What is a heat rash, also known as prickly heat or miliaria, and how does it affect individuals, particularly young children, older adults, and people with medical conditions such as obesity or heart disease?

What is a Heat Rash (Miliaria)?

Heat rash, medically termed miliaria or prickly heat, is a benign skin condition caused by blockage of sweat ducts leading to sweat retention beneath the skin surface, typically triggered by heat, humidity, or fever. 1, 2

Pathophysiology

• Miliaria develops when eccrine sweat ducts become obstructed, preventing sweat from reaching the skin surface and causing it to accumulate at different levels within the skin. 1, 3
• The level of obstruction determines the clinical presentation and classification of miliaria. 2
• Heat, humidity, and hyperthermia are the primary causative factors that lead to excessive sweating and subsequent duct blockage. 1, 2

Clinical Types Based on Obstruction Level

Miliaria Crystallina

• Obstruction occurs in the stratum corneum (most superficial layer), resulting in fragile, clear, fluid-filled vesicles that resemble water droplets on the skin surface. 1, 2
• These vesicles are typically 1-3 mm in size and appear on otherwise healthy-looking skin. 3, 2
• This is the most superficial and mildest form of miliaria. 2

Miliaria Rubra (Prickly Heat)

• Obstruction occurs within the malpighian stratum (deeper in the epidermis), causing red, itchy papules or vesicles. 2, 4
• This form is more symptomatic than miliaria crystallina and can cause significant pruritus. 4

Miliaria Profunda

• Obstruction occurs at or below the dermoepidermal junction (deepest level), representing the most severe form. 2

Clinical Presentation

• The characteristic rash appears as groups of small (1-3 mm) sweat-filled vesicles, with appearance varying based on the type of miliaria. 3, 2
• Lesions typically develop in areas of skin occlusion, heat exposure, or where sweat accumulates (back, shoulders, trunk). 2, 4
• The condition is more common in infants and children but can occur in adults, particularly during febrile illnesses or in hot, humid environments. 1, 2

Key Distinguishing Features

• Unlike urticarial rashes, miliaria lesions are not raised wheals but rather vesicles that are firm to palpation and do not migrate to different locations. 5, 2
• Individual lesions do not blanch and resolve with desquamation rather than disappearing without trace. 2
• The rash does not typically involve the palms and soles. 6

Natural Course and Management

• Miliaria is self-limited and resolves spontaneously within hours to days once the triggering factors are removed. 1, 2
• Resolution is promoted by keeping affected areas cool, well-ventilated, and dry. 1
• After regression, characteristic desquamation (peeling) occurs, particularly after resolution of fever. 2

Special Populations at Risk

• Young children and infants (most common age group). 1
• Hospitalized febrile patients, particularly those with prolonged fever or in intensive care settings. 1, 2
• Individuals exposed to hot, humid climates or thermal stress. 2, 4
• Patients with conditions causing prolonged sweating or skin occlusion. 3, 4

Clinical Pitfalls to Avoid

• Do not confuse miliaria with cholinergic urticaria—while both are triggered by sweating stimuli, cholinergic urticaria presents with itchy wheals that last less than 1 hour and migrate, whereas miliaria presents with fixed vesicles. 7, 5
• In patients with atopic dermatitis, sweat can cause itch through multiple mechanisms including miliaria development, but also through sweat leakage into the dermis and altered sweat composition. 8
• Extensive bullous forms of miliaria can occur rarely in adults and should not be mistaken for other bullous disorders. 3