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Heat rash

OVERVIEW

What is prickly heat?

Prickly heat, also known as miliaria, sweat rash, or heat rash, is a common transient skin condition that occurs in summer or hot environments[1].

Due to high temperatures and humidity, excessive sweating, poor sweat drainage, and blockage of sweat gland ducts by keratin, sweat leaks into and irritates surrounding tissues, leading to papules, vesicles, or small blisters around sweat pores[1].

It can affect both adults and children but is more common in children, especially infants. Generally, keeping the environment cool and the skin clean and dry will allow the rash to resolve on its own[2].

For itching or more severe symptoms, cool or cold compresses or calamine lotion can be applied to relieve discomfort. If fever occurs (rectal temperature above 37.5°C in children) or skin infection develops (with symptoms like localized heat, redness, or pain), medical attention should be sought[3].

SYMPTOMS

What are the common manifestations of prickly heat?

There are three main types of prickly heat, including miliaria crystallina, miliaria rubra, and miliaria profunda.

  1. Miliaria crystallina, also known as white prickly heat

    • Caused by superficial blockage of sweat ducts, it is characterized by superficial, transparent 1–2 mm blisters resembling water droplets, which are easily ruptured upon light rubbing. The blisters may merge and do not cause significant inflammation, so there is no surrounding erythema.

    • The blisters often appear in clusters, most commonly on the head, neck, and upper torso of newborns, as well as the torso of adults. The affected areas are usually not itchy.

    • In hot and humid environments, the blisters may appear in batches over days to weeks and resolve within hours to days (typically 1–2 days), leaving fine scales after drying[1,2].

  2. Miliaria rubra, also known as red prickly heat

    • This is the most common type, usually occurring within the epidermis, where sweat leakage causes inflammation around the sweat ducts. It is characterized by 2–4 mm red papules (non-follicular), which may also present as papulovesicles or pustules, often with erythema at the base.

    • In infants, it most commonly occurs in skin folds such as the neck, armpits, or groin; in adults, it is often found in areas of clothing friction: the upper torso (especially the back), scalp, neck, and skin folds. The face, palms, and soles are generally unaffected.

    • Miliaria rubra is usually accompanied by a burning or stinging itch that worsens with sweating. It may become infected, leading to impetigo or multiple abscesses.

    • Additionally, miliaria rubra has a subtype: miliaria pustulosa (pustular prickly heat) [2]. It presents with small pustules the size of pinheads at the top of the rash. When pustules appear, bacteria may be present. This is common in flexural areas (e.g., the inner side of limbs, such as the palm side of the arms), the perineum, and the head of children[2,4].

  3. Miliaria profunda, also known as deep prickly heat

    • It usually occurs after repeated episodes of miliaria rubra and is caused by deeper blockage of sweat ducts (at the dermal-epidermal junction). Sweat leakage into surrounding tissues leads to firm, red to skin-colored papules 1–4 mm in diameter.

    • Miliaria profunda is rare and most commonly seen in adult males, particularly military personnel stationed in tropical climates. It often appears on the torso but may also affect the limbs. Since sweat is trapped deep in the skin, the affected areas rarely sweat, a condition also known as tropical anhidrosis.

    • The rash is generally asymptomatic or mildly symptomatic, with symptoms becoming more noticeable during sweating. Reduced or absent sweating across the skin may occur, usually without itching. In cases of widespread skin involvement, systemic symptoms such as headache and fever (oral temperature above 37.2 °C in adults) may appear[1,2].

CAUSES

What causes heat rash?

  1. Common causes include: hot and humid environments (most common), intense physical activity (such as long-distance running, playing basketball, etc.), febrile illnesses (such as neutropenic fever), and skin occlusion (such as wearing non-breathable clothing, bandaging, etc.).

    Due to high temperatures and humidity, excessive sweating combined with the failure of sweat to evaporate promptly leads to sweat retention in the sweat gland ducts. This causes excessive hydration of keratinocytes in the skin, resulting in their swelling and blockage of the sweat gland ducts[2,4].

  2. Additionally, the occurrence of heat rash may also be influenced by other factors[2,4]:

    • Medications: Such as clonidine and atropine, which affect the sympathetic nervous system and inhibit sweating, leading to heat rash.

