By: Dr. Szatmari
PDD is a generic term referring to a group of disorders that share certain essential features: qualitative impairments in reciprocal social interaction, qualitative impairments in both verbal and nonverbal communication and a restrictive stereotypic pattern of behaviours. There are a variety of disorders that fall under this category, the most well known of which is autism. A number of other, non-autistic forms of pervasive developmental disorders have also been identified in recent years but have not yet received systematic study. In this article, I would like to expand on the differences between autism and these other, atypical forms of pervasive developmental disorders.
There is little agreement among experts in the field as to how many atypical forms of PDD might exist. Generally speaking, however, atypical forms differ from autism in one of three ways; in their age of onset, in the number of PDD symptoms present or in the types of symptoms present in the child. There is some evidence to suggest that these atypical forms of PDD are much more common than autism itself, having prevalence rates of 20-30 per 10,000, as compared to 4 per 10,000 for autism.
Autistic children virtually always show some disturbance in their development prior to three years of age. The most common symptom first identified is lack of language development, but, in hindsight, parents may identify other, more subtle, social impairments as well. There are, however, some clearly documented cases that demonstrate onset after three years of age. These children develop quite normally up until that point and then demonstrate a regression in either social or language skills or both. This regression may be precipitated by a clear neurological event such as epilepsy or meningitis or can be associated with a vague flu-like illness. Not uncommonly, however, this regression is not associated with any clear-cut precipitant and appears to happen “out of the blue”. As a result, these children are frequently quite handicapped but eventually begin to make slow progress and can regain many useful adaptive skills. In view of the characteristic loss of skills, these children have been referred in the literature as having a “disintegrative pervasive developmental disorder”.
The second commonly encountered, atypical form of PDD consists of those children who have the same age of onset as autism but develop fewer symptoms. Children with this atypical PDD frequently present with between five and seven symptoms (out of the 16 diagnostic features outlined in DSM-IV-R) and a judgment is made by the clinician that these symptoms represent a significant handicap for the child over and above his or her developmental delay.
Apparently, these children differ from autism in a number of other ways as well. For example, this group contains a higher proportion of girls and more frequently presents with various neurological disorders (such as the Fragile X Syndrome and Tuberous Sclerosis) than autistic children. There is also some evidence that this atypical group may have a higher frequency of epilepsy and abnormalities on brain imaging. There also appears to be greater variability in their cognitive development. For example, some of these atypical children may be very bright and there is another subgroup that may be quite profoundly handicapped. Accompanying this variability in cognitive development, there is a corresponding greater variability in outcome. Once again, a subgroup of these children may have a better outcome than the autistic child, whereas another subgroup may have a worse outcome. Clearly this atypical group is quite heterogeneous and further work needs to be done.
The third type of atypical pervasive developmental disorder not only has fewer symptoms than autism but also demonstrates a different pattern of symptoms; i.e., they typically lack certain features that are very characteristic of autistic children. These are the children that are described as having “Asperger’s Syndrome”.
Children with Asperger’s Syndrome usually demonstrate normal language and cognitive development (though the onset of speech may be slightly delayed). They do not, however, demonstrate the unusual language features so characteristic of autistic children; that is, echolalia, neologisms, idiosyncratic use of speech, pronoun reversal etc. They are less socially impaired that autistic children, and develop reasonable patterns of attachment with parents and other family members. It is with peers, however, that the qualitative impairments in reciprocal social interaction become most apparent. In addition, these children develop imaginative play, though the nature of the play is quite repetitive and lacks creativity. Historically these children have been described as developing very elaborate and unusual preoccupations such as an intense interest in insects, science fiction characters, underground subways etc.
Children with Asperger’s Syndrome appear to differ from autistic children on a number of other parameters as well. For example, they may include a larger proportion of boys and may demonstrate fewer neurological disorders than autistic children. In addition, there is some tentative evidence that they have a better outcome than autistic children, even those of comparable cognitive development. On the other hand, a number of families have been described in which one child has Asperger’s Syndrome and another is autistic, This suggests that in some circumstances, autism and Asperger’s Syndrome may have the same etiology.
I would like to emphasize that the data upon which I have drawn these distinctions are very sparse. In contrast to autism, very little systematic research has been done on the atypical forms of pervasive developmental disorders. With more information, much of what I have said may have to be modified. At the present time, there is considerable debate among experts in the field as to how to differentiate these various disorders from autism and whether such divisions actually make any difference. However, as clinicians and parents, it is important that we educate professionals and the public about the different presentations of all the children on the PDD spectrum. In effect, both autistic and atypical forms of PDD require a similar specialized approach in spite of the clinical differences that might exist between them.