Author: Dr. Glen DiPasquale
Comorbidity is described as a situation where two or more conditions that are diagnostically distinguishable from one another tend to occur together. The exact nature of the relationship between comorbid conditions is a matter of some debate in the research literature (Martini, Heath & Missiuna, 1999; Clarkin & Kendall, 1992; Goff, 1992). It is particularly difficult to determine whether one condition is in fact a symptom of the other – causality versus correlation. These important debates aside, research provides support for a number of conditions co-occurring with learning disabilities more often than expected “just by chance”.
The largest body of studies supports a comorbid relationship between learning disabilities and attention deficit disorder (with or without hyperactivity). This extensive research, featuring comorbidity estimates as high as 70%, was summarized recently by Riccio, Gonzalez & Hynd (1994) and Maynard, Tyler & Arnold (1999).
In fact, learning disabilities are sometimes confused with Attention Deficit Hyperactivity Disorder (ADHD). It is important to note that these are two distinct conditions, in spite of the significant level of co-morbidity. ADHD is not a specific learning disability. The distinguishing characteristics of students with ADHD include being more easily distracted, failing to finish assignments, weaker persistence of effort, day dreaming, looking away more often from activities they are requested to do and demonstrating less persistence of effort when completing boring activities (Barkley, Dupaul & McMurray, 1990). As well, children with ADHD have been distinguished from those with LD based on their higher levels of activity and impulsiveness. As mentioned above, a very large percentage of those who have ADHD also have accompanying learning disabilities, while approximately 30% of those who have learning disabilities also have ADHD. Nevertheless, the interventions that benefit people with ADHD and those who have learning disabilities are not the same. Therefore, it is important to diagnose these conditions accurately, before developing an Individual Education Plan for the student.
Confusion sometimes also arises for students who have learning problems arising from an acquired brain injury. While several symptoms of this condition also occur in children with learning disabilities, acquired brain injury is sometimes treated as distinct from learning disabilities. It is important to note that neither ADHD nor acquired brain injury are identified as specific exceptionalities within the Ontario educational system. As a result, many students with these conditions are included under the learning disability designation for the purposes of special education service delivery.
A group of disorders also found frequently to be comorbid with learning disabilities is that involving social, emotional, and/or behavioural difficulties (Kamphaus, Frick & Lahey, 1991; Glassberg, Hooper & Mattison, 1999). Studies suggest that anywhere from 24% to 52% of students with learning disabilities have some form of such a disorder (Rock, Fessler & Church, 1997). This group encompasses diagnoses such as conduct disorder and oppositional/defiant disorder (DeLong, 1995; Shaywitz & Shaywitz, 1991), as well as social adjustment disorder (Lyon, 1996).
Research also suggests that depressive or dysthymic disorders co-occur with learning disabilities (San Miguel, Forness & Kavale, 1996) although the nature of the relationship continues to be controversial (Wiener, 1998).
Research provides significant evidence supporting the co-morbidity of the following disorders with learning disabilities:
- Tourette’s Syndrome (Burd, Kauffman & Kerbeshian, 1992; Shady, Rulton & Champion, 1988; Chase, Friedhoff & Cohen, 1992; Walter & Carter, 1997)
- Schizophrenia (James, Mukherjee & Smith, 1996; Gillian, Johnstone, Sanderson, Cunningham & Muir, 1998);
- Epilepsy (Kerr & Espie, 1997; Espie, Kerr, Paul, O’Brien, Betts, Clark, Jacoby, & Baker, 1997; Laidlaw, Richens & Chadwick, 1993);
- Language/communication disorders (Riccio & Hynd, 1993; Schoenbrodt, Kumin & Sloan, 1997);
- Hearing impairment (Bunch & Melnyk, 1989);
- Visual disabilities (low vision, blindness) (Erin & Koenig, 1997); and
- Developmental co-ordination disorder (Missiuna, 1996; Fletcher-Finn, Elmes, & Strugnell, 1997; Martin, Heath & Missiuna, 1999)