Miliaria, or eccrine miliaria, is a frequently seen skin disease triggered by blocked eccrine sweat glands and ducts, causing backflow of eccrine sweat into the dermis or epidermis. This backflow results in a rash comprising sweat-filled vesicle formation under the skin. Other names for it include “heat rash,” “prickly heat,” or a “sweat rash.” It is most common in warm, humid climates during the summer months. The 3 main types of miliaria are crystallina, rubra, and profunda and are classified by the depth of obstruction of the sweat duct causing clinical and histological differences. The rash is usually self-limited and resolves independently of treatment.
Though miliaria affects all age groups and both genders equally, infants and children are at a higher risk due to eccrine duct immaturity. Sweating is the most common risk factor for miliaria. Therefore, hot or humid conditions and high fevers are associated with miliaria. The following have been identified as causes of miliaria:
Miliaria frequently affects neonates and those individuals suffering from increased sweating and living in hot and humid climates. Though seen in all races and both genders, each type of miliaria affects a slightly different population.
Miliaria crystallina, or sudamina, commonly affects neonates with the greatest incidence at 2 weeks of age or less. It affects between 4.5% to 9% of neonates. It can also be seen in adults who have recently relocated to a warmer climate.
Miliaria rubra, the most common form of miliaria, is frequently seen in neonates between 1 and 3 weeks of age and can affect up to 30% of adults living in hot and humid conditions.
Miliaria profunda is the rarest form of miliaria and is most commonly seen in people who have recurrent episodes of miliaria rubra or individuals exposed to new warm climates such as military persons deployed in tropical climates.
The main cause of miliaria is obstruction of the eccrine sweat duct. This can be due to cutaneous debris or bacteria like Staphylococcus epidermidis which form biofilms. This leads to leakage of sweat back into the epidermis or dermis, resulting in cellular overhydration and swelling which further occludes the duct. More profound involvement of the eccrine gland or duct may lead to rupture. The different types of miliaria have different cutaneous involvement, with miliaria crystallina occurring due to ductal occlusion of the stratum corneum, miliaria rubra occurring due to ductal occlusion in the epidermis at the subcorneal layers, and miliaria profunda resulting from a ductal occlusion in the dermal-epidermal junction, specifically the papillary dermis.
The histology of miliaria is different based on the type, as each different type is classified by the depth of obstruction of the eccrine duct. Miliaria crystallina usually displays subcorneal or intracorneal vesicles of the intraepidermal part of the duct that may contain neutrophils. Miliaria rubra displays epidermal spongiosis with parakeratosis and vesicles in the epidermis communicating with the eccrine duct. There may be inflammatory lymphocytic infiltrate surrounding the duct and superficial vasculature. Miliaria profunda displays intradermal spongiosis of the eccrine duct, and is comparable to miliaria rubra, though shows rupture of the eccrine duct and more significant lymphocytic inflammation. It is PAS-positive, diastase-resistant.
Miliaria is a disorder involving vesicles, papules, and pustules of the skin. It is crucial to take a closer look at the skin and location of the rash in to characterize the rash and arrive at an accurate diagnosis.
Miliaria crystallina shows up as 1- to 2-mm superficial vesicles, affecting both adults and neonates usually younger than 2 weeks old. Because the pathophysiology involves the stratum corneum or the most superficial layer of the outer layer of the skin, the vesicles have a thin superficial layer that results in the vesicles resembling water droplets on the skin that easily rupture. The vesicles are superficial; therefore, an inflammatory response is typically absent. The upper trunk, neck, and head are the most commonly affected sites. The blisters will usually appear within a few days of exposure to risk factors and will resolve within a day after the superficial layer of skin rubs off.
Miliaria rubra is the most prevalent form of miliaria, and due to obstruction of eccrine ducts in the deeper layers of the skin, an inflammatory response is witnessed. This results in larger, erythematous papules and vesicles. One of the critical clinical diagnostic features is the minimal follicular involvement, which helps differentiate miliaria rubra from folliculitis. If pustules are present, then miliaria rubra is called miliaria pustulosa and may indicate a bacterial infection. Because an inflammatory response is involved, patients may experience pruritic and painful symptoms, which may worsen during perspiration. In neonates usually between the ages 1 to 3 weeks, the groin, axilla, and neck are the most commonly affected areas. In adults, miliaria rubra is most likely seen in places where clothes rub on the skin such as the trunk and extremities. The face is usually spared. Superinfection with staphylococci may occur, and when impetigo or multiple abscesses are involved, the condition is called periporitis staphylogenes.
Miliaria profunda, due to a deeper involvement of the skin at the dermal-epidermal junction, results in firm, large, flesh-colored papules that are also not centered around a follicle. The eruption can be asymptomatic but may also be extremely pruritic. It is usually seen in patients with numerous previous episodes of miliaria rubra. The most common location involved in adults is the trunk, but the arms and legs may also be involved. The skin rash usually appears within minutes to hours of perspiration and resolves within an hour of sweating cessation.
In both miliaria rubra and miliaria profunda, anhidrosis may occur in affected areas due to obstructed sweat glands. As such, heat exhaustion may occur due to ineffective thermoregulation in patients with largely affected areas and should be considered in the setting of a skin rash accompanied by hyperthermia.
Miliaria is a clinical diagnosis. Laboratory tests are often inconclusive and not helpful. Dermoscopy has been found to be a useful tool, particularly in people with darker skin, revealing large white globules with surrounding darker halos (white bullseye). When in doubt, a skin punch biopsy would be useful to help with diagnosis. Please refer above to the histopathology section regarding findings. High-definition optical coherence tomography has assisted in finding the obstruction of the eccrine duct.
Because hot, sweaty conditions are the main risk factors for miliaria, general measures to decrease sweating and eccrine duct blockage are warranted in the management of miliaria. This includes cooler environments, wearing breathable clothes, exfoliating the skin, removing skin occluding objects such as band-aids or patches, and treating febrile illnesses.
Specific modalities for treatment of miliaria are unique depending on the type. Miliaria crystallina is usually not treated as it is self-limited and usually resolves within 24 hours. Miliaria rubra treatment is geared towards decreasing inflammation, and therefore mild to mid-potency corticosteroids like triamcinolone 0.1% cream may be applied for one to two weeks. If miliaria pustulosa develops, topical antibiotics such as clindamycin are indicated.
Very little information exists regarding the treatment of miliaria profunda except the general measures listed above. However, results in one study showed improvement of 1 patient with miliaria profunda with the combined use of oral isotretinoin 40 mg per day for 2 months and topical anhydrous lanolin.
Most cases of miliaria resolve spontaneously after decreasing risk factors and moving to a cooler environment.
The most serious complication that may result from miliaria is anhidrosis, leading to poor thermoregulation and heat exhaustion. This may permanently disable a person from work or prevent an active person from continuing exercise or sports. Bacterial superinfections may occur due to the epidermal layer being affected.
Physicians should educate patients about the causes of miliaria, including hot, humid climates, and non-breathable clothes. If a patient is predisposed to miliaria, the clinician should create a personalized plan should for the patient that includes methods of avoiding overheating, which clothes are most appropriate, and how to react when the rash appears. Clinicians should also educate patients on the signs and symptoms of heat exhaustion if they develop anhidrosis secondary to miliaria.
Miliaria is a generally benign skin disorder that occurs in patients of all ages and genders who are exposed to humid, warm climates. It is usually self-limited and resolves spontaneously in response to a cooler, dryer atmosphere. In situations where a rash appearing to be miliaria does not resolve quickly, a consultation with a dermatologist should rule out other more serious rashes that may mimic miliaria.
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