Heat rash, also known as prickly heat, sweat rash and miliaria, is an itchy, red skin rash that often causes a stinging or prickling sensation. It is common in infants but can affect adults too, especially during hot, humid weather.

Heat rash develops when sweat ducts become blocked and perspiration is trapped under the skin. This causes irritation and results in the rash. It is common in a hot or humid climate, although it can also occur in winter.

Symptoms range from superficial blisters to deep, red lumps. Some forms of heat rash can trigger an intensely itchy or prickly feeling. Severe forms of heat rash may need medical care, but the best way to relieve symptoms is to cool the skin and prevent sweating. Heat rash usually goes away on its own.

According to Medilexicon’s medical dictionary:
Miliaria is “an eruption of minute vesicles and papules due to retention of fluid at the orifices of sweat glands.”
The rash can occur anywhere on the body, but most commonly affects areas covered by clothing. People who are overweight or sweat easily are more likely to get prickly heat. Babies and children are also more prone to prickly heat than adults, because their sweat glands are not fully developed.
What are the signs and symptoms of prickly heat or heat rash?
A symptom is something the patient feels and reports, while a sign is something other people, such as the doctor detect. For example, pain may be a symptom while a rash may be a sign.

The symptoms of prickly heat tend to appear after a few days of being exposed to hot temperatures, although sometimes they will not appear for several weeks or months.

The rash

Prickly heat causes a rash to form on the skin. The rash is typically made up of minute spots or bumps surrounded by a patch of red skin. Sometimes the spots look like tiny blisters. This rash may cause:
A stinging prickling sensation
Intense irritation
Itching
Mild swelling
The rash most commonly appears on the:
Abdomen
Armpits
Back
Feet
Groin
Hands
Neck
Upper chest
The symptoms will usually be worse on the areas of the body that are covered by clothing, since clothing can cause friction and will make increase sweating.

Adults usually develop heat rash in skin folds and wherever clothing causes friction. In infants, the rash is mainly found on the neck, shoulders and chest, but it can also occur in the armpits, elbow creases and groin.

Miliaria can be classified according to the level at which obstruction occurs in the sweat glands. There are three types of miliaria. Signs and symptoms for each type vary:

Miliaria crystalline

The mildest form of heat rash affects the sweat ducts in the topmost layer of skin. Miliaria crystallina is marked by:
Tiny, superficial blisters
Clear papules
Bumps that break easily
The blisters in this type of heat rash are not itchy or painful. The condition usually clears on its own, but can come back if hot, humid weather persists. It is common in newborns, but adults can develop it too.

Miliaria rubra

It occurs deeper in the outer layer of skin. Miliaria rubra is sometimes called prickly heat. Adults usually develop miliaria rubra after they are exposed to hot, humid weather. It can also happen in people confined to bed rest. Infants usually develop this type of heat rash between the first and third weeks of life. Typically, signs and symptoms include:
Red bumps
Itchy or prickly feeling in the affected area
Little or no sweating in the affected areas (anhidrosis)
Miliaria profunda

It is a less common form of heat rash. Miliaria profunda occurs mainly in adults who have had repeat bouts of miliaria rubra. This condition affects the dermis, which is a deeper layer of skin. It appears soon after exercise or any activity that causes sweating. Signs and symptoms may include:
Firm, flesh-colored lesions that resemble goose bumps
A lack of perspiration. This may lead to symptoms of heat exhaustion, such as dizziness, nausea and a rapid pulse
Heat rash usually heals on its own and does not require medical care. Seek medical advice if symptoms last longer than a few days, if the rash seems to be getting worse, or if there are signs of infection such as:
Increased pain
Swelling, redness or warmth around the affected area
Pus draining from the lesions
Swollen lymph nodes (glands) in the armpit neck or groin
A fever or chills
What are the causes of prickly heat or heat rash?
Prickly heat occurs when the sweat glands become blocked. Instead of evaporating, perspiration remains trapped beneath the skin, causing inflammation and rash.

With excessive sweat, it is easier for dead skin cells and bacteria on the skin to collect in the sweat glands. If these glands become blocked, the sweat is trapped underneath the skin in tiny swollen pockets. The skin becomes irritated. When the pockets burst and release sweat, this causes a stinging, prickling sensation that gives the condition its name.

