Approach to common skin rashes: Clinical sciences

A skin rash is an area of irritated or inflamed skin that reflects the body’s reaction to a localized or systemic process. It can result from external contact exposure or an internal process causing immune dysregulation or infiltration of inflammatory cells. It’s important to first identify if an adverse medication reaction is the cause of your patient’s rash as this might be potentially life-threatening.

Other possible diagnoses include infectious causes such as folliculitis, scabies, shingles, and tinea corporis; and inflammatory or autoimmune causes such as atopic dermatitis, contact dermatitis, seborrheic dermatitis, acne vulgaris, rosacea, and psoriasis.

Now, if your patient presents with a skin rash, first perform a focused history and physical examination.

Your patient will report a rash that might be red, itchy, scaly, or painful. The physical exam will reveal a rash that might be erythematous, macular, papular, vesicular, or comprised of discrete plaques. At this point, diagnose dermatitis, a nonspecific diagnosis that indicates inflammation of the skin.

Next, assess for a cutaneous adverse medication reaction. These patients report a widespread rash that appeared in as little as one day and up to three weeks after starting a medication such as an antibiotic, NSAID, or antiepileptic.

The physical exam will show an erythematous maculopapular eruption, and there might be dark red or purpuric macular lesions or urticaria.

With these findings, diagnose cutaneous adverse drug reaction.

Here’s a clinical pearl! Urticaria, better known as hives, is a common, usually self-limiting, skin reaction triggered by medications, certain foods, allergens, or stress. Symptoms include itchy, raised red wheals, also known as welts, on the skin’s surface. Acute urticaria may progress rapidly to life-threatening angioedema or anaphylactic shock. The mainstay of treatment is avoiding further exposure to the suspected trigger!

Here's another clinical pearl! Cutaneous adverse drug reactions that carry significant risk for complications or even death include anaphylaxis, Stevens-Johnson Syndrome, toxic epidermal necrolysis, acute generalized exanthematous pustulosis, and drug reaction with eosinophilia and systemic symptoms, otherwise known as DRESS syndrome. So, always do a thorough review for new medications and have a high index of suspicion if your patient’s rash is not improving or is worsening!

On the other hand, if a cutaneous adverse medication reaction is not present, your next step is to assess for infectious causes of a skin rash.

First up is folliculitis. Your patient will report an itchy rash on hair-bearing regions of the skin as well as a recent increase in sweating. They might also report hot tub or swimming pool exposure, or recent use of topical steroids.

The physical exam will reveal papules and pustules around hair follicles with perifollicular erythema. With these findings, diagnose folliculitis!

Here’s a clinical pearl! More severe cases of folliculitis may lead to cellulitis, an infection of the deeper skin tissue, where the skin is erythematous, edematous, warm to the touch, and tender to palpation. If your patient’s rash is worsening, they might be developing cellulitis and will need antibiotics!

Let’s move on to scabies. These patients report a severely itchy rash that’s worse at night; and they may have a history of inadequate access to hygiene or poor nutritional status. Physical exam will reveal serpiginous burrows with erythematous papules and vesicles, most commonly in the hands, feet, wrists, and elbows. With these findings, diagnose scabies!

Here’s a clinical pearl! Scabies is typically a clinical diagnosis but if you’re unsure, you can perform dermoscopy or obtain skin scrapings. Dermoscopy will reveal burrow structures and on skin scrapings you can visualize mites!

And here's another clinical pearl! Regardless of symptoms, all close contacts of patients with scabies should receive treatment with topical permethrin. Moreover, patients and contacts should decontaminate bedding, towels, and clothing when receiving treatment, to prevent reinfection.

Next let’s discuss shingles. Patients may sometimes start with a prodrome of fever and malaise but more commonly the first sign is skin pain and tingling, followed by a painful localized rash. The physical exam will show a unilateral papulovesicular rash isolated to a specific dermatome.

At this point, consider shingles and test the vesicular fluid with a Tzanck smear; a polymerase chain reaction or PCR; or a direct fluorescent antibody or DFA test. Or you can order a varicella-zoster IgM blood test. If the Tzanck, PCR, or DFA are positive; or if the varicella-zoster IgM is positive, diagnose shingles. Keep in mind that a shingles diagnosis can often be made based on clinical findings alone.

Next up is tinea corporis. These patients typically report an itchy, red rash typically on the neck, trunk, or extremities. Physical exam will reveal single or multiple annular or ovoid patches, sharp margins with an erythematous scaly edge, and a ring shape with central clearing.

With these findings, consider tinea corporis.

Obtain skin scrapings and perform a KOH test. If the KOH test reveals septate and long hyphae, diagnose the dermatophyte infection called tinea corporis. A fungal culture can be done if the KOH test is negative or inconclusive, or if the presentation is unusual. Keep in mind that the diagnosis of tinea corporis can also be made on clinical findings alone.

Okay, now if infectious causes of a skin rash are not present, assess for inflammatory or autoimmune causes.