Miliaria

Etiology

Although miliaria affects all age groups, ethnicities, and both genders equally, infants and children are at higher risk due to the immaturity of their eccrine ducts.[3] Sweating is the most common risk factor, making hot or humid conditions and high fevers particularly associated with the development of miliaria.

Other causes of miliaria include:

• Occlusion of the skin: Transdermal drug patches and tight clothing have been associated with miliaria.[4][5]
• Type I pseudohypoaldosteronism: This condition involves mineralocorticoid resistance, leading to sodium loss through eccrine glands, and has been associated with pustular miliaria rubra.[6][7][8]
• Strenuous physical activity: Intense exercise can contribute to the development of miliaria.
• Morvan syndrome: A rare autosomal recessive disorder characterized by hyperhidrosis, among other abnormalities, which predisposes individuals to miliaria.[9][10]
• Medications: Drugs that induce sweating, such as bethanechol, clonidine, and neostigmine, have been associated with miliaria.[11] Additionally, a few cases of isotretinoin-induced miliaria have been reported.[12]

Epidemiology

Miliaria frequently affects neonates and adults with increased sweating, as well as individuals living in hot and humid climates. While miliaria can occur in both genders and across all ethnicities, each type may be more common in specific populations.

• Miliaria crystalline: This condition is also known as sudamina, and it commonly affects neonates, with the highest incidence observed at 2 weeks of age or younger.[13] The rash affects approximately 4.5% to 9% of neonates.[14][15] However, it can also occur in adults who have recently relocated to a warmer climate.[16] 

• Miliaria rubra: This is the most common form of miliaria and is frequently observed in neonates between 1 and 3 weeks of age. This condition can also affect up to 30% of adults living in hot and humid conditions.[3] 

• Miliaria profunda: This is the rarest form of miliaria, most commonly seen in individuals with recurrent episodes of miliaria rubra or individuals exposed to new warm climates, such as military personnel deployed in tropical climates.[17]

Pathophysiology

The main cause of miliaria is the obstruction of eccrine sweat glands or ducts. This obstruction can result from cutaneous debris or bacteria, such as S epidermidis, which forms biofilms.[18][19] The obstruction leads to sweat leakage into the epidermis or dermis, causing cellular overhydration, swelling, and further occlusion of the ducts. In more severe cases, deeper involvement of the eccrine glands or ducts may lead to their rupture.

The different types of miliaria involve varying depths of cutaneous obstruction. Miliaria crystallina occurs with ductal occlusion in the stratum corneum; miliaria rubra involves ductal occlusion in the epidermis at the subcorneal layers; and miliaria profunda results from ductal occlusion at the dermal-epidermal junction, specifically within the papillary dermis.

Histopathology

The histology of miliaria varies by type, as each is classified by the depth of obstruction in the eccrine duct.

• In miliaria crystallina, histology reveals subcorneal or intracorneal vesicles originating from the intraepidermal portion of the duct, which may also contain neutrophils.

• Miliaria rubra is characterized by epidermal spongiosis with parakeratosis and vesicles within the epidermis that communicate with the eccrine duct. This condition may also be associated with an inflammatory lymphocytic infiltrate surrounding the duct and superficial vasculature.

• Miliaria profunda involves intradermal spongiosis of the eccrine duct and is similar to miliaria rubra. However, miliaria profunda differs from miliaria rubra by involving more extensive rupture of the eccrine ducts and more pronounced lymphocytic inflammation. This also shows periodic acid-Schiff positivity and is diastase-resistant under microscopy. 

History and Physical

A comprehensive medical history should be obtained, as it is essential for accurately diagnosing and effectively managing miliaria. Clinicians should begin by inquiring about the onset, duration, and progression of skin symptoms, including the appearance and location of vesicles, papules, or pustules. Notably, it is important to inquire about recent environmental changes, such as relocation to a hotter or more humid climate or increased sweating due to physical activity or illness. The interprofessional healthcare team should also gather information on any recent exposure to irritants or new skincare products, review the patient's medical history for conditions that might affect sweat gland function, and check for any previous episodes of miliaria. Understanding these factors is crucial for distinguishing miliaria from other skin conditions and guiding appropriate treatment.

Clinical Examination of Miliaria

Miliaria is characterized by the presence of vesicles, papules, and pustules on the skin. A thorough examination of the skin, focusing on the characteristics and location of the rash, is essential for an accurate diagnosis.

