Syndrome X

Fragile X syndrome is a genetic disorder that occurs in all ethnic groups, and one out of 700 pregnant women carries the fragile X defect. In 1992, a new test was announced for determining whether a woman carries the fragile X genetic defect. When a woman learns that she carries this defect, she may then elect to undergo amniocentesis or chorionic villus tests to determine whether her fetus also carries the fragile X defect. The incidence of fragile X syndrome is estimated to be approximately one in every 2,000 to 3,000 births; the estimates include one per 1,500 males and one per 2,500 females.

Fragile X appears to be caused by an abnormal number of repeats of a genetic sequence on a segment of the X chromosome. (It was initially characterized as the Martin-Bell syndrome.) Because fragile X has been observed in all ethnic groups, it may be considered one of the most frequent single-gene disorders in humans. It is difficult to diagnose in infants, but the features slowly become evident as the child grows.

Developmental delay and mental retardation are the most significant features of fragile X syndrome. In fact, fragile X syndrome is a common cause of mental retardation--researchers estimate that 20% of all boys with IQ levels between 30 and 55 (severely to moderately retarded) have fragile X syndrome. Overall, the population with fragile X syndrome ranges in mental retardation from profound (IQ below 25) to borderline, with an average IQ (intelligence quotient) in the moderately retarded range (35-55).

Before the onset of puberty, boys with the syndrome have delayed developmental milestones and may display some avoidance behavior similar to autism. They may also exhibit hyperactivity and attention deficit/hyperactivity disorder . Language delay is frequently observed; absence of speech is rare, but a playful, repetitive speech pattern is common.

An adult male with fragile X syndrome is likely to have the following characteristics: a long, narrow face with head circumference above the 50th percentile; prominent jaw and forehead; large and slightly malformed ears; hyperextending joints; high arched palate, and enlarged testicular volume.

In girls, the symptoms appear to be milder than in boys. The mental retardation is less severe, with most female patients falling in the mild-to-borderline retarded range. Emotional problems are reportedly more common in females with fragile X syndrome. An adult female with fragile X syndrome has facial characteristics similar to an adult male--prominent jaw and forehead and large, protruding ears.

Mental retardation caused by fragile X syndrome (or any other cause) often requires that the individual receive lifetime custodial care.

Gale Encyclopedia of Childhood & Adolescence. Gale Research, 1998.