Pregnant 21-year-old with patchy skin eruption

There is a hyperpigmented eruption in the axial and groins with a conspicuously reticulated periphery. The hyperpigmented eruption on her abdomen is also reticulated in its morphology.

The diagnosis is confluent and reticulated papillomatosis (CARP). This rash is characterised by multiple hyperpigmented scaly macules and papillomatous papules that form confluent patches or plaques, particularly on the upper trunk, and has a reticular pattern peripherally. 

The diagnosis is generally clinched clinically, however, a skin biopsy can be done to exclude other differential diagnoses. Histopathology findings include:

• Hyperkeratosis
• Parakeratosis
• Papillomatosis
• Mild focal acanthosis
• Increased melanin pigmentation and melanosomes (hence, causing hyperpigmentation)
• Dermal inflammatory cell infiltrate.

The exact cause of this condition is unclear. It results from disordered keratinisation and an association with skin infection secondary to Dietzia papillomatosis, a gram-positive aerobic actinomycete, has been described.

Genetic factors may also contribute as some familial cases have been previously reported. In these cases, mutations of the gene for protein keratin-16 (K-16) were found.

The first-line therapy for CARP is a course of tetracycline (minocycline or doxycycline) for 6–12 weeks or a macrolide antibiotic (azithromycin or erythromycin) for 4–6 weeks. As this patient is pregnant, macrolides are more appropriate in her case.

A localised rash can be treated with topical treatments such as tacrolimus, retinoids, or calcipotriol ointments. 

For refractory cases and non-pregnant patients, systemic retinoids such as isotretinoin and acitretin can be used.