Heat Rash (Miliaria)

Introduction:  The importance of sweating

The human body has over two million sweat glands, which are distributed over the entire skin surface.  They are particularly numerous on the palms and soles, where 600 - 700 glands per square centimeter are present, but also occur on the face (181 per square centimeter), the arms and legs (108 per square centimeter), and on the trunk (64 glands per square centimeter). The most common type of sweat gland is the eccrine gland. 

In order to understand the structure and function of sweat glands, a little background on skin anatomy is necessary (Figure 1).  The skin consists of three main layers: the epidermis, which is the highly cellular outermost layer and which is itself divided into multiple sublayers; the dermis, which consists mostly of connective tissue, blood vessels, nerves and glands; and the subcutaneous fat, the deepest layer, which serves as insulation and also contains blood vessels, nerves, and hair follicles.
 

Figure 1.  The layers of the skin

The eccrine sweat glands consist of coiled tubes that are located deep in the dermis, near the border with the subcutaneous fat.  Eccrine glands are innervated by nerves from the sympathetic branch of the nervous system.  These nerves use acetylcholine as a neurotransmitter. They are controlled by the hypothalamus, a region of the brain that regulates body temperature. 

When stimulated by heat, exercise, fever or emotion, the hypothalamus signals to the eccrine glands to produce sweat, a dilute solution of sodium chloride, potassium and bicarbonate.  The sweat travels through eccrine ducts that pass through the dermis and epidermis before being released onto the skin surface.  Under conditions of extreme heat, an individual can produce up to 10 liters of sweat per day.

Although many people find sweating unpleasant, sweating is physiologically important. As sweat evaporates from the skin, it cools the skin surface, preventing the body from overheating.  When something goes wrong with this process, due to an inborn lack of sweat glands, drugs that prevent sweating or blockage of the sweat ducts, the body may overheat.


What is Heat Rash?

Heat rash or “miliaria” is a skin eruption that results from blockage of the eccrine sweat ducts. This prevents sweat from reaching the skin surface and causes it to leak into the skin.  The blockage may occur at one of three levels, resulting in three different types of heat rash (Figure 2).

1.  Miliaria crystallina:  In this type of heat rash, also called “sudamina,” blockage of the sweat duct occurs very superficially, in the stratum corneum, a layer of dead skin cells that make up the outermost layer of the epidermis.  This kind of heat rash appears as tiny clear bubbles or blisters on the skin that are easily broken, releasing clear fluid.  There are usually no symptoms.
2.  Miliaria rubra:  In this type of heat rash, also known as “prickly heat,” blockage of the sweat duct occurs in the deeper layers of the epidermis, resulting in leakage of sweat into the epidermis.  The presence of sweat within the epidermis causes inflammation (attracts white blood cells).  Prickly heat looks like red bumps on the skin and causes a burning or itchy, prickling sensation.  Occasionally, these bumps fill with pus and are called “miliaria pustulosa.”
3.  Miliaria profunda.  In this type of heat rash, the sweat ducts are obstructed between the epidermis and the dermis (the layer of skin beneath the epidermis).  Sweat leaks into the upper part of the dermis and subtle, flesh-colored bumps (like goose flesh) are seen.

Figure 2.  The eccrine sweat gland and duct, showing the level of sweat blockage in the various forms of heat rash (miliaria)

 

 What causes heat rash?

Heat rash, as its name would suggest, occurs in susceptible individuals who are exposed to hot, humid conditions.  It is seen in people who spend time in a tropical climate to which they are not acclimated, and in people with high fevers.  This is more likely to occur when the skin is covered with clothing, bandages or sheets that do not allow sweat to evaporate.  It may also be provoked by exercise in tight-fitting clothing that prevents evaporation of sweat.  It is thought that overhydration of skin due to pooled sweat may result in blockage of the sweat ducts.

There is some evidence that bacteria play a role in development of heat rash.  Common skin bacteria such as Staphylococcus epidermidis and Staphylococcus aureus have been found to occur in increased numbers on the skin of patients with heat rash.   A material from the outer layer of Staphylococcal bacterial capsules has been found in the plug obstructing sweat glands.  And in experimental studies of conditions that cause heat rash, antibiotics seem to prevent heat rash, suggesting that certain bacteria may play an important role.

