Cognitive Impairment

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Cognitive Test Scores and Mental Retardation

Intelligence is not one skill but a composite of multiple cognitive processes, including visual and auditory memory, abstract reasoning, complex language processing, understanding of syntax, visual perception, visual motor integration, and visual spatial processing. A variety of standardized intelligence tests are available for use with children at each age level. Scores across a variety of cognitive tasks are summed to form an IQ or, for younger children, a developmental quotient (DQ) (Lichtenberger, 2005). Cognitive assessments of very young infants are limited in their predictive ability because of their reliance on assessment of visual-motor and perceptual abilities. As children mature, more verbal and abstract cognitive processes can be evaluated, and scores more accurately reflects their abilities. Cognitive tests are standardized for diverse large populations, with an IQ score of 100 considered the population mean.

The IQ score is a global score that does not include information about subtle dysfunctions. The full range of cognitive deficits seen in preterm children is not well described by the IQ score, and further cognitive analyses are necessary. Many preterm children have a wide scatter in their cognitive abilities, with excellent performance in some areas but relative weakness in other areas, and these contribute to difficulties in the classroom and at home.

Calculation of a DQ for preterm infants is complicated by whether their age should be calculated from their birth date (i.e., the chronological age) or from their due date (i.e., age corrected for the degree of prematurity). This issue is more important arithmetically the younger the infant is and the lower the gestational age at birth was. For example, a 6-month-old preterm infant born 3 months early who has skills at the normal level for a 3-month-old would have a normal DQ of 100 if it was corrected for the degree of prematurity but would be considered delayed in skill attainment, with a DQ of 50, if the chronological age was used.

For the most part, neuromaturation of the preterm infant in the NICU proceeds along the same timeline as intrauterine development (Allen, 2005a; Saint-Anne Dargassies, 1977). From biological and maturational perspectives, few environmental influences significantly accelerate neuromaturation, and most agree that one should fully correct for the degree of prematurity when preterm infants are evaluated and that this correction should be incorporated for at least the first 2 years of life (Allen, 2002; Aylward, 2002a; Aylward, 2002). Whether or not one corrects for the degree of prematurity may influence IQ scores for up to 8 years (Rickards et al., 1989).

Mental retardation is a disability that originates in childhood and is characterized by significant limitations both in intellectual functioning and in adaptive behavior, as expressed in conceptual, social, and practical adaptive skills (AAMR, 2005). Intellectual functioning is considered subaverage or significantly limited when an individual's IQ score is 2 or more standard deviations below the mean on a standardized intelligence test (generally an IQ less than 70 or 75, depending on the test). Borderline intelligence is when an individual's IQ score is between 1 and 2 standard deviations below the mean (generally, IQs of 70 to 80 or 85).

In a study of children with mental retardation in Norway, children born at 32 to 36 weeks of gestation had a 1.4 times increased risk of mental retardation than full-term children, and this

TABLE 11-2 Mean IQ Scores in Children Born Preterm and Full Term

Rate of

Mean (SD) IQ Score

TABLE 11-2 Mean IQ Scores in Children Born Preterm and Full Term

Rate of

Mean (SD) IQ Score

Year(s) of

Age

Number of

Follow-up

Gestational

Birth

Study

Birth

(yr)

Subjects

(%)

Age (wk)

Weight (g)

Preterm Group

Full-Term Control Group

Marlow et al., 2005

1995

6

241

78

<26

82.1 (19.2)

105.7(11.8)

Carvale et al., 2005

1998

3-4

30

30-34

110.8(10.4)

121 (10.6)

Mikkola et al., 2005

1996-1997

5

103

86

<27

94 (19)

Wilson-Costello et al.,

2005

1990-1998

1.8

143

90

<750

80.3 (20)

1990-1998

269

90

750-1,000

85.3(19)

Wilson-Costello et al.,

2005

1990-1998 1982-1989

1.8

402 51

90 88

<1,000 <750

83.6 (19) 81.4(21)

Wilson-Costello et al.,

2005

1982-1989 1982-1989

1.8

160 211

OO 00 00 00

750-1,000 <1,000

87.9 (19) 86.4(20)

Mikkola et al., 2005

1996-1997

5

172

85

<1,000

96(19)

2

78

91

95(13)

