Se conoce poco sobre los niveles de Autismo y como éstos pueden variar a lo largo de la infancia o desarrollo de una persona.
Un estudio realizado observando a 129 niños con Autismo, a quienes se evaluó anualmente entre sus 2,5 años y 5,5 años mostró variaciones en el diagnóstico. Algunos niños mantenían el mismo nivel, otros moderaban su nivel de Autismo y algunos tenían mejoras sustanciales.
Cómo se miden los niveles de Autismo
Muchos de los diagnósticos están basados en reportes de las familias. Existen cuestionarios específicos como the Social Communication Questionnaire (SCQ), the Social Responsiveness Scale (SRS), Gilliam Autism Rating Scale, Second Edition (GARS-2), Autism Diagnostic Interview—Revised (ADI-R). Además existen evaluaciones basadas en la observación de profesionales como the Childhood Autism Rating Scale (CARS & CARS-2); y the Autism Diagnostic Observation Schedule (ADOS & ADOS-).
Los niveles se definen por la edad, habilidades cognitivas y habilidades de lenguaje. Esas correlaciones reflejan las habilidades un niño y sus caracterísiticas dentro del espectro autista.
Identificando las trayectorias de los niveles de Autismo
The Current Study
Participants were 129 children enrolled in a longitudinal study of early language development in children with ASD. Children between 24 and 36 months of age with suspected or diagnosed ASD were initially recruited from local early intervention programs, developmental medical clinics, and from the community. Children participated in an initial visit at age 2½ and annual follow up visits over the next three years. Participant demographics are presented in Table 1. The participants in this study overlap with the participant samples in (references omitted for purposes of blind review).
Sample Description (n=129)
|Multiracial or Other||12||9|
|Maternal Education (n=128)|
|11 to 12 years||43||34|
|13 to 15 years||39||30|
|16 or more years||46||36|
|Language Loss (n= 111)|
|Intensive Intervention (n=107)|
|Module 1(or Toddler)||115||91|
|Time 4 (n=103)|
|Time 1||30.82 (4.07)||23-39|
|Time 4||66.59 (5.00)||57-79|
|Time 1 (n=127)||7.60 (1.91)||1-10|
|Time 4 (n=103)||7.15 (1.81)||3-10|
|Mullen Developmental Quotient|
|Time 1 (n=111)||76.39 (14.46)||38-115|
|Time 4 (n=103)||76.29 (18.89)||33-108|
|Vineland-II Daily Living Skills|
|Time 1(n=125)||80.09 (9.83)||50-104|
|Time 4 (n=102)||79.55 (10.78)||48-111|
|PLS-4 Auditory Comprehension|
|Time 1(n=125)||60.14 (12.34)||50-117|
|Time 4 (n=100)||81.69 (26.46)||50-129|
|PLS-4 Expressive Communication|
|Time 1 (n=124)||72.92 (11.66)||50-110|
|Time 4||78.76 (25.86)||50-133|
Note. ADOS CSS represent calibrated autism severity scores on the Autism Diagnostic Observation Schedule. PLS-4 indicates the Preschool Language Scale, 4th Edition.
Most children (n = 101) contributed data at three or four time points. A subset of children (n = 65) was not evaluated at the third time point because of a change in study protocol. In addition, a number of families (n = 26) withdrew from the study at some point over the four years. In the full sample, 12 participants contributed data at a single time point. All participants were included—regardless of the number of data points they contributed— because they all helped to characterize variability in CSS. Children whose families did and did not complete the full study did not differ on initial age, maternal education, cognitive and language scores, or CSS, p’s = .15 to .91. Children who were not seen at Time 3 had significantly lower maternal education (p = .009), CSS (p = .049), and PLS-4 Auditory Comprehension standard scores (p = .01) than children who were seen at Time 3. The reason for these differences is unclear since the decision of which children to evaluate at Time 3 was based simply on the timing of their initial visits. It should be noted that the magnitudes of these differences were small to moderate (Cohen’s d = .47 for maternal education; Cohen’s d = .35 for CSS; Cohen’s d = .45 for PLS-4 Auditory Comprehension).
Comprehensive evaluations were conducted at age 2½, 3½, 4½, and 5½ (Time 1-4). Parents or legal guardians provided signed informed consent for their child to participate. All study procedures were approved by the university Institutional Review Board.
At Time 1, best estimate clinical DSM-IV diagnoses were made using all available information and assessment results, including the ADOS (Lord et al., 2002) and a toddler research version of the ADI-R (Rutter et al., 2003). In the full sample, 91% (n = 117) of participants received an initial diagnosis of Autistic Disorder/autism and 9% (n = 12) received a PDD-NOS diagnosis. The ADOS was administered at each subsequent time point, and best estimate clinical diagnoses were made again based on all available information. Among the 103 participants who remained in the study through Time 4, four received a different diagnosis than their initial Time 1 best estimate diagnosis. Specifically, three children with an initial PDD-NOS diagnosis were given an Autistic Disorder/autism diagnosis at Time 4, and one child with a Time 1 diagnosis of Autistic Disorder/autism received a PDD-NOS diagnosis at Time 4.
All measures outlined below were administered annually. Demographic and treatment information was collected via parent questionnaires. Maternal education (range = 11 to 20 years of formal education) was classified as 11-12 years, 13-15 years, or 16+ years; one family did not report this information.
