It is very rare and estimated to affect 1 in 100,000 per year. Because of its rarity the documentation, cases and information are sparse and not a huge amount is known for certain, meaning that EAC could actually be a set of many un-classified skin lesions. It is known to occur at all ages and all genders equally. Some articles state that women are more likely to be affected than men.

Often no specific cause for the eruptions is found. However, it is sometimes linked to underlying diseases and conditions such as:

- Food (including blue cheese or tomatoes).

- Contact Dermatitis (i.e. cleaning agents, fabric softeners, etc.)

- Fungal, Bacterial and Viral infections such as sinusitis, tuberculosis, candidiasis or tinea.

- Drugs including finasteride, etizolam (and benzodiazepines), chloroquine, hydroxychloroquine, oestrogen, penicillin and amitriptyline.

- Cancer (especially the type known as erythema gyratum perstans, in which there are concentric and whirling rings).

- Primary biliary cirrhosis.

- Graves disease.

- Appendicitis.

- Lupus

- Pregnancy (EAC usually disappears/stops soon after delivery of baby).

- Hormone (Contraceptive Pill, Stress, Hormone Drugs)

- Lyme Disease

It is associated with various physiological as well as pathological changes, or may be a normal finding:

- Portal hypertension

- Chronic liver disease (including chronic hepatitis)

- Pregnancy

- Polycythemia

- Thyrotoxicosis

- Rheumatoid arthritis (especially in patients with polycythaemia)

- Eczema and psoriasis

- Deep telangiectasias

- Coxsackievirus A infection (Hand, foot and mouth disease)

- Rocky Mountain spotted fever

- Secondary syphilis

- Kawasaki disease

- Adverse drug reaction: palmoplantar erythrodysesthesia (acral erythema)

Because circulating levels of estrogen increase in both cirrhosis and pregnancy, estrogen was thought to be the cause for the increased vascularity. More recently, nitric oxide has been implicated in the pathogenesis of palmar erythema.

EN is associated with a wide variety of conditions, including:

- Idiopathic

In about 30–50% of cases, the cause of EN is unknown.

- Infection

- Streptococcal infection which, in children, is by far the most common precipitant,

- Primary infection of Tuberculosis

- "Mycoplasma pneumoniae"

- "Histoplasma capsulatum"

- "Yersinia"

- Epstein-Barr virus

- "Coccidioides immitis" (Valley fever)

- Cat scratch disease

- Autoimmune disorders, including

- Inflammatory bowel disease (IBD)

- Behçet's disease

- Sarcoidosis

- Pregnancy

- Medications, including

- Sulfonamides

- Penicillins

- Oral contraceptives

- Bromides

- Hepatitis B vaccination

- Cancer, including

- Non-Hodgkins lymphoma (NHL)

- Carcinoid tumours

- Pancreatic cancer

EN may also be due to excessive antibody production in lepromatous leprosy leading to deposition of immune complexes.

There is an association with the HLA-B27 histocompatibility antigen, which is present in 65% of patients with erythema nodosum.

A useful mnemonic for causes is SORE SHINS (Streptococci, OCP, Rickettsia, Eponymous (Behçet), Sulfonamides, Hansen's Disease (Leprosy), IBD, NHL, Sarcoidosis.

Palmar erythema is reddening of the palms at the thenar and hypothenar eminences.

Erythema nodosum is the most common form of panniculitis. It is most common in the ages of 20–30, and affects women 3–6 times more than men.

About 15 percent of patients with inflammatory bowel disease develop erythema nodosum.

Many suspected aetiologic factors have been reported to cause EM.

- Infections: Bacterial (including Bacillus Calmette-Guérin (BCG) vaccination, haemolytic "Streptococci", legionellosis, leprosy, "Neisseria meningitidis, Mycobacterium, "Pneumococcus, "Salmonella" species, "Staphylococcus" species, "Mycoplasma pneumoniae), "Chlamydial.

