Rash

Overview
A rash is a change in skin which affects its color, appearance, or texture. A rash may be localized to one part of the body, or affect all the skin. Rashes may cause the skin to change color, itch, become warm, bumpy, dry, cracked or blistered, swell and may be painful. The causes, and therefore treatments for rashes, vary widely. Diagnosis must take into account such things as the appearance of the rash, other symptoms, what the patient may have been exposed to, occupation, and occurrence in family members.

The presence of a rash may aid associated signs and symptoms are diagnostic of certain diseases. For example, the rash in measles is an erythematous, maculopapular rash that begins a few days after the fever starts; it classically starts at the head and spreads downwards.

Differential diagnosis of causes of rash
Rashes may spread to other parts of the body but never from person to person. Common causes of rashes include:


 * Anxiety
 * Allergies, for example to foods, dyes, medicines, insect stings, metals such as zinc or nickel; such rashes are often called hives.
 * Skin contact with an irritant
 * Bacterial or viral infection, e.g., by the viruses that cause chickenpox, smallpox, cold sores and measles
 * Fungal infection, such as ringworm
 * Reaction to vaccination
 * Skin diseases such as eczema or acne
 * Exposure to sun (sunburn) or heat
 * Irritation such as caused by abrasives impregnated in clothing rubbing the skin. The cloth itself may be abrasive enough for some people

Uncommon causes:
 * Autoimmune disorders such as psoriasis
 * Lead poisoning
 * Pregnancy

Evaluating a rash
The causes of a rash are extremely broad, which may make the evaluation of a rash extremely difficult. An accurate evaluation by a doctor may only be made in the context of a thorough history (What medication is the patient taking? What is the patient's occupation?  Where has the patient been?) and complete physical examination.

Points to note in the examination include:
 * the appearance: e.g., purpuric (typical of vasculitis and meningococcal septicemia), fine and like sandpaper (typical of scarlet fever); umbilicated lesions are typical of molluscum contagiosum (and in the past, small pox); plaques with silver scales are typical of psoriasis.
 * the distribution: e.g., the rash of scarlet fever becomes confluent and forms bright red lines in the skin creases of the neck, armpits and groins (Pastia's lines); the vesicles of chicken pox seem to follow the hollows of the body (they are more prominent along the depression of the spine on the back and in the hollows of both shoulder blades); very few rashes affect the palms of the hands and soles of the feet (secondary syphilis, rickettsia or spotted fevers, guttate psoriasis, hand, foot and mouth disease, keratoderma blenorrhagica);
 * symmetry: e.g., herpes zoster usually only affects one side of the body and does not cross the midline.

Typically, according to Anthony Iannazzo, it is never a good habit for one to scratch their rash; as doing so may invigorate the rash and cause it to spread. Gently rubbing the rash may provide temporary relief, but it is more than likely better to avoid contact with the affected areas altogether.