Allergic rashes are also referred to as hives or urticaria. These rashes result from an allergic reaction to certain foods or medicines and are more common in people with other allergic disorders or those with a family history of allergy. They present as an itchy skin rash similar to that of an insect bite, with smooth raised reddish blebs known as wheals. They are more common over the arms and legs but may be present on other parts of the body as well. They may increase in size and merge with each other resulting in frequent changes in shape. They may also disappear and reappear at the same place. They characteristically exhibit the blanching phenomenon, wherein they turn pale when some pressure is applied at their center. In some severe cases the allergic rashes may also be accompanied by swelling on the lips, eyes, hands, feet and throat (angioedema). Angioedema may cause difficulty in breathing which may be life threatening.
Allergic rashes may last from a few days to months. Depending on this duration, they can be classified into acute allergic rashes, which may last up to 6 weeks, and chronic allergic rashes, lasting beyond 6 weeks
- Acute allergic rashes may result from allergy to specific foods, medicines, contact with animal dander especially cats, pollen, latex products and insect bites. The common allergy causing foods include shellfish, fish, nuts, wheat, soya, eggs and milk. Medicines that can cause an allergic rash include penicillin, non-steroidal anti-inflammatory drugs (NSAIDs), sleeping pills and blood pressure lowering medications.
- Most chronic rashes are related to auto-immune disorders or infections rather than allergies. Only 5 to 10% of chronic rashes are caused due to allergy.
To confirm the diagnosis, a detailed medical history and history of events around the time of the appearance of the rash are crucial. The doctor will enquire about the foods and medications consumed, history of any insect bite, stress or illness in the near past. Details about the time of appearance of the rash and its duration are also essential.
A few basic laboratory tests such as complete blood count and erythrocyte sedimentation rate (ESR) may be conducted to confirm the allergic reaction. Certain additional tests may also be ordered to rule out causes other than allergy. A skin test may be conducted to confirm the specific allergen that could have triggered the allergic reaction. In more than 50% of allergic rashes cases, the causative agent is not identified.
Acute allergic rash usually resolve by themselves in a few days to a few weeks. The causative allergen or the triggering agent, if identified, should be avoided. The itching and the rashes can be reduced by avoiding hot baths or showers and use of loose comfortable clothing. Allergic rashes usually reduce in cool conditions and tend to flare up in warmer conditions. Alcohol, strong sunlight, stress and even certain medication (aspirin, anti-inflammatory drugs, codeine, and angiotensin-converting enzyme (ACE) inhibitors) may aggravate the rashes. Hence these should be avoided. If your prescribed medications are aggravating your rash consult your doctor to change these medicines. Anti-histamines creams may be applied on the rash to provide symptomatic relief.
Anti-histaminics only block the release of histamine but cannot reduce the effect of histamine once it is released (Histamine is a chemical substance released into the bloodstream by the mast cells, on activation by an allergen, which is responsible for the symptoms associated with the allergic reaction). If anti-histaminics are not effective, a short course of corticosteroids may be prescribed. In severe reactions, especially those involving the throat, emergency epinephrine shot may be given for temporary relief.
In chronic rash the symptoms may last for several months or even years. Consultation with an allergist is crucial for proper diagnosis and treatment of the underlying condition.