Elsevier

Brain and Cognition

Volume 61, Issue 2, July 2006, Pages 195-210
Brain and Cognition

Mathematics learning disability in girls with Turner syndrome or fragile X syndrome

https://doi.org/10.1016/j.bandc.2005.12.014Get rights and content

Abstract

Two studies were carried out to examine the persistence (Study 1) and characteristics (Study 2) of mathematics learning disability (MLD) in girls with Turner syndrome or fragile X during the primary school years (ages 5–9 years). In Study 1, the rate of MLD for each syndrome group exceeded the rate observed in a grade-matched comparison group, although the likelihood of MLD persisting through the primary school years was comparable for all three groups. In Study 2, formal and informal math skills were compared across the syndrome groups, a normative group, and children from the normative group who had MLD. Few differences were observed between the Turner syndrome and normative groups. Despite having rote counting and number representation skills comparable to those in the normative group, girls with fragile X had difficulty with counting rules (e.g., cardinality, number constancy). However, this difficulty did not distingush them from the MLD group. Overall, counting skills appear to distinguish the Turner syndrome and fragile X groups, suggesting that the specificity of math deficits emerges earlier for fragile X than Turner syndrome.

Introduction

Turner syndrome and fragile X syndrome are two X-chromosome associated disorders, both of which are linked to poor math performance (Bennetto et al., 2001, Bruandet et al., 2004, Grigsby et al., 1990, Kemper et al., 1986, Mazzocco and McCloskey, 2005, Rovet, 1993, Rovet et al., 1994, Temple et al., 1996, Temple and Marriott, 1998). Indeed, children with either syndrome are more likely than children from age, grade, and IQ matched comparison groups to meet criteria for math learning disability (MLD) even as early as kindergarten (Mazzocco, 2001). However, it is unclear whether MLD in young children with Turner syndrome or fragile X represents a persistent phenotypic characteristic or a short term delay. In addition, the nature of the math difficulties in either syndrome may vary substantially due to differences in their respective cognitive phenotype (Bennetto et al., 2001, Mazzocco, 1998, Mazzocco, 2001, Mazzocco and McCloskey, 2005, Molko et al., 2003, Rivera et al., 2002, Rovet, 1993, Rovet, 2004, Rovet and Buchanan, 1999, Rovet et al., 1994). Although there is very little research comparing math performance in children with Turner syndrome versus fragile X, there is theoretical support for the notion that the nature of math difficulties may differ because of these contrasting phenotypes. As such, exploring MLD and its manifestation in syndromes with known genetic causes may inform our understanding of variations in underlying sources of math difficulties. Towards that end, the present study was designed to examine both the persistence of early math difficulties during the primary school years and the nature of those difficulties among children with Turner syndrome or fragile X.

Turner syndrome results from the partial or complete loss of one of the two X chromosomes typically present in females. Its prevalence is approximately 1 in 1900 live female births (Davenport, Hooper, & Zeger, in press). One consequence of X monosomy is that the ovaries of females with Turner syndrome fail to develop, resulting in a lack of estrogen production (Ross & Zinn, 1999). Estrogen may influence performance, particularly on verbal and nonverbal memory tasks (Ross, Roeltgen, Feuillan, Kushner, & Cutler, 2000), and may contribute to the cognitive phenotype associated with Turner syndrome (McCauley et al., 1987, Ross and Zinn, 1999, Ross et al., 2000). Females with Turner syndrome do not typically meet criteria for mental retardation; however, they may have learning disabilities, particularly in the area of mathematics (Rovet, 1993).

Fragile X syndrome is the leading known cause of inherited mental retardation. It occurs in approximately 1 in 4000 to 1 in 9000 live births (e.g., Crawford, Acuna, & Sherman, 2001) as the result of a single gene mutation on the long arm of the X-chromosome (Verkerk et al., 1991, Yu et al., 1991). This mutation leads to impaired production of a protein (FMRP) that is important for neural development. Although there is much phenotypic variability in children with the syndrome, most males with fragile X meet criteria for moderate to mild mental retardation (i.e., IQ scores between 36 and 70; Bailey, Hatton, & Skinner, 1998). In contrast, ∼50% of females with fragile X will have mental retardation (Rousseau et al., 1994), whereas the remaining females may have less severe cognitive impairments including learning disabilities, or may have no noticeable effects of the syndrome (Cronister et al., 1991, Hagerman et al., 1999). To investigate the subtle aspects of the cognitive phenotype in fragile X, in the present study we limited participation to those individuals with fragile X without mental retardation, which included only females.

