This patient demonstrated several critical signs that pointed toward the diagnosis of shoe allergic contact dermatitis (SACD). Specifically, her recent history of using new boots for prolonged and repeated hikes raises suspicion of their role as the offending agent. In addition, the subacute and progressive timeline of the presenting symptoms is typical of type IV hypersensitivity reactions. The eczematous and blistering morphology is classic for allergic contact dermatitis. Finally, the distribution of the lesions to the dorsal toe and instep is highly specific for shoe allergen exposure, rather than allergy to a topical medication, such as neomycin-polymyxin B-bacitracin (Neosporin). Altogether, these findings led to the diagnosis of SACD.
In a younger patient, juvenile plantar dermatosis (JPD) would be an important consideration. JPD is most commonly found in prepubertal children who wear occlusive footwear. It is thought to be caused by hyperhidrosis and subsequent dehydration of the feet, which leads to shiny, erythematous scaling and fissuring of the plantar and dorsal aspects of the foot. Similar to SACD, it usually presents bilaterally and symmetrically, sparing the inner web spaces, and is associated with exercise. However, JPD is more painful than pruritic and more dry than wet morphologically, and it presents in school-age children only.[2,3]
In this patient, the first exposure did not produce skin irritation, as would be expected with acute irritant dermatitis. Acute irritant dermatitis occurs in most persons exposed to a substantial concentration of irritant materials. These irritants cause an inflammatory reaction of the skin that varies in severity and presentation depending on the irritant involved, its concentration, contact duration, and precondition of the skin. This type of reaction does not require prior sensitization and occurs within minutes to hours of initial contact. The patient did not notice the rash and itching after putting the boots on, nor did she notice it after taking her boots off at the end of her hike. Instead, the symptoms manifested several days later. Thus, the timing of her presentation helps exclude acute irritant dermatitis.
Fungal infection of the feet, also known as tinea pedis, is a reasonable diagnostic consideration when a patient describes a history of fungal nail infection (onychomycosis), progressive pruritus, and peripheral scaling of an erythematous lesion. The history of moist occlusive footwear and exposure to the outdoors during the hike makes this a tempting choice. To differentiate between tinea pedis and SACD, inspect the web spaces between the toes. This area rarely is exposed to shoes and their allergens, but it is a very common site for fungal infection. Moreover, tinea pedis less commonly involves blistering, prefers the plantar surface of the feet, is characterized by satellite lesions, and typically presents unilaterally or asymmetrically bilaterally.[1,3]
Given this patient's history of mild eczema as a child, the clinical presentation could suggest an atopic dermatitis flare. However, atopic dermatitis usually has vaguely bordered lesions and occurs on the neck, face, trunk, and flexor surfaces of the elbows and knees in persons in her age group. In addition, her childhood eczema resolved at least a decade before her current visit. Thus, the timeline and the bilateral localized lesions discretely found on the dorsal toe and instep militate against this diagnosis.
Psoriasis could be another consideration in the differential diagnosis for this patient. Plaque psoriasis is the most prevalent subtype, classically involving the scalp, elbows, knees, and back, although it can occur anywhere. Palmoplantar psoriasis is typically found on the palms and soles; however, nothing remarkable was seen on the patient's palms upon examination. No plaques were found during the physical examination. Furthermore, the subacute bilateral onset does not match the chronic progression of psoriasis. Given this patient's unremarkable full-body skin check and lack of inflammatory comorbidities, psoriasis is not likely.
Another condition that involves erythematous and excoriated pruritic lesions is scabies. It can be found at almost any site on the body (including the feet), with the exception of the neck and face. However, this patient lacks many of the common features of scabies, such as digital web involvement, serpiginous burrow lines, predominantly nocturnal symptoms, and poor hygiene. The isolated presence of the disease on this patient's feet without burrows upon examination directs away from this diagnosis. To confirm it is not a case of scabies, inspect skin scrapings.
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Cite this: Melba Estrella, Ansley Devore, Alan Snyder;, et. al. 29-Year-Old Hiker Self-Treats Her Rash With Coconut Oil - Medscape - Jul 06, 2023.