X-linked ichthyosis is a genetic skin disorder that affects males. It is an inborn error of metabolism characterized by a deficiency of the enzyme steroid sulfatase. Under normal conditions, this enzyme breaks down (metabolizes) cholesterol sulfate, a member of the chemical family of steroids. Cholesterol sulfate plays a role in maintaining the integrity of the skin. If steroid metabolism is interrupted and cholesterol sulfate accumulates in the skin cells, the skin cells stick together more strongly than usual. The normal shedding of dead skin cells is inhibited and the skin cells build up and clump into scales.
Boys with X-linked ichthyosis appear normal at birth. The skin symptoms generally appear within the first year of life. Brownish scales that adhere to the skin are among the first signs of the disorder. The back and legs are most frequently involved early. The face, scalp, palms and soles, and hollows of the elbows and knees are usually spared.
In about half of adult males, comma-shaped corneal opacities occur in the eyes (seen on exam by an ophthalmologist), but they do not interfere with vision. Symptoms can improve markedly in the summer months and warm humid climates.
A small percentage of males may experience undescended testes (crytpchordism). These men may be at increased risk for contracting malignancies of the testes.
Women who are carriers of X-linked ichthyosis and give birth to sons with the disorder may experience a delay in labor or failure of labor to initiate. The enzyme defect can cause a decrease in production of maternal estriol in late pregnancy, which may affect labor and delivery. Low serum estriol levels detected by prenatal screening suggest the presence of a fetus with X-linked ichthyosis.
X-linked recessive disorders are conditions that are coded on the X chromosome. Females have two X chromosomes; males have one X chromosome and one Y chromosome. Therefore, in females, the normal gene on one X chromosome can mask disease traits on the other X chromosome. Since males have only one X chromosome, if they inherit the gene for a disease present on the X they will express the disease. Men with X-linked disorders transmit the gene to all their daughters, who are carriers, but never to their sons. Women who are carriers of an X-linked disorder have a 50 percent chance of transmitting the carrier condition to their daughters and a 50 percent risk of transmitting the disease to their sons.
X-linked ichthyosis can be diagnosed before birth by amniocentesis or chorionic villus sampling. Low maternal estriol levels can suggest the presence of X-linked ichthyosis.
X-linked ichthyosis is treated by applying skin softening creams and lotions. This can be especially effective after bathing while the skin is still moist. X-linked ichthyosis responds relatively well to topical treatment with alpha-hydroxy acids, which accelerate the shedding of the dead skin cells. Cholesterol containing emollients may also improve the scaling. Alpha-hydroxy acids may sting the skin of babies and young children and should be used cautiously or in combination with another mild emollient product.
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Oral retinoids, drugs derived from vitamin A, can be effective against some forms of ichthyosis. However, X-linked ichthyosis is not considered severe enough to warrant the use of oral retinoids.
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