Well: Think Like a Doctor: 18-Year Rash Solved!

Think Like a Doctor

Solve a medical mystery with Dr. Lisa Sanders.

On Thursday, we challenged Well readers to figure out why a 40-year-old woman had a rash on her hands for nearly two decades. I thought this was a really tough case. Apparently, so did my readers. Nearly 300 of you wrote in with some very thoughtful suggestions, but only five were able to make the diagnosis.

And the correct diagnosis is:

Persistant photo-allergic dermatitis (also known as a persistent light reaction or chronic actinic dermatitis)

The first person to identify this rare cause of a persistent eczema-like rash was Errol Levine, a retired radiologist from South Africa now living in Santa Fe, N.M. He focused on the fact that the patient had moved and now had a longer drive to work. He figured that the unusual distribution of the rash could be caused by her wrapping her fingers around the steering wheel of her car. As you will see, that is exactly the logic used by the doctor who made the diagnosis.

I was impressed by how many of you realized that the rash was a reaction to light. That was something that eluded most of the patient's doctors and the patient herself. And I want to give a shout-out to Marcela Garcia of Munich, who first figured out why the rash had such a strange distribution on her hands. Outstanding work, all of you!

The Diagnosis

This patient's rash was caused by sunlight reacting with a chemical in her skin, setting off an allergic reaction. Here's how that works: Certain substances can be transformed into irritants by light, especially by ultraviolet light. As you may recall from high school science class, ultraviolet, or UV, light is a form of electromagnetic radiation with a wavelength shorter than that of visible light. One type of ultraviolet light, UVB, causes sunburn. Another type, UVA, doesn't cause the skin to burn, but to tan – it's what's used in tanning beds. And, in this case in particular, it's important to know that UVA rays, unlike UVB ones, are able to travel through glass.

In patients with a photo-allergy, sunlight alone doesn't cause a problem, and on its own, a particular substance, either taken by mouth or applied to the skin, is something benign and tolerated. But the two together cause a rash that looks exactly like an allergic reaction (also known as a contact dermatitis) — because that's what it is.

To treat the rash, you have to get rid of either the allergen or the light. Steroid creams and emollients can help but will not cure the rash as long as the chemical and the light are at work causing the reaction.

This strange — and rare — reaction was first recognized in the 1960s when thousands of people developed a sun-triggered allergic rash after using a soap that had been made with an antibiotic called tribromosalicylanilide. Over the next several decades the reaction was observed with a few components of perfumes (musk ambrette and coumarin) and cosmetics (eosin in lipstick). Most of these substances are no longer in use.

These days, the most commonly encountered substance linked to this type of photo-allergy is, oddly enough, sunscreens containing the chemical PABA and its derivatives. Most people are able to use these compounds without a problem, but some will develop an allergic reaction in sunlight. PABA derivatives are not just in sunscreen creams but also in some hairsprays and cosmetics to reduce sun damage caused by ultraviolet light. These products can often be identified by the anti-aging claims made by the manufacturers.

The photo-allergic reaction can look like a sunburn but more often like a typical contact dermatitis. As with this patient's rash, it usually starts with small vesicles filled with clear fluid that itch and eventually become red, raw and shiny or thickened.

But there's another wrinkle in this diagnosis. Most of the time, these allergic reactions are transient. Once the chemical causing the reaction is removed, the injury stops and the skin starts to heal. However, there are some patients for whom the response continues long after use of the triggering chemical ends. No one knows why this happens. For these patients, protection from light is the only effective therapy.

How the Diagnosis Was Made

When Dr. David Grekin, the dermatologist whom the patient turned to, examined her hands, he noticed that her fingertips were free of rash. He'd seen that kind of distribution only once, in an elderly dermatologist friend who had spent most of his life in Hawaii and had hands riddled with evidence of sun damage along the back of his hands, but not the ends of his fingers.

When asked about his hands, the older dermatologist explained that fingers curl under when they are in the normal relaxed position and so are protected from the sun rays that damaged the rest of his skin. As soon as Dr. Grekin saw that this patient's fingers were spared, he thought that the rash may have been caused by the sun.

He didn't tell the patient what he thought she might have. Instead he asked her some questions to help him figure this out.

Do you spend a lot of time outside? Not really.
Do you garden? A little.

Dr. Grekin was a little disappointed. Just being out in the sun wasn't causing the strange distribution. He kept probing. What shielded her fingers from the sun?

Do you play tennis? No.
Do you drive to work? Yes.
How far and in what direction? About 20 minutes, going south to work each morning and north coming home, she told the doctor.
And do you hold the wheel at 10 and 2? (He was referring to the driving-school terms for the hand position they teach.) Yes.

That was it. Now Dr. Grekin was certain he knew the cause of the rash. Your hands are allergic to sunshine, he told the patient. He explained his thought process. Driving south to work in the morning and home north in the afternoon put the driver in the sunny side of the car both ways. The UVA light streaming through the windows was setting off the allergic reaction.

The cure for this rash was to wear gloves when she was outside, he told her, especially when she was driving. Protecting the skin from the sun should prevent further injury and her hands should start to heal. He suggested using gloves made of a material that blocked UVA light – leather or a specialized light-resistant fabric — whenever she was outside.

What was she allergic to? the patient asked. She had changed every cream, every soap, every lotion many times since this rash had started. Dr. Grekin wasn't sure. Indeed, they would probably never know, he told her. But something she had used years earlier had started the reaction and it lived on, long after the trigger was gone, in what is called a persistent light reaction.

No one knows why this reaction continues, but Dr. Grekin has a theory. He suspects that for some reason, the triggering chemical penetrates the skin and becomes permanently imbedded in the deeper layers of the skin — like tattoo ink — so that these people will have a lifetime of sun allergy, even if they never use the offending agent again. For those with this persistent form of injury, only avoiding the light will allow the skin to heal.

A Return to Normal

As soon as the doctor told her that he didn't think her rash was eczema, the patient felt a wave of relief. If it wasn't eczema, maybe it could be cured, she thought. She answered Dr. Grekin's odd questions, anxiously awaiting his diagnosis.

When he suggested that she could eliminate the rash simply by wearing gloves when she drove, she was ecstatic and amazed. All those years of creams and bleeding and bandages could end with a pair of lightproof gloves. She started wearing leather gloves immediately and ordered a lighter pair off the Internet.

She visited a friend who'd known her through many years of the rash and told her of this new diagnosis. The friend was excited but skeptical. She examined the patient's hands carefully and they marked their calendars to check again in six weeks to see if there was any improvement.

They didn't have to wait that long. The very next week the friend declared that the patient's hands looked "as if they belonged to someone else." They were still red but much better. By the end of the month they looked nearly normal.

Now, just over a year after her diagnosis, the patient's hands are fine. The only remnants of the rash are a few faint scars at the knuckles. And, she told me proudly, she's bought only one box of adhesive bandages since she got the gloves — and she hasn't even used them on her hands.

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