Autism Diagnostic Observation Schedule
The ADOS (Lord et al., 2002) is a semi-structured, standardized assessment of social interaction, communication, and behaviors relevant to ASD. Modules are selected based on an individual’s expressive language and developmental level. A preliminary research version of the Toddler module (Luyster et al., 2009) was used for participants under 30 months of age at Time 1.
A raw score was calculated for each ADOS administration, based on the revised algorithms (Gotham et al., 2007). Each ADOS raw algorithm score was then converted to a CSS between 1 and 10 based the child’s age and the ADOS Module he or she received (i.e., the respective calibration cell for each data point; see Gotham et al., 2009). For participants who received the Toddler module at Time 1, we followed the same procedure as Gotham and colleagues (2009) by recording the corresponding items to Module 1 algorithms. Scores of 1 to 3 indicate a non-spectrum classification; scores of 4 to 5 indicate an autism spectrum classification; and scores of 6 to 10 indicate an autism classification. CSS ranged from 1-10 (see Table 1).
Mullen Scales of Early Learning
The Mullen Scales of Early Learning (Mullen, 1995) is comprehensive developmental measure designed for children between birth and 68 months of age. The Mullen is comprised of five scales (Receptive Language, Expressive Language, Gross Motor, Fine Motor, & Visual Reception); only the Visual Reception and Fine Motor scales were administered. The Visual Reception scale measures visual discrimination and visual memory and includes items that require children to remember pictures and match objects and letters. The Fine Motor scale measures visual-motor ability, including object manipulation and writing readiness. This scale includes items that require children to imitate block structures, copy shapes, and cut with scissors. It was not possible to obtain T-scores for all participants at each time point, either because children’s raw scores were too low or because their ages were outside the range for which the Mullen manual provides normative data. For this reason, a developmental quotient was derived by averaging age equivalent scores from the Visual Reception and Fine Motor scales, dividing by the child’s chronological age, and multiplying by 100 (see Bishop, Guthrie, Coffing, & Lord, 2011).
Vineland Adaptive Behavior Scales, Second Edition
The Survey Interview Form of the Vineland Adaptive Behavior Scales, Second Edition (Vineland-II; Sparrow, Cicchetti, & Balla, 2005), is a semi-structured caregiver interview that assesses an individual’s adaptive behaviors in four broad domains: Communication, Daily Living Skills, Socialization, and Motor Skills. The Vineland-II was designed for use with individuals from birth through age 90. Domain-level standard scores and subdomain-level age equivalent scores are available. An overall Adaptive Behavior Composite score can also be obtained. Because we were interested specifically in daily living skills, the standard score from this domain was used in the analyses.
Preschool Language Scale, Fourth Edition
The Preschool Language Scale, Fourth Edition (PLS-4; Zimmerman, Steiner, & Pond, 2002) is an omnibus measure of receptive and expressive language skills for children between birth and 6 years, 11 months. The PLS-4 Auditory Comprehension subscale and Expressive Communication subscale measure receptive and expressive language, respectively. The PLS-4 assesses a variety of language skills, including vocabulary and grammar. The Auditory Comprehension and Expressive Communication subscales provide raw scores, age equivalent scores, and standard scores; a total language score that combines the receptive and expressive scores is also available. The standard scores from the Auditory Comprehension and Expressive Communication subscales were used in the analyses unless otherwise noted.
Other Variables of Interest
This variable was created based on parent responses on the ADI-R (Rutter et al., 2003) and represents whether the child had a parent-reported language loss of three or more words for at least one month at some point during development. Only “definite” losses (i.e., coded a “2” on the ADI-R) were included. Among the participants for whom language loss data was available (n = 111), 28% were reported to have had a definite language loss.
Intensive Behavioral Intervention
Parents completed questionnaires about children’s intervention services at each visit and at 6-month intervals between visits. Because the available information about intervention services was somewhat limited and highly variable, a broad, dichotomous variable was derived that differentiated children who had ever received intensive autism intervention (i.e., 20 or more hours per week) over the course of the larger longitudinal study from those who had not. Among the participants for whom complete intervention data were available (n = 107), 66% received 20 or more hours per week of intensive, in-home autism-specific therapy at some point over the course of the longitudinal study.
To identify trajectory classes of autism severity, a series of latent class growth models (LCGMs; Muthén & Muthén, 2000) allowing for 2, 3, 4, and 5 latent classes was estimated using the Mplus software, Version 6.12 (Muthén & Muthén, 1998-2011). The analysis assumed a fixed occasion design, with time (Time 1-4) as the independent variable and CSS as the dependent variable. In an LCGM, the intercept and linear growth parameters are allowed to vary between, but not within, the latent classes. We also explored models with a quadratic term added, but such models failed to converge in most instances, likely due to the limited number of measures per child (maximum of 4). Models were estimated using restricted maximum likelihood estimation with robust standard errors, and the residual variance of CSS was constrained to equality across the four time points both within and across classes. Models allowing for different numbers of classes were compared using Akaike’s Information Criterion (AIC) and the sample-size adjusted Bayesian Information Criterion (SSBIC);1 lower AIC and SSBIC values are indicative of a relatively better fit.
