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Dermatology Practice Today

When a Rash is Not a Rash

Published November 15, 2013 8:36 AM by Raymond Shulstad
Every day thousands of people go to their practitioner's offices, walk-in clinics and emergency rooms complaining of a "rash". Some of them itch, some of them don't. Some have been present for a few days, some for many years. Some come and go sporadically without warning, some present acutely, some appear like clockwork at specific times of the year or after exposure to specific irritants. 

Most of these conditions are either related to exposure to an allergy or irritant or are in the atopic dermatitis spectrum. Most practitioners can recognize and distinguish the urticarias and things such as scabies relatively easily and treat them appropriately. For the majority of patients, many exanthems will resolve without treatment; for those that don't, a topical steroidal cream or ointment will resolve or improve them.

Sometimes though, something that appears to be a rash is only a cutaneous manifestation of something that has the potential to be significantly more ominous. One such example of this is mycosis fungoides or cutaneous T-cell lymphoma. This form of lymphoma can look exactly like an atopic dermatitis and have the same symptomology (redness, pruritis, sporadic and transient).  The difference is that this condition, if it advances, can be lethal.

So, how can you tell the difference if they all look the same? In a word, BIOPSY! If an elderly person presents to your clinic with complaints of a long standing rash, treated for multiple years with minimal success with topical steroids and the erythema seems to be confined to photo-protected areas of the body, or those covered with clothing, take a sample.

It is unlikely that atopic dermatitis will acutely begin in the fifth or sixth decades of life with no previous history. It also usually presents on the antecubital and posterior knees. Mycosis fungoides usually presents in the fifth or sixth decades of life and ultraviolet light suppresses rash. Therefore, the exposed areas of skin are spared and as mentioned earlier, the trunk or covered regions of the skin are the most common sites of presentation.

If you take a biopsy and it is consistent with mycosis fungoides, don't panic. Refer them to hematology/oncology for evaluation and management. The majority of cases do not progress to the more dangerous stages and can be controlled with topical steroids and ultraviolet treatments. I would advise anyone who sees a person with rashes on the body that appear in the sun covered regions to keep this disease in mind and familiarize themselves with it.

2 comments

Great article. Thanks. Would like to know if jock itch or athletes feet must itch. What are the differential dx's when there are limited available tests and no microscope. I check with the UV light on a ophthalmoscope (we don't have a woods lamp) to r/o erythrasma. I usually treat with tolnaftate and education. If no results, I try mycolog. If still no results, I try diflucan weekly x one mo (only antifungal po med available here)

Elizabeth Stewart-jones, adult med - NP, corrections December 5, 2013 12:32 PM
sussex DE

Thank you Raymond for the reminder and a wonderful write up.%0d%0aIn my field, lamictal rashes are another tricky rash. It could show up as sores just on the scalp, rash only on the crotch area, acne, eczema patches, psoriasis patches, etc. ANY skin change and the lamictal is stopped. Pt's forget to mention a sore on the scalp, etc.  Just an FYI.

Dr. Kelly gardiner, psych - PhDc, PMHNP December 3, 2013 5:28 PM
detroit MI

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    Occupation: Physician assistant and nurse practitioners
    Setting: Various dermatology settings
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