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Ichthyosis

Ichthyosis

Definition

Derived from two Greek words meaning "fish" and "disease," ichthyosis is a congenital (meaning present at birth) dermatological (skin) disease that is represented by thick, scaly skin.

Description

The ichthyoses are a group of skin diseases caused by an abnormality in skin growth that results in drying and scaling. There are at least 20 types of ichthyosis. Ichthyosis can be more or less severe, sometimes accumulating thick scales and cracks that are painful and bleed. Ichthyosis is not contagious. Some forms of ichthyosis are inherited while others are acquired in later life as a symptom of systemic disorders.

The most common form of ichthyosis, accounting for 95% of all cases of ichthyosis, is called ichthyosis vulgaris (vulgaris is the Latin word for "common"), and occurs in approximately one person in every 250. It is inherited in an autosomal dominant manner. The most rare types of ichthyosis occur in fewer than one person in one million and are inherited in an autosomal recessive manner. Ichthyosis occurs regardless of the part of the world the child is from, or the ethnic background of the parents.

Acquired ichthyosis is extremely rare; neither its incidence in the United States nor its incidence in the rest of the world are known as of 2003.

Both inherited and acquired ichthyoses affect males and females equally.

Causes and symptoms

Inherited ichthyoses

Depending on the specific type of ichthyosis, the inheritance can be autosomal recessive, autosomal dominant, X-linked recessive, X-linked dominant, or sporadic. Autosomal recessive means that the altered gene for the disease or trait is located on one of the first 22 pairs of chromosomes, which are also called "autosomes." Males and females are equally likely to have an autosomal recessive disease or trait. Recessive means that two copies of the altered gene are necessary to express the condition. Therefore, a child inherits one copy of the altered gene from each parent, who are called carriers (because they have only one copy of the altered gene). Since carriers do not express the altered gene, parents usually do not know they carry the altered gene that causes ichthyosis until they have an affected child. Carrier parents have a 1-in-4 chance (or 25%) with each pregnancy, to have a child with ichthyosis.

Autosomal dominant inheritance also means that both males and females are equally likely to have the disease but only one copy of the altered gene is necessary to have the condition. An individual with ichthyosis has a 50/50 chance to pass the condition to his or her child.

The skin is made up of several layers, supported underneath by a layer of fat that is thicker or thinner depending on location. The lower layers contain blood vessels, the middle layers contain actively growing cells, and the upper layer consists of dead cells that serve as a barrier to the outside world. This barrier is nearly waterproof and highly resistant to infection. Scattered throughout the middle layers are hair follicles, oil and sweat glands, and nerve endings. The upper layer is constantly flaking off and being replaced from beneath by new tissue. In ichthyosis, the skin's natural shedding process is slowed or inhibited; and in some types, skin cells are produced too rapidly.

The abnormality in skin growth and hydration called ichthyosis may present with symptoms at birth or in early childhood. Ichthyosis can itch relentlessly, leading to such complications of scratching as lichen simplex (dermatitis characterized by raw patches of skin). Either the cracking or the scratching can introduce infection, bringing with it discomfort and complications.

Acquired ichthyoses

The mildest form of acquired ichthyosis is called xeroderma, or dry flaky skin. It is not associated with any systemic diseases. Xeroderma occurs most often on the lower legs of middle-aged and elderly adults during cold weather, or on the lower legs of people who bathe too often. It is characterized chiefly by mild or moderate itching.

Ichthyosis may also be an early symptom of such disorders as AIDS, lymphoma, hypothyroidism, or leprosy. In these cases the ichthyosis is most noticeable on the patient's trunk and legs.

A few rare cases of acquired ichthyosis have been attributed to the use of certain drugs, specifically cimetidine (Tagamet), triparanol (Metasqualene), dixyrazine (a phenothiazine derivative used as an antipsychotic), nicotinic acid (vitamin B3, butyrophenone antipsychotics (Haldol, Inapsine, Orap), and clofazimine (Lamprene).

