Ichthyoses at a Glance
- A heterogeneous group of skin diseases characterized by generalized scaling, and often areas of thickened skin.
- Most types are inherited, and these usually present at birth. However, some forms are acquired.
- Scales may vary in size, color, and body site.
- May be accompanied by erythema, abnormalities in other parts of skin and adnexal structures.
- May be associated with systemic findings, such as failure to thrive, increased susceptibility to infection, atopic dermatitis, neurosensory deafness, neurologic and other disease.
- Pathology is usually nonspecific with the exception of epidermolytic hyperkeratosis, neutral lipid storage disease, Refsum disease, and acquired ichthyosis associated with sarcoidosis.
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
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.
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
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 these disorders ...