Heat Rash

Inflammatory sweat retention eruption (prickly heat) in which the eccrine duct is blocked in the mid to lower epidermis, so sweat leaks into the surrounding epidermis and causes spongiotic inflammation.

• Duct obstruction follows heat and humidity, fever, exercise, occlusive clothing or dressings, bed rest, heavy bedding, thick ointments and, in infants, overwrapping and incubator use. Keratin plugs and a Staphylococcus epidermidis extracellular polysaccharide biofilm contribute to the blockage.
• It is the commonest form of miliaria. It affects up to a third of infants in hot climates, and also military recruits, athletes, bedridden patients and people newly arrived in the tropics.
• Drugs that increase sweating or provoke it can contribute: bethanechol, clonidine, isotretinoin, doxorubicin and opioids.
• The disease resolves in days once sweating is controlled. Repeated attacks can progress to deep obstruction with reduced sweating.

• Discrete, red, 1 to 4 mm papules and papulovesicles, with a prickling, stinging or itchy sensation, appearing in crops during sweating. Lesions are not centred on hair follicles.
• Sites: the trunk, neck, skin folds, waist under belts, areas under tight straps and clothing, and in infants the neck, upper chest and diaper area. The face and palms are usually spared.
• Pustules (miliaria pustulosa) form when there is secondary bacterial infection or inflammation.
• Differential diagnosis: bacterial or Malassezia folliculitis, candidiasis, contact dermatitis, viral exanthem, impetigo, scabies, drug eruption, Grover disease, atopic dermatitis and heat urticaria.
• Red flags: temperature of 40 C or more, headache, dizziness, nausea, cramps, confusion, stopped sweating or collapse (heat exhaustion or heat stroke); in an infant, lethargy, poor feeding and persistent fever.
• Complications: secondary infection, sleep disturbance, post-inflammatory pigmentation, transition to miliaria profunda with anhidrosis, and heat illness when a large body area is affected.

• Clinical, from the morphology, distribution and heat or occlusion history.
• Swab pustules for bacterial culture, and perform potassium hydroxide preparation if candidiasis or Malassezia is suspected.
• In patients with heat symptoms record core temperature, pulse, blood pressure and mental state. Check electrolytes, urea, creatinine, glucose and creatine kinase if heat exhaustion or heat stroke is possible.
• Biopsy is rarely needed and shows a spongiotic vesicle at the intraepidermal duct, with a keratin plug.

• Cooling is the treatment:
  • Move to a cool, air conditioned or well ventilated place, use fans, and reduce activity until the rash settles, usually in 2 to 5 days.
  • Wear loose, light cotton clothing. Remove tight and synthetic garments.
  • Take cool showers or apply cool compresses, and pat dry without rubbing.
  • Avoid occlusive creams, oils and plastic dressings.
  • Keep skin folds dry and separated, and drink enough fluid.
  • Reposition bedridden patients often, and use breathable mattress covers.
• Symptom relief:
  • Calamine lotion 2 to 3 times daily, or menthol 0.25 to 1% in aqueous cream, for itch.
  • Hydrocortisone 1% cream twice daily for 3 to 5 days for inflamed itchy areas. Avoid prolonged use, and avoid on the face in infants.
  • Cetirizine 10 mg orally once daily (child 2 to 5 years 2.5 mg, 6 to 11 years 5 mg) if itch disturbs sleep.
• Secondary bacterial infection: mupirocin 2% ointment 3 times daily for 5 to 7 days for localised lesions. For extensive disease give flucloxacillin 500 mg orally 6 hourly for 7 days (child 12.5 to 25 mg/kg 6 hourly), or clindamycin 300 to 450 mg orally 8 hourly in penicillin allergy.
• Candidal overgrowth: clotrimazole 1% or nystatin 100,000 units/g cream 2 to 3 times daily for 1 to 2 weeks.
• Heat exhaustion or heat stroke is an emergency. Remove clothing, spray with cool water and fan, place ice packs on the neck, axillae and groin, give oral fluids if alert, and start intravenous 0.9% sodium chloride 1 to 2 L over 1 to 2 hours adjusted to response. Aim for a core temperature below 39 C within 30 minutes, and arrange emergency transfer.
• Review drugs that provoke sweating or impair sweat flow with the prescriber.
• Prevention: acclimatise to heat over 1 to 2 weeks, dress infants lightly, avoid overwrapping, keep bedroom temperature comfortable and shower after exercise.
• Special groups:
  • Infants: lukewarm baths, light cotton clothing, no ointments, and hydrocortisone 0.5 to 1% only for 3 to 5 days. Fever with rash and poor feeding needs medical assessment.
  • Pregnancy: cooling, calamine and brief hydrocortisone; use cefalexin or flucloxacillin for infection.
  • Elderly and bedridden: inspect folds and pressure areas, and keep skin dry.
  • Diabetes and obesity: treat secondary infection early and optimise glucose control.
• Follow up: review if there is no improvement in 1 week, or if infection, sweat failure or systemic symptoms appear. Refer to dermatology for recurrent or persistent disease.