Chromosome 18q- Syndrome
NORD is very grateful to Shashikant Kulkarni, PhD, Director of CytoGenomics and Molecular Pathology, Director of Clinical & Molecular Cytogenetics, Department of Pathology, Washington University School of Medicine, for assistance in the preparation of this report.
Synonyms of Chromosome 18q- Syndrome
- 18q Deletion Syndrome
- 18q- Syndrome
- Chromosome 18 Long Arm Deletion Syndrome
- Chromosome 18, Monosomy 18Q
- Del(18q) Syndrome
- Monosomy 18q Syndrome
- No subdivisions found.
Chromosome 18q- syndrome (also known as Chromosome 18, Monosomy 18q) is a rare chromosomal disorder in which there is deletion of part of the long arm (q) of chromosome 18. Associated symptoms and findings may vary greatly in range and severity from case to case. However, characteristic features include short stature; mental retardation; poor muscle tone (hypotonia); malformations of the hands and feet; and abnormalities of the skull and facial (craniofacial) region, such as a small head (microcephaly), a "carp-shaped" mouth, deeply set eyes, prominent ears, and/or unusually flat, underdeveloped midfacial regions (midfacial hypoplasia). Some affected individuals may also have visual abnormalities, hearing impairment, genital malformations, structural heart defects, and/or other physical abnormalities. Chromosome 18q- syndrome usually appears to result from spontaneous (de novo) errors very early during embryonic development that occur for unknown reasons (sporadically).
As noted above, associated symptoms and findings may vary from case to case. However, many infants with the disorder have a low birth weight and growth delays after birth, resulting in short stature. In addition, Chromosome 18q- syndrome is often characterized by low muscle tone (hypotonia); sudden episodes of uncontrolled electrical activity in the brain (seizures); moderate to severe delays in the acquisition of skills requiring the coordination of mental and physical activities (psychomotor retardation); and varying degrees of mental retardation. Evidence suggests that most individuals with the disorder are affected by profound, severe, or moderate mental retardation. However, mild mental deficiency has been reported in some cases. In addition, some affected children may have behavioral problems, such as abnormally increased activity (hyperactivity), aggressive behavior, and tantrums.
Chromosome 18q- syndrome is also typically associated with malformations of the skull and facial (craniofacial) region. Characteristic craniofacial findings may include an unusually small head (microcephaly); flat, underdeveloped (hypoplastic) midfacial regions; deeply set eyes; a "carp-shaped" mouth; and/or relative protrusion of the lower jaw (mandibular prognathism). Some affected individuals may also have a broad nasal bridge; incomplete closure (clefting) or unusual narrowness of the roof of the mouth (palate); and/or an abnormal groove in the upper lip (cleft lip).
Chromosome 18q- syndrome is also often characterized by additional eye (ocular) defects, such as vertical skin folds that may cover the eyes' inner corners (epicanthal folds); involuntary, rhythmic, rapid eye movements (nystagmus); and/or abnormal deviation of one eye in relation to the other (strabismus). Associated ocular defects may also include abnormally small eyes (microphthalmia); partial absence of ocular tissue from the colored region of the eyes (coloboma of the iris); clouding of the normally transparent front region of the eyes (corneal opacities); defects of the retinas and optic disks; and/or other ocular abnormalities. (The retina is the nerve-rich membrane upon which images are focused at the back of the eye; its specialized nerve cells convert light into nerve impulses that are transmitted to the brain via the optic nerve. The optic disk, also known as the "blind spot," is the region where fibers of the retina become part of the optic nerve.) Such ocular defects may result in varying degrees of visual impairment.
Some individuals with Chromosome 18q- syndrome may also have malformations of the ears. These may include unusually prominent ears and/or abnormally narrow (stenotic) or absent (atretic) external ear canals, with associated hearing impairment.
