Knowledge in the area of developmental disabilities has been expanding rapidly. One area that has received particular attention is the topic of related comorbid conditions. This phenomenon is not exclusive to the field of developmental disabilities. However, research with this population is of recent origin. The purpose of this paper is to review the origins of this field including some of the notable developments and potential future trends.
Developmental disabilities consist of a range of conditions including intellectual disabilities (ID), autism spectrum disorder (ASD), cerebral palsy (CP), sensory impairments, and developmental coordination disorders among others (Boot et al., 2012, Chen et al., 2012, Meyns et al., 2012, Szumski and Karwowski, 2012 and van Gent et al., 2012). These disorders are well defined and have been the subject of assessment and treatment research for decades. This state of affairs is largely due to the severity and chronic nature of these conditions. However, these developmental disabilities may be moderated over time, and various physical and psychological supports can be put into place to assist in establishing more typical functioning.
Early on the development of the psychological and educational treatments was very limited as were assessment methods. Similarly, the understanding of etiology and symptom expression was in its infancy. Persons with ASD were characterized as young children (Kanner, 1943). In addition, it was believed that the condition was caused by poor parenting and occurred primarily among well-educated families (Kanner, 1943). An impediment to the comorbid psychopathology field in developmental disabilities was a protracted debate over whether children could experience depression (Costello, 1980, Lefkowitz and Burton, 1978 and Timimi, 2004). Many professionals suggested that childhood depression did not exist. Since that time a cultural revolution has occurred with respect to these attitudes. Massive amounts of data are accumulating which contradicts this position. Similarly, ADHD and ASD are now recognized as disorders which co-occur at high rates (Mayes et al., 2012 and Montes and Halterman, 2007). This research has been advanced very recently since the DSM-IV did not allow for a comorbid diagnosis of ADHD with ASD.
The field is maturing at an exponential rate. The basic theoretical foundation of these changes is the recognition of the complexity of human behavior. Developmental disorders overlap with a variety of conditions (Amr et al., 2012 and Kishore et al., 2004Matson and LoVullo, 2008, Matson et al., 1997b and Matson et al., 1999). Additionally, researchers are becoming aware not only that co-occurring disorders exist within the context of ID and ASD, but also that these developmental conditions put individuals at an increased risk of comorbid psychopathology (Smith and Matson, 2010a, Smith and Matson, 2010b and Smith and Matson, 2010c). It is critical to be able to accurately diagnoses these comorbidities in order to plan and implement comprehensive intervention packages (e.g. Matson et al., 2005 and Rojahn et al., 2003).
Becoming increasingly pertinent is the concern of what specific symptoms exemplify overlapping disorders, where the cutoff points occur in differential diagnosis, and how to most accurately measure symptoms and diagnose comorbid conditions. As a result, a good method of tracking the field's development is to analyze the number, type, and complexity of assessment methods that have been developed on this topic. For the current review, the focus will be restricted to ID and ASD.
The fields of ID and ASD have seen rapid developments in how they are viewed conceptually. Initially both disorders were seen as unitary constructs. Given this theoretical model, the notion of comorbid psychopathology fell outside the scope of clinical thinking and practice. As the field evolved, research began to appear suggesting not only that a range of comorbid psychopathology existed, but that a wide range of emotional problems were evident in this group of individuals. The amount and breadth of research in this growing area is expanding very rapidly.
Researchers now have a consensus that comorbid psychopathology is a frequent phenomenon among individuals with autism. No one knows with certainty what are likely to be the major trends in the future. Nonetheless, past trends of ten point to potential future behavioral trajectories. The most likely scenario is that the amount of research will increase in this field. The marked increase in publications thus far, for example, has led to a huge increase in the number of journals devoted to developmental disabilities during the last two decades. Also, the comorbid psychopathology field will continue to develop subspecialty areas. At this point, anxiety disorders and ADHD among persons with autism have developed to a point where each has become a subspecialty area.
The study of ASD in particular has seen a breathtaking increase in interest, particularly in the last decade. This interest is likely to continue for some time, and is further bolstered by various new public and private funding sources. The funding supports the development of research for both research and clinical services.
On the other hand, ID as a whole occupies a much smaller piece of the research pie compared to decades past. ASD in particular but also Developmental Coordination Disorder, Cerebral Palsy, and other developmental conditions have become more prominent in the research. As research has led to better assessment and diagnostic practices, persons previously diagnosed as ID now often fall under the category of ASD. This phenomenon is referred to as diagnostic substitution. This phenomenon is likely to continue as the adjustment of boundaries between various developmental disabilities evolves. These modifications are a characteristic of the field which will be with us for some time to come as trends in diagnosis and the development of additional knowledge about these various disorders continue to emerge. The field of ID has been a leader in the development of understanding comorbid psychopathology among persons with developmental disabilities. Similarly, the study of many specific developmental disabilities has mirrored the advances in the ID field, with research into comorbidities rapidly advancing. This trend is likely to continue. The future of research in this area is as a result both interesting and promising.
