Considering Coexisting Conditions or Comorbidity
(an LD definition support document) - 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)
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