Erythroderma is generalized exfoliative dermatitis, which involves 90% or more of the patient's skin. The most common cause of erythroderma is exacerbation of an underlying skin disease, such as psoriasis, contact dermatitis, seborrheic dermatitis, lichen planus, pityriasis rubra pilaris or a drug reaction. Primary erythroderma is less frequent and is usually seen in cases of cutaneous T-cell lymphoma, in particular in Sézary's disease.

The most common causes of exfoliative dermatitis are best remembered by the mnemonic device ID-SCALP. The causes and their frequencies are as follows:

- Idiopathic - 30%

- Drug allergy - 28%

- Lymphoma and leukemia - 14%

- Atopic dermatitis - 10%

- Psoriasis - 8%

- Contact dermatitis - 3%

- Seborrheic dermatitis - 2%

Differential diagnosis in patients with erythroderma may be difficult.

A stinging and burning sensation with rash is often felt and noticed, but itching is less common. Often the rash is steroid responsive, initially improving with application of topical steroid. The redness caused by perioral dermatitis has been associated with variable level of depression.

Initially, there may be small pinpoint papule either side of the nostrils. Multiple small (1-2mm) papules and pustules then occur around the mouth, nose and sometimes cheeks. The area of skin directly adjacent to the lips, also called the vermillion border, is spared and looks normal. There may be some mild background redness and occasional scale. These areas of skin are felt to be drier and therefore there is a tendency to moisturise them more frequently. Hence, they do not tolerate drying agents well and the rash can be worsened by them.

Perioral dermatitis is also known by other names including rosacea-like dermatoses, periorofacial dermatitis and periorificial dermatitis.

Unlike rosacea which involves mainly the nose and cheeks, there is no telangiectasia in perioral dermatitis. Rosacea also has a tendency to be present in older people. Acne can be distinguished by the presence of comedones and by its wider distribution on the face and chest. There are no comedones in personal dermatitis.

Other skin diseases which may resemble perioral dermatitis include:

- Rosacea

- Acne vulgaris

- Seborrheic dermatitis

- Allergic contact dermatitis

- Irritant contact dermatitis

Erythroderma (also known as "Exfoliative dermatitis," "Dermatitis exfoliativa") is an inflammatory skin disease with erythema and scaling that affects nearly the entire cutaneous surface.

In ICD-10, a distinction is made between "exfoliative dermatitis" at L26, and "erythroderma" at L53.9.

Nummular dermatitis is characterized by chronic or relapsing itchy coin-sized ovoid-shaped red plaques. They can occur on the trunk, limbs, face, and hands.

Perioral dermatitis is a type of skin rash. Symptoms include multiple small (1–2 mm) bumps and blisters with sometimes background redness and scale, localized to the skin around the mouth and nostrils. Less commonly the eyes and genitalia may be involved. It can be persistent or recurring and resembles particularly rosacea and to some extent acne and allergic dermatitis. The term dermatitis is a misnomer because this is not an eczematous process.

The cause is unclear. Topical steroids are associated with the condition and moisturizers and cosmetics may contribute. The underlying mechanism may involve blockage of the skin surface followed by subsequent excessive growth of skin flora. Fluorinated toothpaste and some micro-organisms including candida may also worsen the condition, but their roles in this condition is unclear. It is considered a disease of the hair follicle with biopsy samples showing microscopic changes around the hair follicle. Diagnosis is based on symptoms.

Treatment is typically by stopping topical steroids, changing cosmetics, and in more severe cases, taking tetracyclines by mouth. Stopping steroids may initially worse the rash. The condition is estimated to affect 0.5-1% of people a year in the developed world. Up to 90% of those affected are women between the ages of 16 and 45 years, though it also affects children and the elderly, and has an increasing incidence in men.

