Dermatitis is the name given to inflammatory lesions in the skin, irrespective of whether they involve dermis or epidermis; in most cases both components of the skin are involved.

Some patterns of inflammatory skin disease have characteristic features, which enable them to be identified more accurately.
Other patterns are caused by micro-organisms such as bacteria or other organisms, but many are non-specific.

ECZEMA

The most common pattern of non-specific dermatitis is traditionally called eczema, and may have many causes.
Non-specific (eczematous) dermatitis may be acute, sub-acute or chronic

In acute dermatitis the skin becomes red, itchy and tender, with tiny blisters called vesicles forming in the epidermis {\I (Fig. 21.1).} The vesicles burst, discharging clear, yellow fluid, and then crust over.
The reddening of the skin is due to a chronic inflammatory cell infiltrate around blood vessels in the upper dermis; leakage of fluid from the vessels may produce swelling of the upper dermis, which may cause the lesion to be slightly raised above the level of normal skin. 

Vesicles develop because fluid accumulates between the epidermal cells (spongiosis), eventually forming small fluid-filled collections.
Some of the chronic inflammatory cells around the vessels in the upper dermis may migrate into the epidermis. The vesicles enlarge until they burst onto the surface.
Because the lesion is itchy in the acute stage, it is almost invariably scratched by the patient. Repeated trauma (scratching) to lesions of acute eczema leads to chronic dermatitis.

CHRONIC DERMATITIS IS USUALLY THE RESULT OF CHRONIC TRAUMA TO ACUTE DERMATITIS

Chronic non-specific dermatitis is usually the result of chronic trauma to acute dermatitis lesions.
The skin is thickened, often cracked, and covered by thick, opaque scale.
This scale is a greatly thickened layer of surface keratin (hyperkeratosis), which overlies an epidermis markedly thickened by increase in the number of cells in the various layers (particularly stratum spinosum and the granular layer).
Such thickening, termed acanthosis, is a common feature of chronic inflammatory skin diseases of many types. The epidermis also shows elongation and accentuation of the rete ridge system, an arrangement that, in normal skin, is designed to resist shearing forces. The dermis shows increased fibrosis, with prominent thick-walled vessels.

Constant scratching and picking at itchy inflammatory lesions in the skin leads to localized lesions with greatly thickened epidermis and thick horny keratin scale on top, together with localized fibrous thickening of the dermis Called prurigo nodularis, the lesions often continue to be picked and traumatized so that there is surface ulceration, a lesion sometimes called picker's nodule.

The term sub-acute dermatitis is sometimes used to describe skin inflammation in which there are features of chronic dermatitis (acanthosis, hyperkeratosis, dermal fibrosis, etc.), but with the addition of active spongiosis and vesicle formation.

The cause of non-specific dermatitis usually has to be inferred from the distribution and nature of the rash rather than from histological differences

Among the most common types of non-specific dermatitis seen in community practice is atopic dermatitis.
Often starting in infancy and childhood but persisting into adult life, this pattern is associated with a strong family history, e.g. the patient and other family members may have asthma, hayfever or a predisposition to urticarial skin rashes.

Gravitational dermatitis affects the ankle and lower leg of patients with varicose veins, and is sometimes called varicose eczema.
The dermatitis features are superimposed on chronic changes in the skin due to inadequate venous drainage, e.g. thick-walled dermal vessels or extravasation of red blood cells (leading to brown pigmentation by haemosiderin).

Irritant contact dermatitis, which is due to contact of the skin with strong agents such as detergents and alkalis, most commonly affects the hands. The disease is particularly common in housewives who do not wear gloves when handling strong detergents, and in some people who have occupational exposure.

Allergic contact dermatitis is usually a reaction to metals such as nickel, materials in cosmetics, dye mixtures and rubber. It tends to occur in people who have a probable inherited predisposition, and the lesions are localized to the site of contact, e.g. around the wrist in response to nickel in the back or strap of a watch.

In {seborrhoeic dermatitis, the reddened and inflamed skin is covered by thick waxy or white scale. In infants this is largely confined to the scalp (cradle cap), whereas in adults the face is also involved. Obese elderly people may develop this pattern in skin creases. This condition has recently been linked with Pityrosporum yeasts.

Some types of specific dermatitis have characteristic histological and clinical appearances

Lichen planus is a common inflammatory skin disease that often affects the flexor aspects of the forearm, wrist and ankle.
It also occurs on the trunk, as well as affecting various mucosal surfaces, particularly in the mouth and vulva; it occasionally arises on the penis.
On the skin the lesions are raised, itchy papules, often purplish red in colour, which sometimes form flat and shiny raised plaques, The cause is unknown and the lesions often persist for many months or even years.
When each lesion resolves, it frequently leaves a pigmented patch of flat skin. The main histological abnormality in lichen planus is damage to the basal layer of the epidermis, destroying both basal cells and any contained melanocytes.
It is this destruction of basal melanocytes that allows melanin to drop down into the dermis, where it accumulates in dermal macrophages, giving a healed lesion a brown discoloration. 
Associated with the destruction of the basal layer is a characteristic pattern of lymphocytic infiltration, which is closely applied to the dermoepidermal junction in the upper dermis. This pattern of inflammation is called lichenoid and can occasionally be seen in other skin conditions.
Lichen planus affecting the buccal and genital mucosae has a tendency to lead to separation of the epidermis from the underlying sub-mucosa, as a result of destruction of the basal layer of the epidermis.
The ensuing erosion leaves naked areas of inflamed sub-mucosa; this change rarely occurs in skin.
Another variant of lichen planus, follicular lichen planus, causes destruction of the basal layer of hair follicles, which may result in hair loss; follicular lichen planus is one of the causes of inflammatory alopecia .