History and clinical findings: A 44-year-old man presented at a dermatologist with a 2 months history of a blue-brown reticular macule on the right thigh that had appeared spontaneously. It was neither painful nor itching and showed no growth or further colour change.
Investigations: Punch biopsy, antinuclear antibodies, CrP, immune electrophoresis, hepatitis serology, urine diagnostics showed normal results.
Diagnosis: On specific inquiry the patient, a long-distance truck driver, reported to rest his laptop during driving breaks always on the right thigh. We diagnosed a "laptop dermatitis".
Conclusion: Consider external mechanical or thermal triggers if skin changes are unilateral. Thermal isolation from permanent heat exposure prevents an erythema ab igne reliably.
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