106(10)

Doyle et al., 2005

1991-1992 1985-1987

8

224 206

93 97

<1,000

94.9(15.8) 94.2(16.9)

104.9(14.1)

1979-1980

8

87

98

<1,000

96.3(15)

Taylor et al., 2000

1982-1986

11

60

82

<750

78(17.4)

99.1(18.1)

Taylor et al., 2000

1982-1986

11

55

85

750-1,499

89.5(14.4)

Halsey et al., 1996

1984-1986

7

30

60

<1,000 1,500-2,500

95.4(18.7) 109.7(15.1)

112.4(19.6)

McCarton et al., 1997"

1984-1985

8

874

89

<2,001 2,001-2,500

88.3,89.5 96.5, 92.1

intervention trial

Halletal., 1995

1984

8

324

87

<1,000 1,000-1,499

90.4(11.1) 93.7(13.6)

102.5(12.4) 101.1

Lefebvre et al., 2005

1976-1981

18

59

75

<1,500

12-16

<1,000

9.4(12)

108(14)

Saigal et al., 2000c

1977-1982

40 110 150

750-999

<1,000

86(20) 91(18) 89(19)

102(13)

Hack et al., 2002

1977-1979

20

242

78

<1,500

87

90

NOTE: SD=standard deviation.

NOTE: SD=standard deviation.

TABLE 11-3 Proportion of Survivors of Preterm Birth with and without Cognitive Impairment

Percentage of Subjects with Scores:

Year(s) of

Percentage of Subjects

Number of

Gestational Birth

2 SDs below the

1-2 SDs below the

TABLE 11-3 Proportion of Survivors of Preterm Birth with and without Cognitive Impairment

Year(s) of

Percentage of Subjects

Number of

Gestational Birth

2 SDs below the

1-2 SDs below the

Study

Birth

Age (yr)

Monitored

Subjects

Age (wk)

Weight (g)

Mean

Mean

Normal

Hintz et al., 2005

1996-1999

1.5

89

121

<24

52

26

22

1996-1999

80

315

24

44

25

31

Hintz et al., 2005

1996-1999

1.5

89

436

<25

47

24

29

1993-1996

80

102

<24

38

36

26

Hintz et al., 2005

1993-1996

1.5

89

239

24

40

28

32

1993-1996

80

341

<25

40

30

30

Vohr et al., 2005

1997-1998

1.6

80

910

22-26

<1,000

37

1993-1996

716

38.5

1993-1994

665

41.8

Vohr et al., 2005

1997-1998

1.6

80

512

27-32

<1,000

23

1995-1996

533

25.50

1990-1994

444

29.90

Mikkola et al., 2005

1996-1997

5

85

103

<28

12

Marlow et al., 2005"

1995

6

78

241

<26

21

25

54

Wood et al., 2005"

1995

2.5

92

19

<24

27

31

42

69

24

30

40

30

Wood et al., 2005

1995

2.5

92

143

25

30

31

39

231

22-25

30

34

36

Piecuch etal., 1997b

1990-1994

1.6

95

86

24-26

23

Piecuch etal., 1997b

1990-1994

1-6

95

18

24

39

33

28

30

25

30

23

47

1-6

95

38

26

11

18

71

Emsley etal., 1998

1990-1994

1.5-10

100

96

23-25

15

1984-1989

96

13

Msalletal., 1993

1983-1986

4

97

149

<28

10

St S

Ph O

00

9

OS

9

OS

ci

00

<N

9

9

OS

9

9

OS

1

1

99 99

29 00 00 99

vo 00

risk increased to 6.9-fold for children born at less than 32 weeks of gestation (Stromme and Hagberg, 2000). The risks of mental retardation in children born preterm compared with those in children born with normal birth weights increase from 2.3-fold for children with birth weights of 1,500 to 2,499 grams to 12-fold for children with birth weights of less than 1,500 grams, 15-fold for children with birth weights of less than 1,000 grams, and 22-fold for children with birth weights of less than 750 grams (Resnick et al., 1999; Stromme and Hagberg, 2000). Nonetheless, children born at less than 32 weeks of gestation or with birth weights of less than 1,500 grams comprised only 4 percent of children with mental retardation.