Following model selection, children were assigned to an autism severity trajectory class based on their posterior probabilities of class membership. Posterior probability values range from 0 – 1 and represent the likelihood that each child belongs to a particular class; values close to 0 indicate a low likelihood that a child would be assigned to a particular class, and values close to 1 indicate a high likelihood of being assigned to that class. For example, a child might have a posterior probability of .002 for belonging to one class and .998 for belonging to another class. Children were placed in the class with the highest posterior probability.
Our second objective was to determine whether demographic variables and experiential factors were predictive of class membership. IBM SPSS Statistics for Windows, Version 21 (IBM Corp, 2012), was used to perform multinomial logistic regression analyses in which class membership was the categorical outcome variable and each factor of interest was a predictor. Each predictor was tested in a separate model because we were interested in the zero order associations between each factor and class membership. Strength of prediction was evaluated using McFadden’s R2, a Pseudo R2 value, with higher values indicating better prediction. McFadden’s R2 represents the relative goodness-of-fit of a model, or its substantive significance; unlike linear regression, it should not be interpreted as the proportion of variance in the outcome variable explained by the predictor(s).
Our third objective was to determine whether trajectories of nonverbal cognition, daily living skills, receptive language, and/or expressive language differed within each trajectory class. To address this aim, we estimated a series of multi-level linear growth models predicting each of the four outcomes of interest using Hierarchical Linear and Nonlinear Modeling (HLM) software, Version 7 (Raudenbush, Bryk, & Congdon, 2010). A multi-level approach allowed us to investigate class differences in the random intercepts and random slopes of these outcomes, while appropriately handling the longitudinal nature of the data (i.e., repeated measures across children). In each of the four models, time (Time 1-4) was a Level-1 predictor and latent class membership was a Level-2 predictor of both intercept and slope. Time was centered at Time 1, when children were approximately 2½ years of age. In each model, we first tested the main effect of between-class differences in intercept and slope. If an omnibus χ2 test indicated significant class differences, planned pairwise contrasts were conducted. Type 1 error was controlled using the Bonferroni-Holm method.2 Effect sizes of significant between-class differences in intercept and slope were quantified using a measure analogous to Cohen’s d.3Go to:
Initial CSS Validation
Because the CSS metric has undergone independent validation (de Bildt et al., 2011; Shumway et al., 2012), we examined the issue of validity prior to conducting our primary analyses. Combining all data points, we first compared distributions of CSS and ADOS raw scores across calibration cells based on age and language level (see Figure 1). Consistent with prior work (Gotham et al., 2009; de Bildt et al., 2011; Shumway et al., 2012), the CSS represented a more uniform distribution than the raw scores across the calibration cells.
The distribution of calibrated severity scores (a) and raw algorithm scores (b) on the Autism Diagnostic Observation Schedule, separated by calibration cells based on age and language level.
Second, regression analyses revealed that CSS were consistently more weakly associated with a number of demographic variables and experiential factors than raw algorithm totals, confirming their relative independence from phenotypic and demographic characteristics (Gotham et al., 2009; Shumway et al., 2012; de Bildt et al., 2011). Regression analyses predicting raw algorithm scores and CSS were conducted with nonverbal cognition (Mullen developmental quotient) and language (PLS-4 total language standard score) in the first block, and demographics (gender, race/ethnicity, maternal education, and age) in the second block. At Time 2, the full model explained 39% of the variance in raw algorithm scores, but only 14% of the variance in CSS. This pattern was consistent across all four time points; it confirmed the intended properties of the CSS and supported their use in subsequent analyses.
Trajectory Classes of Autism Severity
LCGMs were estimated containing 2, 3, 4, and 5 classes. In each model, the dependent variable was CSS, and the independent variable was time (Time 1-4). The four-class model had the lowest AIC and SSBIC, indicating that it provided the best fit to the data (see Table 2). The four latent trajectory classes that emerged are presented in Figure 2. Interestingly, the four classes closely resembled the four primary classes identified by Gotham and colleagues (2012). To maintain consistency, each class was named on the basis of its qualitative and quantitative features—Persistent High, Persistent Moderate, Worsening, and Improving—using the same terminology adopted by Gotham et al. Children were assigned to the class with the highest posterior probability. The average posterior probabilities and the number of children assigned to each latent class are presented in Table 3, along with intercept and slope values for each class. Most children were assigned to either the Persistent High class (36%) or the Persistent Moderate class (42%), with fewer children assigned to the Worsening class (8%) and Improving class (14%).
Individual trajectories of calibrated severity scores for children assigned to the Persistent High trajectory class (a; n = 47), the Persistent Moderate class (b; n = 54), the Worsening class (c; n = 10), and the Improving class (d; n = 18). The dashed line indicates the mean trajectory within each class.
Latent Trajectory Class Model Comparison
Note. AIC indicates Akaike’s Information Criterion. SSBIC indicates sample-size adjusted Bayesian Information Criterion. The four-class model had the lowest AIC and SSBIC values, indicating the best fit to the data.
Autism Severity Trajectory Classes
|n (%)||Mean Posterior|
|Persistent High||47 (36.4%)||.90||9.18||−0.24|
|Persistent Moderate||54 (41.8%)||.78||7.12||−0.05|
Note. Children were assigned to the class with the highest posterior probability. Mean Posterior Probability values are presented for classifying children into each of the four classes. Intercept is the mean calibrated severity score at Time 1 (age 2½).
