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Mentally retarded individuals ten to develop additional mental illness diagnoses at three to four times at the rate of the normal population. When more than one disorder is diagnosed at a time, the set of diagnoses are known as co-morbid disorders.
Mentally retarded individuals are prone to Attention Deficit Hyperactivity Disorder, or ADHD. While ADHD is diagnosed in 8-15 percent of the general population of children, between 17 and 52 persons of adults with mental retardation may qualify for the diagnosis. The range provided here is large because studies disagree on the prevalence of ADHD with in the mentally retarded population. ADHD is difficult to detect amidst the other prominent symptom of mental retardation.
ADHD has behavioral characteristics that include inattention, impulsivity, and fidgeting. The incidence of co-morbid depression in the mentally retarded population is difficult to determine with precision, as mentally retarded individuals have difficulty communicating their moods during evaluation. Depression is possible for mentally retarded individuals starting at any age of life. Children may become depressed upon realizing that they are different from their peers and unable to keep up. Depression may manifest itself in adolescents and adults through disturbance in sleep and eating routines, social withdrawal, and anxiousness. Bipolar disorder, another mood disorder, is between two and three times more frequent in the mentally retarded population than the regular population.
Mentally retarded individuals and individuals with pervasive Development Disorder alike often develop Stereotypic Movement Disorders, which involve unnecessary repetitive movements that interfere with everyday functioning. The movements may take on a self-injurious form and restraints may be required at times to keep affected individuals from hurting themselves.
Dementia
Typically refers to gradual degeneration of mental function that interferes with every day functioning. Memory loss is significant and affected individuals often misplace objects, lose track of conversations, get lost while driving, or have difficulty completing tasks. Dementia may be a result of a medical condition or head trauma. Typically, individuals who have mental retardation do not have a heightened incidence of dementia later in life; however, Alzheimer’s disease is common among middle-aged individuals with Down syndrome. Alzheimer’s disease occurs with greater frequency and at a far earlier age than normal with in the Down population. Down patients who develop Alzheimer’s disease typically pas away with in ten years of their diagnosis. Traumatic brain injury, such as may occur due to a fall or car accident, can lead to a diagnosis of mental retardation if the injury occur prior to the age of eighteen or to a diagnosis of dementia if it occurs after age eighteen.
Schizophrenia is estimated to be present in about 3 percent of individuals with mental retardation, most of whom go undiagnosed. This is in comparison to a rate of 0.8 percent with in the general population.
Down syndrome
Down syndrome or trisomy 21 (British Down’s syndrome) is a genetic condition resulting from the presence of all or par of an extra 21st chromosome. Down syndrome is characterized by a combination of major and minor abnormalities of body structure and function. Among features present in nearly all cases are impairment of learning and physical growth, and a recognizable facial appearance usually identified at birth. It is named after John Langdon Down, the British doctor who first described it in 1866.
Individuals with Down syndrome have lower than average cognitive ability, normally ranging from mild to moderate retardation. Some individuals may have low intelligence overall, but will generally have some amount of developmental disability, such as a tendency toward concrete thinking or naivete. There is also a small number of individuals with Down syndrome with severe to profound mental retardation. The incidence of Down syndrome is estimated at 1 per 800 to 1 per 1000 births.
The common physical features of Down’s syndrome also appear in people with a standard set of chromosomes. They include a simian crease (a single crease across one or both palms), almond shaped eyes, shorter limbs, speech impairment, and protruding tongue. Health concerns for individuals with Down syndrome include a higher risk for congenital heart defects, gastro-esophageal reflux disease, recurrent ear infections, obstructive sleep apnea, and thyroid dysfunctions.
Early childhood intervention, screening for common problems, medical treatment where indicated, a conductive family environment, and vocational training can improve the overall development of children with Down syndrome. While some of the genetic limitations of Down syndrome cannot be overcome, education and proper care, initiated at any time, can improve quality of life.
