Are seasonal allergies the cause of this pruritic rash?

Case 1: For 2 months, a 35-year-old woman has been troubled by a bilateral pruritic eruption on her neck. The condition did not respond to a 3-week course of oral terbinafine. The patient has a history of childhood asthma; her only current medication is an oral contraceptive. She has had a cat for the past 2 years. She has not used any new shampoos or conditioners.

What is the most likely diagnosis?

A. Contact dermatitis.

B. Atopic dermatitis.

C. Tinea corporis.

D. Seborrheic dermatitis.

E. Lichen simplex chronicus.

Case 1: This patient has atopic dermatitis, B, from an unknown cause. The rash responded to moisturizers and a moderately potent topical corticosteroid.

Contact dermatitis is a possible diagnosis; patch testing would be warranted if the symptoms persisted. Tinea corporis would have responded to the oral antifungal. Seborrheic dermatitis preferentially affects the scalp along the hairline and the area directly behind the ears. Lichen simplex chronicus would be considered if the rash persisted. However, it generally affects the nuchal area rather than the lateral neck.

Case 2: For several months, a 42-year-old woman was bothered by persistent asymptomatic erythema on her face.After doing some Internet research, she self-diagnosed rosacea and decided to treat it with a cut aloe vera leaf in an attempt to reduce the inflammation. Instead, she found that the rash became pruritic, as well as more erythematous and papular.

What does this look like to you?

A. Contact dermatitis to aloe vera.

B. Rosacea flare.

C. Pityrosporum folliculitis.

D. Staphylococcal infection.

E. Polymorphous light eruption.

Case 2: The increase in erythema and papules and the onset of pruritus--in conjunction with the initiation of a new topical agent--suggested contact dermatitis to aloe vera, A. The diagnosis was confirmed when the exacerbation resolved after the aloe was discontinued. Patch testing could have been used if the diagnosis had remained unclear. The underlying rosacea was treated with metronidazole.

A flare of rosacea is not typically pruritic. Pityrosporum folliculitis can be pruritic, but the lesions are follicular, unlike those seen here. A staphylococcal infection would be painful. Polymorphous light eruption (PMLE) may present as pruritic papules on skin surfaces after exposure to the sun or to tanning beds, but the patient had no history of such exposure. Generally, PMLE is a diagnosis of exclusion.

Case 3: A 12-year-old boy presents with a symmetric pruritic rash on his arms and legs that has coincided with the onset of his seasonal allergies. He has taken montelukast and cetirizine for years. He participates in sports year-round and sometimes takes more than 1 shower a day. The family owns a dog and a ferret.

What do you suspect?

A. Adverse drug reaction.

B. Contact dermatitis.

C. Atopic dermatitis flare.

D. Psoriasis.

E. Tinea corporis.

Case 3: This patient has an atopic dermatitis flare, C. The synchronous occurrence of atopic dermatitis and a respiratory condition is seen occasionally. The patient's dermatitis was exacerbated by excessive bathing, a habit common among adolescents. His condition improved with a regimen of fewer showers, moisturizers, and a moderately potent topical corticosteroid.

Neither montelukast nor cetirizine is likely to produce a cutaneous drug reaction such as the one seen here. The pattern and location of the patient's rash are not typical of contact dermatitis. Psoriasis is scalier than this patient's rash. It is unlikely that a dermatophyte eruption would arise so quickly and be so extensive. 

Case 4: A 10-year-old girl who plays soccer at school has had pruritic papules on her elbows for several weeks. She has not been exposed to any new detergents or new clothing. Her family has a cat and a dog. No one else in the family has similar lesions, although her brother has seasonal allergies.

What type of eruption is this?

A. Scabies.

B. Psoriasis.

C. Atopic dermatitis.

D. Lichen spinulosus.

E. Granuloma annulare.

Case 4: The patient has atopic dermatitis, C. This condition generally presents in children aged 2 to 12 years as pruritic papules in the flexural areas. The rash can be controlled with moisturizers and low-potency topical corticosteroids.

Scabies would generally be more widespread after a few weeks. Psoriasis is more scaly and patchy. Lichen spinulosus may erupt on the elbows in atopic patients, but it typically is asymptomatic. Granuloma annulare is also asymptomatic.