Other causes are:
Spending long periods of time in bed due to an illness or a long-term health condition. Immobility and illness can make increase sweating
Dressing in too much clothing in winter
Sitting too close to a fire or heater
Certain factors seem to play a role in blocked sweat ducks, including:
Immature sweat ducts. A newborn’s sweat ducts are not fully developed. They can rupture more easily, trapping perspiration beneath the skin. This usually happens in hot weather, but can occur anytime infants are dressed too warmly. Also newborns who have high fevers or are in incubators can develop blocked sweat ducts.
Tropical climates. Hot, humid weather is particularly conducive to miliaria.
Physical activity. Intense exercise, hard work or any activity that causes extensive perspiration can lead to heat rash.
Certain fabrics. People who always wear clothing that does not allow perspiration to evaporate normally can develop heat rash.
Medications. Certain prescription medications have been linked to heat rash:
Bethanechol (Urecholine), which treats bladder problems
Clonidine (Catapres), a high blood pressure drug sometimes used to treat attention-deficit/hyperactivity disorder (ADHD)
The acne medication isotretinoin (Accutane)
Doxorubicin (Adriamycin), a chemotherapy drug
Overheating. Overheating in general. Bundling up too much in winter, sleeping under an electric blanket can lead to heat rash.
Heavy creams and ointments. These products can block the sweat ducts.
What are the risk factors of heat rash?
A risk factor is something which increases the likelihood of developing a condition or disease. For example, obesity significantly raises the risk of developing diabetes type 2. Therefore, obesity is a risk factor for diabetes type 2.

Factors that increase the risk of heat rash include:
Being a newborn. Though heat rash can affect anyone, newborns are most susceptible.
Being in a tropical climate. People living in the tropics are far more likely to have heat rash than are people in temperate climates.
Participating in intense physical activities. Anything causing profuse sweat, especially if not wearing moisture-wicking clothing, can trigger heat rash.
What are the complications of heat rash?
Heat rash usually heals without problems, but complications sometimes occur:
Infection. Occasionally, heat rash becomes infected with bacteria, causing inflamed and itchy pustules. This can occur in children in diapers. However, it should not be confused with diaper rash, which results from irritation to tender skin, not from blocked pores.
Heat exhaustion. In hot weather, people with miliaria profunda are at risk of heat exhaustion. This is because their sweat ducts are blocked. Heat exhaustion can cause:
A rapid pulse
Dizziness
Headache
Low blood pressure
Nausea
Untreated heat exhaustion can lead to heatstroke, a life-threatening condition.

No tests are needed to diagnose heat rash; only a visual medical exam is necessary.
What is the treatment for prickly heat or heat rash?
Prickly heat is not a serious condition and rarely requires any specific treatment. Most rashes will disappear naturally after a few days.

However, there are a few recommendations that may help to ease the symptoms:
After bathing, let your skin air-dry, instead of toweling off.
Anhydrous lanolin, which may help prevent duct blockage and stop new lesions from forming.
Avoid excessive heat and humidity. Try to spend some time in the shade, or try using a small fan. Being exposed to heat will only increase sweating, and could make the rash worse.
Avoid using creams and ointments, which can block pores further.
Calamine lotion can be used if the skin feels sore and irritated. It will help cool and soothe the skin. It is available at most pharmacies. Use cool compresses to calm itchy, irritated skin.
Hydrocortisone cream or Topical steroids can be used if the rash feels particularly itchy and irritated. Low-strength hydrocortisone cream is available at local pharmacy. However, avoid using this cream on the face and always follow the instructions on the package.
Keep your skin cool. Taking a cool bath or shower will help prevent sweating and will soothe the skin. Avoiding sweating even for a few hours each day by staying in an air-conditioned room can get considerable relief.
Limiting physical activity that can cause more sweating.
Menthol and/or camphor based preparations can ease itching sensation.
Wear loose cotton clothing. Avoid wearing synthetic fibers, such as nylon and polyester. They trap heat more easily than natural fibers. Wearing lightweight clothing made of fabrics that “breathe” is helpful.
If skin is kept cool and dry, heat rash will usually clear up in seven to 10 days. In cases where the rash has developed into open blisters or pustular lesions a doctor should be consulted since more aggressive, medically monitored treatment may be required.
Prevention
To help prevent heat rash:
Avoid tight fitting clothes that can irritate tender skin.
Avoid using creams or ointments. They do not prevent heat rash and can block pores.
Bathe in cool water with a nondrying soap that does not contain fragrances or dyes.
Frequent cool showers or cool baths with mild soap can help to prevent heat rash.
In summer, dress in soft, lightweight, cotton clothing. And avoid overdressing in winter. Children should wear the same amount of clothing that an adult would wear to stay comfortable.
Keep sleeping area cool and well ventilated.
When weather is hot, stay in the shade or in an air-conditioned building or place a fan at a safe distance to gently circulate the air.