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The ichthyoses are a heterogeneous group of skin diseases characterized by generalized scaling. Scaling reflects altered differentiation of the epidermis. In this chapter, the terms epidermal differentiation, keratinization, and cornification will be used synonymously. The name ichthyosis is derived from the Greek ichthys, meaning “fish,” and refers to the similarity in appearance of the skin to fish scale. Both inherited and acquired forms are found. Early reports of ichthyosis in the Indian and Chinese literature date back to several hundred years bc, and the condition was discussed by Willan in 1808.1
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Ichthyosis can present at birth or develop later in life. It can occur as a disease limited to the skin or in association with abnormalities of other organ systems. A number of well-defined types of ichthyosis with characteristic features can be reliably diagnosed. However, because of the great clinical heterogeneity and the profound effect of the environment on scaling, a specific diagnosis can be challenging in certain patients and families.
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Siemens introduced genetic concepts into the ichthyoses.2 Wells and Kerr classified the ichthyoses on a genetic basis3 and separated X-linked recessive ichthyosis from ichthyosis vulgaris (IV).4 Gassman developed the concept of retention versus hyperproliferation hyperkeratosis5. Van Scott, Frost, and Weinstein subsequently proposed a classification of the ichthyoses based on differences in rates of epidermal turnover, characterizing them as either disorders of epidermal hyperproliferation or disorders of prolonged retention of the stratum corneum.6 Subsequently, Williams and Elias proposed a classification that lists the disorders of cornification in which clinical, genetic, or biochemical data suggest a distinct disease.7 Traupe reviewed the ichthyoses and suggested classification on a clinical level.5
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Genetic approaches to understanding the ichthyoses have revealed the gene defects underlying many of these genodermatoses.8 A new classification is evolving based on the underlying molecular and genetic bases of these disorders. Knowing which gene is mutated directs us to the underlying pathophysiologic process. Understanding and describing these disorders on the basis of common molecular processes leads to more rational approaches to understanding their pathophysiology and treatment. A listing of the more common and the better understood hereditary ichthyoses according to pattern of inheritance and clinical features is shown in Tables 49-1, 49-2, and 49-3. Grouping ...