Defining Eczema

Eczema is a common skin disease. Eczema is dry, rough, red, itchy, skin dryness, crusting, flaking, blistering, cracking, oozing, or bleeding. The most common symptom that people complain about is the itch that is a side effect of eczema. It's sometimes called the 'itch that rashes', meaning that once you start scratching, you develop a rash.

To understand eczema, we must begin with an understanding of the composition of the skin itself. The skin is comprised of three layers: epidermis, dermis and fat.

The outer layer of the skin is the epidermis, which contains sheets of epithelial cells called keratinocytes. These keratinocytes are produced at the junction between the epidermis and the second layer of skin, the dermis. The epidermis is supported from below by the dermis. The epidermis contains many layers of closely packed cells. The cells nearest the skin’s surface are flat and filled with a tough substance called keratin. The epidermis contains no blood vessels – these are all in the dermis and deeper layers. The epidermis is thick in some parts (one millimeter on the palms and soles) and thin in others (just 0.1 millimeter over the eyelids). Dead cells are shed from the surface of the epidermis as very fine scales, and are replaced by other cells, which pass from the deepest (basal) layers to the surface layers over a period of about four weeks. The dead cells on the surface take the form of flattened, overlapping plates, closely packed together. This layer is known as the stratum corneum and is remarkably flexible, more or less waterproof and has a dry surface so that it is inhospitable to microorganisms.

The second layer of the skin or dermis is made up of connective tissue, which contains a mixture of cells that give strength and elasticity to the skin. This layer also contains blood vessels, hair follicles and roots, nerve endings, and sweat and lymph vessels and glands. The elements of the dermis all carry messages or fluids to and from the epidermis so it can grow, respond to the outside world and react to what goes on inside the body.

Underneath the dermis is a layer of fat, which acts as an important source of energy and water for the dermis. It also provides protection against physical injury and the cold.

In eczema, the main problems occur in the epidermis where the keratinocytes become less tightly held together. As a result, they become vulnerable to external factors such as soap, water and more aggressive solvents such as those used as part of work or hobbies. These solvents dissolve some of the grease and protein that contribute to the natural barrier of the skin. Once this process has begun, the skin may become inflamed as a reaction to minor irritation such as rubbing or scratching. This, in turn, makes the eczema worse and a cycle of irritation, inflammation, and deterioration of eczema becomes established.

As part of this cycle, the skin becomes less effective as a barrier. It is less effective at preventing damage from solvents and abrasive materials acting from the outside, and it is also more likely to lose body moisture from within. In a small patch of eczema, this can mean just a few vesicles (very small bubbles in the skin) bursting and leaking water. As the eczema gets worse, the fluid may come from the dermis and include blood from broken capillaries. When severe eczema covers a large percentage of the body surface, it is possible to lose substantial amounts of body fluid, blood and protein through the skin. In addition to these materials, the body can lose heat from the skin, which can become important in people who are physically infirm. The barrier function of the skin is reduced further when scratching occurs and breaks are gouged in the skin by fingernails. As with solvents, this fuels the eczema and is termed the 'itch–scratch cycle.' When skin becomes broken and there is a mix of blood, fluid, and protein on the surface, there is a high chance of infection. This infection is usually bacterial and will add to the symptoms and severity of the eczema.

The epidermis is where the environment collides with the body's immune system. Usually the immune system reacts only to parts of the outside world that present a danger, such as insect bites. In many people with eczema, however, the immune system reacts more vigorously than usual to a wider range of normally harmless influences such as animal dander (small particles of hair or feathers), pollen and house-dust mite. As these trigger allergic reactions, these substances are known as allergens. The immune system tries to destroy allergens by releasing a mixture of its own irritant substances, such as histamine, into the skin. The result is that the allergen may be altered or removed, but at the expense of causing soreness and making the skin fragile so other problems can develop, such as bacterial infection or damage from scratching.