Diagnosis

A dermatologist will often make the diagnosis of ichthyosis based on findings from a clinical examination. However, a skin biopsy, or DNA study (from a small blood sample) is necessary to confirm the diagnosis. Evaluation for associated problems is done by a complete physical medical examination.

For some types of ichythyosis, the abnormal gene has been identified and prenatal testing is available. At present this is true for the autosomal recessive congenital ichythoses, which include: lamellar ichthyosis (LI), autosomal recessive lamellar ichthyosis (ARLI), congenital ichthyosiform erythroderma (CIE), and non-bullous congenital ichthyosiform erythroderma (NBCIE).

There are four different genes that have been located for the autosomal recessive congenital ichthyoses. Testing, however, is available for only one gene, known as transglutaminase-1 (TGM1). This gene is located on chromosome 14. Once a couple has had a child with ichthyosis, and they have had the genetic cause identified by DNA studies (performed from a small blood sample), prenatal testing for future pregnancies may be considered. (Note that prenatal testing may not be possible if both mutations cannot be identified.) Prenatal diagnosis is available via either chorionic villus sampling (CVS) or amniocentesis. CVS is a biopsy of the placenta performed in the first trimester of pregnancy under ultrasound guidance. Ultrasound is the use of sound waves to visualize the developing fetus. The genetic makeup of the placenta is identical to the fetus and therefore the TGM1 gene can be studied from this tissue. There is approximately a one in 100 chance for miscarriage with CVS. Amniocentesis is a procedure done under ultrasound guidance in which a long thin needle is inserted through the mother's abdomen into the uterus, to withdraw a couple of tablespoons of amniotic fluid (fluid surrounding the developing baby) to study. The TGM1 gene can be studied using cells from the amniotic fluid. Other genetic tests, such as a chromosome analysis, may also be performed through either CVS or amniocentesis.

Acquired ichthyosis is usually diagnosed in the course of identifying the underlying disorder. With the exception of acquired ichthyosis related to lymphoma, a doctor cannot tell the difference between inherited and acquired ichthyosis by examining skin samples through a microscope.

Treatment

Most treatments for ichthyosis are topical, which means that they are applied directly to the skin, not taken internally. Xeroderma is eaily treated by minimizing bathing and applying an emollient cream or mineral oil after bathing while the skin is still moist. Some forms of ichthyosis require two forms of treatmenta reduction in the amount of scale buildup and moisturizing of the underlying skin. Several agents are available for each purpose. Reduction in the amount of scale is achieved by keratolytics. Among this class of drugs are urea, lactic acid, and salicylic acid. Petrolatum, 60% propylene glycol, and glycerin are successful moisturizing agents, as are many commercially available products. Increased humidity of the ambient air is also helpful in preventing skin dryness.

Because the skin acts as a barrier to the outside environment, medicines have a hard time penetrating, especially through the thick skin of the palms of the hands and the soles of the feet. This resistance is diminished greatly by maceration (softening the skin). Soaking hands in water macerates skin so that it looks like prune skin. Occlusion (covering) with rubber gloves or plastic wrap will also macerate skin. Applying medicines and then covering the skin with an occlusive dressing will facilitate entrance of the medicine and greatly magnify its effect.

Secondary treatments are necessary to control pruritus (itching) and infection. Commercial products containing camphor, menthol, eucalyptus oil, aloe, and similar substances are very effective as antipruritics. If the skin cracks deeply enough, a pathway for infection is created. Topical antibiotics like bacitracin are effective in prevention and in the early stages of these skin infections. Cleansing with hydrogen peroxide inhibits infection as well.

Finally, there are topical and internal derivatives of vitamin A called retinoids that improve skin growth and are used for severe cases of acne, ichthyosis, and other skin conditions. Tazarotene (Tazorac), a retinoid that was originally developed to treat psoriasis and acne, appears to give good results in treating ichthyosis with fewer side effects than other retinoids.

Prognosis

This condition requires continuous care throughout a lifetime. Properly treated, in most cases it is a cosmetic problem. There are a small number of lethal forms, such as harlequin fetus.

