Axillary granular parakeratosis was first described in 1991, and is believed to be a contact reaction, confined to the axillae with distinctive histopathologic features.
The name ‘‘intertriginous granular parakeratosis’’ was introduced later, because of reports of cases involving areas other than the axillae,
namely, the inframammary and groin regions. Additional cases have been described involving nonintertriginous skin, which has included the chest, lower back, buttocks, and most recently the trunk involving only the follicular epithelium.
Hence, granular parakeratosis is the most appropriate name for this disorder.
The disorder is observed in all age groups, although it has been described more frequently in women older than the age of 40. In reported cases, the ages range from as young as 6 months to 83 years.
The axilla is the most commonly affected site, either unilaterally or bilaterally.
The morphology of the lesions can be scaly to keratotic patches and plaques, but the early lesions are small keratotic papules on an erythematous base that coalesce to form plaques.
If symptomatic, the patient can experience a burning sensation or pain, but the most frequently reported symptom has been mild to severe pruritus.
The clinical differential diagnosis includes Hailey-Hailey disease, contact dermatitis, psoriasis, pemphigus vegetans, and Darier-White disease, especially when nonintertriginous skin is involved. A skin biopsy, however, shows distinctive histopathologic
changes.
Typically, the epidermis is mamillated or gently papillated.
There is variable degree of psoriasiform hyperplasia, dilated capillaries, and a thickened cornified layer with unique features, namely, the presence of nucleated cornified cells containing basophilic granules that are normally confined to the granular layer (and thus the name
granular parakeratosis).
These zones may alternate with compact orthokeratosis and parakeratosis without the presence of the basophilic granules. A defect
in the processing of profilaggrin to filaggrin in the granular layer is believed to the reason for the retention of keratohyaline granules within corneocytes.
No therapeutic regimen has been consistently effective, but because the disorder is believed to be an acquired defect in keratinization, retinoids, in both oral and topical forms, and vitamin D analogs have been preferentially used.
Therapeutic success has been reported using topical corticosteroids, systemic and topical antifungals, ammonium lactate
12% lotion, and cryotherapy.
Spontaneous resolution of the lesions has been noted in a few cases.
The etiology of granular parakeratosis has been speculated to be a peculiar contact reaction to deodorant, antiperspirant, or other personal hygiene products, such as soaps and creams, based on resolution of the eruption once the personal hygiene products have been iscontinued in some of the cases.
In the few cases in which the subjects were patch tested, however, no subjects had a positive reaction to any component of deodorant or fragrance. In addition, there have been cases in which cessation of the implicated products did not result in resolution and there were cases in which there was no history of using any such products. Friction, hyperhidrosis, and obesity have been speculated to have a contributory role in the development of granular parakeratosis. In a few cases, fungi have been present in the lesions of granular parakeratosis, namely Candidal yeasts and dermatophytes.
The pathogenic role of these fungi is unclear because they were present in only a few cases, and the sites where granular parakeratosis occur are frequent sites for these infections.
The true nature of this disorder still remains elusive.

Tomado de JAAD

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Granular parakeratosis.
Actas Dermosifiliogr . 2007 Jun;98(5):355-7.

Granular parakeratosis is a rare entity that results from an acquired disorder of keratinization. Clinically presents as dark erythematous plaques, occasionally pruritic, that usually involve the axilla and other intertriginous areas. The pathology is characteristic and consists of thickening of the stratum corneum with compact parakeratosis and retention of keratohyaline granules, whereas the stratum granulosum is preserved. The etiology is unknown although some factors such as irritating physical or chemical agents have been implicated. Treatment response is variable. We report a new case in a 50-year-old woman with brownish and hyperkeratotic plaques on both axillae, of two years duration, with a compatible pathology that showed a favorable response to tacalcitol.