Heat rash (prickly heat)

Heat rash, also called prickly heat or miliaria, is a self-limiting skin condition caused by blockage of sweat ducts, leading to sweat leakage into the epidermis or dermis and subsequent inflammation. It typically occurs in hot, humid environments or in situations of excessive sweating.

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Types and Pathophysiology

The type depends on the level at which the sweat duct obstruction occurs:

1. Miliaria crystallina

   • Blockage in the stratum corneum

   • Presents as superficial clear vesicles without inflammation

2. Miliaria rubra (most common “prickly heat”)

   • Blockage in the deeper epidermis

   • Presents as red, itchy, prickling papules or papulovesicles

3. Miliaria pustulosa

   • Secondary bacterial infection of miliaria rubra lesions, resulting in pustules

4. Miliaria profunda

   • Blockage in the dermo-epidermal junction

   • Presents as firm, flesh-coloured papules and usually occurs in individuals repeatedly exposed to heat

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Risk Factors

• Hot, humid weather

• Excessive physical activity or overheating

• Occlusive clothing or dressings

• Fever (especially in infants)

• Prolonged bed rest

• Use of thick creams or ointments that block sweat ducts

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Clinical Features

Miliaria rubra (prickly heat)

• Multiple small (1–3 mm) red papules or papulovesicles

• Itchy or prickling sensation

• Commonly affects areas where sweat accumulates: neck, upper chest, back, abdomen, groin, armpits, under breasts

Miliaria crystallina

• Tiny, clear superficial vesicles without redness or itching

• Often seen in infants with fever

Miliaria pustulosa

• Lesions contain pus; usually due to Staphylococcus aureus superinfection

Miliaria profunda

• Firm, skin-coloured papules; may reduce sweating and cause heat intolerance

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Diagnosis

• Clinical diagnosis based on history and appearance

• No laboratory tests required unless infection suspected (then swab for bacterial culture)

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Management

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General Measures (all types)

• Move to a cooler, less humid environment

• Wear light, loose-fitting, breathable clothing

• Avoid excessive sweating

• Keep affected skin clean and dry

• Use fans or air conditioning to reduce heat exposure

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Topical Treatments

Miliaria rubra

• Calamine lotion: Apply 2–4 times daily to relieve itching

• Menthol-containing cooling lotions: Soothing and antipruritic

• Low-potency topical corticosteroid (e.g., hydrocortisone 1% cream): Apply thinly 1–2 times daily for up to 1 week for significant inflammation and itching

• Topical antibacterial (e.g., fusidic acid 2% cream 3 times daily for 5–7 days) if mild secondary infection suspected

Miliaria pustulosa

• Topical antibiotic as above or oral antibiotic if extensive infection

  • Flucloxacillin: Adults 500 mg orally every 6 hours for 5–7 days

  • Penicillin allergy: Clarithromycin 500 mg orally every 12 hours for 5–7 days

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Systemic Treatments

• Oral antihistamines for itch relief:

  • Cetirizine: Adults 10 mg once daily; children per weight-based dosing

  • Loratadine: Adults 10 mg once daily; children per weight-based dosing

• Oral antibiotics only if secondary bacterial infection is widespread or severe

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Prevention

• Avoid overheating and excessive sweating

• Use lightweight, breathable fabrics

• Take cool showers and allow skin to air-dry

• Avoid heavy creams/ointments in hot weather

• Keep indoor humidity moderate to low

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Prognosis

• Usually resolves within 1–2 days after removal from heat exposure

• Recurrent exposure to heat without preventive measures can cause repeated episodes

• Miliaria profunda may persist longer and is prone to recurrence in susceptible individuals