Last Updated: 2023-10-12
Author(s): Steybe T., Navarini A.A.
Dermatitis hidrotica; Dew drops; Hidroa; Heat flares; Heat pimples; Prickle heats; Red dog; Sweat blisters; Sweat frizzles; Sweat blisters
Miliaria is a common, usually pruritic exanthema caused by obstruction or autoinflammatory processes of eccrine sweat glands and their ducts.
Although miliaria affects all age groups and both sexes equally, infants and children are more at risk due to the immaturity of the excretory ducts. Sweating is the most common risk factor for miliaria. Therefore, hot or humid environments and the presence of fever are associated with miliaria. Miliaria cristallina occurs in 4.5% to 9% of newborns and may also occur in adults who have recently moved to a warmer climate. Miliaria rubra, the most common form of miliaria, often occurs in newborns between 1 and 3 weeks of age. It can also affect up to 30% of adults living in hot and humid areas.
Depending on the pathophysiological point of attack, miliaria can be subdivided according to clinical aspects into:
- Miliaria rubra: due to occlusion of the sweat gland excretory duct at the level of the stratum spinosum, usually acute, disseminated, pruritic erythematous papules and vesicles form. Mostly the face and the palms of the hands and feet are affected.
- Miliaria cristallina: due to occlusion in the area of the stratum corneum, mostly truncal localised, pinhead-sized, clearly filled vesicles develop.
- Miliaria alba: pathophysiologically similar to Miliaria cristallina, with yellowish-cloudy contents of the vesicles clinical assignment to Miliaria alba.
- Miliaria pustulosa: due to occlusion and also rupture of the excretory structures at the level of the dermato-epithelial junctional zone, multiple, densely standing, itchy pustules develop increasingly in the area of the intertriges (due to bacterial overload). Frequent in hyperaldosteronism type I.
- Miliaria profunda: Occlusion in the area of the dermis. Clinically similar to miliaria rubra.
- Miliaria neonatorum (Miliaria of the infant): common in newborns, usually between 1 and 3 weeks old. Mostly the groin, armpit and neck are affected.
The main cause of miliaria is occlusion of the eccrine sweat glands or ducts. This can be caused by cutaneous debris or bacteria such as Staphylococcus epidermidis with a biofilm formation. This leads to leakage of sweat into the epidermis or dermis, resulting in cellular hyperhydration, swelling and further obstruction of the excretory ducts. If the eccrine glands or excretory ducts are more severely affected, they may rupture.
Causes of miliaria are:
- Occlusion of the skin: transdermal drug patches and tight clothing have been associated with miliaria.
- Pseudohypoaldosteronism type I: mineralocorticoid resistance leads to sodium losses via the eccrine glands.
- Strenuous physical activity and sweating
- Morvan syndrome: a rare autosomal recessive inherited condition that, among other abnormalities, causes hyperhidrosis and predisposes to miliaria.
- Medications: Drugs that stimulate sweating, such as bethanechol, clonidine and neostigmine, have been associated with miliaria. In addition, a few cases of isotretinoin-induced miliaria have been observed.
Clinical diagnosis. Dermoscopy has proven to be a useful tool, especially in people with pigmented skin, as it usually reveals large white papules with surrounding halos (white bullseye)
Trunk, rarely extremities. Face, palms and feet mostly omitted.
The histology of miliaria differs according to type, as it is classified according to the depth of occlusion of the eccrine duct. Miliaria cristallina shows subcorneal or intracorneal vesicles from the intraepidermal part of the duct and may contain neutrophils. Miliaria rubra shows epidermal spongiosis with parakeratosis and vesicles in the epidermis communicating with the eccrine duct. It may be associated with an inflammatory lymphocytic infiltrate surrounding the ductus and superficial vasculature. Miliaria profunda involves intradermal spongiosis of the eccrine duct and is similar to miliaria rubra. Miliaria profunda differs from miliaria rubra by further rupture of the eccrine ducts and more severe lymphocytic inflammation. It is periodic acid-Schiff (PAS)-positive and microscopically diastase-resistant.
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