Cognitive Effects of Sickle Cell Disease in the Absence of Cerebrovascular Accident

Many children with SCD who have no history of CVA also exhibit cognitive decrements on general IQ measures and on measures of specific cognitive abilities relative to both the general child population and demographically matched comparison groups (Brown et al. 1993a; Schatz et al. 2002a). A meta-analysis conducted by Schatz et al. (2002a) documented that children with SCD and no CVA often scored lower on general IQ measures than healthy children, with 51% of studies reporting significant differences. On average, school-age children with SCD scored 4-5 standard score points lower than controls, with their age-adjusted scores falling in the low average range of ability. Research that examined specific cognitive domains found more robust and consistent differences, with 71% of studies reporting significant decrements in at least one cognitive domain (Schatz et al. 2002a). Medium-size effects corresponding with clinically significant decrements have been found most frequently in attention, processing speed, and executive functioning skills (Bernaudin et al. 2000; Brown et al. 1993a; Fowler et al. 1988; Knight et al. 1995; Noll et al. 2001; Schatz 2004; Wang et al. 2001a), and several studies have also documented verbal, language, and memory deficits (Bernaudin et al. 2000; Brown et al. 1993a; Knight et al. 1995; Noll et al. 2001; Schatz 2004; Steen et al. 2005; Wang et al. 2001a; Wasserman et al. 1991).

Cognitive decrements in children with SCD and no history of stroke appear to begin early in life and progress in severity throughout childhood and adolescence (Schatz and Roberts 2007; Thompson et al. 2002; Wang et al. 2001a). Thompson et al. (2002) documented an initial decrease in cognitive functioning between 12 and 24 months of age in children with SCD, and Schatz and Roberts (2007) found early deficits in attention and executive functioning in a sample of young preschool children with SCD, particularly in those with higher-risk SCD genotypes. Furthermore, a longitudinal study conducted with school-age children showed progressive effects of SCD on cognitive ability from early childhood throughout middle and late childhood (Wang et al. 2001a). Cognitive decrements in children ages 6-18 years became significantly larger over time in several domains in this study, including verbal IQ, math achievement, and processing speed, with scores decreasing an average of 0.5, 0.9, and 0.2 standard score points, respectively, per year (Wang et al. 2001a). Across these areas of ability, findings suggest that many children with SCD exhibit abilities in the lower end of the average range during early and middle childhood, with scores subsequently falling below the average range of their same-age peers by adolescence.

Cognitive declines in children with SCD are often clinically and practically significant. Decreases in cognitive scores reflect that children with SCD are falling further behind their peers in abilities that are central to learning and academic progress. Several studies found that children with SCD showed high rates of enrollment in special education services relative to peers and were likely to have academic attainment problems throughout school (Fowler et al. 1988; Nettles 1994; Schatz 2004). Results from a study comparing children who have SCD with demographically matched peers showed that children with SCD were much more likely to exhibit academic difficulties in terms of requiring remedial instruction or grade retention (i.e., 31% of patients vs. 14% of controls; Schatz 2004).

The causes of cognitive decrements and academic difficulties in children with no history of CVA are not yet clear, although research suggests that both disease-related and psychosocial factors may play a role in determining cognitive outcomes. In young children, studies have identified associations between cognitive performance and disease subtype (Schatz and Roberts 2007), parenting factors (Thompson et al. 2002), and socioeconomic variables (Tarazi et al. 2007). In older children, socioeconomic status has also been related to cognitive performance, although it explains only some of the variability in cognitive outcomes (Brown et al. 1993a; Fowler et al. 1985; Schatz 2004).

Results from studies examining disease-related variables related to cognitive outcomes in schoolage children suggest that anemia severity is a key factor in determining cognitive scores, because children with more severe anemia consistently exhibited larger cognitive decrements across multiple domains (Bernaudin et al. 2000; Brown et al. 1993 a; Steen et al. 1999, 2003). One explanation for the link between anemia and cognition is that mild, chronic cerebral hypoxia occurs with more severe anemia, leading to diffuse structural and functional brain abnormalities (Steen et al. 1999). Studies conducted by Steen et al. (1999, 2003, 2004, 2005) have used quantitative MRI techniques, which have revealed abnormalities in gray matter structures that are significantly associated with anemia severity and cognitive scores. Investigations using positron emission tomography (Powars et al. 1999), perfusion MRI (Kirkham et al. 2001; Oguz et al. 2003), and single photon emission computed tomography (Al-Kandari et al. 2007) have also documented localized metabolic and perfusion problems in children and adolescents with SCD that may help explain cognitive difficulties.

Studies to date have not shown significant effects of number of hospitalizations or school absences on cognitive or achievement test scores in SCD (Brown 1993; Fowler et al. 1985; Wang et al. 2001a). Other measures of illness severity, such as incidence of pain or fatigue levels, are more difficult to quantify and have not been adequately studied. Overall, further research is required to examine the multiple biological, behavioral, and psychosocial factors that may interact to determine cognitive outcomes in children with SCD.

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