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Joe R. Monroe, PA-C, MPAS, DEPARTMENT EDITOR
An “infection” that antibiotics won’t cure
Joe R. Monroe, PA-C, MPAS
The author practices in the dermatology department of the Springer Clinic, Tulsa, Okla, and is the founder and President-elect of the Society of Dermatology Physician Assistants. He has indicated no relationships to disclose relating to the content of this article.
 A 29-year-old woman presented to the dermatology clinic with what she called an “infection” on her anterior thigh (see Figure 1). It had been present for 10 days and had not responded to oral and topical antibiotics. The patient reported that the “infection” itched terribly but did not hurt. It was getting worse, which is what prompted the referral to dermatology.
Upon further questioning, the patient said that when this “itchy, blistery rash” first erupted on her thigh, she began to apply “triple-antibiotic ointment.” Despite this self-treatment, the condition worsened considerably right away. The patient visited her primary care physician, who switched the treatment to topical bacitracin/neomycin/polymyxin B (Neosporin) and started her on cephalexin, 500 mg four times daily, with no salutary effect. The patient was quite well in other respects, denied recent foreign travel, had not experienced any trauma to this area, and had never experienced this condition before. She was afebrile and had not had myalgia or malaise with this rash.
On physical examination, a nontender, sharply-demarcated 12-cm rectangular area of almost solid blistering and erythema was noted. The results of potassium hydroxide (KOH) testing of the blister roof were negative for fungal elements.
The most likely cause of this rash is
- Staphylococcal infection
- Fungal infection
- Contact dermatitis
- Herpes zoster
Discussion
The correct answer is contact dermatitis, which in this case was probably due to exposure to neomycin, one of the ingredients in triple-antibiotic ointment and Neosporin (which are essentially the same thing). This patient scenario is exceedingly common. The rash probably started from some other cause, such as poison ivy or other contact dermatitis, but the initial trigger was rendered moot by the neomycin-containing ointment. Suppurative bacterial infection, such as that caused by staphylococci, will not typically manifest as a bullous process and will be painful rather than pruritic, as in this case. The complete nonresponse to what should have been adequate antibiotic therapy was instructive; the clinician was able to rule out bacterial infection as the cause of this patient’s rash. Fungal infections (tinea corporis) can certainly be vesicular and pruritic, but the negative KOH test result mitigated against that as a likely diagnosis, as did the acute onset and sharply-marginated area of involvement—attributes much more consistent with contact dermatitis. Therefore, fungal infection is also an incorrect diagnostic choice. A herpes zoster (shingles) lesion is certainly blistery, but it is almost always painful and it would be unlikely to manifest in this shape or be confined to this distribution. Herpes zoster is also an incorrect diagnosis.
Treatment for this patient consisted of discontinuing the offending topical medication, applying a mid-strength topical corticosteroid cream (triamcinolone 0.1%) three times daily, and using aluminum acetate (Burow’s solution) soaks twice daily to dry the lesion up. The rash cleared within 5 days.
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