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Xerotic eczema is the inflammatory end stage of xerosis. This process begins when, because of insufficient and on the surface of the skin, epidermal water loss resulting in evaporation exceeds replenishment from below. In this setting, epithelial cells shrink to the point where islands of cells separate in a manner similar to that seen on the dirt bed of a dried-up lake. The term "xerotic eczema," as opposed 10 simple xerosis, is used when the cracks and fissures are (leep enough to cause visible inflammatory changes.
Factors that reduce surface lipid and thus enhance water loss include aging, excess bathing, and excess rubbing of the skin. All of these factors are particularly troublesome for atopic individuals, since they constitutionally have skin that is drier than normal. Environmental factors also adversely influence water loss from the skin. Thus low humidity, especially when building air is heated in the winter, shifts the equilibrium toward increased water loss. The presence of air flow such as occurs with wind and fan-driven air enhances evaporation and thus aggravates the effect of low humidity.
Therapy
The therapy of xerotic eczema depends on reducing the rate of water loss from the skin. This is accomplished by reducing the removal of natural lipid and by the addition of artificial lipid. Lipid loss can be lessened primarily by changing bathing habits. Patients should bathe less frequently (every other day), use cooler water, decrease the use of soap, and pat, rather than rub, the skin dry. Artificial lipid is added through the process of lubrication . Moisturizing creams ("hand creams") should be applied 4 to 8 times/day on the hands and twice daily on the trunk and extremities. Lubricants are particularly helpful when applied to wet skin immediately after bathing, since they then trap additional moisture before evaporation occurs. Bath oils are often recommended, but they are not easily used in the showers that most people prefer, and in any event, their overall role in skin lubrication is minor.
Inflammation, when present, must also be treated. This is accomplished through the twice daily application of mid-potency steroids. In the setting of xerosis an ointment base is frequently preferable to a cream base. Rarely, short-term use of systemic steroids is also necessary.
Scratching, when present, must also be controlled, lest further epithelial destruction occur.
Pruritus disappears quickly after treatment in most individuals. No new lesions develop, and all evidence of infestation ordinarily disappears within 7 to 14 days. Approximately 20% of patients, however, are apparent treatment failures. In about half of these patients new lesions continue to appear, and one must assume that either therapy has been inadequately carried out or that reinfection has occurred. Such patients and all of their contacts should be retreated. In the other half, no new lesions develop, but itching and scratching persist at the site of old lesions. These individuals, most of whom are genetically atopic, have developed an itchÂscratch cycle and will continue to scratch indefinitely unless topical steroids and antihistamines are used to break up the cycle.
Scabies, if left untreated, persists for years. During this time there is a gradual resolution of old lesions and subsequent development of new lesions. This prolonged course accounts for the colloquial name of the disease" the 7-year itch."
Pathogenesis
Scabies is due to an infestation with the human variety of the mite Sarcoptes scabiei. The female of the species burrows within the stratum corneum, depositing eggs that over a 3-week period mature into adult mites. The adult mite is just at the threshold of visibility and can sometimes be recognized as a tiny red or brown-red dot at the end of an intact burrow.
Transmission of the disease ordinarily depends on direct person-to-person contact. In a small percentage of cases, however, the contagion occurs through the use of shared clothing or bed linen. Infestation occurs in individuals at all ages and from all socioeconomic groups, but for unknown reasons it rarely develops in blacks. Once present, the disease spreads by scratching; mites and eggs are transferred from one location to another via fingernail contamination.
Early in the course of infestation there is little in the way of host inflammatory reaction, but after several weeks, allergic sensitization with accompanying pruritus and inflammation takes place. The role of immunologic response in the resolution of scabies has not been adequately studied, but the occurrence of epidemics at 20- to 30-year intervals suggests that the development and waning of herd immunity may be important from an epidemiologic standpoint.
Therapy
For many years, lindane (gamma-benzene), sold under the trade names Kwell and Scabene, has been the treatment of choice for scabetic infestations. It is quite effective and relatively cheap, but systemic absorption does occur and is associated with the potential problem of neurotoxicity. Expression of this potential problem only occurred when lindane was used for infants, but for this reason its use has been generally supplanted by 5% permethrin (Elimite). Crotamiton (Eurax) and precipitated sulfur have also been used historically, but neither is as effective as lindane or permethrin.
Both lindane and permethrin are used in a similar manner. Patients and all of their close contacts are instructed to bathe and then apply a thin layer of the medication over the entire body. This is left in place for 8 to 12 hours, at which time the patient bathes a second time. Clothing or bed linen used prior to or during therapy is washed with soap and water in a normal fashion. No special attention is needed for furniture and other inanimate objects. A single treatment carried out in this manner will result in clearing of 80% of patients. As mentioned above, an additional 10% will require retreatment because of problems with therapeutic compliance or reinfection. The remaining 10% will require the additional use of steroids and antihistamines in order to break up the itch-scratch cycle.