INTRODUCTION
As the weather warms, both seasonally and longitudinally, the role of heat in eliciting or exacerbating various dermatoses becomes especially relevant. While much attention has been given to skin cancers and photodermatoses related to ultraviolet radiation, the broader dermatologic consequences of heat exposure remain underappreciated. Heat-induced dermatoses, ranging from benign but uncomfortable eruptions to severe inflammatory conditions, represent a spectrum of overlooked conditions that carry significant implications for diagnosis, management, and health equity, particularly among vulnerable populations. This review explores the diagnostic and therapeutic nuances of common dermatoses linked to heat, including miliaria, inducible urticarias, erythema ab igne, contact burns, and select heat-aggravated inflammatory conditions.
Miliaria
Miliaria is a benign condition caused by obstruction of eccrine sweat glands or ducts, often due to cutaneous debris or bacteria, resulting in sweat backflow into the epidermis or dermis.
It is characterized by the depth of ductal involvement, giving rise to three distinct clinical forms: miliaria crystallina, rubra, and profunda. Miliaria crystallina (MC) is the most superficial form, occurring within the stratum corneum, while miliaria rubra (MR) involves obstruction in the subcorneal epidermal layers, and miliaria profunda (MP) occurs at the dermal-epidermal junction.1
Neonates are often affected by miliaria as their immature sweat duct systems are prone to dysfunction, manifesting as MC and MR. Other predisposing factors include febrile states and environmental contributors such as high humidity and occlusive clothing. Diagnosis of miliaria is primarily clinical, based on the characteristic morphology and distribution of lesions. Clinical presentation mirrors the location of involvement: MC presents as superficial vesicles; MR as larger erythematous papules or vesicles; and MP as firm, fleshcolored papules. Secondary bacterial infection should be suspected when pustular changes are present, referred to as miliaria pustulosa.1
Management of miliaria emphasizes preventive measures, including maintaining a cool ambient environment, opting for breathable fabrics, gently exfoliating the skin, removing occlusive items like bandages or patches, and treating underlying fevers. Directed treatment of miliaria depends on the type. MC typically self-resolves within 24 hours, while MR can be managed with 0.1% triamcinolone cream twice daily for 1 to 2 weeks. For MP, limited data suggest improvement with oral isotretinoin 40 mg daily for 2 months combined with topical anhydrous lanolin. If miliaria pustolosa occurs, topical clindamycin 1% can be utilized.1
Cholinergic and Heat Urticaria
Inducible urticarias, particularly those triggered by heat, represent a challenging and often underrecognized subset of temperaturesensitive dermatologic conditions. Two subtypes, cholinergic urticaria (CholU) and heat urticaria (HU), occur in response to increases in temperature, though they are distinct entities.2,3 CholU is triggered by internal increases in core body temperature, such as those induced by exercise, emotional stress, or hot water exposure. Lesions are typically pinpoint papular wheals with a surrounding erythematous flare, often accompanied by intense sensations of itching or pain. HU, in contrast, is induced by direct application of heat, such as warm bath water (reported in 85% of HU cases), hot air, solar thermal energy, and hot objects like water bottles. HU manifests as larger, well-demarcated wheals typically confined to the area of contact.3,4 When sunlight is a trigger, care must be taken to distinguish HU from solar urticaria.4 HU may be accompanied by systemic symptoms in over half (53%) of patients, including headache, dizziness, abdominal cramps, and dyspnea. Mucosal involvement, angioedema, and syncope have rarely been reported.4 CholU has been associated with angioedema, respiratory symptoms, and anaphylaxis.2
The diagnostic work-up of HU includes heat provocation testing, though methods vary and often lack precise temperature thresholds, prompting the development of a novel Peltier-based energy device to standardize identification of the threshold for urticaria onset.3 In CholU, exercise or passive warming tests are employed to reproduce symptoms - the absence of symptoms during physical exertion testing supports a diagnosis of HU over CholU.2
First-line treatment for both HU and CholU consists of trigger avoidance and second-generation H1-antihistamines at up to fourfold the standard dose.2-4 However, complete symptom resolution is rare in HU with antihistamines alone (even at the highest doses). Omalizumab has shown success for refractory HU, with three patients achieving full remission in recent case reports using 300 to 450 mg biweekly or monthly.3 Additional management options for HU include indomethacin (25 mg QID), ketotifen (unspecified dose), and omalizumab ± antihistamines for CholU. Though not
Miliaria
Miliaria is a benign condition caused by obstruction of eccrine sweat glands or ducts, often due to cutaneous debris or bacteria, resulting in sweat backflow into the epidermis or dermis.