On the basis of data for preterm children born in the late 1980s and 1990s, survivors born preterm with the lowest gestational ages and birth weights have the highest risk of mental retardation and borderline intelligence (Tables 11-2 and 11-3). A recent large study of infants born at less than 26 weeks gestation in 1995 in the British Isles and evaluated at age 6 years reported that 21 percent had an IQ 2 or more standard deviations below the test mean and 25 percent had borderline intelligence (i.e., IQs 1 to 2 standard deviations below the test mean), whereas for the controls born full term the rates were 0 and 2 percent, respectively (Marlow et al., 2005).

Studies that compare preterm children's performance on intelligence tests against published test norms may underestimate their cognitive disadvantage. Although cognitive tests are standardized on the basis of a mean IQ of 100 for normal populations, there is a tendency for the mean IQ score in normal or control populations to drift upward over time. Marlow et al. (2005) noted a mean cognitive score of 106 in their full-term classmate controls. With restandardization, the percentage of children born before 26 weeks of gestation who had cognitive scores 2 standard deviations or more below the full-term comparison group's mean score rose from 21 to 41 percent.

Children born full term with normal birth weights and raised in similar environments have generally served as comparison groups in studies of the outcomes of preterm birth. In a 1989 meta-analysis, 4,000 children born with birth weights of less than 2,500 grams had a mean IQ that was 5 to 7 points lower than the mean for 1,568 controls who were born full term (Aylward et al., 1989). In more recent studies of children with birth weights of less than 1,500 or 1,000 grams, the preterm children have mean IQ scores that were 10 to 17 points, or 1 standard deviation, below those for the full-term controls (Breslau et al., 1994; Doyle and Anderson, 2005; Grunau et al., 2002; Halsey et al., 1996; Hansen and Greisen, 2004; Whitfield et al., 1997).

A 2002 meta-analysis of 16 case-control studies of children aged 5 years old or older and born from 1975 to 1988 noted significantly lower cognitive scores for 1,556 children born preterm compared with those for 1,720 controls born full term, with a weighted mean difference of 10.9 (95 percent confidence interval = 9.2 to 12.5) (Bhutta et al., 2002). When only studies that excluded severely neurologically impaired children born preterm were analyzed, the weighted mean difference was 10.2 (95 percent confidence interval = 9.0 to 11.5).

Many studies have noted a trend toward lower mean cognitive scores with decreasing gestational age and birth weight categories (Tables 11-2 and 11-3) (Bhutta et al., 2002; Doyle and Anderson, 2005; Hall et al., 1995; Halsey et al., 1996; McCarton et al., 1997; McCormick et al., 1992; Saigal et al., 2000c; Taylor et al., 2000; Wilson-Costello et al., 2005).

There is some controversy as to the consistency of individual DQ and IQ scores over time. Artifacts of intelligence testing contribute to this confusion. One small study of infants with birth weights of less than 1,000 grams found lower Bayley cognitive scores at 18 to 20 months from term than at 8 months from term, and this was associated with infant behavioral characteristics and family income (Lowe et al., 2005). The Bayley test items are, by necessity, heavily weighted toward visual motor abilities during the first year, but during the second year language concepts can be evaluated. In a study with 200 children with birth weights of less than 1,000 grams, Hack and colleagues (2005a) noted a significant improvement between the Bayley scores at age 20 months and cognitive scores at age 8 years (means, 76 and 88, respectively). The proportion of children with cognitive impairment (i.e., IQ scores 2 or more standard deviations below the test mean) decreased from 39 percent at age 20 months to 16 percent at age 8 years. This difference could be an artifact of the use of different tests at different ages. Ment and colleagues (2003) reported an increase in vocabulary test scores of 10 or more points in 45 percent of children with birth weights of less than 1,500 grams when they were retested at age 96 months after initial testing at age 36 months.

Further complicating the interpretation of these differences is the upward drift of IQ scores as a function of increased time from standardization (Flynn, 1999). Improvement in IQ scores with age is most common in children born preterm who have no neurological injuries or impairment and whose mothers have high levels of educational attainment (Hack et al., 2005; Koller et al., 1997; Ment et al., 2003). Despite improvements in their IQ scores with age, these children had more academic problems than children with stable IQ scores in the average range (Hack et al., 2005).

