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.
Presently the DSM-III-R criteria
for autism require that 8 out of the 16 diagnostic features be
present to make a diagnosis of autism (though this is likely to
change in DSM-IV). Children with this atypical PDD frequently
present with between five and seven symptoms 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.
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
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 abut 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.
Dr. Peter Szatmari
Newslink is a publication of:
The Autism Society of Ontario
Co-director, PDD Team
Chedoke-McMaster Hospitals
Associate Professor, Dept. of Psychiatry
McMaster University, Hamilton
300 Sheppard Avenue West
Suite 302
Willowdalae, Ontario
Canada, M2N 1N5
PH: (416) 512-9880
FAX: (416) 512-8026
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