INTRODUCTION — Eccrine miliaria (miliaria) is a common, transient cutaneous disorder caused by blockage within the eccrine sweat duct. It is also known as "sweat rash," "prickly heat," or "heat rash." There are three main types of miliaria (crystallina, rubra, and profunda), which are distinguished by clinical appearance as well as histologic findings (picture 1A-D). The type of miliaria that develops depends upon the level at which the duct is blocked.
The clinical features, diagnosis, and management of miliaria will be reviewed here.
EPIDEMIOLOGY — Miliaria occurs most commonly under conditions of sweating. It is found in both children and adults:
●Miliaria crystallina, also known as sudamina, is very common in neonates. Incidence peaks at approximately one week of age. Reports of frequency range from 4.5 percent  to 9 percent . It has also been reported in adults. Congenital miliaria crystallina has been described.
●Miliaria rubra is the most common type. It has been reported in 4 percent of neonates  and in up to 30 percent of people of all ages . Miliaria pustulosa is an uncommon variant of miliaria rubra that has been most frequently reported in neonates . (See 'Clinical manifestations' below.)
●Miliaria profunda, also known as tropical anhidrosis or mamillaria, is rare. It is most common in adult males, especially military personnel stationed in tropical climates .
Subclinical miliaria has been postulated to be an initial step in the development of atopic dermatitis; however, this is controversial .
ETIOLOGY — Occlusion and/or inflammation of the eccrine sweat duct are the cause of all types of miliaria. Overhydration of the corneocytes is thought to cause them to swell and block the duct. Skin cells and other debris can also block the duct. Deeper involvement of the duct and gland can cause inflammation and rupture.
Neonates are particularly susceptible, mainly to miliaria crystallina and miliaria rubra. This has been postulated to be due to underdevelopment and/or delayed patency of the eccrine sweat duct after birth .
Anything that causes sweating can lead to miliaria in infants, children, and adults. Common causes include:
●Hot and humid environments
●Strenuous physical activity
●Febrile illness 
●Occlusion of the skin
Hot and humid environments are particularly common causes, especially soon after moving to a tropical climate. The condition usually resolves with acclimatization. Occlusion of the skin with nonporous clothing  or bandage wrappings may cause miliaria, as may transdermal medication patches .
Other associations have been described:
●Medications – Miliaria crystallina has occurred with medications that affect follicular epithelia, particularly isotretinoin . Miliaria crystallina occurring after chemotherapy, particularly in patients with febrile neutropenia, has been reported [11,12].
In addition, it is postulated that innervation of the eccrine sweat duct may play a role in the induction of sweating and the development of miliaria . Sweating is under the control of the sympathetic autonomic system, but the postsynaptic neurotransmitter is acetylcholine. Sweat has been demonstrated to be inhibited by anticholinergic agents and stimulated by cholinergic drugs. Miliaria has been reported in patients treated with alpha-sympathomimetic drugs such as clonidine, and in patients treated with opiates, beta-blockers, atropine, bethanechol, and other medications that influence the sympathetic or cholinergic systems.
●Cutaneous diseases that disrupt the epidermis – Miliaria crystallina following staphylococcal scalded skin syndrome, Stevens-Johnson syndrome, and toxic epidermal necrolysis has been reported .
●Disorders associated with increased salt in the sweat – Miliaria crystallina has occurred in patients with hypernatremia but without fever; direct desiccation of the corneocyte by excess sodium is thought to be causative . In addition, miliaria rubra has been reported in type I pseudohypoaldosteronism, in which increased concentration of salt in the eccrine sweat has been noted .
●Ultraviolet or ionizing radiation – Ultraviolet exposure may disrupt the duct and has been associated with miliaria crystallina . Radiotherapy also has been reported to precipitate miliaria .
●Cutaneous flora – Bacteria, particularly staphylococci, have been found in lesions of miliaria, and treatment with antibiotics has been helpful in prevention of experimentally induced miliaria . The periodic acid-Schiff-positive material in pathologic specimens of miliaria rubra is reported to be an extracellular polysaccharide substance produced by Staphylococcus epidermidis . (See 'Histopathology' below.)
●Skin maceration – Miliaria rubra has occurred in association with maceration of the skin, especially the lower legs, from wading through water . Miliaria localized to the nasal bridge, related to use of a respirator mask, has been reported in health care workers .
•Morvan syndrome – Morvan syndrome is a rare autosomal recessive disease characterized by hyperhidrosis, which may cause miliaria. Pain, neuromyotonia, weight loss, insomnia, and hallucinations also occur in this syndrome .
•Pseudohypoaldosteronism type I – Pseudohypoaldosteronism (PHA) type I is a rare group of salt-wasting disorders characterized by resistance to aldosterone. PHA type I is usually diagnosed in the neonatal period with hyponatremia, hyperkalemia, and metabolic acidosis. Cutaneous symptoms may include miliaria rubra, folliculitis, and atopic dermatitis .
