There are hundreds of conditions that have been shown to causes rashes. Some of the most common causes include allergies, autoimmune disorders, infections, irritants, poisonous plants, and skin disorders.
Allergies: An allergic reaction, which occurs when the body's immune system overreacts to a harmless substance (like pollen or dust mites), may cause a skin rash. Common triggers of allergic reactions include insect stings (bees, wasps, etc.), medications (like antibiotics and seizure medications), foods (especially peanuts, seafood, and eggs), and latex (like rubber gloves and condoms).
Autoimmune disorders: Autoimmune disorders, which occur when the immune system mistakenly identifies the body's own cells as foreign invaders (like bacteria), may also cause skin rashes. Rashes are a common feature of autoimmune disorders such as rheumatoid arthritis and lupus..
Infections: Many infections caused by bacteria, fungi, or viruses may cause skin rashes. Infections such as chickenpox, Lyme disease (bacterial infection), ringworm (parasitic infection), shingles (chickenpox in adults), measles, fifth disease (flu-like illness that causes reddening of the cheeks), and herpes (viral infection) are among the most common causes of rashes.
Irritants: Rashes may develop if the skin becomes irritated. For instance, some patients develop rashes after exposure to heat or sun. Some babies develop diaper rash if the diaper rubs against the skin or is not changed frequently enough.
Poisonous plants: Exposure to poison ivy, poison oak, and poison sumac plants may lead to an itchy skin rash. These plants produce oil called urushiol, which may trigger an allergic reaction. Patients who are allergic to these plants may experience a rash that consists of swollen, itchy, red bumps and blisters that appear wherever the oil has touched the skin.
Skin disorders: Other potential causes include skin disorders like acne (pimples), psoriasis, rosacea, and atopic dermatitis (eczema).
General: While the duration of a skin rash varies depending on the cause, most rashes resolve either on their own or with treatment within one to 14 days. Some rashes, including those caused by conditions like lupus and rosacea, are long-term but they can be managed with medications. There are many different forms of rashes that vary in their appearance, location, severity, and duration. The skin may be red and/or itchy. If the skin itches, it is called pruritus. If a butterfly-shaped, red rash develops on the cheek and nose, it is called a malar rash.
Dermatologists classify rashes based on their physical appearance. The main types of rashes include discoid rashes, macular rash, macularpapular rash, papular rash, papulosquamous rash, and vesicular rash.
Discoid rash: A discoid rash is characterized by red and scaly patches of skin on the face and scalp that can lead to scarring and temporary hair loss
Macular rash: When the skin has areas of flat, red spots, it is called a macular rash.
Macularpapular rash: A macularpapular rash occurs when a patient has areas of flat, red spots combined with areas of small, raised, solid bumps on the skin.
rash: A papular rash causes small, raised, solid bumps on the skin.
Papulosquamous rash: A papulosquamous rash occurs when the patient has a combination of pimples and dry, flaky skin.
Vesicular rash: A vesicular rash occurs when a patient has small, raised, fluid-filled blisters.
Other symptoms: A rash is usually just one symptom of an underlying medical condition. Some of the most common symptoms associated with rashes include fever and enlarged lymph nodes.
General: A rash is easily identified during a physical examination. However, because it is a symptom of an underlying medical condition, the cause must be identified in order to treat it. During a physical examination, a healthcare provider will take a careful medical history to determine the underlying cause. Medical tests may be necessary.
Medical history: The healthcare provider will ask whether the patient has a history of allergies, infections, skin diseases, or has been exposed to chemicals or irritants. A healthcare provider will also ask about the patient's daily activities. For instance, individuals who spend time in wooded areas have an increased risk of developing rashes from exposure to poisonous plants.
Physical examination: The healthcare provider will ask when the rash first developed. It is important to know if it started after the patient ate a new food, tried a new skin product, or took a new drug, herb, or supplement.
The location and pattern of the rash is also important. For instance, a heat rash will only be present on areas of the skin that were exposed to the sun. Certain patterns are associated with specific diseases. For instance, patients who have lupus often develop a butterfly-shaped rash on their cheeks and nose. Reactions to poisonous plants often have a streaky pattern where the plant brushed against the skin.
Depending on the underlying cause, certain rashes typically last longer than others. For instance, a rash caused by a viral infection called roseola usually only lasts a couple of days, whereas fifth disease (flu-like illness) may cause a rash for a week. Patients with lupus will have symptoms that persist until they receive treatment.
The healthcare provider will also take into account other symptoms (if any) that accompany the rash. For instance, if patients also have a fever and enlarged lymph nodes, an infection may be suspected.
