Picture Quiz

Case history

A 51 year old woman was seen because of a three day history of itchy bumps on her back. She said that she woke three days before with itching on her back and noticed red bumps on her back only. She had no idea how the rash started and had no previous history of a similar rash. She was otherwise in good health, had no fever, chills, or other systemic symptoms, and was not taking any medications. She had no history of allergy to medications, foods, or other substances. She said that she had not had any unusual foods and had not started to use any new soaps, laundry detergents, or topical preparations. She had not worn any new clothing in the past week. She had not gone camping or hiking in the past month. Examination showed an erythematous papular eruption with possible pustules confined to the back (see figure). There were no burrows between the fingers, no nail changes, and no scaling.

Questions

(1) What essential question remains to be asked to make the diagnosis?
(2) What is the diagnosis?
(3) What features of this rash distinguish it from other conditions in the differential?
(4) What is the appropriate treatment of this rash?

Answers

(1) "Have you recently spent time in a hot tub or spa?"
(2) Pseudomonas folliculitis, also called "hot tub folliculitis."
(3) Perifollicular lesions with some pustules.
(4) In most cases, the rash will resolve spontaneously within 10 days. In severe cases, an oral antipseudomonal antibiotic may be prescribed. Oral diphenhydramine hydrochloride may be given for itching.

Discussion

The patient has pseudomonas folliculitis, which is also called "hot tub folliculitis." She had been in her friend's hot tub twice just before the rash broke out. In pseudomonas folliculitis, perifollicular pustules typically begin appearing three days after exposure to a contaminated hot tub. Although "hot tub" folliculitis is usually caused by Pseudomonas aeruginosa, a culture is not needed to establish this diagnosis. This can be a difficult diagnosis to make unless the pattern is recognised and the essential question about hot tub exposure is asked. Other sources of this infection include contaminated loofah sponges and diving suits. Folliculitis can also be caused by other bacterial organisms, such as Staphylococcus aureus and Streptococcus pyogenes. A specimen may be taken from some pustules for bacterial culture if the causative agent is uncertain. If the rash is scaly, a scraping for a potassium hydroxide preparation may be helpful to look for Pityrosporum species (a superficial yeastlike organism that can cause folliculitis).

Differential diagnosis

The differential diagnosis for a pruritic erythematous papular eruption is vast. Some of the more common possibilities include viral exanthema, scabies, various fungal infections, and allergic reactions to medication, foods, soaps, laundry detergents, topical preparations, plants, or clothing. Careful examination with an optical loupe to determine that the lesions were visible around hair follicles and that some were pustular helped to make this diagnosis. The primary morphological characteristics of pseudomonas folliculitis include papules, pustules, urticarial plaques, macules, and vesicles.

Treatment

In most cases, folliculitis caused by Pseudomonas aeruginosa will resolve spontaneously within 10 days.¹ In severe cases, an oral antipseudomonal antibiotic, such as a fluoroquinolone, may be prescribed. In the case described here, the doc- tor and patient decided together to allow the folliculitis to resolve on its own. The patient did not want to use an antibiotic unless it was absolutely necessary. The physician suggested that the patient might use oral diphenhydramine hydrochloride (Benadryl) to treat the itching, if needed. Prevention requires meticulous cleaning of the hot tub and appropriate water chemical management. The patient called the doctor a week later to report that the rash had disappeared, but some residual discoloration still remained. The doctor reassured her that the residual discoloration, which is due to postinflammatory hyperpigmentation, should fade during the coming months.

Richard Usatine, assistant dean of student affairs, University of California, Los Angeles
Email: rusatine@ucla.edu


studentBMJ 2001;09:43-84 March ISSN 0966-6494

1. Habif T. Clinical dermatology: a color guide to diagnosis and therapy. 3rd ed. St Louis: Mosby,1996: 258-9.