What are the other Names for this Condition? (Also known as/Synonyms)

• Miliaria
• Sweat Rash

What is Heat Rash? (Definition/Background Information)

• Heat Rash, also known as Miliaria, is an itchy and slightly uncomfortable rash that occurs typically during hot-humid weather, due to excessive sweating. It is a commonly observed condition in children and adults
• The cause of Heat Rash typically occurs from blocked sweat glands arising from several factors such as tight clothing and intense physical activity. In this condition, there is an occlusion of the eccrine sweat glands
• Depending on their appearance, there are three forms of Heat Rash:
  • Miliaria crystallina: It involves the superficial layers of the skin (stratum corneum) and causes the production of clear vesicles without associated redness (erythema). It is the least severe form of Heat Rash
  • Miliaria rubra: The condition involves the deeper layers of the skin lining (epidermis) causing leakage of sweat and associated inflammation resulting in the formation of red papules. It is the most common form of Miliaria
  • Miliaria profunda, also known as tropical anhidrosis: It involves the deeper layers of skin (specifically, the dermo-epidermal junction). This form usually occurs after the individual has multiple recurrent episodes of Miliaria rubra. It is a rare condition that is often noted in adult males
• The signs and symptoms generally depend on the type of Heat Rash and may include the presence of small blisters or red bumps, itchiness, an absence of sweating, heat intolerance, and fever, in some cases
• Heat Rash is generally not a serious condition. It usually goes away without the need for any treatments. In a vast majority of cases, the prognosis is excellent, though rarely, this may also depend on the type of Heat Rash

Who gets Heat Rash? (Age and Sex Distribution)

• Heat Rash can affect individuals of all ages. However, it is mostly seen in newborns and young children
• Miliaria crystallina generally affects newborns, while Miliaria rubra and Miliaria profunda mainly affects adults
• There is no gender preference noted and both males and females are affected
• The condition is seen worldwide across all races and ethnic groups

What are the Risk Factors for Heat Rash? (Predisposing Factors)

The risk factors for Heat Rash include:

• Continuously lying on one side of the body for a prolonged period, such as observed in those who are bedridden
• Athletes who undertake intense physical activity
• Residing in or traveling to hot and humid climates; living in tropical regions
• Working outdoors in hot and humid conditions, such as farmhands, construction workers, etc.
• People who work in hot-humid (indoor) environments such as mine shafts or certain industries
• Presence of high levels of the bacterium Staphylococcus epidermidis increases one’s risk. S. epidermidis is a bacterium that safely lives on the surface of the skin
• Underdeveloped sweat ducts
• Wearing non-porous synthetic clothing, especially during hot periods
• Frequent skin infections
• Individuals who are overweight or obese are at a higher risk for Heat Rash
• Cancer treatments such as radiotherapy
• It may occur as a side effect of certain medications and may include:
  • Induction chemotherapy
  • Drug-induced hyperhidrosis, or the excessive sweating occurring from using certain medications
  • Stevens-Johnson syndrome
  • Toxic epidermal necrolysis
• Genetic disorders such as Morvan syndrome and pseudohypoaldosteronism type I
• Repeated onset of Miliaria rubra may result in the more severe form called Miliaria profunda

It is important to note that having a risk factor does not mean that one will get the condition. A risk factor increases one’s chances of getting a condition compared to an individual without the risk factors. Some risk factors are more important than others.

Also, not having a risk factor does not mean that an individual will not get the condition. It is always important to discuss the effect of risk factors with your healthcare provider.

What are the Causes of Heat Rash? (Etiology)

Blocked pores that trap sweat under the skin cause Heat Rash. 

• Blockage can occur at different skin layers that result in different forms of the condition
• Visible signs of inflammation due to Heat Rash emerge when the trapped sweat leaks into surrounding skin and forms tiny pockets

The sweat glands may be blocked due to many factors including:

• Tight restrictive clothing
• Intense physical activity
• Presence of a high amount of the bacterium Staphylococcus epidermidis that produces a sticky substance

A study has shown that people who develop Heat Rash when sweaty have around three times as much Staphylococcus epidermidis bacteria on their skin, compared to those who do not. Hence experts believe that increased levels of S. epidermidis on skin may cause the development of a superimposed skin infection called Miliaria pustulosa.

It is important to note that Heat Rash is not an infectious condition, and it cannot be transmitted from one individual to another.

What are the Signs and Symptoms of Heat Rash?

The signs and symptoms of Heat Rash/Miliaria depend on the severity of the condition, and the specific type that is present.

