Hypohidrotic ectodermal dysplasia (HED) is a rare inherited multisystem disorder that belongs to the group of diseases known as ectodermal dysplasias. Ectodermal dysplasias typically affect the hair, teeth, nails, sweat glands, and/or skin. HED is primarily characterized by partial or complete absence of certain sweat glands (eccrine glands), causing lack of or diminished sweating (anhidrosis or hypohidrosis), heat intolerance, and fever; abnormally sparse hair (hypotrichosis), and absence (hypodontia) and/or malformation of certain teeth. Many individuals with HED also have characteristic facial abnormalities including a prominent forehead, a sunken nasal bridge (so-called "saddle nose"), unusually thick lips, and/or a large chin. The skin on most of the body may be abnormally thin, dry, and soft with an abnormal lack of pigmentation (hypopigmentation). However, the skin around the eyes (periorbital) may be darkly pigmented (hyperpigmentation) and finely wrinkled, appearing prematurely aged. In many cases, affected infants and children may also exhibit underdevelopment (hypoplasia) or absence (aplasia) of mucous glands within the respiratory and gastrointestinal (GI) tracts and, in some cases, decreased function of certain components of the immune system (e.g., depressed lymphocyte function, and rarely cellular immune hypofunction), potentially causing an increased susceptibility to certain infections and/or allergic conditions. Many affected infants and children experience recurrent attacks of wheezing and breathlessness (asthma), respiratory infections; chronic inflammation of the nasal passages (atrophic rhinitis), scaling, itchy (pruritic) skin rashes (eczema), and/or other findings.
HED is usually inherited as an X-linked recessive genetic trait and is caused by a mutation in the ectodysplasin-A (EDA) gene; in such cases, the disorder is fully expressed in males only. However, females who carry a single copy of the disease gene (heterozygote carriers) may exhibit some of the symptoms and findings associated with the disorder. These may include absence and/or malformation of certain teeth, sparse hair, and/or reduced sweating. HED can also be inherited as an autosomal dominant or autosomal recessive genetic trait, caused by mutations in the EDAR or EDARADD genes. In such cases, the disorder is fully expressed in both males and females.
HED is characterized by lack of or diminished sweating (anhidrosis or hypohidrosis), abnormally sparse hair (hypotrichosis), and/or absence (hypodontia) and/or malformation of certain teeth. In addition, affected individuals often have characteristic facial abnormalities, irregularities of the skin, abnormalities of the mucous membranes lining the respiratory and gastrointestinal (GI) tracts, an increased tendency to develop certain infections and allergic conditions, and/or other abnormalities. The range and severity of the symptoms and findings associated with HED varies from case to case.
A primary feature of HED is a lack of or diminished sweating (anhidrosis or hypohidrosis), resulting from underdevelopment of or partial or complete absence of certain sweat glands (eccrine glands). Because affected infants and children are unable to sweat appropriately when exposed to warm environments, they can experience repeated episodes of heat intolerance and “unexplained” high fevers that may remain unexplained until the disorder is diagnosed. In individuals with HED, exertion can result in elevated body temperature (hyperpyrexia). Eating hot foods may also cause extreme discomfort. In some cases, without appropriate treatment, episodes of hyperpyrexia may result in life-threatening complications; particularly during the first two years of life.
Abnormal sparseness of hair (hypotrichosis) is also a primary characteristic of HED, and is due to incomplete formation and reduced numbers of hair follicles. Scalp hair is usually blond or lightly pigmented; abnormally sparse, short, and fine; and, in some cases, stiff, dry, and unruly. Abnormal bald patches on the scalp (alopecia) may also be present. In addition, the eyebrows and eyelashes are often scanty or absent, although, in some cases, the eyelashes may appear normal. After puberty, male patterns of hair growth (e.g., moustache and beard) can be normal, while in other cases, facial and pubic hair growth may be sparse. In affected males and females, pubic and underarm (axillary) hair is typically scant. In some cases, hair may be absent from the arms, legs, and/or trunk.
The third primary characteristic typically associated with HED is the absence (hypodontia) and/or malformation of teeth. In most cases, the majority of the primary (deciduous) and secondary (permanent) teeth are absent. The teeth most often present include front teeth (central incisors), teeth normally located next to the incisors (canines), and/or, in some cases, one or more molars. In most cases, the teeth that are present are widely spaced, with front teeth being pointed or cone shaped. In some rare cases, individuals with HED may lack all upper and/or lower teeth (edentulous or anodontia). Some individuals can be missing all the teeth in one jaw and have some in the other jaw.
As a result of missing teeth the bony ridge of the jaws (alveolar process) that holds the teeth in place often fails to form properly. In addition, due to hypodontia, the lips may protrude outward (everted) and appear abnormally thick, the gums may be abnormally small or degenerated (atrophic), and the normally exposed red portion of the upper and lower lips (vermilion border) may not be noticeable.