Chromosome 18q- syndrome is also often associated with distinctive abnormalities of the hands and feet, including long, thin, tapered hands; abnormal skin ridge patterns on the fingers and palms; abnormal placement of the thumbs and certain toes; and/or deformities in which the feet are twisted out of shape or position (clubfeet). Some affected individuals may also have rib malformations, hip deformities, and/or other skeletal defects. In addition, abnormal dimples may be present in certain regions, including over the knuckles and the sides of the knees.
Individuals with Chromosome 18q- syndrome may also have genital abnormalities. In affected females, there may be underdevelopment of the skin folds surrounding the vaginal opening (hypoplastic labia). In males with the disorder, genital malformations may include undescended testes (cryptorchidism); an abnormally small penis (micropenis) and scrotum; and/or abnormal placement of the urinary opening (hypospadias), such as on the underside of the penis.
Additional physical abnormalities have also been reported in association with the disorder, such as widely spaced nipples; deficiency of a particular antibody (i.e., immunoglobulin A [IgA]) that helps the body to fight certain infections; kidney (renal) defects; and/or other findings. In addition, in over 30 percent of cases, congenital heart defects may be present. Such heart defects have included an abnormal opening in the fibrous partition (septum) that normally separates the upper chambers (atria) or the lower chambers (ventricles) of the heart (atrial or ventricular septal defects); abnormal narrowing (stenosis) of the opening between the pulmonary artery and the right ventricle (pulmonary stenosis); or patent ductus arteriosus (PDA). In PDA, the channel that is present between the pulmonary artery and the aorta during fetal development fails to close after birth. (The pulmonary artery transports oxygen-poor blood from the right ventricle to the lungs, where the exchange of oxygen and carbon dioxide occurs. The aorta, the major artery of the body, arises from the left ventricle and supplies oxygen-rich blood to most arteries.) In some individuals with Chromosome 18q- syndrome, additional physical abnormalities may also be present.
Chromosome 18q- syndrome is a chromosomal disorder in which there is deletion (monosomy) of part of the long arm (q) of chromosome 18. Chromosomes are found in the nucleus of all body cells. They carry the genetic characteristics of each individual. Pairs of human chromosomes are numbered from 1 through 22, with an unequal 23rd pair of X and Y chromosomes for males and two X chromosomes for females. Each chromosome has a short arm designated as "p" and a long arm identified by the letter "q." Chromosomes are further subdivided into bands that are numbered. For example, "18q21" refers to band 21 of the long arm of chromosome 18.
Evidence suggests that individuals with characteristic features of the disorder have deletions from within band 18q21 (e.g., 18q21.3) or 18q22 (e.g., 18q22.2) that may extend to the end (or "terminal") of chromosome 18q (qter). In some cases, the deletion could be interstitial; that is, in the middle of the chromosome. In addition, in some cases, only a certain percentage of an affected individual's cells may have the deletion, while other cells may have a normal chromosomal makeup (a finding known as "chromosomal mosaicism"). The range and severity of symptoms may depend on the specific size of the deletion and the percentage of cells with the chromosomal abnormality. Reports suggest that those with 18q- mosaicism tend to have less severe symptoms and findings.
In most cases, Chromosome 18q- syndrome appears to be caused by spontaneous (de novo) errors very early in embryonic development. In such instances, the parents of the affected child usually have normal chromosomes and a relatively low risk of having another child with the chromosomal abnormality.
Less commonly, the deletion may result from a "balanced translocation" in one of the parents. Translocations occur when portions of certain chromosomes break off and are rearranged, resulting in shifting of genetic material and an altered set of chromosomes. If a chromosomal rearrangement is balanced, meaning that it consists of an altered but balanced set of chromosomes, it is usually harmless to the carrier. However, such a chromosomal rearrangement may be associated with an increased risk of abnormal chromosomal development in the carrier's offspring.
Rare cases have also been reported in which the disorder has appeared to result from a parental chromosomal inversion or other chromosomal rearrangements. An inversion is characterized by breakage of a chromosome in two places and reunion of the segment in the reverse order.
