heat rash / miliaria rubra

see also:

• acute rashes in adults
• urticaria

• an annoying “prickly” rash mainly involving flexures wear sweat tends to collect in response to prolonged hot humid conditions
• common in those travelling to tropical areas when it mainly affects areas covered by clothing
• may affect most of the body if overheating occurs due to excessively warm electric blanket left on all night
• occurs when sweat is trapped in the skin
• it is NOT contagious

• prolonged heat and sweating:
  • tropical hot humid conditions especially if not acclimatised
  • intense exercise / activity
  • excessive clothing
  • occlusion of the skin with non-porous dressings or synthetic clothing against the skin
  • excessively warm prolonged electric blankets
    • in particular, do not use electric heated throw rugs to lie on while sleeping if not designed for this as they tend to be too hot for that purpose
  • febrile conditions
  • babies are particularly at risk as they cannot control their temperature as well as adults and children can
  • hospitalised or bedridden patients lying on waterproofed mattresses or mattress-protectors
• may also be caused by:
  • radiotherapy
  • drug-induced hyperhidrosis
  • adverse reaction to medication such as induction chemotherapy
  • Stevens-Johnson syndrome (SJS) and Toxic Epidermal Necrolysis (TEN)
  • genetic disease such as Morvan syndrome and pseudohypoaldosteronism type I

• similar to cholinergic urticaria but generally lasts 1-3 days and resolves when hot humid conditions are avoided
• itchy, red, 2–4 mm, non-follicular papules and papulovesicles, often with background erythema
• in adults, often affects the upper trunk, scalp, neck and flexures, particularly in areas of friction with clothing.
  • if due to an excessively warm bed, rash is likely to be widespread on trunk, inner aspect of upper arms and thighs in particular
• in children, involves the trunk and the skin folds of the neck, axilla or groin.
• the inflamed bumps of miliaria rubra may fill with pus and this form is called miliaria pustulosa
• less commonly the dermis is involved resulting in miliaria profunda with flesh-colored 1-3mm firm, asymptomatic deep papules / bumps that look like goose bumps and may break open

• dehydration
• impaired thermoregulation
• hyperhidrosis in not affected areas
• secondary Staph infection

• tinea cruris
• bacterial folliculitis
• acne
• acute generalised exanthematous pustulosis (AGEP)
  • a severe cutaneous adverse reaction (SCAR) to a prescribed drug
  • incidence of 3–5 cases per million population per year
  • onset of is usually within 2 days of exposure to the responsible medication
  • starts on the face or in the armpits and groin and then becomes more widespread
  • persists for one to two weeks and then the skin peels off
  • aka toxic pustuloderma
• toxic erythema of newborn
• Grover disease (transient acantholytic dermatosis)
  • a common acquired itchy truncal rash characterised by acantholysis
  • most often affects Caucasian men over 50 years of age with sundamaged skin
  • risk factors include sun-exposure, sweating, fever, malignancy, and being hospitalised or bedridden.
  • a Grover-like rash has been reported during the febrile phase of COVID-19.
  • increasingly reported with the use of BRAF-inhibitors
  • often starts quite suddenly and may be more common in winter
  • most common sites affected are central back, mid chest, and upper arms and is usually very itchy
  • duration is variable although spontaneous self-resolution in 2-4 weeks is typical
  • it is often relapsing and seasonal
  • skin biopsy may be needed for diagnosis

• avoid hot conditions
• wear loose clothing
• change sweaty clothes often
• use lightweight bedding
• cool damp cloths or cool baths helps short-term itching but excessive showering or bathing should be avoided as this can reduce the natural oils that protect the skin and may make it worse
• keep the skin dry
• avoid fabrics which irritate your skin, like wool or scratchy fabrics.
• drink plenty of water to avoid dehydration
• consider ice packs
• don't scratch but just pad it if you have to
• avoid perfumed shower gels or creams
• consider calamine lotion
• consider antihistamines