The ichthyoses represent a large group of cutaneous disorders linked by the common finding of abnormal epidermal differentiation. These disorders are characterized by the cutaneous scaling, which is said to resemble the scales of a fish. Scaling can be localized or generalized and can be associated with a variety of additional cutaneous and/or systemic manifestations. In patients with ichthyosis, the barrier function of the skin is compromised and has a decreased ability to protect against bacterial or chemical assault and to prevent transepidermal water loss. Ichthyosis vulgaris (
H00735) is the most frequent type. X-linked ichthyosis (
H00134) occurs almost exclusively in boys. Autosomal recessive congenital ichthyosis (
H00734) is genetically very heterogeneous and several different genes have been identified. Mutations in keratin genes are the cause of the keratinopathic ichthyoses (
H00691), such as epidermolytic ichthyosis.
