The differentiation between pervasive developmental disorders (PDD) and ADHD may appear obvious. However, many children who have these disorders in various degrees of severity also exhibit symptoms of ADHD and even respond well to psychostimulants. This is especially true of Asperger's syndrome, which is more elusive from a diagnostic point of view than the other syndromes belonging to this group. Since the main characteristics of Asperger's syndrome include severe and persistent disturbances in social interactions and development of limited and repetitive behavioural patterns, interests and activities, these children show a significant clinical impairment in important functional areas such as the social or occupational sphere. An example of the confusion in this area is given by the work of Ghaziuddin et al. , which describes comorbidities of Asperger's syndrome and shows that the most common comorbidity in these children is that with ADHD.
A study that examined the development of children who were later diagnosed with PDD not otherwise specified (PDD-NOS) compared to children with ADHD found that in early childhood it was very hard to detect any significant differences between the groups . In an examination of the social functioning of children suffering from ADHD, high functioning autism or PDD-NOS , the children with autism were characterized by the highest (least normative) scores on social functioning scales; the next highest scores were of children with PDD-NOS, and the lowest were those of ADHD children. On the other hand, on the ''acting-out'' scale, the highest scores were given to children with ADHD, whereas on the ''social insight'' scale there was no difference between children with ADHD and those with PDD-NOS.
Another group of researchers  compared children with PDD-NOS, ADHD or other mental disorders and healthy controls according to their ability to recognize emotions and theory of mind (ToM). Children with PDD-NOS and with ADHD showed difficulties in emotion recognition and ToM in ways that could not be distinguished from one another. In contrast, children who exhibited behavioural disorders or depression did not show such difficulties and responded like healthy children. A distinction between the two groups of ADHD and PDD-NOS could only be made for second-order functioning in the ToM.
From a therapeutic point of view, it was also found that children suffering from PDD responded as well to stimulants as did children with ADHD . The main observable response was relief from restlessness and excess movement. At the same time, it was found that attention improved as well.
Another important differential diagnosis is that between children with PDD and those suffering from PTSD. Some PTSD characteristics in infancy (overarousal and hypervigilance on the one hand, and detachment, recoiling from people and introversion on the other) are similar to those of PDD. Much further research is needed in this sphere.
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