Correction: Earlier versions of this article, including the May 17, 2011 print edition of The Washington Post, incorrectly referred to aquagenic pruritus as aquagenic pruritis. This article has been corrected.

The problem started so gradually that at first Carrell Grigsby didn’t pay it much attention. A dozen years ago the professional photographer, who lives in Austin, experienced periodic bouts of ferocious itching, mostly on her arms and legs, which occurred without warning and vanished within a few hours.

Grigsby knew she was allergic to mold and pollen. But unlike attacks involving those allergens, these episodes were different: Her skin looked normal, and scratching didn’t alleviate the itch. Even the sensation was unusual. “It felt like tiny ants were crawling under my skin,” Grigsby recalled.

By 2005, she seemed to experience an attack nearly every Saturday, sometimes at home while getting ready to photograph a wedding, at other times in the middle of a shoot. Grigsby worried that her job was stressing her out, but wondered how: She enjoyed photographing weddings and didn’t feel anxious.

Several times, Grigsby said, the itching got so bad she jumped in her car and drove to a nearby emergency room, hoping the doctor on duty could figure out what was wrong. But while sitting in the waiting room the problem would go away; feeling foolish, Grigsby would leave before seeing a doctor, relieved that she didn’t have to describe symptoms that sounded psychosomatic.

Many months after her last trip to the ER, Grigsby learned the improbable reason for her intense bouts of prickly itching.

At first Grigsby, now 64, thought her problem might be an intense reaction to a soap or skin lotion. She switched brands many times, then made an appointment with her allergist; he performed skin tests intended to induce the episodes, but could not replicate the itching. He told her he suspected the problem was an exacerbation of her mold allergy.

The doctor prescribed various antihistamines and, to retard the growth of mold in her home, told her to use large fans and lights in areas prone to dampness.

None of the measures worked, and the episodes became more frequent.

Grigsby bought new towels. She tried to stay out of her bedroom closet, fearing it harbored mold spores. She even used a different bathroom in her house. When nothing helped, she began to think her problem was stress-related.

In 2006, Grigsby consulted her internist, who after three visits referred her to a dermatologist, Jay Viernes.

During their first appointment, on a Friday afternoon in October, Viernes peppered Grigsby with questions, most focused on the timing of her attacks and possible triggers.

He then advised her to take only cool showers and to call him Monday to report whether the water temperature had made a difference.

“When we train, we learn that itching is a big cause of complaint with patients,” Viernes said. Unexplained itching, he noted, has numerous causes. Most cases are harmless and clear up on their own with little or no treatment, but itching can also be a symptom of serious illness, including several cancers and a blood disorder.

Viernes said that his first thought was that Grigsby might have cholinergic uticaria, a form of itchy hives triggered by heat, although she did not have the classic red blotches characteristic of hives. A second, more exotic diagnosis occurred to him, one he spent part of that weekend researching, just in case.

On Monday, Grigsby reported that cool water actually worsened the itch, eliminating his first hypothesis.

Answer in the shower

“I know what you have,” Viernes told her. Grigsby was displaying the classic signs of a condition called aquagenic pruritus, a debilitating skin condition caused by water — not when she drank it, but when it touched her skin.

“I’d never seen a case, and because it’s so rare I had to look it up and see what to do,” recalled Viernes, who said he had learned about the disorder during his residency training in the early 1990s.

Aquagenic pruritus is a poorly understood and often misdiagnosed condition that causes intense and often prickly itching concentrated on the arms and legs, according to the Office of Rare Diseases at the National Institutes of Health. For unknown reasons, the face, head, palms, soles and mucous membranes are typically unaffected.

“A lot of patients think they’re crazy, or doctors don’t believe them,” Viernes said. “These patients sometimes develop hygiene problems and can be severely depressed.” Most sufferers are middle-aged or elderly males and the disorder appears to run in families.

For many, the attacks begin within minutes of exposure to water of any temperature and can last from 10 minutes to two hours. The skin displays no sign of a problem.

The cause of the disorder is unknown, although some researchers believe it may be linked to the release of acetylcholine, a chemical that transmits signals from nerves to muscles. Before a diagnosis is made, other causes of unexplained itching should be ruled out.

Grigsby said she was floored by the diagnosis, which explained the timing of her puzzling attacks. Those that occured while she was getting ready for a wedding were the result of the shower she invariably took before an assignment. And the attacks that sometimes occurred during photo shoots could be explained by her trademark shot: Grisby stood nearly up to her knees in a popular lake to photograph families seated on a dock.

“I was extremely relieved emotionally that this wasn’t all in my head,” she said.

Viernes gave Grigsby the e-mail address of an online support group of people with aquagenic pruritus — — which she said has been a major source of information and comfort. She quickly tried a remedy suggested on the site and found that it relieved the itching: hot showers followed by the use of a hair dryer on affected areas.

“It left me with normal-feeling skin for the first time in a decade after a shower,” she recalled. Viernes also prescribed a topical cream made of capsaicin, the active ingredient in hot peppers, which studies have found to be effective for some aquagenic pruritus patients; Grigsby found that it burned too much to use.

An avid golfer, Grisby noticed that when she had a tan the episodes diminished. In 2009, after experimenting with a tanning bed at her health club, she began thrice-weekly phototherapy sessions at Viernes’s office.

Light treatment is known to work for other skin disorders, including psoriasis and eczema, and Viernes had found studies demonstrating its success in the treatment of aquagenic pruritus. Narrow-band ultraviolet light used in phototherapy poses less of a risk of skin cancer than does light from conventional tanning beds, Viernes said. After six momths, her itching was nearly gone.

But going to a doctor’s office three times a week posed a hardship with her unpredictable work schedule, so in 2010 Viernes asked her health insurer to pay for a home phototherapy unit.

After several months of wrangling, the insurer approved the $5,700 machine, which was installed in Grisby’s bedroom; she uses it when she needs it, popping in for four-minute treatments on her arms, legs and torso as often as every three days.

These sessions, combined with the hot-shower-and-hair-dryer regimen, have enabled her to live a largely itch-free life. And although she has learned how to manage the problem, Grigsby is perplexed about why she developed it in the first place.

“That,” she said, “is the really big mystery.”

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