Over 60% of the young children with autistic disorder or PDD-NOS in the present study exhibited hypotonia and/or hyporeflexia. In our previous study of 6–18 year-old children with autistic disorder (Haas et al., 1996
), 33% of the children were hypotonic and none of the children exhibited hyporeflexia. These results suggest that muscle tone and reflexes are affected in young children with PDD (both with and without mental retardation), and improve with age. Our finding that young children with autistic disorder more commonly display stereotypies during clinical exam than young children with PDD-NOS affirms that stereotypies are a more common feature of autistic disorder and appear to be related to severity of symptoms.
The observed rate of hypotonia (65%) is also higher than described in the Rapin (1996)
study (i.e., approximately 25% of the children with autistic disorder). While that was a slightly older sample of pre-schoolers, this discrepancy may also be related to differences in identification of hypotonia. The assessment of hypotonia is fairly subjective. We do have the advantage of having results from the same neurologist in both of our studies, but we do not have data on the reliability of identification of hypotonia across observations within a child or across participants within or across examiners.
It is difficult to localize hypotonia or apraxia to one particular brain area because these neurological signs can be associated with more diffuse abnormalities. The presence of more prominent motor dysfunction at the younger ages is interesting in the light of our previously reported cerebral and cerebellar anatomic abnormalities and more prominent involvement of frontal brain regions in these same children (Akshoomoff et al., 2004
; Carper & Courchesne, 2005
; Courchesne et al., 2001
). Some motor abnormalities in autism appear to be associated with involvement of the basal ganglia, supplementary motor, and anterior cingulate regions (Hardan, Kilpatrick, Keshavan, & Minshew, 2003
; Minshew, Sung, Jones, & Furman, 2006
). FMRI studies of motor task performance suggest activity patterns of cerebral and cerebellar systems are atypical in high functioning individuals with autism, despite normal task performance (Muller, Kleinhans, Kemmotsu, Pierce, & Courchesne, 2003
; Muller, Pierce, Ambrose, Allen, & Courchesne, 2001
Several of the tasks in the neurological exam could not be administered to the young children in this study due to their young age, lack of understanding or limited cooperation. This included several of the tasks that were found to be abnormal in the previous study of older autistic children, such as tests of graphesthesia, stereognosis, alternating movements, and sequential movements (Haas et al., 1996
). In the study by Rapin (1996)
, the limited ability to perform skilled, purposive limb movements was more common among mentally retarded children with autistic disorder (75%) than children with autistic disorder who had IQs in the normal range (30%). “Limb apraxia” was also reported for 57% of the nonautistic mentally retarded children. Whilst these skills may be delayed in the young children with PDD in our study, we found it difficult to confirm during the neurological examination.
In the present study, we found 32% of young children had an abnormal EEG but there were only 2 subjects (3%) with reported clinical seizure activity. As ours is a young population, this finding highlights the fact that epilepsy tends to develop in later years in autistic individuals. This rate of EEG abnormality is higher than the 15–20% reported previously (Giovanardi Rossi et al., 1995
; Tuchman & Rapin, 1997
). Since these previous studies were limited to or included a large proportion of older individuals, these results suggest that a proportion of young children with EEG abnormalities will eventually develop epilepsy. Our study did not show any correlation between the level of neurological dysfunction observed and the severity of the autism or presence of EEG abnormality, a finding in keeping with other published studies (McVicar & Shinnar, 2004
The main aim of conducting a neurological examination in a young child who appears to have autistic features is to rule out other potential neurological disorders and/or comorbid medical conditions (Filipek et al., 1999
,). Our results indicate that while many aspects of the neurological examination may be normal in the young child with PDD who has no known history of neurological injury or dysfunction, subtle neurological deficits may be present upon examination. The results indicate that neurological findings of hypotonia and poor coordination are detectable in a large proportion of young children with PDD but may diminish over time for many children. Nevertheless, some children with PDD continue to exhibit hypotonia as well as motor skills deficits (Haas et al., 1996
; Klin, Sparrow, Marans, Carter, & Volkmar, 2000
; Rapin, 1996
Our results also indicate that a sizeable proportion of young children with PDD have an abnormal EEG in the absence of any clinical seizure activity. As we only conducted prolonged sleep EEG recordings (Digitrace or video EEG) in 7 of our 60 cases (12%) we cannot state the true incidence of EEG abnormality in our study population, as prolonged sleep recordings may be needed to identify epileptiform abnormalities in this population (Shinnar et al., 2001
; Tuchman & Rapin, 2002
). It is also important to note that it may be very difficult for clinicians to recognize clinical seizures in children with PDD. Some of the characteristics of seizure activity may be interpreted as autistic behavior (Minshew et al., 2005
). It is certainly important for clinicians to consider the possibility of seizures or seizure activity for children who have a history of regression at any age. There are currently no guidelines on medical treatment for epileptiform EEG without clinical seizures, with the exception of treatment for Landau-Kleffner syndrome (Tuchman and Rapin, 2002
). It seems prudent to re-evaluate young children who appear to have PDD and EEG abnormalities, particularly if they demonstrate limited clinical benefit from early intervention programs and have limited language and cognitive development.