Many individuals with HED have additional, characteristic facial features, including a prominent forehead (frontal bossing); underdeveloped nostrils (hypoplastic alae nasi) and a low or sunken nasal bridge (so-called “saddle nose”); and underdeveloped, sunken cheeks (malar hypoplasia).
Distinctive skin changes may also be present. Many affected newborns have unusual scaling or peeling of the skin, while many children develop itchy (pruritic), scaling skin rashes (eczema). In the majority of individuals with HED, the skin on most of the body is unusually thin and soft and can lack normal pigmentation (hypopigmentation). However, the skin around the eyes (periorbital) may be darkly pigmented (hyperpigmentation) and finely wrinkled, appearing prematurely aged. The skin may be extremely dry due to underdevelopment (hypoplasia) or absence (aplasia) of oil-secreting glands (sebaceous glands). Additionally there may be abnormalities in the skin ridge patterns (dermatoglyphic patterns) on the fingers, toes, palms of the hands, and/or soles of the feet. Some individuals with the disorder have unusually thin and brittle nails.
In many individuals with HED, mucous glands within the membrane lining the respiratory and gastrointestinal (GI) tracts (e.g., in the lung, pharynx, larynx, trachea, upper esophagus, stomach, intestines) are underdeveloped (hypoplastic) or absent (aplastic). There are several rare HED forms or subtypes that have abnormally decreased function of certain components of the immune system (e.g., depressed lymphocyte function, cellular immune hypofunction). The immune system works to protect the body against invading microorganisms, toxins, and other substances that are recognized as foreign to the body. In many infants and children with HED, such mucous gland abnormalities and/or immune system irregularities cause an increased susceptibility to certain infections and/or allergic conditions.
Salivary glands can also be underdeveloped (hypoplastic), leading to abnormal dryness of the mouth and an altered sense of taste or smell. In addition, some individuals with HED are unable to produce tears due to underdevelopment of the glands that secrete tears (hypoplastic lacrimal glands), hypoplasia of the ducts through which the tears pass (lacrimal ducts), and/or abnormal narrowing of the small openings in the inner corners of the eyelids where tears normally drain (stenotic lacrimal puncta). Eye (ocular) abnormalities may be present, including loss of transparency of the lens of the eyes (cataracts) and/or clouding of the portion of the eyes through which light passes (corneal opacities).
Females who carry a single copy of the mutated EDA gene for X-linked HED (heterozygote carriers) may have no symptoms or physical abnormalities or may have some of the characteristics associated with the disease. Approximately 70% of female carriers show symptoms that are typically milder than those associated with the fully expressed disorder. Female carriers of X-linked HED may have dental abnormalities such as absence of certain teeth (hypodontia) and/or abnormally small, pointed, conical teeth; sparse hair (hypotrichosis); reduced sweating; and/or irregular dermatoglyphic patterns. In some cases, abnormalities of the breasts and nipples have been reported, and approximately 80 percent of carriers may experience difficulties nursing.
In the majority of reported cases, HED is inherited as an X-linked recessive genetic trait and caused by a mutation of the EDA gene. The protein regulated (encoded) by this gene is a type II membrane protein that acts as a homotrimer (a protein with three identical units of polypeptide) and may be involved in cell-cell signaling during early embryonic development when ectodermal organs are beginning to be formed. The ectodermal germ cell layer normally forms the nervous system, tooth enamel, epidermis of the skin, lining of the mouth, anus, nose, sweat glands, hair, and nails, but a mutated EDA gene will disrupt the normal function of a number of these characteristics.
Human traits, including the classic genetic diseases, are the product of the interaction of two genes, one received from the father and one from the mother.
X-linked recessive disorders are conditions that are coded on the X chromosome. Females have two X chromosomes, but males have one X chromosome and one Y chromosome. Therefore, in females, disease traits on the X chromosome may be masked by the normal gene on the other X chromosome. Since males only have one X chromosome, if they inherit a gene for a disease present on the X, it will be expressed. 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 risk of transmitting the carrier condition to their daughters, and a 50 percent risk of transmitting the disease to their sons. Thus, in summary, when HED is inherited as an X-linked recessive trait, the disorder is fully expressed in males only and it is transmitted through the maternal X chromosome.
In some females who inherit a single copy of the disease gene (heterozygote carriers) for HED, the disease may not be “masked” by the normal gene on the other X chromosome. As a result, in such cases, some females exhibit some of the symptoms associated with the disorder.
Researchers also have reported cases in which HED appears to be inherited as an autosomal dominant or recessive genetic trait, but these patterns of inheritance are less common. A mutation in the EDAR gene can have an autosomal dominant or autosomal recessive pattern of inheritance, while a mutation in the EDARADD gene has an autosomal recessive pattern of inheritance. Autosomal dominant inheritance means one copy of the altered gene in each cell is sufficient to cause the disorder. Autosomal recessive inheritance means two copies of the gene in each cell are altered. Most often, the parents of an individual with an autosomal recessive disorder are carriers of one copy of the altered gene but do not show signs and symptoms of the disorder. In such cases, the disorder is fully expressed in both males and females. The existence of an autosomal recessive form of HED is supported by reports in the medical literature of severely affected females with the fully expressed disorder and multiple affected family members with parents who are closely related by blood (consanguineous). If both parents carry the same disease gene, there is a higher than normal risk that their children may inherit the two genes necessary for the development of the disease.