- Fungal (Coccidioides immitis)

- Parasitic ("Trichomonas" species, "Toxoplasma gondii), "

- Viral (especially Herpes simplex)

- Drug reactions, most commonly to: antibiotics (including, sulphonamides, penicillin), anticonvulsants (phenytoin, barbiturates), aspirin, antituberculoids, and allopurinol and many others.

- Physical factors: radiotherapy, cold, sunlight

- Others: collagen diseases, vasculitides, non-Hodgkin lymphoma, leukaemia, multiple myeloma, myeloid metaplasia, polycythemia

EM minor is regarded as being triggered by HSV in almost all cases. A herpetic aetiology also accounts for 55% of cases of EM major. Among the other infections, "Mycoplasma" infection appears to be a common cause.

Herpes simplex virus suppression and even prophylaxis (with acyclovir) has been shown to prevent recurrent erythema multiforme eruption.

Normally, exfoliation is restricted to a particular area and normal skin will replace the exfoliated parts, so no treatment is needed. Since keratolysis exfoliativa is caused by friction, detergents, and solvents, these factors should be avoided. Creams, especially those with silicone and lactic acid are also helpful. In severe cases, photochemotherapy is an option.

Keratolysis exfoliativa normally appears during warm weather. Due to excessive sweating and friction, in for example athletic shoes, the skin can start to exfoliate. Other factors that can cause exfoliation are detergents and solvents.

Another very common cause has been reported from salt water fishermen, who often suffer from these symptoms. It is not sure whether it is from the salt water or whether it is from some bacteria from fish.

Erythema ab igne was once commonly seen in the elderly who stood or sat closely to open fires or electric heaters; however, erythema ab igne has been reported in both young and elderly individuals. Women have a higher incidence of erythema ab igne than men. Although wide use of central heating has reduced the overall incidence of erythema ab igne, it is still sometimes found in people exposed to heat from other sources such as heating pads, space heaters, hot water bottles, and electronic devices.

Because the eruption is transient and self-limiting, no treatment is indicated.

It can be caused by infection, massage, electrical treatment, acne medication, allergies, exercise, solar radiation (sunburn), cutaneous radiation syndrome, mercury toxicity, blister agents, niacin administration, or waxing and tweezing of the hairs—any of which can cause the capillaries to dilate, resulting in redness. Erythema is a common side effect of radiotherapy treatment due to patient exposure to ionizing radiation.

The cause of PPE is unknown. Existing hypotheses are based on the fact that only the hands and feet are involved and posit the role of temperature differences, vascular anatomy, differences in the types of cells (rapidly dividing epidermal cells and eccrine glands).

In the case of PPE caused by PLD, the following mechanism has been demonstrated: sweat deposits and spreads the drug on the skin surface; then the drug penetrates into the stratum corneum like an external agent; palms and soles have high density of sweat glands, and their stratum corneum is approximately 10 times thicker than the rest of the body, and becomes an efficient long-term reservoir for the penetrating PLD, which was deposited on the skin before.

Erythema disappears on finger pressure (blanching), while purpura or bleeding in the skin and pigmentation do not. There is no temperature elevation, unless it is associated with the dilation of arteries in the deeper layer of the skin.

Acral erythema is a common adverse reaction to cytotoxic chemotherapy drugs, particularly cabozantinib, cytarabine, doxorubicin, and fluorouracil and its prodrug capecitabine.

Targeted cancer therapies, especially the tyrosine kinase inhibitors sorafenib and sunitinib, have also been associated with a high incidence of acral erythema. However, acral erythema due to tyrosine kinase inhibitors seems to differ somewhat from acral erythema due to classic chemotherapy drugs.

Annular erythema of infancy is a skin condition reported in children roughly six months in age, characterized by transitory skin lesions that resolved without treatment within eleven months.

Alpha-1 antitrypsin deficiency panniculitis is a panniculitis associated with a deficiency of the α-antitrypsin enzyme.