CLINICAL MANIFESTATIONS — The clinical features of miliaria crystallina, miliaria rubra, and miliaria profunda are reviewed below:
●Miliaria crystallina results from superficial blockage of the sweat duct (within the stratum corneum) and is characterized by superficial, clear 1 to 2 mm vesicles that resemble water droplets (picture 1A-B, 1E-G). The vesicles may coalesce. Due to their superficial location, the vesicles rupture easily and do not cause a significant inflammatory response. Hence, there is no surrounding erythema.
The vesicles are often widely spread and are most common on the head, neck, and upper trunk of neonates and on the trunk of adults. The involved areas are generally asymptomatic. The vesicles appear in crops within days to weeks of appropriate environmental exposure and resolve within hours to days with superficial desquamation that is described as "branny." Congenital cases have been reported .
●Miliaria rubra results from blockage of the duct, usually within the epidermis, although it can occur in the dermis. Leakage of sweat causes inflammation in the periductal tissue. Miliaria rubra is characterized by erythematous 2 to 4 mm papules (picture 1C, 1H-I). They may be papulovesicular or pustular, and background erythema is often seen. It is important to note that the papules are not follicularly based.
Miliaria pustulosa is a variant of miliaria rubra in which a pustule forms at the top of the eccrine sweat duct. The "hypopyon sign" occurs when pustules develop a transverse fluid level with clear sweat/water above and pus below.
In infants, miliaria rubra is most common in the skin folds of the neck, axilla, or groin. In adults, miliaria rubra is often seen in areas of friction with clothing: upper trunk (especially the back), scalp, neck, and flexural areas. The face, palms, and soles are spared. Rare cases of confluent patches similar to sunburn have been reported .
Unlike miliaria crystallina and miliaria profunda, miliaria rubra is typically associated with itching or stinging that is worsened by sweating. Anhidrosis can occur in affected areas. Miliaria rubra can become superinfected, usually with staphylococci, resulting in impetigo or multiple abscesses known as periporitis staphylogenes .
●Miliaria profunda is caused by blockage deeper within the sweat duct (at the dermal-epidermal junction). It usually occurs after repeated episodes of miliaria rubra. Escape of sweat into the surrounding tissues results in erythematous to skin-colored, firm papules 1 to 4 mm in diameter (picture 1D). The papules are nonfollicular.
Seen most commonly in adults, miliaria profunda usually occurs on the trunk but can be seen on the extremities as well. Because sweat is obstructed deeply within the skin, affected areas show little or no sweating. The eruption is typically asymptomatic. It may also be subtle and become more visible when the patient sweats.
A rare type of miliaria profunda, "giant centrifugal miliaria profunda," has been described in infants as well as a granulomatous variant .
When miliaria rubra or miliaria profunda is severe or recurrent, sweat glands may temporarily malfunction or be completely destroyed; anhidrosis in affected areas may result . This can be accompanied by compensatory hyperhidrosis in nonaffected areas, particularly the face, axillae, and inguinal areas. It can also lead to inefficient thermoregulation and may be accompanied by hyperthermia, heat exhaustion, weakness, malaise, dyspnea, tachycardia, and cardiovascular collapse.
HISTOPATHOLOGY — Histopathologic findings in miliaria reflect the depth of occlusion of the sweat duct:
●Miliaria crystallina – Displays an intracorneal or subcorneal vesicle within the acrosyringium of the sweat duct; a sparse neutrophilic infiltrate may be seen.
●Miliaria rubra – Characterized by spongiotic vesicles in the stratum malpighii that communicate with the sweat duct; surrounding inflammation is predominantly lymphocytic within the dermis and periductal tissues (picture 2). There is often a periodic acid-Schiff-positive, diastase-resistant cast within the eccrine duct. Bacteria are often observed as well. Neutrophils predominate within the sweat retention vesicle in miliaria pustulosa.
●Miliaria profunda – Demonstrates similar histologic findings as miliaria rubra along with rupture of the intradermal duct and significant lymphocytic infiltrates and spongiosis of the intraepidermal portion on the duct (picture 3).
DIAGNOSIS — The diagnosis is usually made based upon the clinical appearance in a neonate or in a patient who has a recent history of sweating. In miliaria crystallina, the physical examination should reveal multiple, superficial clear vesicles without associated inflammation. The erythematous papules, papulovesicles, or pustules of miliaria rubra should not be follicularly based, a feature that aids in distinguishing this disorder from folliculitis. The skin-colored papules of miliaria profunda also should be nonfollicular.
Biopsy to confirm the diagnosis may be necessary in severe or recurrent cases. In vivo imaging studies using high-definition optical coherence tomography are able to localize the blockage of the sweat duct . However, this procedure is not necessary for diagnosis in the clinical setting.
DIFFERENTIAL DIAGNOSIS — The differential diagnosis of miliaria includes other disorders that may present with multiple small vesicles, papules, or pustules on the skin. In general, the clinical evaluation is sufficient to distinguish miliaria from these disorders. Microbiological studies, skin biopsy, or other tests can be helpful when needed:
●Viral infections (herpes simplex virus or varicella (picture 4))
●Fungal infections (Candida or Pityrosporum (picture 5A-B))
●Bacterial folliculitis (especially staphylococci, Pseudomonas) (picture 6)
●Viral exanthems (picture 7)
●Acne (including neonatal acne (picture 8))
●Erythema toxicum neonatorum (picture 9)
●Drug eruptions (such as acute generalized exanthematous pustulosis [AGEP] (picture 10)) (see "Acute generalized exanthematous pustulosis (AGEP)")
●Arthropod assault (see "Insect and other arthropod bites")
●Miliaria crystallina is self-limiting and asymptomatic.
