There is great interest in understanding the earliest manifestations of autism. To date, most accounts of early development in autism have relied on retrospective analysis of home videotapes of infants later diagnosed with autism. Recently, a few prospective accounts of the emergence of autism symptoms during infancy have appeared. One goal of such research is to identify young infants at risk for autism so that very early intervention might prevent, or at least ameliorate, the development of autism symptoms.
Studies of young infants at risk for autism, researchers have examined several autism symptoms known to be present by early preschool age in children with autism. Many of these symptoms pertain to behavioral skills present in typically developing infants during the first 2 years of life. A number of behavioral symptoms reliably distinguish preschool-age children with autism from those with developmental delay. These include impairments in social orienting, joint attention, imitation, responses to emotional displays of others, symbolic play, and language (e.g., Charman and Baron-Cohen 1997
; Charman et al 1998
; Dawson et al 1998a 1998b
; Mundy et al 1986
; Stone et al 1997
). Relatively few controlled studies, however, have examined how children with autism below age 3 differ from children with related disabilities. Charman et al (1998)
found that 20-month-olds with autism were more impaired in joint attention, responses to another's distress, pretend play, and imitation, compared with those with language delay. Other studies found that 24-month-olds with autism performed fewer joint attention gestures, including pointing and showing, and had more impaired language and imitation skills than typically developing and language-impaired children (Lord and Paul 1997
; Stone et al 1997
Retrospective studies of home videotapes have allowed investigators to observe the early social, language, motor, and play behaviors of infants who later receive a diagnosis of autism and to examine developmental differences between infants later diagnosed with autism and typically developing infants (Mars et al 1998
; Osterling and Dawson 1994
) and infants later diagnosed with mental retardation (Baranek 1999
; Osterling, Dawson, Munson 2002
). In one such study, Osterling and Dawson (1994)
examined videotapes of first birthday parties and demonstrated that 1-year-olds later diagnosed with autism could be distinguished from 1-year-old typically developing infants. How often a child looked at the face of another person (“gaze”) correctly classified the greatest number of children (77%). When gaze was combined with the behaviors of showing, pointing, and orienting to name (i.e., social orienting), 91% of the infants with typical development and autism were correctly classified. These results were replicated by Mars et al (1998)
who used blind scoring to evaluate home videotapes of first birthday parties of 25 infants later diagnosed with autism and 25 typically developing infants. Again, the variable “looks at faces” was found to be a powerful discriminator between the two groups, as well as joint attention (e.g., pointing) behaviors.
A subsequent home videotape study compared 1-year-olds later diagnosed with an autism spectrum disorder (ASD) with 1-year-olds later diagnosed with mental retardation and 1-year-olds with typical development. This study showed that 1-year-olds with ASD could be distinguished not only from typical 1-year-olds, but also from 1-year-olds with mental retardation (Osterling et al 2002
). This is important because of the high comorbidity of autism and mental retardation. In this study, the infants with ASD were less likely to look at others and to orient to their names than infants with mental retardation. Joint attention behaviors, however, did not distinguish between ASD and developmental delay at 1 year of age, suggesting that other behaviors related to attending to people and other's speech might be important in distinguishing ASD from developmental delay at very young ages. In yet another home videotape study, a failure to orient to name was the best discriminator between 8- and 10-month-olds with ASD versus typical development (Werner et al 2000
). In addition, some infants failed to display communicative babbling. Werner and Dawson (2005)
used home videotapes of infants later diagnosed with autism to validate the phenomenon of autistic regression in the second year of life. One-year-old infants whose parents reported early onset of autism symptoms (i.e., symptoms evident in the first 12 months of life) displayed fewer joint attention and complex babbling and words at 12 months compared with infants with autistic regression.
A few prospective studies of infants later diagnosed with autism have now been published. Dawson and colleagues (2000)
published the first case study of a 1-year-old infant with autism. Because of early feeding difficulties, this infant was evaluated and closely monitored by a neurologist and occupational therapist from birth, allowing a fairly detailed accounting of his early development. Dawson and colleagues noted that, except for feeding difficulties associated with oral motor problems in the first few months of life, autism symptoms were not apparent until after 6 months, when the infant became less socially responsive. By 13 months, many symptoms were apparent, however, including language delay, social aloofness, and stereotyped motor behaviors. Klin and colleagues (2004)
later reported a case study of an infant from 12 to 20 months of age, noting delays in language and joint attention at 12–14 months, and, by 15 months of age, impaired toy exploration and a restricted range of interests. Two larger studies of infants at risk for autism have been conducted. Zwaigenbaum et al (2005)
studied 65 infants from aged 6 to 36 months who were at genetic risk for autism by virtue of having an older sibling with autism. By 12 months, siblings who were later diagnosed with autism showed poor eye contact and visual tracking, difficulty disengaging attention, and impairments in social orienting, imitation, and social smiling and interest. Other characteristics noted included passivity followed by distress reactions at 6 months, fixation on particular objects, decreased expression of positive affect, and delayed language onset. In the second study of 87 infants at genetic risk for autism, Landa and Garrett-Mayer (2006)
reported no symptoms at 6 months of age, but by 14 months they observed delays in language and motor development. In summary, it appears that symptoms of autism are not readily apparent by 6 months of age; however, by 8–12 months, several behavioral symptoms associated with autism can be observed.
In contrast, few early biological markers of autism have yet been identified. One such marker is an atypical pattern of growth in head circumference characterized by small-to-normal head size at birth followed by an accelerated pattern of growth in head circumference that appears to begin at about 4 months (Courchesne and Pierce 2005
; Gilberg and de Souza 2002
; Redcay and Courchesne 2005
). Courchesne and colleagues (2003)
reported an increase in head circumference of 1.67 SD between birth and 6–14 months. In a meta-analysis using head circumference (converted to brain volume), brain volume measured from magnetic resonance imaging, and brain weight from autopsy studies, Redcay and Courchesne (2005)
found that brain size changes from 13% smaller than control subjects at birth to 10% greater than control subjects at 1 year, and only 2% greater by adolescence. Sparks et al (2002)
reported significantly larger total cerebral volume in 3- to 4-year-old children with autism compared with chronological- and mental-age-matched children with developmental delay and chronological-age-matched children with typical development. It is interesting to note that the timing of the onset of accelerated head growth between 4 and 12 months slightly precedes, then overlaps with the onset of behavioral symptoms.
The purpose of this report is twofold: first, we report longitudinal data on head circumference taken in the first 3 years of life from a group of children later diagnosed with ASD with the goal of examining the longitudinal patterns of head circumference growth during the first years of life in autism. Previous reports primarily have relied on cross-sectional data. We report evidence indicating that there exists a period of accelerated head growth during the first year, followed by a slowing in rate of head circumference growth at about 12 months in autism such that the rate of head circumference growth in the second year is not significantly different from the normal population rate of growth at that age. Second, we review the available studies examining early behavioral development in infants with autism and suggest that the period of accelerated head growth precedes and then overlaps with the onset of symptoms, whereas the period of deceleration coincides with the period of behavioral decline or worsening of symptoms in children with autism.