What is the most likely diagnosis for a patient with recurrent foot and ankle pain characterized by bright red discoloration, a severe burning/tingling sensation ('feet fell asleep ×1000'), worsening with heat and walking, and lack of response to prior thyroid therapy?

Erythromelalgia

This patient has erythromelalgia, a rare microvascular disorder characterized by episodic burning pain, bright red discoloration, and warmth of the extremities triggered by heat and walking. 1, 2

Diagnostic Confirmation

The clinical presentation is pathognomonic for erythromelalgia:

• Intense burning pain described as "feet fell asleep x1000" with bright red discoloration of feet and ankles 3, 1
• Heat-triggered exacerbation (worse in summer, provoked by walking) with symptom relief from cooling 1, 2
• Episodic nature with normal examination between attacks, which is typical and increases risk of missed diagnosis 1, 2

The diagnosis is primarily clinical, based on history and examination during symptomatic episodes, as no specific diagnostic test exists. 2

Critical Laboratory Workup

Your planned laboratory evaluation is appropriate and follows guideline recommendations:

Immediate Priority: Rule Out Myeloproliferative Disorders

• CBC with differential and platelet count is mandatory because myeloproliferative disorders (essential thrombocythemia, polycythemia vera) underlie approximately 5% of erythromelalgia cases and can precede clinical diagnosis by a median of 2.5 years 1, 2, 4
• If CBC shows thrombocytosis or polycythemia, JAK2 testing is appropriate as you planned 1
• Even if initial CBC is normal, repeat monitoring every 6-12 months is essential because myeloproliferative disorders may remain subclinical for years after symptom onset 1, 2

Additional Screening

• Fasting glucose and HbA1c to exclude diabetes as a contributing factor 1, 2
• ANA and ENA panels to rule out autoimmune etiologies such as systemic lupus erythematosus 1, 2
• Thyroid panel, Vitamin B12, Folate, Vitamin D are reasonable given the association with small fiber neuropathy 1

Neurophysiological Testing

Referral to neurology for thermoregulatory sweat testing is strongly recommended as it is the most sensitive functional assessment, showing abnormalities in approximately 90% of erythromelalgia patients. 1, 2

• Quantitative sudomotor axon reflex testing (QSART) provides additional evaluation of small-fiber autonomic function 1, 2
• Electromyography and nerve conduction studies reveal abnormalities in nearly 50% of patients, supporting mixed large and small fiber involvement 1, 2
• Avoid skin biopsy for epidermal nerve fiber density as it shows decreased density in only a minority of cases and should not be used as a primary diagnostic tool 1, 2

Genetic Testing

• SCN9A gene testing is appropriate if labs are negative, particularly given you should assess for family history or early-onset disease 1, 2
• Approximately 5% of cases are familial with autosomal-dominant inheritance due to SCN9A mutations encoding Nav1.7 sodium channels 1, 2

Treatment Algorithm

Critical Pitfall: Amlodipine May Worsen Symptoms

Starting amlodipine 2.5 mg is potentially problematic. Calcium-channel blockers are listed as potential drug-induced triggers for erythromelalgia and should be reviewed as causative agents, not used as treatment. 1, 2

First-Line Therapy: Topical Agents

Topical medications are considered first-line pharmacologic therapy and may be all that is needed to manage this condition: 3, 1

• Midodrine 0.2% cream three times daily is the evidence-based preferred topical for erythema control 1
• Lidocaine patches (4% OTC or 5% prescription) applied to affected areas for 12-24 hours daily to reduce pain 1
• Alternative: compounded amitriptyline-ketamine mixture applied up to three times daily 1

Systemic Therapy Based on CBC Results

• If CBC reveals thrombocytosis or myeloproliferative disorder: initiate aspirin 325 mg daily for at least 1 month, as this is particularly effective in this subgroup and results in dramatic improvement 3, 1, 4
• For aspirin allergy, consider alternative NSAIDs (ibuprofen, indomethacin, naproxen) 1

Second-Line Systemic Options (If Topicals Fail)

If topical agents are insufficient after 4 weeks of adequate trial: 1

• Gabapentin for neuropathic pain component 3
• Duloxetine for small fiber neuropathy 3
• Low-dose naltrexone (LDN) as you planned 3

Allow a minimum of 3-4 months for systemic medications before declaring treatment failure. 1

Essential Lifestyle Counseling

Critical pitfall: Patients commonly use excessive cooling measures that cause tissue damage. 3, 1

• Limit cooling measures (ice packs, cold water immersion) to maximum 10 minutes, four times daily to avoid tissue damage, ulceration, or "immersion foot" 3, 1
• Prohibit continuous fan use as abrupt cessation causes wind-burn-like exacerbation of symptoms 3, 1
• Elevate affected extremities during painful episodes for symptomatic relief 3, 1
• Recommend swimming rather than walking/running to obtain cooling without risking skin injury 1
• Counsel that engaging in usual lifestyle activities and exercise does not worsen long-term outcomes, despite triggering episodes 3

Follow-Up and Referral

• One-month follow-up is appropriate to assess response to topical therapy and review laboratory results 1
• If symptoms remain refractory after adequate trials of topical and systemic therapies, refer to a multidisciplinary pain rehabilitation center for comprehensive management 3, 1
• Maintain periodic CBC monitoring every 6-12 months even after normal initial results 1, 2

Key Diagnostic Pitfalls to Avoid

• Do not dismiss a normal initial CBC; myeloproliferative disorders may emerge years after symptom onset 1, 2
• Recognize that intermittent symptoms with normal examinations between episodes increase risk of missed or delayed diagnosis—this is typical of erythromelalgia 1, 2
• The condition is significantly underdiagnosed: estimated prevalence is 15/100,000, yet only 27% of affected individuals receive a diagnosis despite seeking medical attention 5