●Miliaria rubra tends to resolve with removal of the cause (acclimatization, recovery from illness, removal of occlusive clothing) but can recur. Recurrent episodes can result in miliaria profunda and anhidrosis.
●Although miliaria profunda is likely to resolve without treatment, lasting damage to the sweat gland can occur and result in large areas of anhidrosis, compensatory hyperhidrosis, and heat-related morbidities.
MANAGEMENT — The management of miliaria consists of measures designed to minimize exposure to factors that may stimulate or exacerbate miliaria.
General measures — Patients with all types of miliaria may benefit from the following measures to minimize sweating and obstruction of eccrine sweat ducts:
●Move patient to a cooler environment, if possible
●Wear breathable clothing (such as cotton) that does not occlude the skin
●Remove occlusive bandages in the affected area and use more porous alternatives, if needed
●Treat fever with antipyretics
In addition, daily gentle exfoliation with a rough cloth during bathing or showering may help to improve miliaria by removing debris (eg, corneocytes, sebum, etc) that may occlude the eccrine duct. The use of soap in patients with miliaria rubra is controversial because of the potential for inducing additional skin irritation, and in our experience, soap is helpful if not combined with aggressive scrubbing.
In particular, recurrences of miliaria rubra should be minimized to reduce risk for the development of miliaria profunda. Although asymptomatic, miliaria profunda often leads to areas of transient or permanent anhidrosis. It is unclear whether very close adherence to measures to reduce sweating reduces risk for permanent anhidrosis in patients with miliaria profunda.
Adjunctive pharmacologic interventions — In addition to routine measures, adjunctive medical interventions are sometimes used in an attempt to improve symptoms or accelerate resolution of miliaria rubra or miliaria profunda. These interventions are not required for the resolution of miliaria, and data to confirm efficacy are lacking:
●Miliaria rubra – Adjunctive interventions for miliaria rubra are aimed at reducing inflammation and minimizing potential contributory effects of bacteria (see 'Etiology' above):
•Topical corticosteroids – Mild to mid-potency topical corticosteroid creams or lotions, such as hydrocortisone 2.5% or triamcinolone 0.1% twice a day for one to two weeks, may decrease pruritus and hasten resolution of inflammation.
•Topical antibiotics – Topical antibiotics may reduce inflammation and decrease the amount of bacteria within papules or pustules. This may be particularly beneficial for patients with miliaria pustulosa. Antibiotics targeting staphylococcal and streptococcal organisms are used, such as clindamycin or erythromycin.
The use of oral antibiotics (eg, tetracyclines, sulfonamides, or macrolides) for their anti-inflammatory properties and antibacterial effects is controversial.
●Miliaria profunda – There is little information on adjunctive medical therapies for miliaria profunda. Combination treatment with oral isotretinoin (40 mg per day for two months) and application of topical anhydrous lanolin prior to exercise was associated with improvement in miliaria profunda in one patient .
INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.
Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)
●Basics topics (see "Patient education: Heat rash (prickly heat) (The Basics)")
SUMMARY AND RECOMMENDATIONS
●Miliaria is a common, transient skin disorder caused by blockage within the eccrine sweat duct. (See 'Etiology' above.)
●Miliaria can occur in children and adults. Miliaria is most commonly seen in the neonatal period and in hot environments. Miliaria can also occur in association with fever. (See 'Epidemiology' above and 'Etiology' above.)
●Miliaria is divided into three types, depending on the level at which the duct is compromised:
•Miliaria crystallina is the most superficial form of miliaria. Clear vesicles form at the orifices of eccrine sweat ducts (picture 1A-B, 1E-G). Miliaria crystallina is most common in neonates.
•Miliaria rubra is the most common form of miliaria. It is characterized by erythematous, nonfollicular papules and most commonly involves the upper trunk (picture 1C, 1H-I). Miliaria pustulosa is an uncommon subtype. Miliaria rubra can occur at any age and may be accompanied by pruritus. Miliaria rubra may lead to damage of the eccrine gland and miliaria profunda.
•Miliaria profunda is uncommon. Blockage deep within the duct causes asymptomatic, skin-colored papules (picture 1D). When damage to the sweat gland from miliaria profunda or miliaria rubra is severe, anhidrosis may develop in affected areas resulting in hyperthermia and heat exhaustion. (See 'Clinical manifestations' above.)
●Treatment of miliaria revolves around minimizing exposure to factors that stimulate sweating and occlusion of eccrine sweat glands. The following measures are recommended:
•Movement of the patient to a cooler environment, if possible
•Wearing of breathable clothing (such as cotton) that does not occlude the skin
•Removal of occlusive bandages in the affected area and use of more porous alternatives, if needed
•Treatment of fever with antipyretics (see 'Management' above)