The ichthyoses are 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 or appear in childhood; however, some forms are acquired.
Scales may vary in size, color, and body site.
The may be accompanied by erythema, abnormalities in adnexal structures, and palmoplantar keratoderma.
The may be associated with systemic findings, such as failure to thrive, increased susceptibility to infection, atopic dermatitis, neurosensory deafness, and neurologic and other disease.
Histopathology is usually nonspecific with few notable exceptions.
Early genetic testing can aid in the diagnosis and anticipation of potential systemic abnormalities.
This chapter discusses the heterogeneous group of disorders known as the ichthyoses that share the common feature of generalized scaling and most commonly arise at birth or childhood but can be acquired later in life. Ichthyoses commonly result from genetic mutations in a diverse group of genes, including membrane transporters, lipid biosynthesis enzymes, and structural proteins, among many others. Ensuing epithelial barrier compromise, hyperproliferation, and hyperkeratosis become evident as scaling. Associated systemic abnormalities are often attributable to noncutaneous functions of mutant genes.
The hallmark of ichthyosis is scale, which reflects altered differentiation of the epidermis. Because the Greek word for horn (scale) is keras and the Latin is cornu, in this chapter, the terms epidermal differentiation, keratinization, and cornification are 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 1798.1
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 without the aid of genetic testing.
CLASSSIFICATION OF THE ICHTHYOSES
Siemens introduced genetic concepts into the ichthyoses.2 Wells and Kerr classified the heritable ichthyoses3 and separated X-linked recessive ichthyosis from ichthyosis vulgaris (IV).4 Gassman developed the concept of retention versus hyperproliferation hyperkeratosis.5 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