Chromosomal analysis and genetic counseling are typically recommended for parents of an affected child to help confirm or exclude the presence of a balanced translocation or other chromosomal rearrangement in one of the parents.
Chromosome 18q- syndrome appears to affect females more frequently than males by a ratio of approximately three to two. Since the disorder was originally reported in the medical literature in 1964, more than 80 cases have been recorded.
Symptoms of the following disorders may be similar to those of Chromosome 18q- syndrome. Comparisons may be useful for a differential diagnosis:
Chromosome 18, Monosomy 18p is a rare chromosomal disorder in which all or part of the short arm (p) of chromosome 18 is deleted. The disorder is typically characterized by short stature, variable degrees of mental retardation, speech delays, craniofacial malformations, and/or additional physical abnormalities. Associated craniofacial defects may vary greatly in range and severity. However, such features commonly include an unusually small head (microcephaly); a broad, flat nose; a "carp-shaped" mouth; large, protruding ears; widely spaced eyes (ocular hypertelorism); and/or other abnormalities. In about 10 percent of cases, Monosomy 18p may be associated with holoprosencephaly, a condition in which the forebrain (prosencephalon) fails to divide properly during embryonic development. Holoprosencephaly may result in varying degrees of mental retardation, other neurologic findings, and/or extremely variable midline facial defects, such as the presence of a single, central front tooth (maxillary incisor); closely spaced eyes (hypotelorism); an abnormal groove in the upper lip (cleft lip); incomplete closure of the roof of the mouth (cleft palate); and/or, in severe cases, absence of the nose and/or cyclopia. Cyclopia is characterized by fusion of the eye cavities (orbits) into a single cavity containing one eye. In some individuals with Monosomy 18p, additional physical abnormalities may be present, such as a short, webbed neck; a broad chest with widely spaced nipples; relatively small hands and feet; and/or an unusually small penis (micropenis) and/or undescended testes (cryptorchidism) in affected males. Monosomy 18p is usually caused by spontaneous (de novo) errors very early in embryonic development. (For further information, please choose "monosomy 18p" as your search term in the Rare Disease Database.)
Chromosome 18 Ring is a rare disorder in which there is deletion of chromosomal material from both ends of the 18th chromosome and joining of the ends to form a ring. Associated symptoms and findings may vary greatly in range and severity, depending upon the amount and location of lost genetic material and other factors. However, many individuals with the disorder are affected by mental retardation, low muscle tone (hypotonia), growth retardation, and/or craniofacial malformations. Such craniofacial features often include an unusually small head (microcephaly); widely spaced eyes (ocular hypertelorism); vertical skin folds that may cover the eyes' inner corners (epicanthal folds); a highly arched palate; a "carp-shaped" mouth; and/or low-set, malformed ears and/or narrow (stenotic) or absent (atretic) ear canals, with associated hearing impairment. The disorder may also be characterized by additional physical abnormalities, such as deficiency of a particular antibody (i.e., immunoglobulin A [IgA]) that helps to fight certain infections; rib and/or spinal column (vertebral) defects; abnormal deviation (clinodactyly) of the fifth fingers; webbing of the neck; structural malformations of the heart (congenital heart defects); and/or kidney (renal) defects. A few severe cases have also been reported in which Chromosome 18 Ring has been associated with holoprosencephaly. Chromosome 18 Ring is usually caused by spontaneous (de novo) errors very early in the development of the embryo that appear to occur randomly for unknown reasons (sporadically). (For further information, please choose "chromosome 18 ring" as your search term in the Rare Disease Database.)
Additional chromosomal disorders may have features similar to those potentially associated with Chromosome 18q- syndrome. Chromosomal testing is necessary to confirm the specific chromosomal abnormality present. (For further information on such disorders, choose the name of the specific chromosomal disorder in question or use "chromosome" as your search term in the Rare Disease Database.)
