Chapter Two
Prevalence, Causes, Issues, and Comorbid Disorders

In this chapter, we provide a brief overview of intellectual disability (ID) or intellectual developmental disorder (IDD), the different types of etiology for the disorders (chromosomal abnormalities/genetic origins and environmental toxins/teratogens), and the common types of intellectual disabilities associated with each factor: chromosomal (Down syndrome, Williams syndrome, Prader-Willi syndrome, Angelman syndrome) and environmental (fetal alcohol syndrome, teratogens, alcohol, lead-based paint, etc.). We also supply information on prevalence rates and descriptions of the common characteristics associated with each disorder type. The remainder of the chapter will focus on such topics as distinguishing between an intellectual disability and a developmental delay and other differential diagnoses, such as autism spectrum disorders (ASD), learning disabilities (such as dyslexia, dyscalculia, dyspraxia/developmental coordination disorder, dysgraphia), or other early onset disorders, such as language impairments (including late language emergence (LLE), specific language impairment (SLI, or selective mutism). Some of the more common comorbid disorders will be addressed in Chapter 3.

Etiology of Intellectual Disabilities: Subtypes

Intellectual disabilities can have a wide range of etiology including innate factors (genetic factors and biological) that are often outside of one's control, and environmental factors that are often preventable (toxins in the environment, or teratogens). As a result of the Human Genome Project and advances in human genetics, our knowledge and understanding of the influences that gene variants can have on human development and the potential for increased risks for vulnerability for some conditions have increased significantly. As noted by Khoury, Burke, and Thomson (2000), "Risks for almost all human diseases result from the interactions between inherited gene variants and environmental factors, including chemical, physical, and infectious agents and behavioral or nutritional factors. which raises the possibility of targeting disease prevention and health promotion efforts to individuals at risk because of their genetic make-up (p 5)." At the time of publication of their book on genetics and public health, 872 genes had been identified that could increase the risk of having an intellectual disability (Khoury et al., 2000).

In this section, we will discuss two broad areas of etiology: an intellectual disability due to genetic conditions and chromosome abnormalities, and an intellectual disability resulting from environmental factors.

Intellectual Disabilities Due to Genetic and Chromosomal Abnormalities

Individuals whose intellectual disability results from biological causes have either inherited a genetic defect or develop a chromosomal abnormality (genes combine or mutate or are compromised in some way). The following are some of the most common types of intellectual disability that result from genetic and chromosomal causes.

Down Syndrome

With a worldwide annual prevalence rate of 1 in every 1,000 births and a US prevalence rate of 1 in 800 births, Down syndrome (DS) is one of the most common chromosome abnormalities found in humans (Weijerman & de Winter, 2010). DS results from anomalies with chromosome 21. There are several ways that this gene can be compromised: An extra chromosome 21 may be evident, or the chromosome itself may be damaged in some way. Those with DS who have an extra chromosome are called Trisomy 21. The extra chromosome can cause a host of physical problems and cognitive challenges (Grant, 2006).

Individuals with DS are not a homogeneous group, and characteristics can differ widely, since not all features of DS are present in every case. However, some of the more typical traits include: short stature; short, broad hands and feet; round face; almond-shaped eyes; low muscle tone; flat facial features; and a protruding tongue (Perkins, 2009). In addition to unique physical characteristics, children with DS may also experience problems with motor skills (poor coordination and fine motor development) and speech (grammar, expressive language and articulation). Grammatical skills can be very weak (Fowler, 1990), and expressive language disabilities have been reported to be evident in as high as 83% to 100% of children with DS (Miller, 1999). Many parents of children with DS report that articulation problems often result in outsiders not being able to understand what the child is saying (Kumin, 1994). Despite these limitations, children with DS can be socially engaging and affectionate and have strong imitative skills, although they can also exhibit occasional stubborn tendencies. Individuals with DS may be vulnerable to a number of medical issues. Almost 50% of those born with DS also have congenital heart disease (Freeman et al., 1998) while between 38% and 78% will suffer from hearing loss (Roizen, Walters, Nicol, & Blondis, 1993).