Slope is the mean change in calibrated severity score per year.
Table 4 presents descriptive statistics of CSS by class and by time point. At all four time points, the mean CSS was 7 for the Persistent Moderate class and 9 for the Persistent High class, indicating the general stability of the CSS means in these two classes. The range of mean CSS in the Worsening class was 4 to 6 across the four time points; the range of mean CSS in the Improving class was 5 to 6. Although their names suggest definite patterns of change in CSS over time, it is important to note that the Worsening and Improving classes were both characterized by mean CSS in the mild to moderate range.
CSS Characteristics by Trajectory Class and by Time Point
|Persistent High||9.33 (.92)|
|Persistent Moderate||7.19 (1.30)|
|6.59 (1.14)4-9||7.07 (1.24)|
Note. CSS represent calibrated autism severity scores on the Autism Diagnostic Observation Schedule.
To further characterize the trajectory classes, we examined the number of children in each group whose final CSS decreased, increased, or stayed the same, compared to their initial CSS. (Note that children with data at only one time point (n = 12) could not be categorized in this way.) Based on the mean trajectories, we expected that most children in the Worsening class would have final CSS that exceeded their initial CSS, and vice versa for children in the Improving class. We also anticipated that there would be roughly similar numbers of children who worsened or improved slightly in the Persistent High and Persistent Moderate classes, since mean trajectories for these classes were generally stable. The majority of children in the Worsening class (80%) had higher CSS at their final visit than their initial visit; no children showed improving CSS in this group. As would be expected, the majority of children in the Improving class (61%) had lower CSS at their final visit than their initial visit; approximately one-third of children in this class had the same CSS at both visits, and only one child had an increased CSS at the final visit. The proportions of children with increased, decreased, or identical CSS were generally similar in the Persistent Moderate and Persistent High classes; roughly 40% of children in these two classes had decreased CSS, and 23-33% of children had increased CSS, with the remainder receiving the same CSS at both time points.
Impact of Demographic Variables and Experiential Factors on Trajectory Class
Next, a series of multinomial logistic regression analyses was conducted to determine which demographic variables and experiential factors were related to latent trajectory class. In each model, class membership was the categorical dependent variable, and a demographic variable or experiential factor was the predictor variable. The Persistent High class was designated as the reference category.
Consistent with our initial hypotheses, autism severity class membership was not significantly associated with gender, χ2(3) = 3.94, p = .27, McFadden’s R2 = .01, race/ethnicity (Caucasian vs. other), χ2(3) = 4.58, p = .21, McFadden’s R2 = .02, maternal education, χ2(6) = 6.29, p = .39, McFadden’s R2 = .02, or Time 1 chronological age, χ2(3) = 4.98, p = .17 McFadden’s R2 = .02. These results indicate that children were not more or less likely to be placed within a particular trajectory class of autism severity on the basis of these factors. Additionally, language loss was not a significant predictor of class membership, χ2(3) = .77, p = .86, McFadden’s R2 < .01, meaning that children’s autism trajectory class assignment appears largely unrelated to whether their parents reported a loss of language ability early in life.
Next we tested the association between intensive intervention (i.e., having ever received intensive autism services vs. having never received them) and trajectory class membership. Results indicated a significant main effect of intensive intervention services on class membership, χ2(3) = 24.43, p < .01 McFadden’s R2 = .09. Specifically, children who had ever received intensive autism intervention services were more likely to be assigned to the Persistent High class than each of the other classes: Persistent Moderate, b = 1.35, Wald χ2(1) = 4.72, p = .03; Improving, b = 3.07, Wald χ2(1) = 16.87, p < .01; and Worsening, b = 2.62, Wald χ2(1) = 8.44, p < .01.
Thus, the results of the second objective have shown that children who ever received intensive autism intervention were more likely to show a Persistent High trajectory of autism severity than any other pattern. Class membership was not significantly related to gender, race/ethnicity, maternal education, age, or language loss.
Skill Trajectories across Autism Trajectory Classes
Finally, a series of multi-level models was estimated to determine whether trajectories of nonverbal cognition, daily living skills, receptive language, and/or expressive language differed across the four trajectory classes (see Figure 3). Tests were conducted to identify main effects of class membership on intercept and slope for each variable of interest; if a main effect was significant, it was followed with pairwise contrasts, using the Bonferroni-Holm method to control Type 1 error rate. The intercepts and slopes of the functional skill trajectories for each class are presented in Table 5.
Mean functional skill trajectories for nonverbal cognition (a), daily living skills (b), and receptive language (c) and expressive language (d) on the Preschool Language Scale, 4th Edition, within each autism severity trajectory class.
Intercepts and Slopes of Functional Skill Trajectories by Autism Severity Trajectory Class
(n = 47)
(n = 54)
(n = 10)
(n = 18)
|Mullen Developmental Quotient|
|Vineland-II Daily Living Skills|
|PLS-4 Auditory Comprehension|
|PLS-4 Expressive Communication|
Note. PLS-4 indicates the Preschool Language Scale, 4th Edition.