Difficulties with mathematics in Turner syndrome or fragile X are seen throughout the life span, including the early primary school age years (Grigsby et al., 1990, Grigsby et al., 1996, Kovar, 1995, Mazzocco, 1998, Mazzocco, 2001, Mazzocco et al., 1993, Miezejeski and Hinton, 1992, Rovet, 1993), the later school years (Buchanan et al., 1998, Mazzocco, 1998, Rivera et al., 2002, Rovet, 1993, Rovet et al., 1994, Temple et al., 1996), and adulthood (Bennetto et al., 2001, Bruandet et al., 2004, Grigsby et al., 1990, Mazzocco et al., 1993). Rovet (1993) found that 55% of girls with Turner syndrome between the ages of 6 and 16 years met criteria for MLD, either alone or in combination with reading disability (RD). (In Rovet’s study, MLD was defined as performance below the 25th percentile on the Arithmetic subtest of the Wide Range Achievement Test-Revised.) This percentage exceeded the rate observed in the comparison group of sixth graders, of whom only 26% met criteria for a learning disability in mathematics, reading, or both. Using an even more conservative criterion than Rovet (i.e., quotient scores below the 10th percentile on the Test of Early Math Ability-second edition; TEMA-2), Mazzocco (2001) reported that 43% of girls with Turner syndrome met criteria for MLD. This percentage was significantly higher than the 10% observed among a gender, grade, age, and IQ matched comparison group of girls without Turner syndrome.

There are fewer studies of MLD in girls with fragile X syndrome; however, among girls with fragile X, Mazzocco (2001) reported that five of the nine girls with fragile X (56%) included in her initial study met criteria for MLD, defined as performance below the 10th percentile on the TEMA-2. This prevalence rate was not significantly different from the prevalence rate of 20% observed among an age, grade, and full scale IQ matched comparison group of girls without fragile X. However, when the MLD criterion was broadened to include performance below the 12th percentile on the TEMA-2, the group difference became significant: 87% of the girls with fragile X scored below the 12th percentile compared to 20% in the comparison group.

Although together these studies indicate greater prevalence of MLD in Turner syndrome or fragile X relative to comparison groups, the studies were cross-sectional, and were not designed to address whether MLD in girls with Turner syndrome or fragile X persists over time, or whether it reflects a transitory delay. Over the school age years, children can vary as to whether they meet criteria for MLD (Francis et al., 2005, Shalev et al., 1998, Silver et al., 1999). For example, among a relatively normative sample of children recruited from a large suburban school district, Mazzocco and Myers (2003) found that approximately 44% of primary school age children meet investigator-defined criteria for MLD during at least one of their primary school age years. However, approximately 66% continued to meet this criterion for one or more of the remaining primary school years, whereas the remainder did not meet the criterion more than once (Mazzocco & Myers, 2003). Individual variability over time has led some researchers to suggest that the criteria for MLD must be met at more than one point in time, if the classification of MLD is to be valid (Geary, 2004, Geary et al., 2000).

Given the elevated prevalence of math difficulty in both Turner syndrome and fragile X, it is important to assess whether the early MLD observed in girls with Turner syndrome or fragile X persists over time at a rate that matches or exceeds the frequency reported for the general population (Mazzocco & Myers, 2003). Determining the persistence of MLD in Turner syndrome and fragile X will contribute to understanding the degree to which children with either syndrome are at risk for MLD. In addition, the longitudinal design of the present study extends previous, cross sectional findings that suggest persistent difficulty with math across development.

Awareness of the prevalence and persistence of MLD serves to describe the extent to which math difficulties occur in a given population, but this information is insufficient for uncovering the nature of MLD. Mathematical competence depends both on conceptual knowledge of mathematical domains and the relevant procedural knowledge that is used for problem solving in those domains (Geary, 2005). This knowledge is supported by multiple cognitive systems including executive controls (e.g., working memory function, such as attention and inhibition), and language and visuospatial systems (see Geary, 2005 for a detailed summary). As such, MLD could reflect deficits in conceptual or procedural knowledge in mathematics, or it may reflect deficits in the underlying cognitive domains (Geary, 1993, Geary, 2005). It is unclear which of these alternatives pertains to Turner syndrome and fragile X, because cognitive phenotypes for both disorders include difficulty with mathematics, and deficits in working memory and visuospatial ability. Both alternatives need to be explored to establish the nature of MLD in Turner syndrome and fragile X. The present study was designed to examine whether girls with Turner syndrome or fragile X can be distinguished from each other, or from their peers with no known syndrome, on the basis of mastery of formal and informal math skills such as reading and writing numbers, judging magnitude, counting, and addition facts. Based on the limited studies presented to date, there is evidence that different MLD profiles exist for these two groups, as summarized below.