Miliaria crystalline: This condition appears as 1 to 2 mm superficial vesicles, affecting both adults and neonates, usually younger than 2 weeks old.[20] The pathophysiology involves the most superficial layer of the epidermis, the stratum corneum, resulting in vesicles with a thin superficial layer that resembles water droplets on the skin and easily ruptures.[11][21] As these vesicles are so superficial, an inflammatory response is typically absent. The most commonly affected areas are the upper trunk, neck, and head. The rash usually appears within a few days of exposure to risk factors and resolves within a day after the superficial layer of skin rubs off.[22]

Miliaria rubra: This condition is the most prevalent form of miliaria and is characterized by the obstruction of eccrine ducts in the deeper layers of the skin, which triggers an inflammatory response. This leads to the formation of larger, erythematous papules and vesicles. A key clinical feature that helps differentiate miliaria rubra from folliculitis is the minimal involvement of hair follicles. When pustules are present, miliaria rubra is referred to as miliaria pustulosa, which may indicate a bacterial infection. Due to the inflammatory response, patients may experience pruritic and painful symptoms that can worsen during perspiration, leading to increased irritation. In neonates, typically between 1 and 3 weeks old, the groin, axilla, and neck are the most commonly affected areas. In adults, miliaria rubra is often seen in areas where clothing rubs against the skin, such as the trunk and extremities, while the face is usually spared. Superinfection with staphylococci can occur, and when associated with impetigo or multiple abscesses, the condition is known as periporitis staphylogenes.[23] 

Miliaria profunda: Miliaria profunda results in large, firm, flesh-colored papules that are not centered around hair follicles due to their deeper involvement at the dermal-epidermal junction. The eruption can range from extremely pruritic to asymptomatic. This condition is often observed in patients with a history of multiple episodes of miliaria rubra. The rash typically affects the trunk in adults, but it may also involve the arms and legs. The rash usually appears within minutes to hours of sweating and resolves within an hour after sweating ceases.

In both miliaria rubra and miliaria profunda, anhidrosis may occur in the affected areas due to obstructed sweat glands. Consequently, heat exhaustion can result from ineffective thermoregulation in patients with extensive involvement, and this should be considered when a skin rash is accompanied by hyperthermia.

Evaluation

Miliaria is primarily diagnosed clinically, as laboratory tests are often inconclusive and generally not useful. Dermoscopy is a valuable tool, particularly for individuals with darker skin, as it can reveal large white globules with surrounding darker halos (white bullseye).[24] If the diagnosis remains uncertain, a skin punch biopsy may be helpful. High-definition optical coherence tomography can also assist in identifying the obstruction of the eccrine duct.[25]

Treatment / Management

As hot and sweaty conditions are primary risk factors for miliaria, managing the condition involves implementing general measures to reduce sweating and prevent eccrine duct blockage. This includes creating cooler environments, wearing breathable clothing, exfoliating the skin, removing skin-occluding objects such as bandages or patches, and addressing febrile illnesses.

Specific modalities for the treatment of miliaria are unique depending on the type. Miliaria crystallina usually does not require treatment, as it is self-limiting and typically resolves within 24 hours. Miliaria rubra is managed by reducing inflammation; thus, mild-to-mid-potency corticosteroids, such as 0.1% triamcinolone cream, may be applied to the affected areas for 1 to 2 weeks. If miliaria pustulosa develops, topical antibiotics such as clindamycin are used to address the superimposed bacterial infection.

Limited information exists on the treatment of miliaria profunda beyond the general measures previously mentioned. However, studies have indicated that miliaria profunda may improve with a combined regimen of oral isotretinoin (40 mg/d for 2 months) and topical anhydrous lanolin.[26](B3)

Differential Diagnosis

Miliaria clinically resembles and must be differentiated from the following:

• Viral exanthems or viral infections, such as herpes simplex or varicella
• Cutaneous candidiasis or other fungal skin infections
• Folliculitis, whether bacterial or pityrosporum
• Neonatal acne or erythema toxicum neonatorum
• Drug rashes, particularly acute, generalized, exanthematous pustulosis
• Grover disease
• Arthropod bites
• Lymphocytoma cutis or cutaneous T-cell pseudolymphomas

Prognosis

Most cases of miliaria resolve spontaneously once risk factors are reduced and the patient is moved to a cooler, less humid environment. All types of miliaria typically resolve relatively easily, except for miliaria profunda, which can have a variable prognosis. While symptoms often improve with treatment, individuals may be prone to recurrent episodes, especially in hot climates. Chronic cases can potentially cause permanent damage to sweat glands, leading to anhidrosis and an increased risk of heat-related illnesses.

Complications

The most severe complication of miliaria, particularly miliaria profunda, is anhidrosis, which impairs thermoregulation and can lead to heat exhaustion. This can permanently impact a person's ability to work or prevent an active individual from continuing exercise or sports. Opportunistic bacterial superinfections may also occur in miliaria pustulosa and rubra due to changes in the affected epidermal layer caused by this skin condition.