Crozet Annals of Medicine: Hot Legs

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Left leg rash

Spring on a college campus is a beautiful riot of life-affirming re-emergence. The trees softly bud and the daffodils burst into vibrant colors. Frisbees drift over the newly green grass and the long pale limbs of the students reappear after a winter spent hibernating under the cover of warm clothes. The change of wardrobe is the most rapid and reliable sign of warm weather’s return, overnight the shorts, skirts, and sandals come out, as do the masses of students, previously huddled studying in libraries and dorms.

Along with the sudden reveal of the daffodils and crocuses springing from the bare earth, the sudden reveal of so many students’ legs can surprise and even bewilder. I am speaking medically of course.

The rash on this one leg in particular was as bewildering to my patient as it was to me. The young woman had just discovered it on the first really warm day when she cast off her sweatpants and donned a short skirt. She was otherwise in good health, although the past winter had been hard for her with frequent seasonal illnesses. A bout with the flu was followed by strep throat, which was followed by a G.I. bug. She had been housebound for much of the semester with these ailments.  She was feeling better now but was worried about this new rash.

The rash was mostly on her left thigh, with a faint suggestion of a similar rash on her right thigh but it was subtle on the right and I wasn’t sure if I was truly seeing something or not.

On the left thigh, the rash had rather straight borders, which is kind of unusual and seemed to be a clue. The rash had a lacey red-purple pattern to it, “reticulated” we call it in medicine after the Latin word for net. The rash resembled a fishing net. Here was another clue.

Rashes with linear borders are often caused by particular exposures to allergens or irritants.  This is called contact dermatitis. I frequently see rashes that follow the pattern of a gold necklace or a belt buckle. Gold is extremely non-allergenic; in fact, along with platinum it is referred to as a noble metal due to its lack of reactivity with other substances and its non-corroding properties. Gold does not rust. And yet patients get rashes from this non-allergenic metal. The allergy is actually caused by nickel, a common metal additive to gold jewelry and a common allergen.

Eighteen-karat gold is only 75 percent gold. The other 25 percent is other metals, especially nickel. I only know this factoid because it is often difficult to convince patients that their cherished gift is in fact not pure gold. I temper this news with the fact that pure gold, while noble, is far too soft to work into durable jewelry.

Another linear rash is from irritants like the urushiol oil in poison ivy, which tends to have tiny blisters in linear patterns where the plant was brushed along the skin or where the oil was spread by fingers and hands. Soap and water will remove urushiol oil and can prevent a poison ivy reaction if done quickly after the exposure. The fluid in the blisters is made by your body and cannot spread poison ivy further to you or others.

Allergic or irritant contact dermatitis is sometimes painful but is essentially always itchy. It also usually is slightly raised from the surrounding normal skin. My patient’s rash was neither itchy nor painful, nor was it raised. It was not a typical contact dermatitis but it was so similar. It was so linear, it had to be an exposure to something.

I was pondering this and trying to put it together with her ailments of the winter. I went back to see her again and found her in the classic pose of the undergraduate, sitting on the stretcher with her laptop computer open on her lap. Do these kids ever go anywhere without their devices?

Wait, her laptop was exactly over her rash. But laptops are not allergenic. But they are hot. A quick Google search of laptop rashes revealed the source of her rash.

She had “erythema ab igne,” a dermatologic condition caused by long-term exposure to low grade heat such as heating pads or laptops. The phrase is Latin for “redness from fire.” It is also commonly referred to in the modern era as toasted skin syndrome. Chronic exposure to heat can cause changes in the blood vessels in the surface of the skin. They dilate and eventually pigment the skin, likely due to iron deposition from ruptured red blood cells.

Over the long housebound winter my patient had been doing all her schoolwork on her bed with her laptop on her lap. The heat from the processor was concentrated mostly on the left side of her computer hence the rash predominantly on her left thigh.

Fortunately the rash had not progressed to the point of hyperpigmentation and with the advice to work with her laptop on a desk her erythema ab igne would resolve over the next few weeks.

As to why spring came so early this year, I will leave that to the climate scientists. I am enjoying it, though, as you can probably tell.

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