Only specific aspects of mathematics appear to be problematic for girls with Turner syndrome. For example, simple arithmetic, number comprehension and production, counting, and some aspects of understanding quantity, such as number comparison and estimation, are intact among adults with Turner syndrome (Bruandet et al., 2004). Similarly, some basic aspects of number sense, including counting (Mazzocco, 2001), reading and writing numbers, and magnitude judgments are age appropriate among school age girls (Temple & Marriott, 1998). Yet individuals with Turner syndrome, as a group, perform more poorly on measures of mathematics achievement than do their peers (McCauley et al., 1987, Molko et al., 2003, Rovet, 1993). Girls with Turner syndrome also have significantly lower performance on visual-perceptual and visual-motor tasks relative to their age and grade matched peers (Mazzocco, 2001, Rovet and Netley, 1982, Temple and Carney, 1995), which may be related to their math performance (Mazzocco, 1998, Rovet, 1993). Yet, Rovet et al. (1994) did not find a consistent relationship between visual spatial processing and procedural knowledge or math fact retrieval, which lead them to conclude that poor math performance in Turner syndrome is independent of visual spatial abilities.

Other researchers have observed a relationship between visual spatial and mathematical skills. Mazzocco (1998) found that, relative to girls with fragile X, girls with Turner syndrome made more errors associated with visual spatial ability, such as alignment errors on math calculation problems. In addition, Mazzocco (1998) found that visual spatial ability (as measured by the Judgment of Line Orientation test) was a strong predictor of math performance in girls with Turner syndrome, but was the weakest predictor of math performance in girls with fragile X. In a later study of MLD in kindergarteners, Mazzocco (2001) reported that girls with fragile X demonstrated lower performance relative to an age, grade, and IQ matched comparison group on the KeyMath-Revised Numeration subtest, which includes items ascertaining number sense, such as counting. Girls with Turner syndrome did not differ from their comparison group on this subtest. These findings suggest that girls with fragile X can be distinguished based on mastery of basic numerosity concepts (Mazzocco, 2001).

Taken together, the findings summarized above suggest that the two syndrome groups may be characterized by deficits in specific areas of math, such as counting, or by differences in the cognitive processes that underlie specific math skills (Mazzocco & McCloskey, 2005). In the present study, we first examine whether MLD observed in either syndrome persists over the primary school years. We then examine whether performance on specific individual items from three math measures, or composite scores from items reflecting one of several basic number concepts, differs across these two syndrome groups. Of interest is whether either syndrome group represents a model of distinct math deficits.

Section snippets

Study 1

Findings from earlier studies have demonstrated a higher incidence of mathematics learning disability (MLD) in children with Turner syndrome or fragile X, relative to children with neither disorder (e.g., Mazzocco, 2001, Rovet, 1993). The criteria for MLD that were used in these previous studies were based on one-time assessments. In the present study, we examined the frequency with which children met investigator-determined criteria for MLD at two time points during primary school years.

One

Study 2

Although both Turner syndrome and fragile X are associated with poor math performance (Bennetto et al., 2001, Rovet, 1993, Temple and Marriott, 1998), the causes of this math difficulty may distinguish these syndrome groups from each other and from the general population. The present study was designed to compare the performance of girls with Turner syndrome or fragile X to their peers, during the early primary school years. Based on previous findings that girls with Turner syndrome have intact

General discussion

The findings from the present study support previous reports of poor math performance in females with Turner syndrome or fragile X. The results of Study 1 indicate a higher frequency of MLD among girls with either syndrome, relative to the rate of MLD in the general population. Children from either syndrome group who meet the criteria for MLD will continue to do so, at least during their primary school age years, at a rate at least comparable to that observed in the normative group (∼70%). The

Acknowledgments

This work was supported by NIH grant HD R01 03461, and by a grant from the Spencer Foundation, both awarded to Michéle Mazzocco. The data presented and the views expressed are solely those of the authors. The authors thank the children who participated in the study, their parents and teachers, the staff at participating Baltimore County Public School elementary schools, and research assistant Stacy Chung. The authors acknowledge the outstanding contribution made by Gwen F. Myers, Project

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