More than skin-deep: a guide to skin rash diagnoses

Most people have experienced a rash in one form or another – and there are many, many forms! When diagnosing and treating a rash, there are a lot of characteristics to take into account, like where the rash appears, the colour and the texture, and further information is needed such as allergies, recent activity, diet, and environment… it can be overwhelming to wade through all the possibilities. Luckily, celebrated consultant dermatologist Dr Eugene Ong shares his knowledge in this handy guide.

What is a rash?

A rash on the skin could be anything ranging from a flush of redness, a spread of small bumps, or a patch of scaliness to weeping sores and painful itchiness. Any noticeable alteration to the skin that is not usual could be considered a rash. The colour or irritation from a rash may be more difficult to notice for people with darker skin.

If the rash is localised in one place, it could be a superficial reaction due to cleaning agents, allergies, or even a form of sunburn. They should not require intensive treatment and can be addressed with topical medications. Some of them will go away on their own.

If the rash is present in or spreads to multiple parts of the body, it could suggest a more serious issue, like more aggressive fungal infections or autoimmune diseases.

Kinds of rashes

Possible causes of skin rashes that shouldn’t cause concern:

• Friction rash (intertrigo): an inflammation that arises from skin-to-skin friction and hastened by heat and moisture. It will look like a flat, reddish rash, and can commonly occur in places like the armpits, between the thighs, and between folds of skin. 
• Reaction from touch (contact dermatitis): a type of eczema where the skin is itchy, dry, and cracking due to contact with powerful chemical substances like industrial cleaners or superglues, or an allergen.
• Allergies to food, medication, or substances: for example, latex allergy rash can appear as aggressive hives or blisters within 48 hours after exposure. 
• Exposure to sunlight (polymorphic light eruption): a red itchy or burning rash with a slight texture that can appear from a few hours to a few days after being in the sun. It appears on the body where there was sun exposure and can turn into blisters 
• Bed bugs and mites:  raised, itchy and inflamed, like mini mosquito bites.
• Heat rash: caused by overheating, it is characterised by mild swelling and prickly, small raised spots that can appear anywhere on the body.
• Infections of bacterial, fungal, or parasitic origin have rashes that vary in appearance and location.
• Diseases like measles, shingles, or chickenpox will appear as raised blotches or small itchy bumps. They are contagious but preventable with vaccines and once contracted and cured, they cannot be contracted again.

There are chronic skin conditions with rashes as symptoms. The state of these conditions can range from mild to severe and disruptive of daily life, so treatment will vary:

• Eczema is red or purple, crusty and flaky and appears in round patches of the skin, but can be all over the body.
• Psoriasis is dry cracked skin in smaller patches of purple or red with lighter scales of skin.
• Rosacea is reddening or darkening of the skin, commonly of the face, that can come and go.

Serious conditions that are accompanied by rashes:

• Pemphigus vulgaris (PV), which is a chronic autoimmune condition. The rash takes the form of painful, fluid-filled blisters all over the body, but especially on mucous skin like the inside of the mouth and nose and the genitals.
• Stevens-Johnson syndrome (toxic epidermal necrolysis), which is triggered by certain medications such as epilepsy medications, antibiotics, and anti-inflammatory medications. The rash begins as circular patches on the upper body and then spreads all over, evolving into blisters and sores.
• Toxic-shock syndrome, which is commonly associated with using a menstruation product for too long but can also be triggered by infection of a wound. The rash will be raised and rough.
• Staphylococcal scalded skin syndrome (SSSS), where the skin peels all over the body and looks as though it has been scaled. It is caused by a type of Staphylococcal aureus bacteria.

How are skin rashes diagnosed and treated?

The dermatologist will evaluate the cause of the rash by asking about your medical history and the products you use at home (for example, are you using any new cosmetics or cleaning products), or if you’ve tried any new foods, or medications or been in contact with foreign substances. They will review your skin, and might be able to diagnose based on their experience and expertise. After this, should the dermatologist still be unable to diagnose with certainty, they will initiate a testing process of patch tests, allergen tests, skin cultures, blood tests, or biopsies, which may take up to a couple of weeks to receive the lab results.

You should see a dermatologist or a doctor if the rash:

• does not go away after two weeks
• causes pain
• is leaving weeping sores or blisters of blood and fluid
• is in the mucous membranes of the body
• spreads rapidly
• is contagious

Babies and young children have immune systems that are still developing, meaning they are more vulnerable to conditions like measles, impetigo, or scarlet fever. There are benign conditions that could be causing your child’s rash like slapped cheek syndrome or nappy rash, but if your baby is suffering from a rash, it is better to get a professional opinion as it could be urgent.

Once the cause of the rash is discovered, a treatment plan will be devised. For most conditions, dermatologists will prescribe a topical corticosteroid or antihistamines. There are specific medications for fungal and bacterial infections, and some rashes simply indicate that the skin needs to be cooled down and moisturised. More serious conditions might need specialised treatments that could last months and necessitate intensive care.

If you have an unknown skin condition and are seeking professional treatment, Dr Eugene Ong is available for consultation via his Top Doctors profile.