Difficulties in Locating Discrete Diagnostic Categories: The Examples of Developmental Language Disorders and Genetic Syndromes

Introduction

Humanity develops through the chronic interaction between genetic and environmental factors. Many times, this interaction is responsible for the appearance of some developmental disorders, which affect the spoken and written language, as well as the communication skills of individuals. They refer to a significant number of people and have as their starting point childhood. Often, developmental disorders present a continuous spectrum of symptoms. Genes, through the processes they contribute to, result in the creation of phenotypes, which are shared in developmental disorders. Therefore, the same genes are very likely to be involved in different disorders. This is due to the overlapping of phenotypes (Konstantinou & Kosmidou, 2011) and in this way, a genetic overlapping connection between different disorders is created (Nisiotou & Vlachos, 2014). Other times, however, we find no common ground among the disorders. Each disorder has its own unique characteristics, which are not found in another and is a distinct clinical category

Several researchers have attempted to interpret the relationships of developmental disorders at the genetic, neurobiological, and behavioral levels. Recent researches confirm the experience of clinics that the simultaneous presence of two or more disorders in the same person is common, preventing diagnosis and intervention (Nisiotou & Vlachos, 2014). Learning, attention or behavior disorders can coexist, they are interconnected. Clinical research data report an overlap of neurodevelopmental disorders, a continuum of language and cognitive features in individuals with disorders.

Initially, it is argued that developmental disorders, despite their differences, often co-exist due to a strong genetic background (Gilger & Kaplan, 2001). However, it is being investigated whether they coexist due to a different etiology (comorbidity) or whether they belong to a spectrum with a common biological background (Nisiotou & Vlachos, 2014). Clinical data reveal possible genetic overlap due to a common biological factor, multiple involvement of specific genes. They are possibly responsible for the simultaneous appearance of 2 developmental disorders, e.g. dyslexia and Attention Deficit Hyperactivity Disorder (ADHD) (Gayan et al., 2005).

To date, there are three prevailing views regarding developmental disorders. The first speaks of overlap-coexistence between disorders, while the second separates them into distinct clinical categories. The latter view holds that the clinical continuum is impossible to slice into distinct categories. Many scholars reject the third position because they believe that the continuum is not separable. Taking into account several studies that have been carried out, it is believed that each disorder belongs to its own category and has its own distinct characteristics. Of course, some researchers question this position. They argue that some genes may be common to other disorders and therefore some features appear in more than one disorder (Nisiotou & Vlachos, 2014). However, by looking at each disorder individually, we realize the uniqueness of the characteristics of each disorder that make it different from the others.