Characteristics of Down syndrome
Example with Down syndrome may have some or all of the following physical characteristics: oblige eye fissures with small skin folds on the inner corner of the eyes, muscle hypotonia, a flat nasal bridge, a single palmar fold (simian crease), a protruding tongue (due to small oral cavity, poor muscle tone, and an enlarged tongue near the tonsils), a short neck, white spots on the iris known as Brush-field spots, excessive flexibility in joints, congenital heart defects, excessive space between large and second toe, and a single flexion furrow of the fifth finger. Most individuals with Down syndrome have mental retardation n the mild (IQ 50-70) to moderate range (IQ 35-50), with scores for children with Mosaic Down syndrome (explained below) some 10-30 points higher. In addition, individuals with Down syndrome can have serious abnormalities affecting any body system.
Genetics
Down syndrome is a chromosomal abnormality characterized by the presence of an extra copy of genetic material on the 21st chromosome, either in whole (trisomy 21) or part (such as due to translocations). The effects of the extra copy vary greatly from individual to individual, depending on the extent of the extra copy, genetic background, environmental factors, and random chance. Down syndrome occurs in all human populations and analogous effects have been found in other species such as chimpanzees and mice. Recently, researches have been able to create transgenic mice with most of human chromosome 21 (in addition to the normal mouse chromosome). People with Down syndrome often have a simian crease on the palmar surface of the hand. The extra chromosomal material can come about in several distinct ways. A normal human karyotype is designated as 46,XX or 46,XY, indicating 46 chromosomes with an XX arrangement for females and 46 chromosomes with an XY arrangement for males.
Trisomy21
Trisomy 21 (47,XX+21) is caused a meiotic non-disjunction event. With non-disjunction, a gamete (i.e., a sperm or egg sell) is produced with an extra copy of chromosome 21; the gamete thus has 224 chromosomes. When combined with a normal gamete from the other parent, the embryo now has 47 chromosomes, with three copy of chromosome 21. Trisomy 21 is the cause of approximately 95% of observed down syndromes, with 88% coming from the non-disjunction in the maternal gamete and 8% coming from non-disjunction in the parental gamete.
Mosaicism
Trisomy 21 is generally caused before the conception, and all cells in the body affected. However, when some of the cells in the body are normal and other cells have trisomy 21, it is called Mosaic Down Syndrome (46,XX/47,XX+21). This can occur in one of two ways: a non-disjunction event during an early cell division in a normal embryo leads to a fraction of the cells with trisomy 21; or a Down syndrome embryo undergoes non-disjunction and some of the cells in the embryo never revert back to the normal chromosomal arrangement. There is considerable variability in the fraction of trisomy 21, both as a whole and among tissues. Thos is the cause of 1-2% of the observed Down syndromes. There is evidence that mosaic Down syndrome may produce less developmental delay, on average than fully trisomy 21.
Robertsonian translocation
The extra chromosome 21 material that causes Down syndrome may be due to a Robertsonian translocation. In this case, the long arm of chromosome 21 is attached to another chromosome, often chromosome 14 (45,XX,t(14;21q)) or itself (called an isochromosome, 45,XX,t(21q;21q)). Normal disjunction leading to gametes has a significant chance of creating a gamete with an extra chromosome 21. Translocation Down syndrome is often referred to as familial Down syndrome. It is cause of 2-3% of observed cases of Down syndrome. It does not show the mental age effect, and is just as likely to have come from fathers as mothers.
Duplication of portion of chromosome 21
Rarely, a region of chromosome 21 will undergo a duplication event. This will lead to extra copies of some, but not all, of the genes on chromosome 21 (46,XX,dup(21q)). If the duplicated region has genes that are responsible for Down syndrome physical and mental characteristics, such individuals will show those characteristics. This cause is very rare and no rate estimates are possible.
Parental screening
Pregnant woman can be screened for various complications in their pregnancy, or due to risk factors such as advanced maternal age. There are several common non-invasive screens, each has a significant chance of suggesting a fetus with Down syndrome when, in fact, the fetus does not have this genetic abnormality. Such results are called false positives. Screens positives must be verified before a Down syndrome diagnosis is made.
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