Severity of Disability

Many outcomes researchers recognize the limitations of reporting the outcomes for individuals born preterm only in terms of the diagnoses of specific neurodevelopmental disabilities and have defined and reported on the severity of disability as well. For 8-year-olds born in 1991

and 1992 in Australia, severe disability (i.e., severe CP, blindness, or IQ scores 3 or more standard deviations below the mean) occurred in 9 percent of the children with birth weights of less than 1,000 grams and 12 percent of the children with birth weights of less than 750 grams (Doyle and Anderson, 2005). Moderate disability (i.e., moderate CP, deafness requiring hearing aides, or IQ scores 2 to 3 standard deviations below the mean) occurred in 10 percent of those with birth weights less than 1,000 grams and 15 percent of those with birth weights less than 750 grams. Mild disability (i.e., mild CP or IQ scores 1 to 2 standard deviations below the mean) occurred in 25 percent of children weighing less than 1,000 grams at birth and 33 percent of children weighing less than 750 grams at birth. More than half (56 percent) of the children with birth weights of less than 1,000 grams and 40 percent with birth weights of less than 750 grams had no disabilities.

Although a relatively small proportion of children born preterm have multiple disabilities, this group of children faces significant challenges with respect to mobility, academics, and the transition toward independence. Multiple disabilities were seen in 5 percent of kindergartners born at less than 28 weeks of gestation (Msall et al., 1992). By using the definitions of handicap based on the Education for All Handicapped Children Act (P.L. 94-142), 2.5 percent of the parents of children born with normal birth weights reported that their child had multiple handicaps, whereas 5 percent of the parents of children born with birth weights of between 1,501 and 2,500 grams, 12 percent of the parents of children born with birth weights of between 1,001 and 1,500 grams, and 14 percent of the parents of children born with birth weights of less than 1,000 grams reported that their child had multiple handicaps (Klebanov et al., 1994). In a sample of infants born with birthweights less than 1,000 grams, 14 percent of the infants born at greater than 32 weeks of gestation had a severe disability (Mercier et al., 2005). In contrast, severe disability was reported in 100 percent of the infants born at less than 22 weeks of gestation, 48 percent of those born at 22 to 23 weeks of gestation, 37 percent of those born at 24 to 25 weeks of gestation and 25 percent of those born at 26 and 27 weeks of gestation.

The presence of any neurosensory and neurodevelopmental impairment has been investigated in a number of studies of preterm infants. These impairments include: moderate to severe CP, cognitive impairment 2 or more standard deviations below the mean, blindness in both eyes, and hearing loss requiring amplification in both ears. For children born preterm in the 1990s, 28 percent born at 27 to 32 weeks gestation, 13 to 25 percent weighing less than 1,000 grams at birth, 45 percent born at less than 27 weeks of gestation, 58 percent born at 24 weeks of gestation, and 61 percent born at less than 24 weeks of gestation had neurosensory or neurodevelop-mental impairment, whereas 1 percent of controls born full term had such impairments (Hansen and Greisen, 2004; Hintz et al., 2005; Saigal et al., 2000c; Vohr et al., 2005; Wilson-Costello et al., 2005). Hack and colleagues (2002) reported a major neurodevelopmental disability rate of only 10 percent among young adults born with birth weights of less than 1,500 grams.

The probability of survival without a neurosensory impairment or a major disability increases with increasing gestational age. As many as 56 to 77 percent of children born with birth weights less than 1,000 grams and birth weights less than 750 grams survive and are free of major disability (Doyle and Anderson, 2005; Hack and Fanaroff, 1999; Hansen and Greisen, 2004; Piecuch et al., 1997a,b; Saigal et al., 1990; Wilson-Costello et al., 2005). However, among children born at less than 26 weeks gestation, only 20 percent were free of disability at age 6 years, with 34 percent of these children having a mild disability, 24 percent having a moderate disability, and 22 percent having a severe disability (Marlow et al., 2005). This is similar to the 21 per cent of survivors born at gestational ages less than 25 weeks who had no impairments reported by Hintz and colleagues (2005).

Some use the term "intact survival," calculated as the number of survivors who are "normal" divided by all live births. Generally, "normal" means no major cerebral palsy, mental retardation, or severe sensory impairment (i.e., no neurosensory impairment). As a concept, intact survival is most useful at the limit of viability, when the rate of mortality is so high. Two recent regional studies have reported survival without major disability: 0 to 0.7 percent for those with gestational ages of less than 23 weeks, 6 percent to 35 percent for those with gestational ages of 23 weeks, 13 to 42 percent for those with gestational ages of 24 weeks, and 31 to 56 percent for those with gestational ages of 25 weeks (Doyle, 2001; Wood et al., 2000). This concept of survival without disability may be useful for discussions with prospective parents as they face the impending delivery of their infant at less than 26 weeks gestation (see Appendix C).

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