The main effect of class membership on baseline nonverbal cognition (intercept) was significant, χ2(3) = 11.17, p = .01. Pairwise comparisons revealed that Time 1 nonverbal cognition was significantly lower in the Persistent High class than in the Improving class, χ2(1) = 7.81, p < .01, d = .92. The difference between initial nonverbal cognition in the Persistent High and Worsening classes was marginal, χ2(1) = 6.46, p = .01, d = 1.00. There were no significant differences in slope of nonverbal cognition across the four classes, χ2(3) = 3.42, p = .33.
The main effect of class membership on Time 1 daily living skills (intercept) was significant, χ2(3) = 15.71, p <.01. Pairwise comparisons revealed that Time 1 daily living skills were significantly lower in the Persistent High class as compared to each of the other classes: Improving, χ2(1) = 8.43, p < .01, d = 1.15; Worsening, χ2(1) = 11.93, p < .01, d = .92; and Persistent Moderate, χ2(1) = 12.06, p < .01, d = .91. There were no significant differences in slopes across the four classes, χ2(3) = 2.92, p > .50.
The main effect of class membership on Time 1 receptive language (intercept) was marginal, χ2(3) = 7.33 p = .06; no pairwise contrasts were significant. The main effect of class membership on receptive language growth (slope) was significant, χ2(3) = 21.22 p < .01. Pairwise contrasts revealed that the Persistent High class had a significantly lower slope than the Improving class, χ2(1) = 16.09, p < .01, d = 1.55, and the Worsening class, χ2(1) = 9.54, p < .01, d = 1.95. This means that children in the Improving and Worsening classes showed significantly higher rates of growth in receptive language than children in the Persistent High class.
The main effect of class membership on Time 1 expressive language (intercept) was nonsignificant, χ2(3) = 1.97, p > .50, but there was a significant main effect of class membership on slope, χ2 (3) = 53.74, p < .01. Pairwise contrasts revealed that the Persistent High Class had a significantly lower slope than each of the other classes: Improving, χ2(1) = 49.94, p < .01, d = 2.08; Worsening, χ2(1) = 9.35, p < .01, d = 2.10; and Persistent Moderate, χ2(1) = 6.82, p < .01, d = .75. In addition, the Persistent Moderate class had a significantly lower slope than the Improving class, χ2(1) = 18.41, p < .01, d = 1.33. These results indicate that children in the Persistent High class showed significantly slower rates of growth in expressive language than children in all other classes; children in the Persistent Moderate class also showed significantly slower expressive language growth than children in the Improving class.
In summary, the results of the third objective indicated that children in the Persistent High Class tended to have lower functional skills than children in the other classes, either in baseline level (intercept) or in rate of growth over time (slope). There were significant class differences in baseline levels of nonverbal cognition and daily living skills, but not in rates of growth over time. We found precisely the opposite case for language skills—namely, that there were trajectory class differences in receptive and expressive language growth, but not in baseline language levels. With one exception (Persistent High vs. Persistent Moderate expressive language slope), all pairwise comparisons had values for d above .9, indicating large effects.Go to:
Trajectory Classes of Autism Severity
This study identified four distinct trajectory classes of autism severity—Persistent High, Persistent Moderate, Worsening, and Improving—in a heterogeneous sample of young children with ASD seen at four time points across early childhood. Despite differences in study design and participant characteristics—namely the younger age and higher language level of the current sample—these trajectory classes are very similar to those identified by Gotham et al. (2012) in a group of children with ASD from ages 2 to 15. Although the current sample was recruited more recently than the sample in the study by Gotham et al., the descriptive characteristics of the CSS (i.e., mean, median, standard deviation, and ranges) in each sample were quite similar (K. Gotham, personal communication, December 18, 2012), which may help to explain the similarities in the latent trajectory classes that emerged. Three of the classes (Persistent High, Worsening, and Improving) are also similar to trajectory classes identified by Lord et al. (2012) in toddlers with ASD, using ADOS raw algorithm scores. The fact that these studies have identified similar trajectory classes of autism severity in different age groups of children with ASD provides strong continuity within the literature and demonstrates the robustness of these developmental trajectories, regardless of whether children were assessed during toddlerhood, from toddlerhood to early school age, or through adolescence. Importantly, the participant sample in Gotham et al. was a subset of the original CSS calibration sample; this study replicates and extends their findings in an independent sample of children with ASD.
In both the current study and the study by Gotham et al. (2012), approximately 80% of children were assigned to either the Persistent High or Persistent Moderate trajectory class, with fewer children assigned to the Worsening or Improving classes (8% and 14% of the current sample, respectively). In conjunction, these findings suggest that a vast majority of children with ASD present with levels of autism severity that are consistently moderate or severe, with little change in overall severity level during early development. Although individual children’s CSS varied somewhat across repeat ADOS administrations, the mean CSS within the Persistent High and Persistent Moderate Classes changed very little over the four-year period (see Figure 2a and 2b). This points to relatively consistent autism symptom presentation within these classes and supports the stability of the CSS scoring metric, despite considerable changes in children’s ages and language levels. Note, however, that consistent presentation of autism symptoms does not mean that children are also showing consistent delays in other developmental domains; in fact, as discussed below, many children gained considerable functional skills over this 3-year period.