In some cases, Chromosome 18q- syndrome may be suggested before birth (prenatally) by specialized tests such as ultrasound, amniocentesis, and/or chorionic villus sampling (CVS). During fetal ultrasonography, reflected sound waves create an image of the developing fetus, potentially revealing certain findings that suggest a chromosomal disorder or other developmental abnormalities. With amniocentesis, a sample of fluid that surrounds the developing fetus is removed and analyzed, while CVS involves the removal of tissue samples from a portion of the placenta. Chromosomal analysis performed on such fluid or tissue samples may reveal the presence of Monosomy 18q.
The disorder is usually diagnosed or confirmed after birth (postnatally) based upon a thorough clinical evaluation, detection of characteristic physical findings, and chromosomal analysis. Specialized tests may also be performed to help detect and/or characterize certain abnormalities that may be associated with the disorder.
The treatment of Chromosome 18q- syndrome is directed toward the specific symptoms that are apparent in each individual. Such disease management may require the coordinated efforts of a team of medical professionals, such as pediatricians; surgeons; neurologists; eye specialists; hearing specialists; physicians who diagnose and treat disorders of the skeleton, muscles, joints, and related tissues (orthopedists); heart specialists (cardiologists); and/or other health care professionals.
In some cases, physicians may recommend surgical correction of certain craniofacial, skeletal, genital, and/or other malformations associated with the disorder. In addition, for those with congenital heart defects, treatment with certain medications, surgical intervention, and/or other measures may be required. The specific surgical procedures performed will depend upon the severity and location of the anatomical abnormalities, their associated symptoms, and other factors.
For individuals with ocular abnormalities, corrective lenses, surgery, and/or other measures may be advised to help improve vision in some cases. In addition, in those with hearing impairment, recommended treatment may include surgical measures and/or the use of specialized hearing aids or additional supportive techniques that may aid communication.
In some cases, anticonvulsant medications may be administered to help prevent, reduce, or control seizures. In addition, for individuals with low levels of certain antibodies (i.e., IgA deficiency), disease management may include regular monitoring and appropriate, supportive measures to help prevent and aggressively treat infections.
Early intervention may also be important in ensuring that affected children reach their potential. Special services that may be beneficial include special education, physical therapy, and/or other medical, social, and/or vocational services. Genetic counseling will also be of benefit for affected individuals and their families. Other treatment for this disorder is symptomatic and supportive.
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Chromosome 18q- Syndrome Resources
(Please note that some of these organizations may provide information concerning certain conditions potentially associated with this disorder [e.g., mental retardation, congenital heart defects, craniofacial abnormalities, ocular defects, hearing impairment, etc.].)
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Buyse ML. Birth Defects Encyclopedia. Dover, Mass: Blackwell Scientific Publications, Inc; 1990:381-84.
Brkanac Z, et al. Identification of cryptic rearrangements in patients with 18q- deletion syndrome. Am J Hum Genet. 1998;62:1500-06.
Mahr RN, et al. Neuropsychiatry of 18q- syndrome. Am J Med Genet. 1996;67:172-78.
Strathdee G, et al. Interstitial deletions are not the main mechanism leading to 18q deletions. Am J Hum Genet. 1994;54:1085-91.
Izquierdo NJ, et al. Anterior segment malformations in 18q- (de Grouchy) syndrome. Ophthalmic Paediatr Genet. 1993;14:91-94.
Kline AD, et al. Molecular analysis of the 18q- syndrome--and correlation with phenotype. Am J Hum Genet. 1993;52:895-906.
Fujimoto S, et al. 18q-syndrome with cleft lip and palate. A clinically diagnosed case. J Craniomaxillofac Surg. 1991;19:61-63.
Kanazawa O, et al. A case of 18 q-syndrome associated with status epilepticus. No To Hattatsu. 1989;21:470-74.
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Report last updated: 2009/04/10 00:00:00 GMT+0
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