X-linked HED is a rare disorder that is fully expressed in males only. However, females who carry a single copy of the disease gene (heterozygote carriers) may exhibit milder symptoms associated with the disorder. In those rare cases when HED is inherited as an autosomal recessive genetic trait, males and females are affected in equal numbers. Because the vast majority of cases of HED are thought to be X-linked, it is suspected that approximately 90 percent of affected individuals are male.
HED is thought to occur in approximately 1 in 5,000-10,000 newborns.
Although some symptoms and findings associated with the disorder are present shortly after birth such as heat intolerance, unexplained fever, and/or extensive peeling of the skin, the characteristic facial abnormalities may not be apparent in affected infants. Therefore, the disorder often is not recognized in affected infants and children until associated dental and hair abnormalities become apparent.
In most cases, HED is diagnosed during early childhood when characteristic dental and hair abnormalities become apparent and prompt further testing. Such diagnosis is based upon a thorough clinical evaluation, identification of characteristic physical findings, a detailed patient and family history, and specialized laboratory testing. In some cases, during the newborn period, heat intolerance, unexplained fevers, and/or extensive skin peeling may lead to an earlier diagnosis.
Specialized diagnostic testing may include microscopic examination of small samples of skin tissue removed from the palm, confirming partial or complete absence of eccrine sweat glands. In some cases, other types of sweat testing may be used to determine the reduction or absence of perspiration. One such test that is particularly helpful in detecting females who carry a single copy of the disease gene for X-linked HED (heterozygotes) consists of the application of an iodine-in-alcohol solution over the entire back, followed by the application of a corn starch/castor oil suspension. During such testing, sweat glands become highlighted by a black dot. In heterozygous females, characteristic streaks will appear on the back in the shape of a “V”, demonstrating those areas that are devoid of sweat glands. Another method frequently used is the counting of sweat pores by direct observation. In cases of X-linked HED, direct observation reveals no sweat pores in affected males and decreased numbers of sweat pores in female carriers. In males and females with the autosomal recessive form of HED, such a count will also reveal decreased number of sweat pores.
Additional diagnostic tools are available and may include a test in which the sweat glands are stimulated by a drug called pilocarpine through the use of direct current (iontophoresis) and the resulting perspiration is measured and analyzed. In some cases, application of the substance o-phthalaldehyde may be applied directly to the skin (topically) of the palm. Such testing may reveal absence or reduction of sweating in affected individuals and female carriers.
In addition, dental x-rays to verify the absence of certain teeth and to further characterize associated dental abnormalities play an essential role in helping to confirm a diagnosis of HED or identify carrier status.
Molecular testing for mutations in the EDA, EDAR, and EDARADD genes is available to confirm the diagnosis. Carrier testing is available if the disease-causing mutation(s) have been identified in an affected family member.
The treatment of HED is directed toward the specific symptoms that are apparent in each individual. Treatment may require the coordinated efforts of a team of specialists who need to systematically and comprehensively plan an affected individual's treatment. Such specialists may include pediatricians or internists, physicians who treat disorders of the skin (dermatologists), dental specialists, physicians who diagnose and treat disorders of the ears, nose, and throat (otolaryngologists), allergists, and/or other health care professionals.
If possible, it is recommended that individuals with HED live in a cool climate. Physicians may carefully monitor affected infants and young children and recommend supportive measures to help prevent episodes of severely elevated body temperature (hyperpyrexia). For children and adults with the disorder, preventive and protective measures should include avoidance of physical exertion, protection from high temperatures, and, during warm weather, large amounts of dietary fluids, cooling by water such as use of cool cloths and sponge baths, air conditioning, and/or other supportive measures.
Early dental intervention and restoration is also important. Artificial teeth and/or other devices (prosthetics) may be used to replace absent teeth. Braces, bridges, dental surgery, and/or other corrective measures may be used to help correct dental abnormalities and ensure appropriate nutrition. In addition, in affected individuals with alopecia, hairpieces or wigs may be helpful.
Physicians may recommend that impacted nasal secretions be carefully removed on a regular basis to help prevent or limit the severity of rhinitis. Physicians may also regularly monitor affected infants and children to help prevent respiratory infections and to ensure prompt, aggressive treatment should such infections occur.
In affected individuals with impaired tear secretion (alacrima), the use of artificial tears may help to prevent possible corneal damage.
Early intervention is important to ensure that children with HED reach their potential. Special services that may be beneficial to affected children may include special education and special social support, and/or other medical, social, and/or vocational services.
Genetic counseling will be of benefit for affected children and their families. Other treatment is symptomatic and supportive.
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