Common skin reactions to treatment with topical 5-FU for actinic keratosis include erythema, ulceration, and burning. In this case, however, the skin disruption opened the door to secondary impetigo.
Secondary skin infections are a known risk of treatment with topical 5-FU. The agent inhibits thymidylate synthetase, an enzyme involved in the synthesis and repair of DNA. The inhibition of this enzyme can lead to skin disruption with erosion, desquamation, and the risk of superimposed skin infections, as was seen with this patient.1
Impetigo is a common skin infection affecting the superficial layers of the epidermis and is most commonly caused by gram-positive bacteria, such as Staphylococcus aureus or Streptococcus pyogenes.2 Secondary impetigo, also known as impetiginization, is an infection of previously disrupted skin due to eczema, trauma, insect bites, and other conditions. This contrasts with primary impetigo, which results from a direct bacterial invasion of intact healthy skin. While impetigo predominantly affects children between the ages of 2 and 5 years, people of any age can be affected.2 Impetigo characteristically manifests with painful erosions, classically covered by honey-colored crusts. Thin-walled vesicles often appear and subsequently rupture.
Treatment options for impetigo include both topical and systemic antibiotics. Topical therapy is preferred for patients with limited skin involvement, while systemic therapy is indicated for patients with numerous lesions. Mupirocin and retapamulin are first-line topical treatments. Systemic antibiotic therapy should provide coverage for both S. aureus and streptococcal infections; cephalexin and dicloxacillin are preferred. Doxycycline, trimethoprim-sulfamethoxazole, or clindamycin can be used if methicillin-resistant Staphylococcus aureus is suspected.3
This patient was advised to try warm soaks (to reduce the crusting) and to follow that with the application of white petrolatum bid. The patient was also prescribed doxycycline 100 mg orally bid for 10 days. At the 1-month follow-up, there was some residual erythema, but the impetigo and crusting had resolved. The actinic keratoses had resolved, as well.
Image courtesy of Daniel Stulberg, MD. Text courtesy of Tess Pei Lemon, BA, University of New Mexico School of Medicine and Daniel Stulberg, MD, FAAFP, Department of Family and Community Medicine, University of New Mexico School of Medicine, Albuquerque