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Recessive X-linked ichthyosis
Recessive X-linked ichthyosis (RXLI) is a genodermatosis belonging to the Mendelian Disorders of Cornification (MeDOC) and characterized by generalized hyperkeratosis and scaling of the skin.
RXLI affects almost exclusively males. It is the second most common type of ichthyosis with an estimated prevalence of 1 / 2,000 to 1 / 6,000 males.
Onset may occur within the first days of life with the development of generalized non-erythematous, polygonal, loosely adherent scales. These scales later evolve into grayish or blackish adherent scales that are pronounced on the trunk, the extensor and flexor sites of the extremities, and the neck (giving the ''dirty-neck'' appearance). Skin's folds, palms and soles, are normally spared. Scaling improves with age and during summer. Delayed birth (insufficient cervical dilatation) may be observed. Extracutaneous manifestations like testicular maldescent and/or corneal opacities are possible for rare cases, if the ichthyosis is part of a disease syndrome (syndromic RXLI; see this term).
X-linked ichthyosis is an epidermal lipid metabolism anomaly due to inactivating mutations or deletions in the steroid sulfatase STS gene (Xp22.3). STS codes for a lipid hydrolase of the stratum corneum that participates in the regulation of permeability barrier homeostasis and desquamation by catalyzing the hydrolysis of steroid sulfates (e.g. cholesterol sulfate CSO4, sulfated steroid hormones). STS deficiency leads to increased amounts of CSO4 that inhibit epidermal serine proteases, which in turn results in decreased desquamation of corneocytes with retention hyperkeratosis. There are some much more rare syndromic RXLI cases that are due to contiguous gene deletion affecting neighbouring genes of the STS gene. This can be observed in Kallman syndrome, hypergonadotropic hypogonadism, ocular albinism type 1 (see these terms), or hypertrophic pyloric stenosis.
Diagnosis is based on clinical findings and family history (scaling in male relatives, history of delayed birth). It is confirmed by biochemical (serum protein electrophoresis, STS activity test of fibroblasts or leukocytes) and molecular/cytogenetic analyses (polymerase chain reaction (PCR), fluorescent in situ hybridization (FISH)). Histology or ultrastructure of the skin is helpful for the differentiation of ichthyosis vulgaris.
Differential diagnosis includes ichthyosis vulgaris, autosomal recessive congenital ichthyosis (ARCI), namely lamellar ichthyosis, syndromic RLXI, or multiple sulfatase deficiency (see these terms).
Maternal urine and serum steroid measurements may show decreased estrogen levels. Therefore, RXLI may be detected in utero, when maternal estriol levels are measured for prenatal screening for Down syndrome and other disorders.
XLRI is transmitted as an X-linked recessive trait: it affects males and is inherited through female carriers. Female patients have rarely been reported.
Management and treatment
Treatment consists in hydrating and softening the skin with the use of lubricating bath oils and emollients containing humectants and keratolytics (e. g. urea, lactic acid, and glycolic acid). For adult patients systemic retinoids are an option, e. g. during winter, when the ichthyosis is often more severe.
RXLI represents a benign form of ichthyosis. The ichthyosis is life-long but hyperkeratosis and scaling may improve with age. Life expectancy is normal.