Miliaria crystallina: It occurs close to the surface of skin and is the mildest form of Heat Rash. This form of Heat Rash is characterized by:

• Small clear blisters that resemble droplets of water on skin 
• 1-2 mm sized clear crystals 
• Blisters that break easily
• Absence of itching or pain sensation

Miliaria rubra (prickly heat): It occurs in the outer layer of skin (epidermis) and is the most common form noted. This form of Heat Rash is characterized by:

• Itchy or prickly feeling
• Red bumps or blisters that are 2-4 mm in size
• Generally, the neck and shoulder region is affected
  • In children, the trunk, neck, armpits, and groin are usually involved
  • In adults, the upper trunk, scalp, neck, and flexural folds are commonly involved
• There is little sweating around the affected area
• Intolerance to heat
• Fatigue

Miliaria profunda: It occurs in the dermis (middle layer of the skin) and is an uncommon form. This form of Heat Rash is characterized by:

• Presence of 1-3 mm sized rashes, mostly in the trunk and extremity
• The presence of firm rashes
• Flesh-colored rashes that may resemble goose bumps
• In some, bigger lumps on skin may be noted
• Slight itchiness
• Increased heart rate
• Lack of sweat
• Fever
• In many cases, this form of Heat Rash is asymptomatic

Adults with Heat Rash may present fever.

Miliaria pustulosa: It is also known as yellow/white rash. This form is characterized by pustules on skin.

How is Heat Rash Diagnosed?

In most individuals, Heat Rash is easily diagnosed and does not have the requirement for more detailed tests and/or invasive exams. A healthcare provider or dermatologist will be able to visibly determine the presence of the condition.

Nevertheless, the following tests and exams may be undertaken, if necessary:

• A complete physical examination and a thorough medical history
• Dermoscopy: Dermoscopy is a diagnostic tool where a dermatologist examines the skin using a special magnified lens
• Skin or tissue biopsy: A skin or tissue biopsy is performed and sent to a laboratory for a pathological examination, who examines the biopsy under a microscope. After putting together clinical findings, special studies on tissues (if needed) and with microscope findings, the pathologist arrives at a definitive diagnosis
• Cultures of skin biopsies, if necessary
• Blister smears may be undertaken to rule out herpes simplex infection

Note: A skin/tissue biopsy is normally not necessary to diagnose Heat Rash but may be performed to rule out other causes or similar conditions/infections.

A variety of conditions may exhibit similar signs and symptoms. Hence, a differential diagnosis to eliminate other conditions or infections may be considered, before arriving at a definitive diagnosis. These include:

• Acne
• Acute generalized exanthematous pustulosis (AGEP) or pustular drug eruption
• Bacterial folliculitis
• Fungal infections
• Grover disease
• Herpes simplex infection
• Toxic erythema of newborn

Many clinical conditions may have similar signs and symptoms. Your healthcare provider may perform additional tests to rule out other clinical conditions to arrive at a definitive diagnosis.

What are the possible Complications of Heat Rash?

Heat Rash will usually clear by itself without any complications. In some rare cases, the potential complications may include:

• Increased sweating (hyperhidrosis) may occur in non-affected body areas
• Secondary bacterial infection of the rash, usually by Staph bacteria
• Heat exhaustion (especially with Miliaria profunda): If left untreated, heat exhaustion can infrequently lead to death

How is Heat Rash Treated?

The goal of treatment for Heat Rash is to cool the body and reduce sweating. Most forms of Heat Rash will disappear once the body has sufficiently cooled down. 

Cooling the body may be achieved by:

• Sitting next to a fan or air conditioner/cooler
• Bathing in cold water
• Stopping any physical activity
• Cold water compresses

In case of severe Heat Rashes, topical creams may be used. Such creams include:

• Hydrocortisone, a weak steroid, to reduce inflammation
• Anhydrous lanolin, to reduce pore blockage
• Calamine lotion, to reduce itching
• Antibiotics may be prescribed for secondary infections, if any

Over-the-counter ointments (not prescribed by healthcare providers) should be avoided as they may cause blockage of the pores and worsen the condition.

How can Heat Rash be Prevented?

Certain preventive measures for Heat Rash include:

• Regularly changing positions of individuals who are confined to a bed or chair
• Sleeping in a well ventilated area
• Using unscented soaps and detergents
• Wearing light, loose fitting cotton fabrics in humid weather
• Avoiding creams or makeup that block pores
• Treating skin infections promptly and adequately
• Review treatments using medications that cause Heat Rash

What is the Prognosis of Heat Rash? (Outcomes/Resolutions)

• In a vast majority of cases, Heat Rash has an excellent prognosis, as it goes away on its own
• Severe cases of Heat Rash may need appropriate treatments using topical applications
• Rarely, some serious cases can result in conditions, such as heat exhaustion, that may require urgent medical attention

Additional and Relevant Useful Information for Heat Rash:

Cleaning the skin too hard with strong chemicals or soaps may aggravate the skin condition. Care must be taken avoid strong soaps and chemicals that can potentially worsen the condition.