    • Skin conditions: Such as staphylococcal scalded skin syndrome, which damages the epidermal structure and may also trigger heat rash.

    • Diseases that increase sweat salt content: Such as hypernatremia, where excessive salt directly dehydrates keratinocytes, promoting heat rash.

    • Ultraviolet or ionizing radiation: Radiation therapy or UV exposure can damage sweat gland ducts, causing heat rash.

    • Skin microbiota: For example, overgrowth of Staphylococcus bacteria produces extracellular polysaccharides that facilitate heat rash formation.

    • Skin maceration: Such as prolonged exposure to water (e.g., wading), which may also lead to heat rash.

    • Genetic disorders: Such as Morvan syndrome, characterized by excessive sweating, which can cause heat rash.

Who is most prone to heat rash?

Both adults and children can develop heat rash, but it is more common in children, especially infants. This may be due to underdeveloped or obstructed eccrine ducts in newborns[2]. Additionally, different types of heat rash are more prevalent in specific groups.

  1. Miliaria crystallina: Most common in infants, peaking at 1 week of age, and also seen in individuals with high fever (oral temperature 39.1°C–41.0°C), prolonged bed rest, debilitation, or excessive sweating[2,4].

  2. Miliaria rubra: More common in young children, homemakers, and those working in high-temperature environments[1].

  3. Miliaria profunda: Typically seen in patients with recurrent severe miliaria rubra, most frequently in adult males, particularly military personnel stationed in tropical climates[2,4].

Is heat rash contagious?

No. There is no evidence to suggest that heat rash is contagious.

Is heat rash hereditary?

No. There is no evidence to suggest that heat rash is inherited through genes or chromosomes.

DIAGNOSIS

How is Miliaria Diagnosed?

When diagnosing miliaria, doctors primarily consider whether it is the hot season or if there is a recent history of sweating, along with evaluating the patient's clinical symptoms. Specific details are as follows[2]:

  1. For miliaria crystallina, physical examination should reveal multiple superficial, transparent vesicles without inflammation.

  2. For miliaria rubra, non-follicular red papules, papulovesicles, or pustules are present.

  3. For miliaria profunda, firm red to skin-colored papules are observed, which should also be non-follicular.

In severe or recurrent cases, a biopsy may be required to confirm the diagnosis.

What Diseases Can Miliaria Be Confused With? How to Differentiate Them?

Miliaria can be confused with summer dermatitis, folliculitis, eczema, and acne. While all these conditions may present with papules, they can generally be distinguished by their clinical features, as follows:

  1. Miliaria: Lesions commonly appear on the head, face, and skin folds (e.g., armpits, groin), primarily in children, especially infants. It typically manifests as dense, pinhead-sized papules, papulovesicles, or small pustules and often improves when the weather cools[2,4].

  2. Summer dermatitis: Lesions mainly occur on the trunk and limbs, primarily in adults, presenting as large erythematous patches with papules or papulovesicles, accompanied by intense itching[4].

  3. Folliculitis: Lesions are usually localized to hair follicles on the face, neck, buttocks, or genitals, presenting as purulent inflammation confined to follicular openings[1].

  4. Eczema: Lesions often appear on the face, ears, hands, feet, lower legs, genitals, or anus, characterized by symmetrical erythema, papules, and papulovesicles. Acute cases may show exudation with severe itching, while chronic cases feature lichenification, with symptoms fluctuating and recurring[1].

  5. Acne: Lesions predominantly affect the cheeks and forehead, usually symmetrically distributed.

    • Most common in adolescents aged 15–30, presenting as comedones, papules, pustules, nodules, cysts, or scars at follicular openings. It is not strongly related to temperature changes[1].

    • Infantile acne (before age 3) is rare, more common in boys, appearing as acne-like lesions (comedones, papules, pustules) around 3–4 months of age. It usually resolves spontaneously by 6–12 months due to declining androgen levels[5,6].

Definitive diagnosis should be made by a specialist. Self-medication should be avoided to prevent delaying proper treatment.

TREATMENT

Which department should I visit for prickly heat?

