Unilateral hyperpigmented flexural lesion in the left axilla.

A 58-year-old man presented to the clinic with a new pigmented patch on his left axilla. The lesion had been present for 7 months and was pruritic only initially. At presentation, the lesion was asymptomatic. Physical examination found a 3-cm hyperpigmented poorly demarcated patch with erythematous borders and several surrounding pigmented macules in the left axilla (Fig 1). The right axilla was unaffected, and no lesions were present in the oral cavity, neck, trunk, extremities, groin, or other flexural creases. His only medication was lisinopril for hypertension. Skin biopsy was performed with the findings in Fig 2.

Fig 1

Fig 2

Question 1. Considering clinical and histopathologic images, what is the most likely diagnosis?

Fixed drug eruption

Acanthosis nigricans

Erythrasma

Postinflammatory hyperpigmentation

Lichen planus pigmentosus-inversus (LPP-inversus)

Answers:

Fixed drug eruption – Incorrect. Fixed drug eruption is a common dermatologic adverse reaction to various prescribed drugs. Agents implicated in fixed drug eruptions include tetracyclines, nonsteroidal anti-inflammatory drugs, trimethoprim, fluoroquinolones, pseudoephedrine, cetirizine, and phenytoin, none of which this patient was taking.

Acanthosis nigricans – Incorrect. Acanthosis nigricans presents with symmetric, hyperpigmented, velvety plaques commonly on the neck and axillae. This condition is often seen in patients with diabetes, obesity, and/or an internal malignancy. Histologically, there will be hyperkeratosis, papillomatosis, and hyperpigmentation of the basal layer of the skin.

Erythrasma – Incorrect. Erythrasma is a superficial infection of the skin caused by Corynebacterium minutissimum, a gram-positive, non–spore-forming bacillus. This condition is diagnosed via a Wood's lamp examination of the skin showing a red or coral color. A biopsy would not show lichenoid inflammation.

Postinflammatory hyperpigmentation – Incorrect. Postinflammatory hyperpigmentation is an acquired hypermelanosis resulting from cutaneous injury or inflammation. This patient had no prior inflammatory process in her axilla, and the pathology findings showed active inflammation.

LPP-inversus – Correct. LPP-inversus is a rare form of lichen planus, which presents as ovoid, brown, or gray macules and patches in sun-exposed or intertriginous areas, with or without pruritus. Skin biopsy of the lesion confirms this diagnosis. Histologically, LPP-inversus has almost an identical appearance to classic lichen planus (ie, hyperkeratosis, acanthosis, interface dermatitis, and saw-tooth rete ridges), with the addition of dermal melanophages.

Question 2. Which of the following features is characteristic of lichen planus pigmentosus inversus?

Pruritus

Erythema

Bilateral presentation

Spontaneous resolution

Acute onset

Answers:

Pruritus – Incorrect. Contrary to ordinary lichen planus, LPP-inversus does not tend to be pruritic.

Erythema – Incorrect. LPP-inversus presents as brown or gray macules and patches.

Bilateral presentation – Correct. Most presentations of LPP are bilateral; however, there are some reports of a unilateral presentation. Unilateral LPP in the axilla is extraordinarily rare, with a total of 6 previous cases reported in the English-language medical literature. All 6 cases reported had lesions restricted to a single site.

Spontaneous resolution – Incorrect. The clinical course of LPP-inversus can vary with some cases spontaneously resolving or reducing in size and others persisting for years.

Acute onset – Incorrect. LPP-inversus has an insidious onset.

Question 3. What is a potential treatment strategy for this condition?

Clobetasol

Doxycycline

Q-switched neodymium yttrium-aluminum-garnet laser

Botulinum toxin

Adalimumab

Answers:

Clobetasol – Correct. The patient was treated with 2 weeks of daily clobetasol followed by daily tacrolimus 0.1%. The lesions persisted at a 3-week follow-up appointment, although reduction in size was noted. In the inverse variant, caution should be exercised when using high-potency steroids in intertriginous areas, given increased penetration and higher risk of atrophy and striae.

Doxycycline – Incorrect. LPP-inversus is not caused by a bacterial source, thus antibiotics would not provide relief.

Q-switched neodymium yttrium-aluminum-garnet laser – Incorrect. No surgical intervention, to include laser therapy, is indicated for LPP-inversus, which is a benign condition with no malignant potential.

Botulinum toxin – Incorrect. Botulinum toxin is not found to be beneficial in treating LPP-inversus.

Adalimumab – Incorrect. Biologics are not found to be beneficial in treating LPP-inversus. However, other immunomodulators, such as calcineurin inhibitors (tacrolimus and cyclosporine) can be used to treat LPP-inversus.