Infectious & Pediatric Rashes
Crusts, dermatomes, and exanthems: reading rashes by pattern
Bacterial skin infections
Impetigo. A superficial, highly contagious infection of children that favors the face and forms honey-colored crusts. Non-bullous impetigo is caused by Staphylococcus aureus or Streptococcus pyogenes; bullous impetigo comes from staphylococcal exfoliative toxin. Treat limited disease with topical mupirocin and extensive disease with oral cephalexin.
Streptococcal impetigo can be followed by post-streptococcal glomerulonephritis, but not by rheumatic fever, which follows only pharyngitis.
| Feature | Cellulitis | Erysipelas |
|---|---|---|
| Depth | Deep dermis and subcutis | Upper dermis and superficial lymphatics |
| Appearance | Ill-defined erythema, warmth, tenderness | Sharply demarcated, raised, bright-red plaque |
| Organisms | Staph aureus and Strep | Strep pyogenes |
| Systemic features | Variable | Fever, rapid onset |
| Treatment | Strep-active antibiotic (cephalexin); cover MRSA if purulent, and drain abscesses | Strep-active antibiotic such as penicillin |
Pain out of proportion to the exam, rapid spread, crepitus, dusky bullae, or systemic toxicity suggests necrotizing fasciitis, which needs emergent surgical debridement plus broad-spectrum antibiotics.
Fungal infections and scabies
Tinea (dermatophytes). Trichophyton, Microsporum, and Epidermophyton cause an annular scaly plaque with central clearing and an active, raised border, named by site: corporis (body), cruris (groin), pedis (feet), capitis (scalp), and unguium (nails, onychomycosis). KOH prep shows septate, branching hyphae. Topical azoles or terbinafine suffice for skin, but tinea capitis and onychomycosis need oral terbinafine or griseofulvin because topicals do not reach the hair follicle or nail.
Topical steroids blunt the inflammation of tinea and let it spread (tinea incognito). An annular rash that worsens on a steroid cream needs a KOH prep.
Cutaneous candidiasis. Candida albicans produces beefy-red plaques with satellite pustules in moist intertriginous zones (skin folds, diaper area, under the breasts). KOH shows pseudohyphae and budding yeast. Risks include diabetes, obesity, antibiotics, and immunosuppression. Treat with topical nystatin or an azole and keep the area dry.
In the groin, candidiasis involves the scrotum and has satellite lesions; tinea cruris spares the scrotum and has a scaly advancing edge.
Scabies. Sarcoptes scabiei causes intense pruritus that is worse at night, with burrows in the finger web spaces, wrists, waist, and genitals; household contacts itch too. Crusted (Norwegian) scabies, with thick, mite-laden crusts, occurs in the immunocompromised and is highly contagious. Treat with topical permethrin or oral ivermectin, covering the whole household at once and washing bedding and clothing; the itch can persist for weeks after successful treatment.
Viral skin infections
Herpes simplex. Recurrent grouped vesicles on an erythematous base, oral or genital. A Tzanck smear shows multinucleated giant cells (in both HSV and VZV infection); PCR of vesicle fluid is the preferred confirmatory test.
Herpes zoster. VZV reactivation produces painful vesicles in a dermatomal band that does not cross the midline, often after a prodrome of pain in that dermatome. Treat with acyclovir or valacyclovir, ideally within 72 hours of rash onset, to speed healing. Postherpetic neuralgia is the main complication, and the recombinant zoster vaccine (Shingrix) is recommended from age 50, and from age 19 in adults who are or will be immunocompromised, even after a prior episode of zoster.
Vesicles on the tip of the nose (Hutchinson sign) signal zoster in the nasociliary branch of V1 and a high risk of eye involvement; arrange urgent ophthalmology review.
Warts and molluscum. Warts (HPV) are rough, hyperkeratotic papules studded with thrombosed capillaries (black dots). Molluscum contagiosum (a poxvirus) produces dome-shaped, flesh-colored, umbilicated papules in children or sexually active adults and is widespread in HIV. Both are self-limited; options include cryotherapy and curettage.
Childhood viral exanthems
Fever, then rash. The prodrome, the timing of the rash relative to the fever, and one signature finding name the virus.
| Disease | Cause | Signature features | Key complication |
|---|---|---|---|
| Measles | Measles virus | Cough, coryza, conjunctivitis; Koplik spots, then a cephalocaudal (head-to-toe) rash | Pneumonia, encephalitis; give vitamin A |
| Rubella | Rubella virus | Mild illness with postauricular lymphadenopathy | Congenital rubella if acquired in pregnancy |
| Roseola | HHV-6 | About 3 days of high fever, then a rose-pink truncal rash as the fever breaks | Febrile seizures |
| Fifth disease | Parvovirus B19 | Slapped-cheek face, then a lacy, reticular body rash | Aplastic crisis in sickle cell disease |
| Varicella | VZV | Itchy lesions in different stages at the same time | Bacterial superinfection; later zoster |
| Hand-foot-mouth | Coxsackievirus A16 | Oral ulcers plus vesicles on palms, soles, and buttocks; summer and fall | Self-limited |
Toxin-mediated and reactive eruptions
| Feature | SSSS | TEN |
|---|---|---|
| Typical patient | Infants and young children | Adults exposed to a culprit drug |
| Cause | Staph exfoliative toxin cleaving desmoglein-1 | Drug-induced epidermal necrosis |
| Level of split | Superficial granular layer | Full thickness, at the dermoepidermal junction |
| Mucosa | Spared | Involved |
| Clues | Fever, perioral crusting, radial fissures; Nikolsky positive | Painful skin, mucosal erosions; Nikolsky positive |
| Management | Antistaphylococcal antibiotic (nafcillin) and supportive care; good prognosis | Stop the drug, burn-unit care; high mortality |
Erythema nodosum. A hypersensitivity panniculitis producing tender red nodules on the anterior shins that fade like a bruise without ulcerating or scarring. Associations: streptococcal infection, sarcoidosis, inflammatory bowel disease, tuberculosis, coccidioidomycosis and other fungal infections, and drugs (including estrogen-containing contraceptives). Treat with NSAIDs and address the underlying cause.
Erythema nodosum with bilateral hilar adenopathy, fever, and arthritis is Löfgren syndrome, an acute and usually self-limited presentation of sarcoidosis.
Pityriasis rosea. A single scaly herald patch appears first; days later, a generalized eruption of oval scaly plaques follows skin cleavage lines on the trunk in a Christmas-tree pattern. It may be linked to HHV-6/7 and resolves on its own in 6–8 weeks.
Pityriasis rosea spares the palms and soles. A similar rash involving the palms and soles needs an RPR to exclude secondary syphilis.
Id reaction (autoeczematization). A disseminated, pruritic eczematous eruption distant from a primary infection, classically vesicles on the hands during tinea pedis. It is a hypersensitivity response that clears once the primary infection is treated.