Prickly heat (sweat rash)

Now that summer is finally here, prickly heat or sweat rash can sometimes make life a little uncomfortable. When we are hot, our bodies activate our sweat glands to produce sweat to cool us down. In order for sweat to cool our body, it needs to evaporate off the surface of our skin. Sometimes, however these sweat glands get blocked. Also known as miliaria, prickly heat occurs when perspiration is trapped under the skin, causing the skin to turn red and a sensation of warmth, stinging or prickling. It commonly occurs in hot, humid or tropical climates, but is also seen in patients in hospital, and in newborn babies.

Prickly heat can be quite common in newborns within their first week of life as they have immature sweat ducts that can rupture more easily, and sweat can get trapped under the skin. Hospitalised or bedridden patients are prone to sweat rash, especially if lying on mattress protectors or waterproof mattresses. Intense physical activity (in particular if clothing doesn’t allow sweat to evaporate), wearing synthetic clothing or using non-porous dressings as well as suffering from a fever can also contribute to developing prickly heat.

Typically, prickly heat causes superficial clear blisters on the surface of the skin that pop easily and can look like beads of sweat, but can range to deep, red lumps. The blisters usually appear on the head, neck or upper trunk but can also appear in the armpits, elbow creases and groin. In adults the rash is usually in these skin folds or where clothing causes friction. In infants prickly heat tends to break out on their neck, shoulders and chest. The rash may spread further over the body but cannot be passed on to other people.

There are several different types of prickly heat which are classified according to how deep the blocked sweat glands are. Miliaria crytallina is mild prickly heat and affects the sweat glands in the top layer of skin. This rash has clear, fluid-filled blisters and bumps that break easily. Miliaria rubra affects a deeper layer of skin causing red lumps and an itching or prickling sensation. Miliaria pustulosa occurs when the fluid-filled blisters become inflamed and full of pus. The less common miliaria profunda affects the dermis which is the deep layer of the skin. This type of prickly heat causes firm, flesh-coloured lesions that look like permanent goose-bumps.

The best way to treat prickly heat is to avoid the heat, humidity and sweating that caused it, which is not always possible in the middle of summer! It will usually resolve on its own within one or two days by changing to a cooler environment. Make use of air conditioning or fans to circulate the air and create a cooler environment and sleep in a ventilated cool bedroom. Wear loose, lightweight clothes that wick moisture away from the skin. Avoid irritating the skin further, in particular by avoiding using creams and ointments that are petroleum or mineral-oil based as these can block sweat glands even more. Bathe or shower in cool water and let your skin air dry instead of removing the water with a towel.

Cool water compresses can give relief to inflamed skin, as can calamine lotion to soothe any itching (just make sure to use one with an emollient as calamine on its own can dry the skin). Treat a fever that has caused the prickly heat with an anti-pyretic such as paracetamol. If the rash is more serious, a mild steroid such as hydrocortisone used topically for a few days can help treat any inflammation and itching.

Make sure you see a doctor if the rash is getting worse, your symptoms last longer than a few days or you have any signs of infection such as increasing pain, swelling, redness or warmth of the rash, pus, fever or chills, or swollen lymph glands in the armpit, neck or groin.

This blog provides general information and discussion about medicine, health and related subjects. The information contained in the blog and in any linked mate­ri­als, are not intended nor implied to be a substitute for professional medical advice.