A BURNING CAUSE OF BACK PAIN: SGIM MORNING REPORT

Case

We present the case of a 62-year-old man with a prior diagnosis of sciatica who presents with a painful rash of two months on the posterior left leg and lumbar back pain radiating down the left leg. The pain worsened with prolonged sitting, pressure, and walking. Applying a heating pad, oxycodone, and methylprednisolone alleviated his pain. His MRI was normal six weeks ago. Family history includes maternal unspecified thyroid disease, but no other autoimmune history. He lives alone, is independent with daily activities, and is unemployed. He does not use alcohol or intravenous drugs; he has a 30-year smoking history.

On examination, the patient was in distress on the table from his pain. Vitals were temperature 37°C, blood pressure 117/81, heart rate 77, and BMI 18. He had Type 1 skin with a lace-like pattern of hyperpigmented brown-to-purple macules on the left posterior leg in the sciatic nerve distribution. It was non-tender and non-blanching. He had an unremarkable neurological exam. The remainder of the examination was within normal limits. CBC was normal. CMP was notable for sodium at 132 mmol/L, chloride at 97 mmol/L, and total bilirubin at 1.1 mg/dL. His INR was 0.93 (low). Autoimmune antibodies, HIV, HAV, HBV, and HCV, were negative. Dilute prothrombin time, thrombin time, PTT-LA, dRVVT, and lupus anticoagulant were negative. The left lower extremity’s venous and arterial duplex ultrasounds were without deep vein thrombus or arterial stenosis.

Dermatology evaluated the patient. Upon further questioning, he confirmed using a heating pad for many hours daily for one month before the onset of the rash. A left posterior thigh skin biopsy with Fontana-Masson stain highlighted melanophages within the papillary dermis. Iron staining was negative, and the pathologist identified no vasculopathy changes.

Teaching Points

Erythema ab igne (EAI) is a skin reaction caused by chronic heat exposure through infrared radiation.¹ Historically, EAI occurred with exposure from stoves and coal fires but now results from recurrent heat exposure through electronic devices.² The location of EAI may prompt the clinician to inquire about further health problems, such as back or abdomen pain associated with heating pad use. Women are twice as likely to develop EAI.^2,3 The age range varies and can reflect the use of heating devices for chronic pain in elderly adults.

Vasodilation of the venous plexus from heat leads to the classic findings of EAI.⁴ EAI initially causes epidermal atrophy and vasodilation as well as hemosiderin and melanin deposition in the dermis.² EAI can rarely transform into malignancy, usually taking decades, and a biopsy of the skin is warranted to exclude malignancies that mimic EAI. Severe cases may have telangiectasias, atrophy, and bullae.⁴

EAI should be included in the differential diagnosis for a hyperpigmented reticular rash. Its differential diagnosis includes other reticular rashes: livedo reticularis, livedoid vasculitis, poikiloderma atrophicans vascular, dermatomyositis, malignancies, vasculitis, and bullous disease.² EAI will be present only in the setting of chronic heat exposure.

Treatment is removing the heat source. Hyperpig-mentation can be treated with topical tretinoin, hydroquinone, laser therapy, or depigmentation creams.⁵ Despite this, abnormal pigmentation may remain.² EAI has a favorable prognosis with the removal of the causative agent. Patient education is crucial to minimize recurrent heat exposure. Physicians should know the risks associated with heating sources commonly used for chronic pain. EAI is a preventable dermatologic condition that requires healthcare providers to recognize its presentation, acknowledge risk factors, and consider more serious conditions it may mimic.