Dermatology or pediatric dermatology.

Can prickly heat heal on its own?

Yes. It usually subsides quickly when the weather cools down[1]. Different types of prickly heat develop differently[2]:

  1. Miliaria crystallina: Can resolve on its own, usually without noticeable symptoms.
  2. Miliaria rubra: May improve after removing the cause (e.g., removing non-breathable clothing) but can recur. Repeated episodes may lead to miliaria profunda.
  3. Miliaria profunda: May resolve without treatment but can persistently damage sweat glands, leading to extensive anhidrosis, compensatory facial sweating, and heat-related complications (e.g., headaches, dizziness).

How to eliminate prickly heat?

Treatment mainly involves addressing factors that may irritate or worsen prickly heat.

  1. General measures: Focus on reducing sweating and sweat gland blockage. Move to a cooler environment, wear loose, breathable clothing, dry sweat with a towel, bathe with lukewarm water, and gently cleanse with soap to help eliminate prickly heat[2,4].
  2. Topical cooling powders: Such as 1% menthol calamine lotion[4], which has astringent and protective effects. No adverse effects have been reported, and skin peeling is normal and not a cause for concern. Although available over the counter, it is best used under medical guidance.
  3. Other adjunctive medications: Mainly help relieve symptoms and speed up the resolution of miliaria rubra or profunda. Include topical corticosteroids (e.g., 0.05% desonide cream, 0.1% hydrocortisone butyrate cream) and topical antibiotics (e.g., erythromycin ointment, mupirocin ointment)[2].

What medications can be used for prickly heat?

Medications are only adjunctive and not always necessary[2]. For miliaria rubra or profunda, adjunctive treatments may reduce inflammation and minimize bacterial contributions.

  1. Topical corticosteroids: For severe itching or stinging, low- to mid-potency corticosteroid creams or lotions (e.g., 0.05% desonide cream, 0.1% hydrocortisone butyrate cream) can be applied twice daily for 1–2 weeks to relieve itching and speed up inflammation resolution. Long-term use may cause skin pigmentation or secondary infections.
  2. Topical antibiotics: Reduce inflammation and bacterial load in papules or pustules, particularly beneficial for pustular miliaria. Options include erythromycin ointment and mupirocin ointment. Side effects are rare but may include skin irritation (e.g., burning).
  3. Oral antibiotics are currently not recommended.

Is hospitalization needed for prickly heat?

Generally not. Seek medical attention if fever or skin infection (e.g., localized warmth, redness, pain) occurs[2].

DIET & LIFESTYLE

What should I pay attention to in my diet if I have prickly heat?

Try to eat less spicy food (such as chili peppers, garlic, leeks, etc.), as it can worsen sweating.

What should I pay attention to in daily life if I have prickly heat?

  1. Try to stay in a cool environment;
  2. Wear loose, breathable cotton clothing;
  3. Wipe off sweat promptly to keep the skin clean and dry. For example, after sweating from exercise, use a dry towel to dry off immediately;
  4. Use cooler water when bathing, and gently scrub with soap or a towel to help remove debris that may clog sweat ducts;
  5. Avoid scratching the prickly heat to prevent infection. If the itching is unbearable, apply a cold (or ice) compress to the affected area, or apply calamine lotion with 1% menthol multiple times to unbroken skin[2,4];

Can prickly heat powder be used?

It is generally not recommended. Prickly heat powder can clump when exposed to sweat, which may further block sweat ducts.

PREVENTION

How to prevent heat rash?

Heat rash can be prevented. To address the cause, simply lower the temperature and keep the skin dry. Specific measures include:

  1. Improve indoor ventilation and cooling to reduce sweating and promote sweat evaporation;
  2. Wear loose, cotton, or breathable clothing to facilitate sweat evaporation, and change damp clothes promptly;
  3. After sweating from exercise, wipe the skin dry immediately to maintain dryness[4].

How to prevent heat rash recurrence?

The same preventive measures apply. The most important steps are to reduce sweating (e.g., staying in cool environments, avoiding spicy foods like chili and garlic) and keep the skin dry (e.g., wiping sweat with a dry towel promptly, wearing loose, breathable clothing)[2].