Gotham et al. (2012) hypothesized that a persistent, stable, and mild trajectory class of autism severity may emerge in studies of children who were diagnosed with ASD more recently, and at a younger age—like those in the current study. We found, however, no evidence of a persistent mild class of autism severity. Instead, children with more mild CSS fell primarily within the Worsening or Improving classes. At Time 4, no children in the Worsening class had an improved CSS, and only one child in the Improving class worsened, suggesting that these classes were well characterized. Although they represented opposing directions of change in autism severity levels, the most relevant and unifying characteristic of the Worsening and Improving classes may be that they were comprised of children with more mild CSS. Indeed, the LCGM approach to identifying latent classes takes into account not only individual children’s rates of growth, but also their baseline levels of CSS. There were more similarities than differences between the Worsening and Improving classes in functional skill trajectories (see discussion below), suggesting that a slight increase or decrease in mild autism severity level has a relatively limited impact on children’s development of cognition, language skills, and adaptive behaviors. Because a relatively small number of children comprised the Worsening (n = 10) and Improving (n = 18) classes, these findings must be interpreted cautiously. Future work is needed to further quantify the implications of increasing or decreasing trajectories of mild to moderate autism severity.
The current results pointed to persistent and stable trajectories at moderate and high levels of autism severity and less stable trajectories at mild to moderate levels, which suggests that the stability of the CSS metric may depend to some extent on the severity of children’s autism symptom presentation itself. In other words, CSS showed greater longitudinal stability in children whose severity levels were moderate or high than in children whose autism severity levels were relatively mild. One potential explanation may be that the children whose ASD is less severe show more variable symptom presentation on repeat administrations of the ADOS. As discussed by Hus, Gotham, and Lord (in press), for example, restricted behaviors and repetitive interests are marked by the presence of atypical behaviors and can be relatively rare and thus difficult to observe in a context as limited as a single ADOS administration (but see Kim & Lord, 2010). If some children with mild ASD symptoms show evidence of considerable restricted and repetitive behaviors during one ADOS administration, but not another, the resulting CSS may be less stable than for children who consistently show more marked levels of these behaviors. Presentation of atypical social communication behaviors may also vary in children with milder autism symptoms. This increased complexity of quantifying more mild ASD symptoms may also lead to decreased inter-rater reliability on the ADOS.
Further work is needed to clarify the source of variability in mild trajectories of autism severity. Hus et al. (in press) proposed a calibrated metric that provides separate CSS for the ADOS domains of Social Affect and Restricted and Repetitive Behaviors. Although domain-specific trajectories have not yet been examined using this standardized scoring system, Hus and colleagues provided some evidence that the Social Affect and Restricted and Repetitive Behaviors domains may show very different trajectories within individual children—patterns that can be obscured by relying on CSS alone. Examining domain trajectories was outside the scope of the current study—one important avenue for future inquiry is to model longitudinal trajectories of autism severity within each domain.
Impact of Demographic Variables and Experiential Factors on Trajectory Class
Consistent with prior work (Gotham et al., 2012; Lord et al., 2012), children’s assigned trajectory class of autism severity was not statistically related to gender, race/ethnicity, maternal education, or age. It is in some ways encouraging that these factors—which are, in essence, static and unchangeable—do not appear to play a role in children’s presentation of autism symptomatology over early childhood. Because our sample contained relatively limited racial and ethnic diversity, studies with more diverse samples should investigate this issue. Additionally, we found no evidence that early language loss was predictive of a particular trajectory of autism severity, which was also consistent with the findings of Gotham et al. (2012) and Lord et al. (2012).
Children who had ever received intensive autism intervention services were more likely to be placed in the Persistent High class of autism severity than any other class. Although our study is observational and thus cannot speak directly to the causal direction of this relationship, it is our strong suspicion that this finding is an artifact of eligibility criteria for receiving funding for intensive, in-home autism-specific intervention through a state Medicaid waiver program. At the time of this study, the publically funded program was the only means for most families in the state to obtain intensive, in-home intervention for children with ASD, and eligibility criteria were based on level of functional skill impairment, including cognitive, communication, social, and daily living skill deficits. Contrary to this finding, Gotham et al. (2012) identified no trajectory class differences between children who had received high levels of intervention (specifically, over 20 hours of a parent-mediated intervention or over 1667 hours of applied behavior analysis intervention), compared to those children who had less or no intervention. Lord et al. (2012), however, found that more children in the Severe-Persistent class received applied behavior analysis intervention than children with ASD in the Improving and Worsening classes, though this difference was not significant.
Skill Trajectories across Autism Trajectory Classes
Autism severity trajectory classes differed on baseline levels of nonverbal cognition, but not in rates of cognitive growth over time. Nonverbal cognition at Time 1 was significantly lower in the Persistent High class than in the Improving class, and marginally lower in the Persistent High class than in the Worsening class; the Persistent High and Persistent Moderate classes did not differ. In other words, children with more marked deficits in nonverbal cognition during toddlerhood were more likely to show persistent, severe autism symptomatology than more mild autism symptoms that improved or worsened over time. The similarity in growth rates of nonverbal cognition across the four classes demonstrates that the children with ASD in this study generally maintained the extent of delay in nonverbal cognition that they demonstrated early in life, regardless of the trajectory and severity of their autism symptoms. Gotham et al. (2012) found no relationship between baseline nonverbal IQ and severity trajectory class, which may be partially explained by the fact that on average, children in the current study had slightly higher levels of nonverbal cognition than participants in Gotham et al. Lord et al. (2012) found that initial nonverbal IQ did not predict class membership in their sample of toddlers at risk for ASD, but children with ASD in the Improving class showed higher rates of growth in nonverbal mental age than those with Severe Persistent trajectories of autism severity. This finding is interesting because it points to the possibility that examining children’s absolute nonverbal ability may reveal developmental differences related to autism severity, even when standard scores of nonverbal ability do not.