Diagnosis of nummular dermatitis largely clinical. Biopsies are typically not necessary, and cannot be used to rule out other atopic dermatitis or other eczemas. However, patch testing may be employed to rule out irritants (contact dermatitis) as a cause. In children, nummular dermatitis is commonly confused with tinea corporis.

Irritant diaper dermatitis is a generic term applied to skin rashes in the diaper area that are caused by various skin disorders and/or irritants.

Generic "diaper rash" or "irritant diaper dermatitis" (IDD) is characterized by joined patches of erythema and scaling mainly seen on the surfaces, with the skin folds spared.

Diaper dermatitis with secondary bacterial or fungal involvement tends to spread to surfaces (i.e. skin folds), as well as convex surfaces, and often exhibits a central red, beefy erythema with satellite pustules around the border.

It is usually considered a form of irritant contact dermatitis. The word "diaper" is in the name not because the diaper itself causes the rash but rather because the rash is associated with diaper use, being caused by the materials trapped by the diaper (usually feces). Allergic contact dermatitis has also been suggested, but there is little evidence for this cause. In adults with incontinence (fecal, urinary, or both), the rash is sometimes called incontinence-associated dermatitis (IAD).

The term diaper candidiasis is used when a fungal origin is identified. The distinction is critical because the treatment (antifungals) is completely different.

The diagnosis of IDD is made clinically, by observing the limitation of an erythematous eruption to the convex surfaces of the genital area and buttocks. If the diaper dermatitis occurs for greater than 3 days it may be colonized with "Candida albicans", giving it the beefy red, sharply marginated, appearance of diaper candidiasis.

Intertrigo refers to a type of inflammatory rash (dermatitis) of the superficial skin that occurs within a person's body folds. These areas are more susceptible to irritation and subsequent infection due to factors that promote skin breakdown such as moisture, friction, and exposure to bodily secretions such as sweat, urine or feces. Areas of the body which are more likely to be affected by intertrigo include the inframammary fold, intergluteal cleft, armpits, and spaces between the fingers or toes. Skin affected by intertrigo is more prone to infection than intact skin.

The term "intertrigo" commonly refers to a secondary infection with bacteria (such as "Corynebacterium minutissimum"), fungi (such as "Candida albicans"), or viruses. A frequent manifestation is candidal intertrigo.

Intertrigo occurs more often in warm and humid conditions. Generally, intertrigo is more common in people with a weakened immune system including children, the elderly, and immunocompromised people. The condition is also more common in people who experience urinary incontinence and decreased ability to move.

The signs and symptoms of dandruff are an itchy scalp and flakiness. Red and greasy patches of skin and a tingly feeling on the skin are also symptoms.

Discoid eczema (nummular eczema, exudative eczema, microbial eczema) is characterized by round spots of oozing or dry rash, with clear boundaries, often on lower legs. It is usually worse in winter. Cause is unknown, and the condition tends to come and go. (ICD-10 L30.0)

Venous eczema (gravitational eczema, stasis dermatitis, varicose eczema) occurs in people with impaired circulation, varicose veins, and edema, and is particularly common in the ankle area of people over 50. There is redness, scaling, darkening of the skin, and itching. The disorder predisposes to leg ulcers. (ICD-10 I83.1)

Seborrhoeic dermatitis' symptoms appear gradually, and usually the first signs are flaky skin and scalp. Symptoms occur most commonly anywhere on the skin of the scalp, behind the ears, on the face, and in areas where the skin folds. Flakes may be yellow, white or grayish. Redness and flaking may also occur on the skin near the eyelashes, on the forehead, around the sides of the nose, on the chest, and on the upper back.

In more severe cases, yellowish to reddish scaly pimples appear along the hairline, behind the ears, in the ear canal, on the eyebrows, on the bridge of the nose, around the nose, on the chest, and on the upper back.

Commonly, patients experience mild redness, scaly skin lesions and in some cases hair loss. Other symptoms include patchy scaling or thick crusts on the scalp, red, greasy skin covered with flaky white or yellow scales, itching, soreness and yellow or white scales that may attach to the hair shaft.