Certain drugs have been reported to cause the miliaria crystallina type of heat rash:

•  drugs that induce sweating: bethanechol, neostigmine, beta choline, clonidine
•  opioid pain relievers
•  isotretinoin (an acne drug)
•  doxorubicin (a chemotherapy drug)

Who gets heat rash?

Heat rash is thought to be equally common in males and females and to affect all races.  Asians, who sweat less than Caucasians and people of African descent, may be less prone to heat rash.

Heat rash is most common in newborn infants, with miliaria crystallina occurring in 4.5% of newborns in a Japanese study at an average age of seven days.  The same study showed that 4% of newborns developed the miliaria rubra type of heat rash, at an average age of 11-14 days.  Miliaria crystallina has even been reported to occasionally be present at birth.  A recent Iranian study found miliaria crystallina and rubra to occur at an overall combined prevalence of 1.3% of newborns.  This may represent an underestimate, as the infants were examined within the first two days of life, which is before miliaria usually occurs.

Heat rash is also common in travelers to tropical areas. Up to 30% of children and adults from temperate climate zones who travel or move to a hot, humid tropical area will initially experience heat rash, usually of the miliaria rubra type.  After several months of acclimation, the heat rash usually resolves.

Individuals who have repeated episodes of miliaria rubra may eventually develop the less common, deeper form of heat rash, known as miliaria profunda.  This type of heat rash is usually only seen in adults.

Patients with the genetic disease type I pseudohypoaldosteronism produce excessively salty sweat and have an increased incidence of miliaria rubra.  The high concentration of sodium chloride in their sweat is thought to damage the eccrine sweat ducts.  This leads to spillage of sweat into the epidermis, from which the characteristic tiny itchy red bumps of miliaria rubra develop.

Heat rash can be a significant problem for military personnel deployed to tropical and desert areas.  Numerous cases of all types of heat rash have been reported in soldiers serving in Iraq and the Gulf Region as well as in American military personnel who served in Southeast Asia during the Vietnam War and the South Pacific during World War II.


What are the symptoms of heat rash?

In the most superficial type of heat rash, called miliaria crystallina, there are usually no symptoms. 

In miliaria rubra, however, affected individuals often report itching, burning and stinging of the affected areas, leading to the common name of “prickly heat.” 

The lesions of miliaria profunda are typically without symptoms.  However, affected individuals may note a lack of sweating on their affected skin and increased sweating on other body areas.  When exercising or spending time in warm environments, they may suffer from increased body temperature and symptoms of heat exhaustion, including dizziness, fatigue, shortness of breath and palpitations.


What does heat rash look like?

In the miliaria crystallina type of heat rash (Figures 3 and 4), the rash appears as tiny clear fluid-filled blisters, usually grouped on the head, neck and upper trunk of infants and on the trunk in adults.  The blisters are fragile and frequently rupture, spilling clear fluid onto the skin. There is no surrounding redness.  The blisters are often confluent, meaning that they run into each other.  As they resolve, they leave behind a fine bran-like scale on the skin, which eventually peels to leave normal skin. 

Figure 3.  Miliaria crystallina.  The typical “water droplet” appearance of miliaria crystallina.  Photograph courtesy of Kenneth E. Greer, MD.

Figure 4. Miliaria crystallina.  A close up.  Photograph courtesy of Kenneth E. Greer, MD.

 

In the miliaria rubra type of heat rash (Figures 5 and 6), the rash appears as tiny red bumps that are not associated with hair follicles.  They usually do not become confluent.  In infants, they are typically located on the neck or in the groin or skin folds.  In adults, they are usually on the neck, scalp, upper trunk, and skin folds, wherever friction occurs between clothing and the skin.  These lesions may become pus-filled and are then referred to as miliaria pustulosa (Figures 7 and 8).

Figure 5.  Miliaria rubra. An infant with widespread bright red bumps characteristic of miliaria rubra.  Photograph courtesy of Kenneth E. Greer, MD.

 Figure 6.  Miliaria rubra.  An adult with miliaria rubra.  Photograph courtesy of Kenneth E. Greer, MD.

Figure 7.  Miliaria pustulosa.  The lesions of miliaria rubra have become pustular (pus-filled).  Photograph courtesy of Kenneth E. Greer, MD.

