Diagnosis: Axillary granular parakeratosis

The woman had axillary granular parakeratosis (GP), a benign condition. First described in 1991, this disease presents with erythematous hyperpigmented and hyperkeratotic papules and plaques in cutaneous folds. In addition to the axillae, intertriginous forms of the disease can develop in the groin, intermammary or submammary region, and abdominal folds. Associated pruritus may develop.

Although the exact cause of axillary GP is unknown, its origin has been linked to excessive use of topical preparations, in particular antiperspirants and deodorants; however, axillary GP has also been found in persons who have not used such agents. Axillary GP is associated with an occlusive environment, increased sweating, and, less frequently, local irritation. Some studies have linked the condition to obesity. 

GP is rare. In a dermatopathology laboratory, during a five-year period, 18 of 363,343 specimens (0.005%) were diagnosed as GP. All of the specimens were taken from the axillae of adult patients; the majority were women. Most patients who present with suspected GP will have a one- to 12-month history of axillary or intertriginous rash. The condition manifests with intertriginous, bilateral or unilateral, brown- or red-crusted patches, papules, or plaques. GP can be confluent or reticulated. Even when patches or plaques are present, discrete papules can also be seen. Slightly erythematous and lichenified plaques have been reported as well. 

In children, GP has been observed in the groin and on the lower back, buttocks, and flanks. 

The differential diagnosis includes acanthosis nigricans, intertrigo, confluent and reticulated papillomatosis, erythrasma, extramammary Paget’s disease, inverse psoriasis, inverse lichen planus, inverse pityriasis rosea, pemphigus vegetans, and benign familial pemphigus. GP is best distinguished from intertrigo by the presence of dry or keratotic scale in areas where maceration is the usual manifestation of an eruption. 

When GP is suspected, evaluation should include a KOH preparation to check for causative fungi or yeast. Examination with a Wood’s lamp will rule out erythrasma. Definitive diagnosis is made on biopsy. Characteristic histologic findings include psoriasiform hyperplasia, a thickened stratum corneum with retention of keratohyalin granules, parakeratosis, a well-developed granular layer, and a sparse lymphohistiocytic inflammatory infiltrate. The retained granular layer can show focal vacuolization.

Effective treatments include topical steroids, pimecrolimus cream, topical retinoids, topical calcipotriene, and ammonium lactate. Prognosis is good with any form of treatment and avoidance of inciting factors. Sometimes GP resists treatment and has a chronic and relapsing course.

Our patient was treated with desonide 0.05% lotion. Her eruption resolved in one month. One year later, the eruption had not recurred.