Skin Rashes: Types and Causes

There are many different types of skin rashes and many different causes of skin rashes. Although most skin rashes are harmless, some do need treatment (which may be tablets, creams or ointments) from your doctor or pharmacist. Some rashes (especially dark red or purple rashes that don't fade with pressure) may even need urgent medical treatment as they can be associated with meningitis and blood infection (septicaemia).

In this article:
How are skin rashes described?
What skin conditions often cause itching?
What are the common causes of skin rashes?

Please Note: This article is only a guide, so if you have any concerns, you must see or call your doctor, especially if:

• The rash doesn't quickly disappear.
• You feel unwell.
• The rash does not fade with pressure (the best way to test this is to press a glass gently against the rash to see if it fades).
• You are not sure what has caused the rash or have any other concerns.

Always follow the instructions given by your doctor or local hospital.

How are skin rashes described?

Skin rashes can be described in the following way:

• Redness of the skin (called erythema). 
• Flat abnormally coloured areas of skin (called macules). Macules are often either red, dark red or purple, brown or white. 
• Solid raised areas which are up to half a centimetre across (called papules). 
• Solid raised areas which are more than half a centimetre across (called nodules). 
• Areas of red raised skin (called plaques) and scales, which have a flaky silvery-white appearance. 
• Reddish-purple lesions which do not fade with pressure (called purpura):
  • If less than one centimetre across then these are called petechiae. 
  • If more than one centimetre across then they are called ecchymoses. 
• Blisters: these are swellings of the skin containing fluid:
  • If a blister is less than half a centimetre across then it is called a vesicle. If filled with yellow fluid (pus) then it is called a pustule.
  • If a blister is larger than half a centimetre across it is called a bulla (plural is bullae).

What skin conditions often cause itching?

Skin disorders that can cause itching include (these will be discussed in the coming weeks):

• Dry skin 
• Eczema 
• Contact dermatitis 
• Urticaria 
• Lichen planus 
• Psoriasis 
• Folliculitis 
• Prickly heat/heat rash

What are the common causes of skin rashes?

The likely causes of skin rashes are classified below based on their apperance:

Red (erythema) but not scaly

• A skin infection called cellulitis. 
• An allergic reaction called urticaria. 
• Reaction to a medicine you are taking. 
• Burns. 
• Viral rashes - eg, measles or rubella (German measles). 
• Vasculitis. This is a condition involving inflammation of blood vessels, which may occur with various illnesses, including rheumatoid arthritis. 
• Erythema nodosum. This is a condition which causes red rounded lumps (nodules), most commonly on the shins. 
• Redness on the palms of your hands may be caused by liver disease, pregnancy or an overactive thyroid gland (hyperthyroidism). 
• A red rash may occasionally be due to an inflammatory condition called systemic lupus erythematosus, especially if it is on the cheeks. 

Young woman with the typical butterfly rash found in lupus
Source: Doktorinternet via Wikimedia Commons

Red (erythema) and scaly

• Psoriasis. This is a condition where there is inflammation of the skin. 
• Eczema. This is sometimes called dermatitis and also involves inflammation of the skin. It may be caused by an allergy and is then called atopic dermatitis/eczema. This may happen in response to some plants - eg, poison oak or ivy. 
• Seborrhoeic dermatitis (in adults). This is a type of skin rash sometimes called seborrhoeic eczema. In babies it is known as cradle cap. 
• Fungal (or 'yeast') skin infection, such as athlete's foot, groin infection (tinea cruris), ringworm, scalp ringworm or infection with candida. 
• Pityriasis rosea. This condition is described as 'self-limiting' and the rash will clear itself naturally. 
• Pityriasis versicolor. This is a rash which is caused by a yeast-like germ. 
• Lupus erythematosus. 
• Lichen planus. This condition mainly affects the skin and causes an itchy rash. 

Macules

• Red macules may be due to a reaction to a medicine or a viral rash - such as measles or rubella - as well as other causes. 
• A brown macule may be a mole but check with your doctor if a mole changes or you are concerned it might be a melanoma. 
• A white macule may be due to a condition which causes pale patches of skin (called vitiligo) or a skin complaint with flaky discoloured areas (called pityriasis versicolor). 
• If a macule is dark red or purple and does not fade when you put pressure on it then it is a purpura (see below) and you need to see a doctor urgently. This is because it could be a sign of meningitis or blood infection (septicaemia). 

Papules

• Common causes of papules include acne, viral wart, seborrhoeic wart, molluscum contagiosum, scabies, insect bites and skin tags. 
• Other causes include psoriasis. 

Purpura and petechiae

• These are dark red or purple and don't fade with pressure. You need to see a doctor urgently because there may be a serious cause that needs urgent treatment, such as meningococcal infection. 
• However, common causes include injury to the skin or repeated coughing. More serious common causes include liver disease such as cirrhosis. 
• Less common causes include vasculitis (eg, Henoch-Schönlein purpura) or a low level of platelets in your blood (eg, thrombotic thrombocytopenic purpura). 

Nodules

• Common causes of a nodule include a sebaceous cyst, lipoma, skin cancer, or a wart. 
• Other causes include rheumatoid nodules (associated with rheumatoid arthritis) and Heberden's nodes (associated with osteoarthritis). 

Blisters

• Skin inflammation, including reactions to medicines, contact dermatitis, eczema. Eczema on your legs may be caused by varicose veins (varicose eczema). 
• Diseases of your immune system - eg, bullous pemphigoid. 
• Viral infections - eg, chickenpox, hand, foot and mouth disease. 
• Skin infection: a germ (bacterial) infection with impetigo or viral infection with herpes simplex (cold sores or genital herpes) or with herpes zoster (shingles). 
• Rarer causes include pemphigus and pemphigoid. 

Pustules

• Skin infection by a virus (eg, cold sores due to herpes simplex virus) or bacterial germs (impetigo). 
• Inflammation - eg, psoriasis. 
• Pustular skin reaction to medicine you are taking. 
• Pustules on your face may be acne or rosacea. 

Ulcers

Ulcers may be due to venous leg ulcers, pressure ulcers, diabetes skin ulcers or cancerous (malignant) skin ulcers.

Reference(s)
1). DermIS. Dermatology Information System.
2). DermNet NZ. Meningococcal disease (includes infection rash images).
3). NICE CKS. Nappy Rash.