Seborrhoeic dermatitis can occur in infants younger than three months and it causes a thick, oily, yellowish crust around the hairline and on the scalp. Itching is not common among infants. Frequently, a stubborn diaper rash accompanies the scalp rash. Usually, when it occurs in infants the condition resolves itself within days and with no treatment.

In adults, symptoms of seborrhoeic dermatitis may last from a few weeks, to years. Many patients experience alternating periods of inflammation. The condition is referred to a specialist when self-care has proven unsuccessful.

Redness around the lips in circumoral distribution with dryness and scale is typical. Chapping may also occur, especially in cold weather.

An intertrigo usually develops from the chafing of warm, moist skin in the areas of the inner thighs and genitalia, the armpits, under the breasts, the underside of the belly, behind the ears, and the web spaces between the toes and fingers. An intertrigo usually appears red and raw-looking, and may also itch, ooze, and be sore. Intertrigos occur more often among overweight individuals, those with diabetes, those restricted to bed rest or diaper use, and those who use medical devices, like artificial limbs, that trap moisture against the skin. Also, there are several skin diseases that can cause an intertrigo to develop, such as dermatitis or inverse psoriasis.

Acneiform eruptions are a group of dermatoses including acne vulgaris, rosacea, folliculitis, and perioral dermatitis. Restated, acneiform eruptions are follicular eruptions characterized by papules and pustules resembling acne.

The hybrid term "acneiform", literally, refers to an appearance similar to acne.

The terminology used in this field can be complex, and occasionally contradictory. Some sources consider acne vulgaris part of the differential diagnosis for an acneiform eruption. Other sources classified acne vulgaris under acneiform eruption. MeSH explicitly excludes perioral dermatitis from the category of "acneiform eruptions", though it does group acneiform eruptions and perioral dermatitis together under "facial dermatoses".

IBS has symptoms very similar to epidermolytic hyperkeratosis (EHK) but is generally milder than EHK. IBS affects only the upper layers of the epidermis whilst EHK affects the suprabasal layer which is deeper in the

skin.

At birth the baby's skin has a red appearance like a sun burn (erythema). Blistering is usually present at birth and may be extensive or localized depending on the severity of the disease.

Over the first few weeks the redness disappears and is replaced by dry, flaking skin on the arms, legs and around the belly button. Other areas of skin appear normal. The skin is fragile and is prone to blistering (caused by mild trauma or sweating). After a few months hyperkeratosis develops with a dark grey or brown, ridged appearance on the ankles, knees and elbows. Palms and soles are generally unaffected. A slightly unpleasant, sweet odour may be present.

A distinctive characteristic of IBS which is not present in other forms of ichthyosis is called the "Mauserung phenomenon" (Mauserung is German for "moulting" and was first described by H.W.Siemens). These are small patches of bare, apparently normal skin in the middle of areas of hyperkeratosis.

As the sufferer ages the flaking and blistering should improve. The hyperkeratosis may grow more severe but more localized and is generally only present on flexural folds of the major joints.

Repeated licking resulting in a cycle of wetting and drying causes the redness, fissuring and scale. IIt can also occur with lip chewing, thumb sucking or excessive drooling.

Wind instrument players may also experience lip licker's dermatitis.

Compulsive licking of lips causing lick lip dermatitis is also seen as psychological disorder.

Persistent and continuous breathing from the mouth can cause dry lips and result in temptation to repeated lick lips.

The intense contact between a musical instrument and skin may exaggerate existing skin conditions or cause new skin skin conditions. Skin conditions like hyperhidrosis, lichen planus, psoriasis, eczema, and urticaria may be caused in instrumental musicians due to occupational exposure and stress. Allergic contact dermatitis and irritant contact dermatitis are the most common skin conditions seen in string musicians.