Figure 8.  Miliaria pustulosa.  Photograph courtesy of Kenneth E. Greer, MD.

 

In the miliaria profunda type of heat rash, the rash appears as tiny flesh-colored bumps, like “goose pimples,” on the trunk and extremities, which appear with exertion or exposure to heat and disappear shortly afterward, usually within an hour.  In more serious and prolonged cases of miliaria profunda, the affected person may have diminished sweating and their skin could become dry and hot after exercise.


How long does heat rash last?

The miliaria crystallina type of heat rash usually resolves within hours to days, once the cause of overheating is removed. For example, once a patient’s fever has gone down or an infant is no longer tightly swaddled, their fragile miliaria blisters will rupture and their skin will return to normal.

The miliaria rubra type of heat rash also generally clears up within days of removal of the stimulus to sweating.  For example, a person who has recently moved to the tropics and developed heat rash will most likely see his lesions get better within a few days of moving to an air-conditioned environment.  However, studies have shown that patients with this type of heat rash may have impaired sweating and heat intolerance for weeks after the rash is gone.


The miliaria profunda type of heat rash develops rapidly but also disappears rapidly, often within an hour of stopping exercise in hot conditions.  As with miliaria rubra, miliaria profunda may also cause significantly decreased sweating and heat intolerance for weeks or months after the rash has disappeared.


Is heat rash a serious illness?

Usually, heat rash is more of an uncomfortable nuisance than a serious illness. However, when sweat does not reach the skin surface, there is a risk of overheating, particularly if the problem persists.  This lack of sweating, called anhidrosis, can potentially cause dangerous overheating and heat exhaustion.  Military personnel serving in tropical and desert areas have developed severe cases of heat rash, leading to a syndrome called “tropical anhidrotic asthenia,” in which diminished sweating leads to fatigue, dizziness, shortness of breath, and palpitations.


What conditions might heat rash be confused with?

Miliaria crystallina:  The clear fluid-filled blisters seen in this type of heat rash may resemble poison ivy, chicken pox, or herpes simplex.  However, the distribution and appearance of the individual miliaria crystallina lesions is distinctive. 

•  Poison ivy differs from miliaria crystallina in that it is very itchy and usually occurs asymmetrically on exposed skin rather than symmetrically on the face and clothed areas of the trunk
•  Herpes simplex differs from miliaria crystallina in that it is painful and is localized to one area of the body, whereas miliaria crystallina is asymptomatic and affects the face and clothed areas of the trunk in a symmetrical fashion. 
•  Unlike chicken pox lesions, which have the appearance of a blister on a pink or red background (said to resemble a “dew drop on a rose petal”), miliaria crystallina lesions occur on a background of normal skin and are often confluent.

Miliaria rubra/pustulosa: 

•  The red bumps and pimple-like lesions of these forms of heat rash may be confused with erythema toxicum neonatorum, a very common rash in newborns that occurs at the same age that miliaria rubra would be expected.  The rash of erythema toxicum tends to be more variable in appearance than the uniform red bumps seen in miliaria rubra and the lesions tend to be more evanescent, with individual lesions disappearing after minutes to hours. 
•  Miliaria rubra may also resemble candidiasis, a superficial yeast infection.  Candidiasis is usually restricted to the skin folds of the neck, groin, and underarms, unlike miliaria, which may affect the entire trunk and limbs. 
•  Miliaria rubra also may be confused with folliculitis, a bacterial infection of the hair follicles that appears as red bumps or pimples on the trunk and limbs.  Folliculitis can be distinguished from miliaria because it occurs exclusively on hair follicles and tiny hairs can be seen protruding from the lesions.

Miliaria profunda:  The goose-flesh appearance of this type of heat rash is fairly distinctive and unlikely to be confused with other medical conditions when occurring in the setting of a hot humid environment.


How can one prevent and treat heat rash?

As the miliaria crystallina type of heat rash has no symptoms and spontaneously gets better, it does not necessarily need to be treated.  However, simple measures such as using fans or air conditioning, removing excess clothing and applying cool washcloths to an affected person can help. 

Since the miliaria rubra form of heat rash can be very uncomfortable and the miliaria profunda form may lead to heat exhaustion, these types should be treated. 