Daily Living Skills
Similar to findings for nonverbal cognition, significant class differences were identified in baseline levels of daily living skills, but not in rates of growth over time. Children in the Persistent High class had significantly lower levels of daily living skills at Time 1 than children in all other classes, meaning that many toddlers who have considerable deficits in skills such as personal care (e.g., toilet training, teeth brushing, and dressing); domestic skills (e.g., helping with chores, cleaning, and cooking); and community living (e.g., talking on the telephone, using the radio or TV, and showing awareness of safety guidelines) also show persistently high levels of autism severity. As Figure 3b illustrates, mean daily living skills standard scores were generally stable over time across all classes, meaning that on average, children did not gain or lose ground from toddlerhood to school age. Gotham et al. (2012) found no class differences in daily living skills at age 2, but children in the Improving class had significantly better daily living skills than children in the other classes at age 6.
Receptive and Expressive Language
Patterns of language development contrasted with patterns of nonverbal cognition and daily living skill development, such that the trajectory classes differed in rates of receptive and expressive language growth over time but not in baseline language levels. As Figures 3c and 3d illustrate, children demonstrated considerable receptive and expressive language delays at Time 1, regardless of the autism severity trajectory class to which they were assigned. An initial deficit in language skills, then, should not be taken as a definite indication that a child will show a particular trajectory of autism severity. We find it particularly encouraging that there was no systematic relationship between autism severity trajectory class and Time 1 expressive or receptive language—despite the fact that the ADOS explicitly accounts only for differences in spoken language (i.e., through selection of the appropriate module).
As Figure 3c indicates, rates of receptive language growth differed drastically across the severity trajectory classes. The Worsening and Improving classes had significantly higher rates of receptive language growth than the Persistent High class, meaning that children with persistent, severe levels of autism symptomatology are also at risk for persistent, severe deficits in language comprehension. Despite the slowed rate of growth in the Persistent High class, all classes demonstrated higher mean receptive language standard scores at Time 4 than at Time 1. This indicates that children not only gained absolute receptive language skills over time, but also gained ground in comparison to their typically developing peers.
In terms of expressive language, all classes had significantly higher rates of expressive language growth than the Persistent High class. Although mean expressive language standard scores increased in most classes from Time 1 to Time 4, mean expressive language standard scores in the Persistent High class decreased, meaning that on average, children in this class became more delayed relative to age expectations over time. In other words, the negative slope for expressive language standard scores indicated not that children in the Persistent High class lost language skills they had previously acquired, but that they fell further behind their typically developing peers over time. On average, expressive language scores for children in the Persistent High class decreased by 2 standard score points per year. One potential interpretation of this finding is that the subset of children with ASD who do not go on to develop functional spoken language are most likely to be those who demonstrate persistently severe symptoms of autism throughout development. Expressive language is a particularly important intervention target for these children, perhaps along with some form of alternative or augmentative communication to help them express their wants and needs through an alternate modality.
Regardless of trajectory class membership, the children with ASD in this study demonstrated severe receptive and expressive language delays at age 2½. Significant class differences in rates of language growth suggest, however, that some children possess learning abilities that allow them to acquire language skills more quickly than others. In fact, by age 5½, children in the Improving and Worsening groups performed within age expectations for receptive and expressive language, whereas children in the Persistent Moderate and Persistent High classes demonstrated continued delays.
What developmental processes underlie the trajectory class differences in rates of language growth? It is possible that children with lower levels of autism severity can better generalize their language abilities to the higher-order tasks that comprise many of the later items on the PLS-4 (e.g., making grammatical judgments, using language to describe quantitative and qualitative concepts, constructing narratives). Other learning abilities that may contribute to superior language skills include statistical learning (i.e., detection of patterns in language; Romberg & Saffran, 2010), increased accuracy and efficiency of spoken language processing (Venker, Eernisse, Saffran, & Ellis Weismer, in press), and better integration of and access to semantic and syntactic representations. Language learning in children with ASD may also be supported by the ability to extend novel words to appropriate categories (McGregor & Bean, 2012) or make effective use of adult feedback during word learning (Bedford et al., 2012).
Prior work has shown that decreases in restricted and repetitive behaviors are associated with increases in receptive and expressive language abilities in young children with ASD (Ray-Subramanian & Ellis Weismer, 2012), indicating yet another reason that autism severity and language may be linked. It is also possible that higher levels of social interest and engagement lead to increased language-learning opportunities and that better general attentional abilities (i.e., sustained, selective, or flexible attention) lead to better language outcomes. Future studies are needed to more precisely identify the mechanisms that underlie optimal language outcomes in this population.
