What is the next step in evaluating and managing a female patient with bright‑red, burning pain of the feet from the ankles down, worsened by heat and walking (suggestive of erythromelalgia), who has a history of thyroid medication use and a prior negative rheumatology workup?

Next Steps for Suspected Erythromelalgia with Prior Negative Workup

Order a complete blood count with differential and platelet count immediately, as myeloproliferative disorders can emerge years after erythromelalgia symptoms begin and are present in approximately 5% of cases. 1

Immediate Laboratory Evaluation

Hematologic Screening (Priority)

• Obtain CBC with differential and platelet count to screen for essential thrombocythemia or polycythemia vera, which can precede clinical diagnosis by a median of 2.5 years 1
• Even if her previous rheumatology workup included normal blood counts, myeloproliferative disorders may remain subclinical for years after symptom onset 1
• Plan periodic CBC monitoring every 6–12 months going forward, as initial normal results do not exclude future development of these conditions 1

Additional Screening Tests

• Check blood glucose and HbA1c to rule out diabetes as a contributing factor 1
• Consider ANA and ENA screening for autoimmune causes such as lupus erythematosus 1
• Review her current medications (including the weight loss medication) to identify potential triggers such as calcium antagonists or ergot derivatives 1

Neurophysiological Testing

Small Fiber Assessment

• Arrange thermoregulatory sweat testing as the most sensitive functional assessment, which shows abnormalities in approximately 90% of erythromelalgia patients 1
• Consider quantitative sudomotor axon reflex testing (QSART) for additional functional evaluation of small fiber autonomic function 1
• Avoid relying on skin biopsy for epidermal nerve fiber density, as it shows decreased density in only a minority of cases and is not a reliable primary diagnostic tool 1

Large Fiber Assessment

• Perform electromyography and nerve conduction studies, which reveal abnormalities in nearly 50% of patients and support the presence of large fiber involvement 1

Genetic Considerations

• Given that her symptoms may have started earlier in life (she mentions previous thyroid medication trials), consider SCN9A genetic testing if there is any family history or if symptoms began in childhood/adolescence 1
• Approximately 5% of erythromelalgia cases are familial with autosomal-dominant inheritance involving SCN9A mutations 1

Immediate Management While Awaiting Results

Lifestyle Modifications

• Counsel her to limit cooling measures to maximum 10 minutes, 4 times daily to prevent tissue damage, ulcerations, and immersion foot 2, 3
• Advise avoiding ice water immersion and continuous fan use, which can cause windburn-like exacerbation when stopped 2
• Recommend elevating extremities during episodes, which may provide relief 2
• Encourage swimming instead of walking/running for exercise, as it provides cooling without tissue damage 2

First-Line Topical Therapy

• Start lidocaine patches (4% OTC or 5% prescription) applied to affected areas for 12-24 hours 3
• Consider compounded midodrine 0.2% cream applied 3 times daily, which is an evidence-based preferred treatment for erythema management 3
• Alternative: compounded amitriptyline-ketamine mixture applied up to 3 times daily 3

Systemic Therapy Consideration

• If CBC reveals thrombocytosis or myeloproliferative disorder, aspirin 325mg daily for at least 1 month is particularly effective and should be started immediately 3, 4
• For aspirin-allergic patients, other NSAIDs (ibuprofen, indomethacin, naproxen) can be considered 3

Critical Pitfalls to Avoid

• Do not dismiss normal initial CBC results—myeloproliferative disorders may emerge years after erythromelalgia onset 1
• Do not allow her to continue excessive cooling with ice water or fans, as this leads to tissue damage despite providing symptom relief 2, 3
• Recognize that her symptoms being intermittent with normal examinations between episodes increases the risk of missed diagnosis 1
• Do not discontinue treatments too quickly—allow 4 weeks for topicals and 3-4 months for systemic medications before declaring treatment failure 3

Referral Considerations

• If symptoms remain refractory after appropriate trials of topical and systemic therapies, refer to a comprehensive pain rehabilitation center for multidisciplinary management 3
• Consider interventional options (epidural infusion, sympathetic blocks) only after exhausting medical management 5, 6