The key to preventing and treating heat rash is avoidance of conditions that lead to excessive sweating.  The following is recommended:

•  Avoid hot, humid environments
•  Stay in air-conditioned buildings or use fans
•  Avoid wearing occlusive (air-tight) clothing
•  Keep infants in lighter weight clothing and not swaddled
•  Avoid exercise in hot weather
•  Take cool showers or apply cool compresses
•  Treat fevers when present
•  Stop drugs known to be associated with heat rash

Other measures that may be helpful are:

•  Application of topical anhydrous lanolin (wool alcohol) to skin prior to exercise
•  Application of calamine lotion to affected skin
•  Use of topical or oral antibiotics, which are thought to help prevent heat rash by decreasing bacteria on the skin surface
•  Avoidance of ointments, which may block sweat ducts and worsen heat rash

If none of the above measures results in improvement, moving to a cooler climate may the only option.


More Information

Book Chapters about Heat Rash

Fealey RD and Sato K. Chapter 82. Disorders of the eccrine sweat glands and sweating.  In Fitzpatrick’s Dermatology in General Medicine, Seventh edition.  New York.  McGraw Hill Medical 2008, pp. 720-730.

Habif, TP.  Miliaria, in Clinical Dermatology, Third edition.  Mosby St. Louis 1996, pp. 186-7.

Web Resources

http://www.emedicine.com/derm/TOPIC266.HTM

http://dermnetnz.org/hair-nails-sweat/miliaria.html
http://www.aocd.org/skin/dermatologic_diseases/miliaria.html

References

Arpey CJ, Nagashima-Whalen LS, Chren MM, Zaim MT: Congenital miliaria crystallina: case report and literature review. Pediatr Dermatol 1992 Sep; 9(3): 283-7.

Haas N, Henz BM, Weigel H: Congenital miliaria crystallina. J Am Acad Dermatol 2002 Nov; 47(5 Suppl): S270-2.

Haas N, Martens F and Henz BM (2004). Miliaria crystallina in an intensive care setting.  Clin Exp Dermatol 29:32-34.

Hidano A, Purwoko R, Jitsukawa K: Statistical survey of skin changes in Japanese neonates. Pediatr Dermatol 1986 Feb; 3(2): 140-4

Holzle E, Kligman AM: The pathogenesis of miliaria rubra. Role of the resident microflora. Br J Dermatol 1978 Aug; 99(2): 117-37

Kirk JF, Wilson BB, Chun W, Cooper PH: Miliaria profunda. J Am Acad Dermatol 1996 Nov; 35(5 Pt 2): 854-6.

Moosavi Z.  Hosseini T. (2006) One-year survey of cutaneous lesions in 1000 consecutive Iranian newborns. Pediatric Dermatology.  23(1):61-3.

Mowad CM, McGinley KJ, Foglia A, Leyden JJ.  (1995).  The role of extracellular polysaccharide substance produced by Staphylococcus epidermidis in miliaria.  J Am Acad Dermatol 33:729-33.

Oumeish OY, Oumeish I, Parish JL.  (2002). Gulf War Syndrome. Clinics in Dermatology 20:401–412

Pandolf, KB, Griffin TB, Munro EH, Goldman RF.  (1980).  Persistence of impaired hat tolerance from artificially induced miliaria rubra.  Am J Physiol 239:R226-R232.

Pandolf, KB, Griffin TB, Munro EH, Goldman RF.  (1980).  Heat intolerance as a function of percent of body surface involved with miliaria rubra.  Am J Physiol 239:R233-R240.


Sato K, Kang WH, Saga K, Sato KT (1989). Biology of sweat glands and their disorders. I. Normal sweat gland function. J Am Acad Dermatol 20:537-563.

Shuster S.  (1997).  Duct disruption, a new explanation of miliaria.  Acta Derm Venereol  77:1-3.

Stillman MA,  Hindson TC,  Maibach HI.  (1971).  The effect of pretreatment of skin on artificially induced miliaria rubra and hypohidrosis.  British Journal of Dermatology.  84(2):110-6.

Sulzberger MB (1968). Induced miliaria (prickly heat), post-miliarial hypohidrosis and some potential sequelae.  Proc Roy Soc Med 62:347-8.

Wenzel FG, Horn TD Nonneoplastic disorders of the eccrine glands. J Am Acad Dermatol 38:1-17.