Keratolysis exfoliativa, (also known as "Lamellar dyshidrosis,", "Recurrent focal palmar peeling," "Recurrent palmar peeling") is a sometimes harmless, sometimes painful skin condition that can affect the focal surface of the fingers and/or the palm or soles of the feet. It is often misdiagnosed as chronic contact dermatitis or psoriasis. It is characterized by dry skin and superficial, air-filled blisters. These blisters can be peeled off very easily and will leave reddish, tender areas. The loss of this horny layer of the skin, which protects the underlying layers, leaves the skin more vulnerable to dryness and cracking.

Frequent, chronic contact of instruments to skin may make it callous by the thickening of stratum corneum. Use of 'thumb position' in cellists may cause callosity of left thumb. Garrod's pads are seen on the dorsal left second and third fingers over the proximal interphalangeal joints in violinists. Drummer's digit is the callosity seen on the lateral phalynx of the left finger. Callosities need treatment only when they are excessive or symptomatic.

Dandruff is a skin condition that affects mainly the scalp. Symptoms include flaking and sometimes mild itchiness. It can result in social or self-esteem problems. A more severe form of the condition, which includes inflammation of the skin, is known as seborrheic dermatitis.

The cause is unclear but believed to involve a number of genetic and environmental factors. The condition may worsen in the winter. It is not due to poor hygiene. The underlying mechanism involves the over growth of skin cells. Diagnosis is based on symptoms.

There is no known cure. The typical treatment is with antifungal cream such as ketoconazole. Dandruff affects about half of adults. Onset is usually at puberty. Males are more often affected than females. Rates decrease after the age of 50.

People with AD often have dry and scaly skin that spans the entire body, except perhaps the diaper area, and intensely itchy red, splotchy, raised lesions to form in the bends of the arms or legs, face, and neck.

AD commonly occurs on the eyelids where signs such as Dennie-Morgan infraorbital fold, infra-auricular fissure, periorbital pigmentation can be seen. Post-inflammatory hyperpigmentation on the neck gives the classic 'dirty neck' appearance. Lichenification, excoriation and erosion or crusting on the trunk may indicate secondary infection. Flexural distribution with ill-defined edges with or without hyperlinearily on the wrist, finger knuckles, ankle, feet and hand are also commonly seen.

Pityriasis amiantacea (also known as "Tinea amiantacea") is an eczematous condition of the scalp in which thick tenaciously adherent scale infiltrates and surrounds the base of a group of scalp hairs. It does not result in scarring or alopecia.

Pityriasis amiantacea was first described by Alibert in 1832. Pityriasis amiantacea affects the scalp as shiny asbestos-like (amiantaceus) thick scales attached in layers to the hair shaft. The scales surround and bind down tufts of hair. The condition can be localised or covering over the entire scalp. Temporary alopecia and scarring alopecia may occur due to repeated removal of hairs attached to the scale. It is a rare disease with a female predilection.

Pityriasis amiantacea can easily be misdiagnosed due its close resemblance to other scalp diseases such as psoriasis, seborrhoeic dermatitis or lichen planus. However in pityriasis amiantacea the scales are attached to both the hair shaft and the scalp. Pityriasis amiantacea may be present with other inflammatory conditions such as atopic dermatitis or seborrhoeic dermatitis and sebaceous scales and alopecia can occur. According to the dermatology text Bolognia this condition is most often seen in psoriasis, but may also be seen in secondarily infected atopic dermatitis, seborrheic dermatitis, and tinea capitis.

Autoimmune progesterone dermatitis may appear as urticarial papules, deep gyrate lesions, papulovesicular lesions, an eczematous eruption, or as targetoid lesions. Autoimmune progesterone dermatitis initially manifests with eye symptoms, e.g. burning, and progresses into rashes. Its relapsing-remitting pattern in women correspond to the progesterone levels during the menstrual cycle, which spike twice a month. It is an extremely rare disease, only perhaps 50 cases have ever been reported.