7.4 The Role of Co-Morbidity in the Identification and Treatment of Dyslexia

It is not uncommon for students with dyslexia to also be diagnosed with other disorders or conditions. Many parents and teachers are probably aware of the number of students who have been diagnosed with ADHD and dyslexia. This co-occurrence of two or more different disorders in the same individual is called comorbidity. A more formal definition of comorbidity is when “two or more disorders co-occur more often that we would expect by chance” (Pennington, 2006; Kain, Landerf & Kaufmann, 2008). Comorbidity is another important factor and consideration in the identification and treatment of dyslexia.

Research in medicine and the neurosciences have shown that attention deficits and dyslexia are frequent comorbid conditions. Current research suggests that approximately 20% to 40% of children with the inattentive subtype of ADHD have reading problems (Wadsworth, Defries, Willicutt, Pennington, & Olson, 2015; Sciberras, Mueller, Efron, Bisset, Anderson, Schilpzand et al., 2014) and that 20% to 40% of children diagnosed with reading problems have been found to also have ADHD (Germano, Gagliano & Curatolo, 2010). Sally Shaywitz has stated that “between 12 percent and 24 percent of those with dyslexia also have ADHD” (Shaywitz, 2008).

Research does not always differentiate between the term “reading disorder” (RD) and dyslexia. This may account for the inclusion of some students with reading difficulties in these studies, even when the origin of their reading struggles may not be dyslexia. While there is a body of research regarding the co-occurrence of dyslexia and autism spectrum disorders (ASD), many of the reported reading disorders associated with ASD are quite different from dyslexia and involve primary difficulties with reading comprehension as a result of the broader language and communication aspects of ASD. However, in much of the current research, the common overlapping deficits named in studies about the relationship between reading disorders and ADHD include naming speed and phonological short-term memory, which are common characteristics of dyslexia.

Current research suggests that the relationship between ADHD symptoms and reading is found to be predominantly in the inattentive type of ADHD rather than in the impulsive-hyperactivity subtype (Hendren, Haft, Black, Cushen-White, & Hoeft, 2018). The research on comorbidity between dyslexia and ADHD is extensive, probably due to its common occurrence. Phonics-based instruction that is systematic and explicit has the greatest evidence of being effective in the treatment of dyslexia. Such reading interventions for students with both dyslexia and ADHD have been shown to be effective regardless of medical intervention for the treatment of ADHD. While medical treatment for ADHD symptoms is effective in the reduction of these symptoms, research does not show that treatment for ADHD alone led to greater improvements in reading (Tamm, Denton, Epstein, Schatschneider, Taylor, Arnold et al., 2017).

Research in other comorbid conditions with dyslexia, especially those that are often identified in school settings, is also frequently reported. These other academic-based, co-occurring disorders tend to be other specific learning disabilities, specifically in the areas of mathematics and written language. While it is known that students with dyslexia often exhibit discrete difficulties with math — such as poor recall of basic math facts or difficulties in aligning numerals within columns — significant delays in mathematics could be more related to dyscalculia, a specific math disorder/learning disability. Reading disorders and dyscalculia have a comorbidity of approximately 40%, according to a recently completed study (Wilson, Andrews, Struthers, Rowe, Bogdanovic & Waldie, 2015). 

Reading disorders have also been shown to co-occur with dysgraphia (writing). The correlation of word reading and writing performance is shown to be around 70%, although a specific rate of actual comorbidity is unknown (Ehri, 2000). Many researchers will explain the overlap of dysgraphia and dyslexia by noting the reciprocal relationship between learning to read and learning to spell. While disordered spelling can significantly impact written communication, the broader issues with both the physical aspects of writing (handwriting and recall of letter formation) and the broader language aspects of syntax and written expression are often symptoms of a specific learning disability in written language (dysgraphia). (See Chapter 8: Dyslexia and Legislation for more information.)

The other most commonly reported comorbid conditions with dyslexia are related to mental health and are psychological in nature. These types of comorbid conditions come in two broad categories: internalizing mental health problems and externalizing mental health conditions. Internalizing conditions are inward-facing difficulties that occur in an individual and tend to not be overtly obvious to others, while externalizing problems involve more-apparent acting out within the environment and are outward facing.

In Chapter 4: School-Based, Supports for Students with Dyslexia, research related to the comorbidity of dyslexia and depression, anxiety, and social-emotional well-being was mentioned, as was the fact that those with dyslexia are reported to have these types of internalizing conditions on the order of two to five times greater than their non-dyslexic peers (Wilson et al., 2009). Students with dyslexia report greater generalized anxiety than their peers without dyslexia, even after controlling for ADHD symptoms (Nelson & Harwood, 2011; Goldston, Walsh, Arnold, Reboussin, Daniel, Erkanli et al., 2007). However, there is little research that would help distinguish generalized anxiety from anxiety specific to reading (reading anxiety).

In addition to comorbidity with anxiety, students (children and adolescents) with dyslexia exhibit higher rates of depression (Mammarelli, Ghisis, Bomba, Caviola, Broggi et al., 2016; Mugnaini, Lassi, La Malfa & Albertin, 2009). There is evidence for a correlation between severe dyslexia and greater depression in younger children (Maughan, Rowe, Loeber & Stouthamer-Loeber, 2003). Like research on the comorbidity of anxiety and reading problems, the existence of depression with dyslexia does not link to comorbidity with ADHD (Carroll, Maughan, Goodman & Meltzer, 2005).

Students with dyslexia also can exhibit externalizing mental health issues as comorbid conditions. These include behaviors associated with impulse control, conduct disorders, and oppositional defiant disorders. It is not clear, however, how much of the higher incidence of externalizing behaviors among students with reading disorders precedes the reading problem or is the emotional result of it. Research is incomplete in documenting any causal relationship between dyslexia and specific conduct disorders. Evidence that behavioral problems demonstrated by students with dyslexia occur across academic and nonacademic settings suggests that externalizing mental health issues are somewhat independent of reading problems (Kempe, Gustafsonn, & Samuelsson, 2011). Current trends in research suggest the possible explanation of the co-occurrence of dyslexia and behavioral disorders is each condition’s comorbidity with ADHD. ADHD commonly occurs with dyslexia, conduct disorders, and oppositional defiant disorder (Levy, Young, Bennett, Martin & Hay, 2015).

Our current understanding of comorbidity further promotes the recommendation of comprehensive and thorough evaluation of students with dyslexia. When a student receives a diagnosis of dyslexia or ADHD, it is very important to complete further assessment to determine the existence or nonexistence of other commonly occurring/comorbid conditions. When students are found to have comorbid conditions that can affect learning, a multimodal treatment plan is considered optimal. Unrecognized comorbidities can result in interventions that are not adequately specific to the student’s profile of learning challenges and needs. 

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