A long, hot bath should be a soothing indulgence, but in rare cases it may instead spur excruciating headaches, according to researchers.

Reporting in the journal Cephalalgia, the doctors describe six cases of so-called bath-related headache - sudden, severe head pain triggered by showering, soaking in a hot tub, or even rinsing the mouth with water.

The phenomenon appears rare, with only 14 cases total reported in the medical literature, and it’s been seen only in middle-aged and elderly Asian women.

Exactly what sets off the headache and why it appears to be limited to this particular population is a mystery, according to Dr. Wai Wo Mak of the University of Hong Kong, the lead author on the report.

Though migraines are often related to women’s menstrual cycles, these bath-related headaches appear unrelated to either monthly hormonal fluctuations or menopause, Mak told Reuters Health.

In all of the cases documented, the researcher pointed out, the women suffered only one cluster of headache attacks before the problem resolved, which took anywhere from a week to a couple months.

The heat of the water, as well as the pressure on the scalp during a shower, seem to be “essential components” of the headache trigger, according to the researchers.

However, those factors don’t fully explain the phenomenon, since cold water resulted in head pain in some women, while others developed headaches while brushing their teeth or even urinating, Mak said.

According to Mak, bath-related headaches fall under the category of “thunderclap” headaches - which, as the name implies, come on suddenly and severely. The mechanism by which thunderclap headaches arise is poorly understood, and the triggers, Mak said, can run the gamut from weight-lifting to sex.

Among the six women described in the report, there was no sign that the headaches were linked with underlying disease in the blood vessels supplying the brain, and in general, bath-related headaches appear to be a largely “benign” condition, according to the researchers.

Still, two patients were found to have reversible constriction in the cerebral arteries during their headache bouts. Such blood vessel spasms, Mak noted, can temporarily raise the risk of Stroke.

SOURCE: Cephalalgia, March 2005.