Some sources distinguish between the following:

- "Erythema marginatum rheumaticum"

- "Erythema marginatum perstans"

The cause of erythema toxicum is thought to be an activation of the immune system. Some neonates are more sensitive than others and develop erythematous spots all over the body. Another theory is hypersensitivity to detergents in bedsheets and clothing is sometimes suspected, but the connection remains unproven.

It is thought to be a benign condition that causes no discomfort to the infant. The rash will generally disappear spontaneously in about 2 weeks.

It occurs in less than 5% of patients with rheumatic fever, but is considered a major Jones criterion when it does occur. The four other major criteria include carditis, polyarthritis, Sydenham's Chorea, and subcutaneous nodules. In this case, it is often associated with Group A streptococcal infection, otherwise known as "Streptococcus pyogenes" infection, which can be detected with an ASO titer.

It is an early feature of rheumatic fever and not pathognomonic of it. It may be associated with mild myocarditis (inflammation of heart muscle). It is also seen in conditions like allergic drug reactions, sepsis and glomerulonephritis.

It often occurs as a harbinger of attacks in hereditary angioedema. In this case it may occur several hours or up to a day before an attack.

Different types of heat sources can cause this condition such as:

- Repeated application of hot water bottles, heating blankets or heat pads to treat chronic pain—e.g., chronic backache.

- Repeated exposure to heated car seats, space heaters, or fireplaces. Repeated or prolonged exposure to a heater is a common cause of this condition in elderly individuals.

- Occupational hazards of silversmiths and jewellers (face exposed to heat), bakers and chefs (arms)

- Resting a laptop computer on the thigh (laptop computer-induced erythema ab igne). In a 2012 review, Riahi and Cohen describe the characteristics of laptop computer-induced erythema ab igne. Temperatures between 43-47 °C can cause this skin condition; modern laptops can generate temperatures in this range. Indeed, laptops with powerful processors can reach temperatures of 50 °C and be associated with burns. Positioning of the laptop on the thighs can allow for direct exposure to the heating elements of the laptop, which include the central processing unit (CPU) and the graphics processing unit (GPU). At least 15 cases have been reported by 2012 with the condition usually affecting the left anterior thigh. In these reports, 9 of the 15 patients were women (60%) with an average age of 25 years at diagnosis.

- In Kashmir, due to the use of a kanger which also causes kangri cancer

- It is a classic finding in chronic pancreatitis and may also be seen in people with hypothyroidism or lymphoedema

Lipodermatosclerosis is a form of panniculitis associated with chronic venous insufficiency that presents with brown indurations on the front of the shins. It may be associated with pain and other signs of chronic venous insufficiency. The exact cause is unknown.

Other forms include:

- Subcutaneous fat necrosis of the newborn, a form of panniculitis occurring in newborns that is usually self-resolving, that may be a result of hypoxic injury to relatively high levels of brown fat.

- Sclerema neonatorum, affecting premature births.

- Weber–Christian disease, a symmetrical form of the disease of unknown origin occurring in middle-aged women.

- Lupus erythematosus panniculitis, panniculitis associated with lupus erythematosus.

- Forms associated with use of high doses of systemic corticosteroids during rapid corticosteroid withdrawal, and from the injection of silicone or mineral oils.

Erythema multiforme is frequently self-limiting and requires no treatment. The appropriateness of glucocorticoid therapy can be uncertain, because it is difficult to determine if the course will be a resolving one.

Erythema multiforme major (also known as "erythema multiforme majus") is a form of rash with skin loss or epidermal detachment.

The term "erythema multiforme majus" is sometimes used to imply a bullous (blistering) presentation.

According to some sources, there are two conditions included on a spectrum of this same disease process:

- Stevens–Johnson syndrome (SJS)

- Toxic epidermal necrolysis (TEN) which described by Alan Lyell and previously called Lyell syndrome[5].