Allergen-specific immunoglobulin (IgE) test: An allergen-specific immunoglobulin E (IgE) test, commonly referred to as radioallergosorbent test (RAST®), may also be used to determine whether the patient is allergic to particular substances. This test is less accurate than a skin test. It is usually performed in patients who have coexisting severe skin diseases (like eczema or psoriasis) that make it difficult to interpret a skin test. During the procedure, a sample of the patient's blood is sent to a laboratory for testing. The allergen is combined with the blood to determine whether the patient has immunoglobulin E (IgE) antibodies to the allergen. Antibodies are substances that identify and bind to foreign invaders in the body. If the patient has IgE antibodies, an allergy is diagnosed.
Blood test: A blood tests may be performed to determine whether the patient has a bacterial or viral infection. A blood test may also be performed if an autoimmune disorder (like lupus) is suspected. A sample of the patient's blood is analyzed for the presence of autoantibodies. These autoantibodies in the blood mistakenly destroy the patient's own body cells if they have the disorder.
KOH (potassium and hydroxide) preparation: A KOH (potassium and hydroxide) preparation test is used to determine whether a fungal infection is causing the rash. During this test, a healthcare provider will gently scrape the skin with a blunt edge (like the edge of a microscope slide). The sample of scraped skin is then combined with a substance called potassium hydroxide (KOH). This solution allows the healthcare provider to see the fungus (if it is present) under a microscope.
This procedure is not painful because only a tiny amount of skin is needed. Patients may feel a slight pressure sensation when the skin is scraped.
Skin biopsy: A skin biopsy may also be performed. During the procedure, a healthcare provider will inject an anesthetic into the skin, which numbs the area. Then a small sample of skin is removed and analyzed under a microscope to determine whether the patient has a skin disorder, such as psoriasis.
Skin test: A skin test may be used to determine whether the rash is caused by an allergic reaction. During the test, the skin is exposed to the suspected allergens (substances that may be triggering an allergic reaction) and observed for an allergic reaction. If the allergen triggers an allergic reaction, the patient will develop reddening, swelling, or a raised, itchy red wheal (bump) that looks similar to a mosquito bite. The healthcare provider will measure the size of the wheal and record the results. The larger the wheal, the more severe the allergy.
Tzanck test: A tzanck test is used to determine whether a viral infection called herpes is causing a rash. The virus may be suspected if the patient has a rash that contains blisters. During the procedure, a small area of the skin is numbed and a blister is opened. The healthcare provider will scrape a small sample of the fluid and skin from the blister, and it will be analyzed under a microscope for the virus. If the virus is present, a positive diagnosis for herpes is made.
Wood lamp: A wood lamp test may be performed if a healthcare provider suspects the patient has a bacterial or fungal infection of the skin. The procedure is performed in a dark room, at a doctor's office. The healthcare provider shines an ultraviolet light onto the patient's rash. The rash is then observed for color changes when it comes into contact with the light. If a bacterial or fungal infection is present, the skin will appear to glow under the light.
Before undergoing this test, patients should not wash their skin because it may alter test results. Skin products, such as soap, deodorant, makeup, or lotion, should not be applied to the skin prior to testing. These products may glow under the light and cause false-positive results. Also, if the room is not dark enough, a patient may receive false-negative results. This means the patient has the infection, even though he/she tests negative.
Anaphylaxis: A skin rash may be a symptom of a severe and potentially life-threatening reaction called anaphylaxis. The most serious symptoms of anaphylaxis include low blood pressure, breathing difficulties, shock, and loss of consciousness, all of which can be fatal. Patients should seek immediate medical treatment if these symptoms develop.
Psychological symptoms: Some patients who have long-term skin conditions like lupus, chronic acne, or rosacea may suffer from low self-esteem.
Secondary infections: Secondary infections may develop if the patient's rash has open sores or blisters. Patients should not scratch irritated skin because it may cause the skin to break. If bacteria or viruses enter the body through the open skin, an infection may develop. Antibiotics are used to treat bacterial infection, antifungals are used to treat fungal infections, and antivirals are used to treat viral infections. Specific treatment and duration depends on the type and severity of infection and overall health of the patient.
Stevens-Johnson syndrome (SJS): A severe skin rash called Stevens-Johnson syndrome (SJS) may develop as an allergic reaction to medication. SJS is potentially life threatening because in severe cases, the lesions can cause significant scarring of the involved organs, which often leads to loss of function of the organ systems. SJS typically starts with a nonspecific upper respiratory tract infection. The patient may experience flu-like symptoms including, fever, sore throat, chills, headache, and general feeling of discomfort for one to 14 days. Some individuals will also experience diarrhea and vomiting. As the disease progresses, skin lesions quickly develop. Patients may experience extensive shedding of the skin. Skin lesions may occur anywhere, but they are most common on the palms, soles of the feet, back of the hands, and extremities. Mucosal involvement may include reddening of the skin, edema (fluid in body tissues), blistering, open sores on the skin, and dead skin.