Gotham et al. (2012) found that children in the Improving and Worsening classes tended to have higher verbal IQ at age 2, with the Improving class showing the highest rate of growth. At age 6, verbal IQ was significantly higher in the Improving class and significantly lower in the Persistent High class than in all other classes. Although it is worthwhile to interpret our findings regarding class differences in language trajectories in reference to the findings of Gotham et al., it should be noted that our findings may contrast due to a number of factors. As we have acknowledged, participants in the current study and that by Gotham et al. differed in age and language levels. Additionally, Gotham et al. did not separately examine receptive and expressive language skills. The fact that we identified qualitatively different patterns of development in receptive and expressive language— particularly the decline in expressive language standard scores for the Persistent High class—underscores the importance of separately examining these aspects of language.
Finally, Gotham et al. (2012) used verbal IQ—most commonly measured by the Mullen Scales of Early Learning, as reported in Gotham et al. (2009)—as a measure of language ability. Although there are similarities between verbal IQ and language skills as measured by the PLS-4 in the current study, these two constructs are not identical (also see Shumway et al., 2012). The Auditory Comprehension and Expressive Communication subscales of the PLS-4 were designed to assess a broader range of language skills than the Mullen, ranging from basic vocabulary and vocal development to making inferences and demonstrating phonological awareness (Zimmerman et al., 2002). The Mullen manual reports correlations ranging from .72 to .85 with the subtests on an earlier version of the PLS (Mullen, 1995), which provides evidence of overlapping but non-identical measures.Go to:
Conclusion and Limitations
In summary, this study identified four discrete trajectory classes of autism severity in early childhood, based on ADOS CSS: Persistent High, Persistent Moderate, Worsening, and Improving. These classes are strikingly similar to the four primary classes identified by Gotham et al. (2012). Important differences in functional skill trajectories by class emerged, including different rates of growth in receptive and expressive language skills. Our findings also indicate that early deficits in nonverbal cognition and daily living skills may be predictive of a persistent and severe trajectory of autism severity. The robustness of these autism severity trajectories across independent samples contributes to our understanding of ASD as a developmental disorder and may offer clinicians empirical information to inform a child’s short-term prognosis.
One strength of this study is that it examined an independent sample of young children with ASD diagnosed no earlier than 2006. One related limitation, however, is that this participant sample was relatively small (n = 129) compared to the sample in Gotham et al. (2012; n = 345). Although a sample size of 129 is adequate for many statistical analyses (e.g., linear regression), when using LCGM one runs the risk of identifying latent classes that contain small subsets of the original sample. For example, the Worsening class in the current study contained only 8 children at the final time point. Despite their relatively small size, the fact that the Worsening and Improving classes emerged statistically in this study and that by Gotham et al. suggests that they should be acknowledged, though replication is critical. Although the current study included considerably fewer participants than Gotham et al. (2012), the posterior probabilities of assigned class membership were similar, ranging from 73 to 90 (M = 79.5) in the current study, and from 68 to 82 (M = 77.5) in the study by Gotham et al.—which suggests that trajectory class assignment was comparably robust across both studies.
Selection of any analytical approach inherently involves both strengths and limitations. One advantage of the LCGM approach (Muthén & Muthén, 2000) used in this study is that it does not assume a Gaussian (normal) distribution of growth trajectory parameters and thus can theoretically accommodate any distribution. LCGM also allows for variability between but not within trajectory classes, leading to more straightforward interpretation of classes than approaches that introduce variability at both levels. As mentioned, however, one potential disadvantage of this approach is that it is sensitive to latent classes that include a relatively small number of participants, and solutions can therefore be unstable when several such classes are present in the data. Population-based studies are required to determine the prevalence rates of autism severity trajectories in the broader population of children with ASD.
The current study included a maximum of four time points per child, which led to convergence problems when attempting to fit LCGMs with effects above linear effects (e.g., quadratic effects). Although the majority of individuals in Gotham et al. (2012) contributed data at two or three time points, one-fourth of the sample contributed between four and eight assessments, which likely helped in their being able to consider trajectories with a quadratic component. Relatedly, the current study used a fixed occasion design with time as a predictor, whereas Gotham et al. used a variable occasion design with age as a predictor. Despite these differences, it is important to note that inclusion of the quadratic term in the Gotham et al. study did not produce a better fitting model, meaning that the final latent class model in both studies included only intercept and linear effects. In addition, the inclusion of a quadratic component in our analyses was viewed as less critical given our focus on a narrower window of time. Future studies including more frequent assessments during early childhood (e.g., every three to four months) may determine whether trajectories of early autism severity measured by CSS are best modeled with both linear and quadratic effects.
A limitation of all observational studies is that definitively determining causation is not generally possible. Although we identified significant relationships between trajectories of autism severity and trajectories of functional skills, it is not possible to say with certainty whether increased autism severity leads to decreased functional skill levels (in our opinion, the more likely interpretation), or whether lower functional skill levels lead to more severe autism symptomatology. In actuality, the relationship between autism severity and foundational developmental skills likely involves complex, bidirectional influences that shift over the course of development. Finally, this study explored only one measure of autism severity: the ADOS CSS. Although the justification for selecting the CSS is clear, future work is may determine whether trajectories of autism severity using other measures align with the current findings.