In this view, EM major, SJS and TEN are considered a single condition, distinguished by degree of epidermal detachment.

However, a consensus classification separates erythema multiforme minor, erythema multiforme major, and SJS/TEN as three separate entities.

Many conditions affect the human integumentary system—the organ system covering the entire surface of the body and composed of skin, hair, nails, and related muscle and glands. The major function of this system is as a barrier against the external environment. The skin weighs an average of four kilograms, covers an area of two square meters, and is made of three distinct layers: the epidermis, dermis, and subcutaneous tissue. The two main types of human skin are: glabrous skin, the hairless skin on the palms and soles (also referred to as the "palmoplantar" surfaces), and hair-bearing skin. Within the latter type, the hairs occur in structures called pilosebaceous units, each with hair follicle, sebaceous gland, and associated arrector pili muscle. In the embryo, the epidermis, hair, and glands form from the ectoderm, which is chemically influenced by the underlying mesoderm that forms the dermis and subcutaneous tissues.

The epidermis is the most superficial layer of skin, a squamous epithelium with several strata: the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. Nourishment is provided to these layers by diffusion from the dermis, since the epidermis is without direct blood supply. The epidermis contains four cell types: keratinocytes, melanocytes, Langerhans cells, and Merkel cells. Of these, keratinocytes are the major component, constituting roughly 95 percent of the epidermis. This stratified squamous epithelium is maintained by cell division within the stratum basale, in which differentiating cells slowly displace outwards through the stratum spinosum to the stratum corneum, where cells are continually shed from the surface. In normal skin, the rate of production equals the rate of loss; about two weeks are needed for a cell to migrate from the basal cell layer to the top of the granular cell layer, and an additional two weeks to cross the stratum corneum.

The dermis is the layer of skin between the epidermis and subcutaneous tissue, and comprises two sections, the papillary dermis and the reticular dermis. The superficial papillary dermis with the overlying rete ridges of the epidermis, between which the two layers interact through the basement membrane zone. Structural components of the dermis are collagen, elastic fibers, and ground substance. Within these components are the pilosebaceous units, arrector pili muscles, and the eccrine and apocrine glands. The dermis contains two vascular networks that run parallel to the skin surface—one superficial and one deep plexus—which are connected by vertical communicating vessels. The function of blood vessels within the dermis is fourfold: to supply nutrition, to regulate temperature, to modulate inflammation, and to participate in wound healing.

The subcutaneous tissue is a layer of fat between the dermis and underlying fascia. This tissue may be further divided into two components, the actual fatty layer, or panniculus adiposus, and a deeper vestigial layer of muscle, the panniculus carnosus. The main cellular component of this tissue is the adipocyte, or fat cell. The structure of this tissue is composed of septal (i.e. linear strands) and lobular compartments, which differ in microscopic appearance. Functionally, the subcutaneous fat insulates the body, absorbs trauma, and serves as a reserve energy source.

Conditions of the human integumentary system constitute a broad spectrum of diseases, also known as dermatoses, as well as many nonpathologic states (like, in certain circumstances, melanonychia and racquet nails). While only a small number of skin diseases account for most visits to the physician, thousands of skin conditions have been described. Classification of these conditions often presents many nosological challenges, since underlying etiologies and pathogenetics are often not known. Therefore, most current textbooks present a classification based on location (for example, conditions of the mucous membrane), morphology (chronic blistering conditions), etiology (skin conditions resulting from physical factors), and so on. Clinically, the diagnosis of any particular skin condition is made by gathering pertinent information regarding the presenting skin lesion(s), including the location (such as arms, head, legs), symptoms (pruritus, pain), duration (acute or chronic), arrangement (solitary, generalized, annular, linear), morphology (macules, papules, vesicles), and color (red, blue, brown, black, white, yellow). Diagnosis of many conditions often also requires a skin biopsy which yields histologic information that can be correlated with the clinical presentation and any laboratory data.