Principles for Participant Selection

The observation that dyslexia tends to run in families was made early on (Orton, 1937). Twin studies have confirmed that genetic factors underlie a large proportion (30–70%) of population variability in reading measures (Francks, MacPhie, & Monaco, 2002). All longitudinal studies published so far are based on genetics/heredity as an early/preschool at-risk factor (H. Lyytinen, Aro, Eklund, Erskine, Guttorm, Laakso, Leppanen, Lyytinen, Poikkeus, & Torppa, 2004). Thus, the classic at-risk factor in dyslexia is familial occurrence (P. Lyytinen, AM, Puolakanaho, Richardson, & Viholainen, 2001; van Alphen, de Bree, Gerrits, de Jong, Wilsenach, & Wijnen, 2004). 

However, in a study by (Rejnö-Habte Selasse, Jennische, Kyllerman, Viggedal, & Hartelius, 2005) only 39.3% of the participants diagnosed with dyslexia had a family history of language impairment. Unpublished data from our own laboratory have similarly shown that only 64.3% of children with a formal diagnosis of dyslexia had identified dyslexia in the immediate family (Helland, 2002).

Thus, data that exclusively rely on heredity as the risk factor may underestimate the actual incidence of children at risk for dyslexia; not all off-spring of dyslexic parents develop dyslexia, and not all dyslexic individuals have dyslexia in their nearest family. It is therefore necessary to define other risk factors in addition to the genetic factor. 

The “grand” theories of dyslexia suggest a diversity of aetiologies pointing to assessable risk factors:  

Auditive, visual, audio-visual modalities (Boder, 1968; Gjessing, 1977, 1986)

Allergies/left-handedness/dyslexia (Geschwind & Galaburda, 1985a, 1985b, 1985c)

The magno-cellular system (Stein & Walsh, 1997)

Genetics (Annett, 1985; McManus, 1991; Pennington, 1990)

Gender (Miles, Haslum, & Wheeler, 1998; Shaywitz, Shaywitz, Pugh, Constable,  Skudlarski, Fulbright, Bronen, Fletcher, Shankweiler, Katz, & Gore, 1995)

Phonological awareness (Snowling & Nation, 1997; Vellutino, 1979)

Cerebellum/motor control (Nicolson & Fawcett, 1999; Tallal, 2006)

Processing impairment (Tallal, 2006)

In her book on dyslexia, Snowling (2000, p 216) concludes by pointing to the “dyslexia spectrum”, indicating that there is not just one, but multiple possible factors explaining dyslexia. Also, although there is no consensus as to the definition of dyslexia, all major definitions state that dyslexia is a constitutional condition (Lyon, 1995; Lyon, Shaywitz, & Shaywitz, 2003; The British Dyslexia Association, 1998; World Federation of Neurology, 1968). Any constitutional impairment should be possible to identify early by combining theoretical knowledge in information collection and test selection. 

Consequently, to identify children at risk for developmental dyslexia it is necessary to have a broader perspective than genetics/heredity. In clinical situations of dyslexia assessment the anamnesis will typically contain questions from all the “grand” theories, formulated as questions on somatics, language development, motor control and dyslexia in the nearest family. 

References: 

Annett, M. (1985). Left, Right, Hand and Brain: the Right Shift Theory. Hillsdale: NJ. Erlbaum.

Boder, E. (1968). Developmental Dyslexia: A Diagnostic Screening Procedure on Three Characteristic Patterns of Reading and Spelling. A Preliminary Report: Claremont Graduate School and Univ. Center, Calif.

Francks, C., MacPhie, I. L., & Monaco, A. P. (2002). The genetic basis of dyslexia. The Lancet Neurology, 1, 483-490.

Geschwind, N., & Galaburda, A. M. (1985a). Cerebral lateralization: Biological mechanisms, associations, and pathology: I. A hypothesis and a program for research. Archives of Neurology, 42(5), 428-459.

Geschwind, N., & Galaburda, A. M. (1985b). Cerebral lateralization: Biological mechanisms, associations, and pathology: II. A hypothesis and a program for research. Archives of Neurology, 42(6), 521-552.

Geschwind, N., & Galaburda, A. M. (1985c). Cerebral lateralization: Biological mechanisms, associations, and pathology: III. A hypothesis and a program for research. Archives of Neurology, 42(7), 634-654.

Gjessing, H.-J. (1977). Lese- og skrivevansker. Dyslexi [Reading and writing impairments. Dyslexia]. Bergen: Universitetsforlaget.

Gjessing, H.-J. (1986). Function analysis as a way of subgrouping the reading disabled: Clinical and statistical analyses. Scandinavian Journal of Educational Research, 30(2), 95-106.

Helland, T. (2002). Neuro-Cognitive Functions in Dyslexia. Variations According to Language Comprehension and Mathematics Skills. Unpublished Doctoral thesis, University of Oslo, Oslo.

Lyon, G. R. (1995). Toward a definition of Dyslexia. Annals of Dyslexia, 45, 3-27.

Lyon, G. R., Shaywitz, S. E., & Shaywitz, B. A. (2003). A Definition of Dyslexia. Defining Dyslexia, Comorbidity, Teachers' Knowledge of Language and Reading. Annals of Dyslexia, 53, 1-14.

Lyytinen, H., Aro, M., Eklund, K., Erskine, J., Guttorm, T., Laakso, M., Leppanen, P., Lyytinen, P., Poikkeus, A., & Torppa, M.(2004). The development of children at familial risk for dyslexia: birth to early school age. Annals of Dyslexia.     2004 Dec;54(2):184-220., 54(2), 184-220.

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Miles, T. R., Haslum, M. N., & Wheeler, T. J. (1998). Gender Ratio in Dyslexia. Annals of Dyslexia, 48, 27-55.

Nicolson, R. I., & Fawcett, A. J. (1999). Developmental Dyslexia: The role of the Cerebellum. Dyslexia, 5, 155-177.

Rejnö-Habte Selasse, G., Jennische, M., Kyllerman, M., Viggedal, G., & Hartelius, L. (2005). Comorbidity in severe developmental language disorders: Neuropediatric and psychological considerations. Acta Pædiatrica, 2005; 94: 471–478, 94, 471-478.

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Snowling, M. (2000). Dyslexia (Second ed.). Oxford: Blackwell publishers.

Snowling, M., & Nation, K. A. (1997). Language, Phonology and Learning to Read. In C. Hulme & M. Snowling (Eds.), Dyslexia: Biology, Cognition and Intervention. London: Whurr Publishers Ltd.

Stein, J. F., & Walsh, V. (1997). To see but not to read; the magnocellular theory of dyslexia. Trends in Neuroscience, 20(4), 147-152.

Tallal, P. (2006). Process Faster, Talk Earlier, Read Better. In G. D. Rosen (Ed.), The Dyslexic Brain. New Pathways in Neuroscience Discovery. Mahwah: Lawrence Erlbaum Associates.

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Orton, S. T. (1937). Reading, Writing and speech Problems in Children. New York: W. W. Norton.

Pennington, B. F. (1990). The genetics of dyslexia. Journal of Child Psychology & Psychiatry & Allied Disciplines, 31(2), 193-201.

van Alphen, P., de Bree, E., Gerrits, E., de Jong, J., Wilsenach, C., & Wijnen, F. (2004). Early language development in children with a genetic risk of dyslexia. Dyslexia, 10, 265-288.

Vellutino, F. R. (1979). Dyslexia: Theory and Research. Cambridge, Mass.: MIT Press.

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