The majority of children with DS will exhibit limitations in intellectual ability. They may be functioning within the intellectual range of 30 to 70, with an average IQ of 50 (Vicari, 2005). Some children with DS, however, are able to score within the upper limits of the low average range (IQ 85-90), which is referred to as "upper level DS." Children who are diagnosed with upper level DS may not meet criteria for a DSM diagnosis of an intellectual disability. Although there is wide variability in IQ levels for individuals with DS, research has demonstrated that overall IQ level progressively declines with age (Pennington, Moon, Edgin, Stedron, & Nadel, 2003).

DON'T FORGET

Although the prevalence rate for Down syndrome is 1 in 800 births, when maternal age hits 40, the influences of maternal and paternal age can increase that risk by almost 10 percent (e.g., 1 in 800 to 1 in 84 births).

There is an increased risk of bearing a child with DS as the mother or father ages. Research has demonstrated that prior to a woman being 35 years of age there is minimal evidence of increased risk for having a child with DS. However, the odds progress rapidly after 35 years of age. For example, a woman at age 20 has 1 in 1,441 chance of having a child with DS. This increases to 1 in 959 by 30 years of age. But by 40 years of age, the chances increase to 1 in 84, and by 50, the increase rises to 1 in 44 births (Morris, Mutton, & Alberman, 2002). Once maternal age reaches 40, then paternal age seems to take on more influence and now accounts for 50% of the influence on birth risk (Fisch et al., 2003).

Prader-Willi Syndrome

The prevalence rate for Prader-Willi syndrome (PWS) is in the range of 1 in 15,000 to 1 in 30,000 births (Cassidy & Driscoll, 2008). Children born with Prader-Willi syndrome will have a defect on chromosome 15. This defect is passed down from the father and results in a number of associated characteristics, including intellectual impairment. Developmental delay is often detected soon after birth due to low muscle tone and low reflexes (Milner et al., 2005).

Cassidy and Driscoll (2008) reported that milestones are very delayed, with attainment taking about twice the time compared to a normally developing child; examples are sitting at age 12 months (instead of 6 months), walking at age 24 months (instead of 1 year), and speaking words at age 2 years (instead of 1 year). Cognitive disabilities in children with PWS will be identified early in the school program. The majority of children with PWS fall within the range of mild intellectual disability (mean IQ: 60 to 70), while 40% will score within the borderline range (IQ: 70 to 80) or within the range of low-average intelligence (IQ: 80 to 90). About 20% will have an IQ score that falls within the moderate range of intellectual disability (IQ: 40 to 60) (Malich, Largo, Schinzel, Molinari, & Eiholzer, 2000).

Physically, children with Prader-Willi syndrome are often short in stature and have small hands and feet (Dykens & Cassidy, 1995). The syndrome is often accompanied by a triad of symptoms, including: maladaptive behavior (impulsivity, temper tantrums, mood swings, aggression, and compulsive eating), obsessive-compulsive characteristics, and skin picking (Dykens & Cassidy, 1995; Wigren & Hanson, 2005). Compulsive eating can result in weight gain, and behavioral controls are often required to combat obesity.

Angelman Syndrome

The prevalence rate for Angelman syndrome (AS) is estimated to be between 1 in 10,000 to 1 in 40,000 births (Clayton-Smith & Laan, 2003). Children born with AS also have a defect on chromosome 15, but this time it is passed on by the mother. Epileptic seizures can be common in children with AS, occurring in about 80% of the population. Seizures, if present, often will have onset between 1 and 5 years of age and resemble epilepsy with febrile convulsions in infancy (Clayton-Smith & Laan, 2003).

Children with AS also may have odd physical features, such as a flattened and microcephalic head. The syndrome was once called the "happy puppet syndrome" to capture the inappropriate smiling/laughter that accompanies the syndrome (Horsler & Oliver, 2006). Researchers have consistently identified four symptom clusters that characterize the syndrome: inappropriate affect (frequent and inappropriate laughing/smiling), developmental delay, speech delay, and movement disorders (Horsler & Oliver, 2006).

Children with AS have severe intellectual disability. Milestones are delayed, some significantly,...