MFF0033: Rash

Medical Fun Facts logo from Gary Lum

In English, the word rash can have a few meanings.

The word rash can describe doing something without due care and consideration of possible consequences.

A rash can be a series of things of the same type especially if unwelcome which happens in a short period of time.

A rash can also be a spot or area of the skin which may be red (or erythematous). A rash can be itchy, a rash can be raised from the surface of the skin or it can be flush with the surface of the skin.

And then there is a rash vest or rashie. If you’re not a beach person, you may not be familiar with a rashie, they are really great to avoid sunburn and rashes from surfing.

So skin rashes are very common. A rash is a reaction in the skin or mucous membrane to an irritant. A rash could be a reaction, for example, a contact dermatitis associated with a new clothes detergent or an old Velcro watch band that may be harbouring some salt and sand can irritate a skin abrasion. You will develop a rash as a reaction to some chemicals, especially chemical irritants like high concentrations of chlorine.

Heat can cause a rash, which is more a burn, but a burn can be regarded as a type of rash. If we accept burns as rashes, sunburn is a rash and other forms of radiation can cause a rash. Sometimes the burn will blister too. Then there is heat rash, heat rash is also called prickly heat and occurs when sweat ducts get blocked and lead to discomfort. It’s seen more in babies than in adults.

Probably the most common forms of rash are those caused by infectious agents, including bacteria, viruses, insects and other parasites.

Rashes may be circular or annular and as the rash grows, sometimes it can form a growing circle with a clear centre, almost like a bullseye. Tinea or ringworm infections are a common cause of annular rashes in Australia.

Some rashes will follow anatomical paths, for example, zoster or shingles is a secondary manifestation after an initial Varicella Zoster viral infection. During the initial infection, the virus moves along nerves back to the dorsal root ganglia and remain there dormant until a trigger occurs and the rash appears following a precise dermatomal pattern. Typically, it’s one dermatome but it can be more than one sometimes.

Some rashes have a pattern, for example, scrot rot in young boys in the tropics who wear nylon underpants and togs. The rash only appears around the area covered by the underpants or togs. I remember having scrot rot as a teenager. I went swimming training most mornings and because I’d also swim in the lunch break I’d just wear my togs through class. As you can imagine, hot sticky summers in Brisbane, perfect for scrot rot. What I learnt was to always change out of my wet togs and wear cotton underwear or go commando. I also learnt the hard way to empty my swimming bag each night. Have you ever seen fungus growing on your togs and towel?

Depending on the type of infection, some rashes start centrally and move outwards like varicella or chicken pox. Chickenpox may start on your chest or back and then it moves out along your limbs and head. Only a small number of rashes are seen on the palms of the hands or soles of the feet, for example, Coxsackie viral infection that causes hand, foot and mouth disease, secondary syphilis, Janeway (no not the Captain of USS Voyager (NCC-74656)) lesions of infective endocarditis, Kawasaki disease, Toxic shock syndrome, Reactive arthritis and meningococcæmia.

Measles has a distinctive macular papular rash and is always found with coryza. Macular refers to a visible spot or lesion while papular refers to a swelling or growth. A macular papular rash is a visible exanthem that can be felt. An exanthem is a rash of the skin while an enanthem is a rash of the mucous membranes, for example, a rash on your tongue.

Some viral infections also cause vesicular rashes. These are rashes made up of vesicles. A vesicle in a fluid filled erythematous eruption of the skin or mucous membrane which will eventually rupture. Sometimes, blisters form from heat or chemical irritants but if the cause is infectious, the vesicular fluid can be infectious.

Some microorganisms like yeasts can form satellite rashes. A good example is groin thrush. Candida albicans can cause of central red papule and around it form slightly smaller satellite lesions as the infection spread. Given these yeasts like warm and moist conditions, you see them often in warm sweaty areas like the groin and axillæ. As I mentioned before it’s good to not wear clingy clothing, especially nylon around your genitalia. That reminds me, after my Christmas special, I have seen on Facebook someone named Jenny Taylor. I wonder if she gets teased much.

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