Name That Rash: Common Causes of Benign Skin Rashes

Accurate diagnosis of skin reactions requires an understanding of the most common causes, as well as detailed observation of morphology, distribution, and timing, according to Aleena Banerji, MD, of Harvard Medical School and Massachusetts General Hospital, who presented “Benign Skin Rashes and Drug Allergy.”

The most common types of cutaneous drug reactions are exanthematous drug reactions, erythema multiforme, toxic epidermal necrolysis (TEN), and Stevens–Johnson syndrome (SJS). The drug categories causing such reactions are most often antimicrobials, anti-epileptic medications, nonsteroidal anti-inflammatory drugs (NSAIDs), and sulfonylurea antidiabetic medications. Banerji encouraged attendees to keep these common factors in mind when establishing diagnosis.

In addition, Banerji highlighted the morphology of individual lesions and the distribution of lesions as two of the most useful factors to consider when forming a differential diagnosis. Morphology, she explained, can be dome-shaped, pedunculated, verrucous, umbilicated, or flat-topped. Regarding distribution, she offered the following categorizations that can help lead to accurate diagnosis:

• photo-distributed: lupus erythematous, photo-drug eruption, dermatomyositis, pellagra, or porphyria cutanea tarda
• wrists and ankles: lichen planus, scabies, contact dermatitis, eczema
• scalp: seborrhea, contact dermatitis, tinea capitis and kerion, discoid lupus, or psoriasis
• mouth: mucous cysts, leukoplakia, Fordyce spots, pyogenic granuloma, skin cancers, or Kaposi sarcoma

Arrangement also is important, she noted. Therefore, clinicians should consider the arrangement of lesions in relation to each other, the color of the lesions, and the consistency and feel. Lesions can be discord/nummular, annular, targetoid, linear, serpiginous, or grouped/clustered.

Banerji provided definitions and photographs to illustrate many relevant dermatologic terms, along with paired example diagnoses. For example, a nodule would raise suspicion of basal cell carcinoma, a wheal might lead to diagnosis of urticaria, a scale is associated with psoriasis, crust might be indicative of impetigo, and a vesicle could mean varicella zoster.

“History, history, history,” Banerji continued, suggesting that physicians take a detailed history, establish a drug timeline, and create a chart to track the date each drug was started and when the rash started, which all help clinicians visualize what might be the cause.

Banerji then pulled all the information together, reviewing photos and details about several specific cutaneous reactions. The most frequent of all skin reactions to drugs, she said, is exanthematic (morbilliform) reaction. It is limited to the skin (not affecting the mucous membranes), initially appearing on the trunk and then spreading to the extremities. The reaction can appear two to three days after drug initiation but usually happens eight to 11 days afterward. It manifests as profuse eruptions of small papules or pruritic lesions, typically caused by beta lactams, sulfonamides, allopurinol, anti-epileptic drugs, and NSAIDs.

An exanthematic reaction resolves without sequelae in a few days to a week after the causative medication is stopped. Treatment may include topical steroids, antihistamines, and reassurance. If the reaction is not too severe and no other reasonable treatment is available, a patient may choose to continue with the offending medication.

The second most common type of cutaneous allergic drug reaction is urticaria, she said. It occurs 24 to 36 hours after ingestion and is most commonly caused by NSAIDS, penicillin, sulfonamides. The lesions—raised, edematous, erythematous wheals—last less than 24 hours, but new lesions can appear. Urticaria may be associated with anaphylaxis, angioedema, and serum sickness.

Banerji concluded by urging participants to document any and all drug reactions in the medical records, including very specific details about morphology, distribution, and timing.