Incidence of moderate to severe ichthyosis in the United States.

M ost statistics on the incidence of hereditary ichthyosis are acknowledged as best-guess estimates. The exceptions are population-based studies for the prevalence of steroid sulfatase deficiency (xlinked ichthyosis) and mutations in filaggrin (ichthyosis vulgaris). We wondered whether the unique International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) code for ichthyosis might be used to gather incidence information from large administrative claims databases. We reasoned that moderate to severe ichthyosis is clinically distinctive and usually diagnosed at birth and that infants would not have manifested ichthyosis vulgaris (the common, mild form of ichthyosis), forms of nonhereditary “acquired” ichthyosis, or other skin diagnoses easily confused with ichthyosis.