This issue of Topics in Language Disorders (TLD) explores a common clinical experience, whereby clients present with more than one condition. This happens so frequently; we suspect it is often taken for granted that vulnerability in one area opens up vulnerability in other areas.

The term comorbidity was coined by Feinstein (1970) to refer to additional, distinct health conditions occurring during the development of an index disease. Within this conceptualization, the index disease is considered as primary, and fully separate, from the co-occurring, or comorbid, conditions that develop alongside it. In the area of medicine, where the nature of disease is well understood, it is possible to draw this distinction. However, it cannot be applied as clearly to areas of health and disease in which disorders are not so well characterized and diagnostic systems are in flux. This is exemplified in the fields of psychology and psychiatry by current debates over the forthcoming Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, diagnostic system. Because of this, it is quite common within these fields to find the notion of comorbidity somewhat relaxed and it is simply used to refer to two or conditions occurring within the same individual (Robins, 1994). Robins (1994) has argued that the study of comorbidity, as defined by diagnostic coexistence, is valuable to the understanding of comorbid conditions because it gives insight into the nature of these conditions. A key point in this issue of TLD is that comorbidity can be used to further researchers' and clinicians' understanding of communication disorders, particularly developmental communication disorders. In the first article (Tomblin & Mueller, 2012), we introduce some of the principal candidate explanations for comorbidity that can then serve as potential hypotheses for this research.

Attention-deficit/hyperactivity disorder (ADHD) is one of the most common co-occurring conditions experienced by children with developmental communication disorders (i.e., speech, language, or reading impairments). Indeed, it is arguable that clinicians working with school-aged children see this so frequently that the phenomenon is accepted as commonplace and uninteresting. A second point to this issue, however, is to demonstrate how the co-occurrence, or comorbidity, of ADHD with communication disorders is important for understanding the nature of these developmental disorders, as well as their clinical management.

The first question we address is whether ADHD co-occurs with various forms of developmental speech and language disorders (including reading) at rates greater than expected by chance. If this is indeed the case, why does this happen? All possible reasons need to be considered; however, a focus of this issue involves the possibility that these disorders have at least partial genetic overlap, which could then explain the overlap at the symptom, or phenotype, level. If this is true, we might find these "different" developmental disorders are not, in fact, so distinctly different but rather have fuzzy boundaries at the symptom level, which continue on down through neurodevelopmental systems and genetics. The clinical implications of such understanding are hard to predict; however, we can be sure that understanding more about the clinical conditions with which we work is better than knowing less. If we see that some of the neuropsychological pathways in ADHD are in fact shared with developmental communication disorders, it is important to incorporate this insight into clinical management.

Our approach to this issue has been to provide a short article (Tomblin & Mueller, 2012) that lays the groundwork for discussing comorbidity, along with some basic concepts in genetics that will be useful for understanding the latter articles. The second article also serves as a backdrop with respect to the current literature on ADHD. In the second article, Mueller and Tomblin (2012a) review current conceptualizations of the disorder with respect to its diagnosis and principal etiological bases. The literature and the research effort directed toward ADHD are large, particularly in comparison with developmental communication disorders. The result is that a number of studies, each involving large numbers of children, have been conducted multiple times, thus allowing researchers to perform interesting and informative meta-analyses. Much of this overview article on ADHD provides a summary of this rich literature. In the subsequent articles, we examine the issue of the comorbidity of ADHD with spoken language impairment (LI; Mueller & Tomblin, 2012b), speech sound disorder (SSD; Lewis et al., 2012), and reading disorder (Boada, Willicut, & Pennington, 2012). Anyone who has spent much time with a caseload of school-aged children should feel at home here, in that each of these conditions frequently overlaps with others.

In the article of Mueller and Tomblin (2012b), we examine the existing literature on ADHD and spoken LI. With the exception of two studies, most of this research has been carried out in clinical samples. That is, these studies started with children identified clinically as having either ADHD or LI and then determined the rate of the other condition. This research has generally shown high rates of ADHD among children with LI, and vice versa. An alternative way of examining comorbidity is to examine its overlap in the general population. This method is advantageous in that it does not rely on a clinical identification and diagnostic system for the identification of cases, which may miss milder and less complex cases and can lead to an overestimation of comorbidity if only severe cases are included. In this article, we provide an analysis of the comorbidity of LI and ADHD in a population-based sample and show that substantial rates of overlap are still found when using this method. We also see that the risk for ADHD, given LI, is no greater than the risk for LI, given ADHD, indicating the possibility that there is something common influencing this overlap.

Lewis et al. (2012) focus on the comorbidity of SSD and ADHD in children with SSD identified via clinical referral. Likewise, they find elevated rates of comorbidity. Their study is important on two counts. First, they saw that family history of speech-language impairment is associated with greater rates of ADHD in children with SSD, something also found in the article of Mueller and Tomblin (2012b), indicating the possibility of a familial basis to comorbidity. Second, Lewis et al. (2012) found that the comorbidity between SSD and ADHD was partly driven by a third comorbid condition, in the form of LI. This does not mean that we should not expect to see high levels of ADHD in children with SSD alone. Rather, this finding is a reminder of the complexity with which we should consider these disorders and their relationship.

Boada et al. (2012) focus on a form of reading impairment, dyslexia, which has been shown to be comorbid with ADHD. Importantly, dyslexia, as it is currently defined, can be independent of LI in that it is diagnosed on the basis of poor word decoding and recognition abilities, as well as by poor spelling. Thus, dyslexia and LI are not diagnosed in the same way. Dyslexia and LI, however, have been shown to be comorbid and could therefore share a common etiology (Kamhi & Catts, 1986). An intriguing finding from the article by Boada et al. (2012) is that there are probably multiple factors that contribute to dyslexia and ADHD and that these overlap. One of these factors, processing speed, is particularly interesting for those who are interested in LI because slowed processing has been proposed as a prominent feature of children with LI (Kail & Salthouse, 1994; Miller, Kail, Leonard, & Tomblin, 2001). An interesting open question is whether such a slowing might account for the comorbidity between LI and ADHD, as well as, perhaps, dyslexia.

This issue is aimed at encouraging further consideration of the importance of comorbidity in research on developmental communication disorders. The field of communication sciences and disorders straddles the worlds of medicine and education. However, whereas notions of diagnosis and comorbidity are central to the field of medicine, in the field of education, they are not. The problem with diagnosis is that it is often viewed as being largely concerned with labeling and classifying, rather than understanding. However, it may be better to view diagnosis as an effort to understand the nature of clinical interest. In this issue, we emphasize how comorbidity allows us to see beyond the boundaries of a label or diagnosis and can be used to gain insight into what are likely related, and perhaps very similar, conditions.

-J. Bruce Tomblin, PhD

-Kathryn Mueller, PhD

Issue Co-Editors

University of Iowa

Iowa City, IA

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