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The ichthyoses are a family of genetic skin disorders characterized by dry, thickened, scaling skin. There are over twenty types that differ by their underlying genetic cause, outward appearance and/or mode of inheritance. Some are primarily limited to the skin, e.g., Ichthyosis Vulgaris (IV), X-linked ichthyosis (XLI), lamellar ichthyosis/congenital ichthyosiform erythroderma (LI/CIE), epidermolytic hyperkeratosis (EHK), and Netherton syndrome, although the consequences of the skin condition may affect the individual in many ways, both physical and social/psychological (see below). In others the genetic effects may be more generalized and affect other parts of the body, e.g., neutral lipid storage disease, Sjögren-Larsson syndrome, keratitis-ichthyosis-deafness (KID) syndrome and several others. In the ichthyoses, most of the body surface is involved in the process. But in other genetic disorders included under the FIRST umbrella, the rough, scaling, thickened skin may be more limited in distribution, and seen only in patches, or on certain parts of the body. Some of these conditions include Darier disease, pachyonychia congenita, the palmoplantar keratodermas (of which there are many genetic forms), and several others. A more complete list of the FIRST family of disorders is included at the end of this brochure.
Most of the FIRST family of disorders are quite uncommon to rare and affect only one person in several tens of thousands, but there are exceptions. Ichthyosis Vulgaris (IV), ("vulgar" means "common" in Latin), may affect as many as one person in every 250. It is often rather mild in severity and therefore likely that some people with IV are undiagnosed and may think they simply have "dry skin". X-linked ichthyosis (XLI) is somewhat less common, usually more severe than IV, and occurs only in males. Estimates of its frequency range between one in every two-to-six thousand male babies. Lamellar ichthyosis/congenital ichthyosiform erythroderma (LI/CIE) itself is a family of disorders, with at least 6 causative genes; but collectively this group is quite uncommon, occurring in perhaps fewer than one in 100,000 births. Epidermolytic hyperkeratosis (EHK) is similarly uncommon and it too is a family of disorders, with at least 3 causative genes. There is a range of severity in LI/CIE and EHK, with some patients having quite severe involvement, while others may have more limited disease (EHK) or be only mildly to moder-ately affected. These differences are due largely to the effects of the individual’s underlying genetic cause (i.e., the specific mutation).
 “Mode of Inheritance”: (e.g., whether inherited as a dominant or recessive trait). The inheritance patterns of the FIRST family of disorders are discussed in more detail in another FIRST publication, “Ichthyosis: The Genetics of its Inheritance.
Next Chapter: Natural Skin Structure & Function and What goes Wrong in the Ichthyoses