It is characterized by the depth of ductal involvement, giving rise to three distinct clinical forms: miliaria crystallina, rubra, and profunda. Miliaria crystallina (MC) is the most superficial form, occurring within the stratum corneum, while miliaria rubra (MR) involves obstruction in the subcorneal epidermal layers, and miliaria profunda (MP) occurs at the dermal-epidermal junction.1
Neonates are often affected by miliaria as their immature sweat duct systems are prone to dysfunction, manifesting as MC and MR. Other predisposing factors include febrile states and environmental contributors such as high humidity and occlusive clothing. Diagnosis of miliaria is primarily clinical, based on the characteristic morphology and distribution of lesions. Clinical presentation mirrors the location of involvement: MC presents as superficial vesicles; MR as larger erythematous papules or vesicles; and MP as firm, fleshcolored papules. Secondary bacterial infection should be suspected when pustular changes are present, referred to as miliaria pustulosa.1
Management of miliaria emphasizes preventive measures, including maintaining a cool ambient environment, opting for breathable fabrics, gently exfoliating the skin, removing occlusive items like bandages or patches, and treating underlying fevers. Directed treatment of miliaria depends on the type. MC typically self-resolves within 24 hours, while MR can be managed with 0.1% triamcinolone cream twice daily for 1 to 2 weeks. For MP, limited data suggest improvement with oral isotretinoin 40 mg daily for 2 months combined with topical anhydrous lanolin. If miliaria pustolosa occurs, topical clindamycin 1% can be utilized.1
Cholinergic and Heat Urticaria
Inducible urticarias, particularly those triggered by heat, represent a challenging and often underrecognized subset of temperaturesensitive dermatologic conditions. Two subtypes, cholinergic urticaria (CholU) and heat urticaria (HU), occur in response to increases in temperature, though they are distinct entities.2,3 CholU is triggered by internal increases in core body temperature, such as those induced by exercise, emotional stress, or hot water exposure. Lesions are typically pinpoint papular wheals with a surrounding erythematous flare, often accompanied by intense sensations of itching or pain. HU, in contrast, is induced by direct application of heat, such as warm bath water (reported in 85% of HU cases), hot air, solar thermal energy, and hot objects like water bottles. HU manifests as larger, well-demarcated wheals typically confined to the area of contact.3,4 When sunlight is a trigger, care must be taken to distinguish HU from solar urticaria.4 HU may be accompanied by systemic symptoms in over half (53%) of patients, including headache, dizziness, abdominal cramps, and dyspnea. Mucosal involvement, angioedema, and syncope have rarely been reported.4 CholU has been associated with angioedema, respiratory symptoms, and anaphylaxis.2
The diagnostic work-up of HU includes heat provocation testing, though methods vary and often lack precise temperature thresholds, prompting the development of a novel Peltier-based energy device to standardize identification of the threshold for urticaria onset.3 In CholU, exercise or passive warming tests are employed to reproduce symptoms - the absence of symptoms during physical exertion testing supports a diagnosis of HU over CholU.2
First-line treatment for both HU and CholU consists of trigger avoidance and second-generation H1-antihistamines at up to fourfold the standard dose.2-4 However, complete symptom resolution is rare in HU with antihistamines alone (even at the highest doses). Omalizumab has shown success for refractory HU, with three patients achieving full remission in recent case reports using 300 to 450 mg biweekly or monthly.3 Additional management options for HU include indomethacin (25 mg QID), ketotifen (unspecified dose), and omalizumab ± antihistamines for CholU. Though not