Adolescents and young adults who were born preterm continue to demonstrate a cognitive disadvantage compared with those who were born full term. When young adults who were born with birth weights of less than 1,000 grams were tested at a mean age of 18 years, they were found to have lower verbal, performance, and full-scale IQ scores than full-term controls: 93 and 106, 97 and 109, and 94 and 108, respectively (p < 0.0001) (Lefebvre et al., 2005). Hack and colleagues (2002) evaluated 20-year-olds who were born with birth weights of less than 1,500 grams and found a mean IQ of 87, whereas the mean IQ was 92 for controls who were born with normal birth weights. Only half (51 percent) had an IQ score above 84, whereas 67 percent of adults who were born full term had IQ scores above 84. When Saigal and colleagues (2000c) compared 12- to 16-year-olds who were born with birth weights less than 1,000 grams with controls who had normal birth weights, preterm children had lower mean IQ scores even when children with IQ scores below 85 or neurosensory impairments were excluded (mean IQ scores 99 and 104, respectively, p< 0.001; for total sample, mean IQ scores were 89 and 102, respectively; p < 0.0001).

Preterm children with no neurological impairments demonstrate not only lower mean cognitive test scores but also more problems with specific cognitive processes than full-term controls (Anderson and Doyle, 2003; Bhutta et al., 2002; Breslau et al., 1994; Grunau et al., 2002; Hack et al., 1993; Mikkola et al., 2005). One study of preschool children who were born at 30 to 34 weeks of gestation and who had no neurological impairments found lower scores not only on the Stanford-Binet IQ test (111 and 121, respectively; p < 0.001) but also on tests of visual perception, visual motor integration, memory for location, sustained attention, and vocabulary, as compared to a matched control group of children born at term (Caravale et al., 2005).

A number of studies have demonstrated that preterm children who were born with birth weights less than 1,000 or 1,500 grams and who had normal IQ scores have more problems with attention, executive function (i.e., organization and planning skills), memory, language, learning disabilities, spatial skills, and fine and gross motor function than controls who were born with normal birth weights (Anderson and Doyle, 2003; Aylward 2002; Goyen et al., 1998; Grunau et al., 2005; Hack and Taylor, 2000; Halsey et al., 1993; Mikkola et al., 2005; O'Callaghan et al., 1996; Ornstein et al., 1991; Rose et al., 2005; Saigal et al., 1991).

School Problems

Difficulty with cognitive processes contributes to the increased risk of school problems seen in children born preterm (Aylward, 2002; Grunau et al., 2002). In a study of 153 children born at less than 28 weeks of gestation, only half were ready and able to enter kindergarten with their peers (Msall et al., 1992). Speech and language delays, attention deficits, and learning disabilities were common. Among 8- to 10-year-old children who were born preterm with birth weights of less than 800 or 1,000 grams, 13 to 33 percent repeated a grade, 15 to 47 percent required some special education support, and 2 to 20 percent were in special education placements (Buck et al., 2000; Gross et al., 2001; Whitfield et al., 1997).

In a longitudinal study of 813 Dutch children born at less than 32 weeks gestation or with birth weights less than 1,500 grams, at age 9 years children born preterm had more school-related problems than the general Dutch population: 32 and 14 percent, respectively, functioned below grade level; 38 and 6 percent, respectively, received special education assistance; and 19 and 1 percent, respectively, were in special education classes (Hille et al., 1994). In adolescence (age 14 years), 27 percent received special education services, whereas 7 percent of their peers received special education services (Walther et al., 2000).

By early adolescence, the children who had been born preterm with birth weights of less than 1,000 grams were 3 to 5 times more likely than the controls born full term to fail a grade and required 3 to 10 times more special education resources than the controls born full term (Klebanov et al., 1994; Saigal et al., 2000c; Taylor et al., 2000). Saigal and colleagues (2005) found progressive increases in school problems with decreasing birth weight category: 13 percent for full-term controls, 53 percent for children born with birth weights between 750 and 1,000 grams, and 72 percent for children born with birth weight less than 750 g. The proportions of children who had been born with birth weights less than 1,000 grams and who were in regular classrooms without grade failures or special education resources were only 42 to 50 percent at 8 to 10 years of age and as low as 36 percent at 18 years of age (Halsey et al., 1996; Klebanov et al., 1994; Lefebvre et al., 2005; Saigal et al., 2000c).

