mg/day) after 3-7 days 2
• If inadequate response after 2-4 weeks at 300 mg/day, increase to 200 mg three times daily (600 mg/day) 2
• This approach addresses the anxiety component as pregabalin has proven anxiolytic effects, unlike duloxetine or tricyclics 4, 5

Option 2: Add Duloxetine to Current Pregabalin (Combination Therapy)

• Guidelines recommend adding a first-line agent from a different class rather than switching when partial response occurs 1, 6
• Start duloxetine 30 mg once daily for 1 week, then increase to 60 mg once daily 1
• Continue pregabalin at current dose initially, then optimize to 300 mg/day as tolerated 6
• Duloxetine shows superior efficacy in neuropathic pain compared to SSRIs, with moderate-quality evidence supporting its use 1
• Monitor for serotonin syndrome during the first 24-48 hours after starting duloxetine, though this combination is commonly used and not contraindicated 6

Why Mirtazapine Is Suboptimal

• Mirtazapine has not been studied for efficacy in neuropathic pain, unlike duloxetine which has Level I evidence 1
• While mirtazapine is safe in cardiovascular disease, the American Heart Association notes "its efficacy in treating depression in patients with CVD has not been assessed" 1
• Mirtazapine causes significant weight gain and sedation, which may worsen quality of life 1
• For neuropathic pain with comorbid anxiety, pregabalin optimization or duloxetine addition provides evidence-based treatment for both conditions simultaneously 1, 4

Alternative First-Line Options If Pregabalin Optimization Fails

Gabapentin (if pregabalin not tolerated at higher doses)

• Start 300 mg at bedtime, increase to 300 mg three times daily by day 3, then titrate by 300 mg every 3-7 days 7
• Target therapeutic dose: 1800-3600 mg/day in three divided doses 7
• Gabapentin has nonlinear pharmacokinetics requiring three-times-daily dosing, making it less convenient than pregabalin 1, 7

Tricyclic Antidepressants (nortriptyline preferred over amitriptyline)

• Start nortriptyline 10-25 mg at bedtime, increase every 4-7 days to goal of 100 mg 1
• Obtain screening ECG before initiating, especially given anxiety (which may indicate cardiac concerns) 1
• Avoid if cardiac disease history, as TCAs carry arrhythmia risk 1

Critical Pitfalls to Avoid

• Do not abandon pregabalin without optimizing to at least 300 mg/day for 2-4 weeks 2, 3
• Do not use gabapentin and pregabalin together—their efficacy in combination has not been evaluated 2
• Avoid NSAIDs entirely in neuropathic pain—they are ineffective and carry cardiovascular/renal toxicity 1
• When titrating pregabalin, monitor for peripheral edema and weight gain, which occur in 6-16% of patients 1, 3
• Allow 2-4 weeks at therapeutic doses before declaring treatment failure, as neuropathic pain relief develops gradually 1, 3

Monitoring Parameters

• Assess pain reduction using 0-10 numeric rating scale at 2-week intervals during titration 3
• Target outcome: ≥30% pain reduction (moderate benefit) or ≥50% pain reduction (substantial benefit) 3
• Monitor for dose-dependent adverse effects: dizziness (13-35%), somnolence (11-25%), peripheral edema (6-16%) 3
• If combining with duloxetine, assess for mental status changes, neuromuscular hyperactivity, or autonomic instability suggesting serotonin syndrome 6

Renal Dosing Adjustment

• Pregabalin requires mandatory dose reduction in renal impairment as it is 95% renally excreted 2, 8
• For CrCl 30-60 mL/min: maximum 300 mg/day in 2-3 divided doses 2
• For CrCl 15-30 mL/min: maximum 150 mg/day in 1-2 divided doses 2
• Check baseline creatinine clearance before dose escalation 2