etiological relationship between reading disability and phonological disorder, The

Annals of Dyslexia, 2002 by Tunick, Rachel A, Pennington, Bruce F

The present study investigated the etiological relationship between two disorders: dyslexia or reading disability (RD) and phonological disorder (PD). These disorders manifest at different ages and have typically been studied by researchers in different disciplines. However, a growing body of evidence suggests that the disorders overlap at symptomatic, cognitive, and etiological levels of analysis. In previous studies, each disorder has been found to be heritable, and there is also evidence that RD and PD run together in families, but no studies to date have documented whether there is a shared genetic influence between the two disorders. The present study examined this question in a sample of RD and non-RD twins. Subjects with a history of PD were identified and the etiological relation between the two disorders was examined. Results indicated that in the present sample, RD and PD are each heritable on their own. Further, even when correcting for RD, the two disorders were found to be coheritable, indicating that the finding of cofamiliality of RD and PD is at least partially driven by genetic influences. The implications of these results for the conceptualization, identification, and treatment of these two disorders are discussed.

Although their diagnostic criteria are quite different, there is considerable evidence for a relationship between dyslexia or reading disability (RD) and phonological disorder (PD). RD is a type of learning disability; the Diagnostic and Statistical Manual [of Mental Disorders] (DSM-IV) (American Psychiatric Association, 2000, p. 65) identifies reading achievement that is considerably below what would be expected by a child's age, education level, and intelligence as the disorder's defining feature. PD, in contrast, is a type of language disorder that involves disordered speech production, including speech sound deletions and substitutions, and typically affects speech intelligibility. According to the DSM-TV, PD is a communication disorder characterized by a "failure to use developmentally expected speech sounds that are appropriate for age and dialect" (APA, 2000).1 These two disorders typically manifest at different ages. PD tends to be diagnosed in children prior to reading age, while RD does not manifest itself until the reading acquisition process has begun. Further, RD and PD tend to be studied by researchers in different disciplines. Psychologists have tended to study RD, while PD has typically been studied by researchers in the field of speech science.

Despite these differences, a growing body of evidence suggests that there is a relationship between RD and PD. The two disorders have been found to overlap at several levels of analysis: at a symptomatic, cognitive, and etiological level, although the degree of relation may vary by subtypes of PD (e.g., Larrivee & Catts, 1999; Snowling, Bishop, & Stothard, 2000).

SYMPTOMATIC OVERLAP

Numerous studies have documented a symptomatic overlap between RD and PD. Several studies have examined the reading ability of subjects with a documented history of PD, and have found elevated rates of RD symptomatology in these samples relative to controls. Lewis and Freebairn (1992) found that at various ages, subjects with a history of PD performed worse than age-matched controls on measures of phonology, reading, and spelling ability. In a follow-up study of adults with a history of PD, Felsenfeld, Broen, & McGue (1992) found that this sample had poorer reading performance in the 11th grade relative to a control group.

Additional evidence for a symptomatic overlap between RD and PD comes from prospective studies of children diagnosed with PD, who at follow-up, exhibit symptoms of RD. Bird, Bishop, and Freeman (1995) found that children with expressive phonological impairments (PD) at age five performed worse than controls on measures of literacy and phonological awareness when followed up at age seven. Similarly, Larrivee and Catts (1999) recently found that children with PD performed worse than controls on tasks of reading achievement.

Several prospective studies of children at family risk for RD have added further support to these findings. Scarborough (1990) followed a group of such children from preschool through the second grade, and found that these children performed worse than controls on tasks tapping both speech and reading abilities. Gallagher, Frith, & Snowling (2000) conducted a study with preschool children at increased family risk for RD. They found higher rates of speech and language development delays in the at-risk children relative to controls; further, these early skill differences were found to be predictive of later reading ability. Elbro, Borstrom, and Petersen (1998) examined early predictors of RD in high- and normal family-risk children. They found that deficits in pronunciation accuracy were among the early language skills that were significantly predictive of later development of RD. Data from a recent longitudinal study (Pennington & Lefly, 2001) which followed children at high and low family risk for RD yielded evidence for higher rates of articulation problems in children at high risk for RD than in low-risk children.