KEY TERMS

Amniocentesis A procedure performed at 16-18 weeks of pregnancy in which a needle is inserted through a woman's abdomen into her uterus to draw out a small sample of the amniotic fluid from around the baby. Either the fluid itself or cells from the fluid can be used for a variety of tests to obtain information about genetic disorders and other medical conditions in the fetus.

Amniotic fluid The fluid that surrounds a developing baby during pregnancy.

Autosomal dominant A pattern of genetic inheritance where only one abnormal gene is needed to display the trait or disease.

Autosomal recessive inheritance A pattern of genetic inheritance where two abnormal genes are needed to display the trait or disease.

Dermatologist A physician that specializes in diagnosing and treating disorders of the skin.

Emollients Petroleum or lanolin-based skin lubricants.

Keratin A tough, nonwater-soluble protein found in the nails, hair, and the outermost layer of skin. Human hair is made up largely of keratin.

Keratinocytes Skin cells.

Keratolytic An agent that dissolves or breaks down the outer layer of skin (keratins).

Retinoids A derivative of synthetic Vitamin A.

Sporadic Isolated or appearing occasionally with no apparent pattern.

X-linked dominant inheritance The inheritance of a trait by the presence of a single gene on the X chromosome in a male or female, passed from an affected female who has the gene on one of her X chromosomes.

X-linked recessive inheritance The inheritance of a trait by the presence of a single gene on the X chromosome in a male, passed from a female who has the gene on one of her X chromosomes, and who is referred to as an unaffected carrier.

Resources

BOOKS

Beers, Mark H., MD, and Robert Berkow, MD, editors. "Ichthyosis." Section 10, Chapter 121. In The Merck Manual of Diagnosis and Therapy. Whitehouse Station, NJ: Merck Research Laboratories, 2004.

PERIODICALS

Fleckman, P. "Management of the Ichthyoses." Skin Therapy Letter 8 (September 2003): 3-7.

Hatsell, S. J., H. Stevens, A. P. Jackson, et al. "An Autosomal Recessive Exfoliative Ichthyosis with Linkage to Chromosome 12q13." British Journal of Dermatology 149 (July 2003): 174-180.

Lefevre, C., S. Audebert, F. Jobard, et al. "Mutations in the Transporter ABCA12 Are Associated with Lamellar Ichthyosis Type 2." Human Molecular Genetics 12 (September 15, 2003): 2369-2378.

Marulli, G. C., E. Campione, M. S. Chimenti, et al. "Type I Lamellar Ichthyosis Improved by Tazarotene 0.1% Gel." Clinical and Experimental Dermatology 28 (July 2003): 391-393.

Okulicz, Jason F., MD, and Robert A. Schwartz, MD, MPH. "Ichthyosis Vulgaris, Hereditary and Acquired." eMedicine November 1, 2001. http://www.emedicine.com/derm/topic678.htm.

ORGANIZATIONS

Alliance of Genetic Support Groups. 4301 Connecticut Ave. NW, Suite 404, Washington, DC 20008. (202) 966-5557. Fax: (202) 966-8553. http://www.geneticalliance.org.

Foundation for Ichthyosis and Related Skin Types (FIRST). 650 N. Cannon Ave., Suite 17, Landsdale, PA 19446. (215) 631-1411 or (800) 545-3286. Fax: (215) 631-1413. http://www.scalyskin.org.

National Organization for Rare Disorders, Inc. (NORD). 55 Kenosia Avenue, P. O. Box 1968, Danbury, CT 06813. (800) 999-6673 or (203) 744-0100. http://www.rarediseases.org.

OTHER

Immune Deficiency Foundation Website. www.primaryimmune.org.

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ichthyosis

ichthyosis (ik-thi-oh-sis) n. any one of a group of genetically determined skin disorders in which there is abnormal scaling of the skin. Xeroderma is a very mild form of the disorder. i. vulgaris the most common form of the disease, which is inherited as an autosomal dominant condition.

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ichthyosis

ich·thy·o·sis / ˌik[unvoicedth]ēˈōsis/ • n. Med. a congenital skin condition that causes the epidermis to become dry and rough like fish scales. DERIVATIVES: ich·thy·ot·ic / -ˈätik/ adj.

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