Many difficulties in school reflect the presence of learning disabilities, which become more apparent as children who had been born preterm progress through their education. A specific learning disability is a term that refers to a heterogeneous group of disorders of one or more of the basic psychological processes involved in understanding or in using spoken or written language. These disorders may manifest as significant difficulties with the acquisition and use of listening, speaking, reading, writing, reasoning, or mathematical skills. The diagnosis of a learning disability requires comparisons of performances on tests of academic achievement, cognition, language, visual motor integration, and perceptual abilities. Although the incidence of learning disabilities varies depending on how they are defined, most estimates indicate that 10 percent or less of the general population has evidence of a specific learning disability.

Ample evidence suggests that many more children born preterm have specific learning disabilities than children of normal birth weight born full term. By school age, despite normal intelligence, children born with birth weights of less than 1,000 or 800 grams have a 3- to 10fold increased risk of problems with reading, writing, spelling, or mathematics compared with the risk for their peers who had been born full term (Aylward, 2002; Grunau et al., 2002; Hall et al., 1995; O'Callaghan et al., 1996; Ornstein et al., 1991; Saigal et al., 2000c). The most consistent academic difficulties associated with preterm birth are arithmetic and reading (Anderson and

Doyle, 2003; Bhutta et al., 2002; Hack et al., 1994; Klebanov et al., 1994; O'Callaghan et al., 1996; Ornstein et al., 1991; Saigal et al., 2000c). The proportion of children born preterm who experience academic difficulties increases with age as the complexity of the schoolwork increases and efficiency becomes an issue in the higher grade levels (Aylward, 2002). In a detailed analysis of the nature of the learning disabilities in 8- to 9-year-olds who had been born with birth weights of less than 1,000 grams, Grunau and colleagues (2002) suggested that the children's problems with visual memory, visual motor integration, and verbal intelligence explained many of their difficulties with arithmetic and reading.

As with other neurodevelopmental disabilities and school problems, the prevalence of learning disabilities increases with decreasing gestational age and birth weight: 7 to 18 percent in children born full term, 30 to 38 percent in children born with birth weights 750 to 1,499 grams, 66 percent in children born at less than 28 weeks of gestation, and 50 to 63 percent in children born with birth weights less than 750 grams (Avchen et al., 2001; Aylward, 2002; Breslau, 1995; Grunau et al., 2002; Hack et al., 1994; Halsey et al., 1996; Hille et al., 1994; Pinto-Martin et al., 2004; Taylor et al., 2000). In a study of 12- to 16-year-olds in three birth weight categories (normal, 750 to 1,000 grams, and greater than 1,000 grams), the proportion with scores 2 or more standard deviations below the mean increased with decreasing birth weight for reading (0, 12, and 23 percent, respectively), spelling (2, 18, and 38 percent, respectively), and arithmetic (5, 32, and 50 percent, respectively) (Saigal et al., 2000c). The parents of more mature preterm children (those with gestational ages 32 to 35 weeks) reported significant problems with mathematics in 29 percent of the children, speaking in 19 percent, reading in 21 percent, and writing in 32 percent (Huddy et al., 2001).

In a study of 20-year-olds, those with birth weights less than 1,500 grams continued to have lower academic achievement scores in mathematics and reading than controls born with normal birth weights (Hack et al., 2002). Fewer of the young adults studied had graduated from high school (74 and 83 percent, respectively). The mean age at graduation was higher for those with lower birth weights (18.2 and 17.9 years, respectively). Fewer men born weighing less than 1,500 grams than men of normal birth weight attended a 4-year college (16 and 44 percent, respectively), but the rates were similar for women (33 and 38 percent, respectively). In a Canadian study of 18-year-olds born with birth weights of less than 1,000 grams, 56 percent obtained a secondary school diploma, whereas 86 percent of the controls who had been born full term did so (Lefebvre et al., 2005). In contrast, a recent study of 23-year-old Canadians weighing less than 1,000 grams at birth and controls born with normal birth weights found no differences in the rates of completion of high school, postsecondary education, or university education or the total number of years of education that they had completed (Saigal et al., 2006).

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