Advances in medical care have resulted in increasing numbers of infants surviving after being born following a very short gestational period.1,2 Physical therapists play an important role in managing these infants at risk of delayed gross motor development.3 Palisano and colleagues4 state that 5 elements are essential to maximize the optimal outcome for children, including examination, evaluation, diagnosis, prognosis, and intervention. Standardized assessment tools are often used in examination and evaluation of children to guard against the bias of subjective clinical judgment.4 Results of standardized assessment tools are also used to justify the eligibility of these children for early intervention services.5 Hence, knowledge of the strengths and weaknesses of a standardized assessment tool can assist therapists in understanding the real implications of the results and avoiding the pitfalls of misdiagnoses, which in turn can assist clinical reasoning in determining how the infants or children should be managed.6
The Alberta Infant Motor Scale (AIMS) is one of the standardized assessments commonly used in early infancy.7 It is a norm-referenced assessment of motor development for infants from birth until the age of 18 months.8 The AIMS was designed to identify infants with motor delays and to evaluate over time the motor development in all infants younger than 18 months.9 The AIMS involves observation of the infant and requires minimal handling or facilitation of the infant during the assessment. The AIMS also highlights the quality of the movements and subtle changes in the observed motor skills.7
The AIMS consists of 58 test items administered in 4 different positions, that is, prone (21 items), supine (9 items), sitting (12 items), and standing (16 items) (Appendix 1).8 Each item represents a motor behavior that is commonly observed in infants who are developing typically, for example, rolling from prone to supine, sitting with arm support, and pulling to standing. The items are arranged according to the developmental sequence of motor skills in each position. An item is credited according to the following 3 criteria: (1) the body parts that are bearing weight; (2) the postural alignment of each body part; and (3) the antigravity movements involved in that item. In each of the 4 positions the least mature and most mature “observed” items are identified for the infant. The items between these 2 items represent the “window” of the movement repertoire for the infant. One point is allocated for each observed item within this window. The raw subscore of the infant comprises the points for each item below the least mature observed items in the window, plus all the observed items within the window in that position. The total score is the sum of all the subscores in the 4 positions. The total score can also be converted into an age-based percentile rank according to the normative data in the manual.8
The reliability study by the authors of the AIMS in 253 infants with normal development showed that the intraclass correlation coefficient (ICC) for interrater reliability was greater than 0.96 between 2 assessors and test-retest reliability ranged from 0.86 to 0.99.8 In another reliability study of a group of 45 infants born preterm with low birth weight (mean gestation = 31.5 wk, SD = 3, and mean birth weight = 1523.4 g, SD = 508.4), the motor performance of the infants was independently scored by 3 trained physical therapists using the AIMS.10 The ICC for intrarater reliability of the total score was greater than 0.98 and the standard error of measurement (SEM) was less than 1.3. The ICC and SEM of interrater reliability of the total scores were greater than 0.97 and less than 1.3, respectively; however, the ICC values for the stand subscores from birth to 7 months corrected age (CA) were about 0.75. The authors argued that the possible explanation for these low ICC values was the limited number of test items to evaluate the standing skills at such an early age. There are only 3 test items for infants who cannot support their own weight in standing. Hence, the range of possible scores is between 0 and 3. The small variability in scores in this subscale would attenuate the ICC values.10
Interrater reliability among 8 experienced early intervention providers was also investigated in a group of 6 infants developing typically.11 The ICCs of interrater reliability of the total AIMS scores ranged from 0.98 to 0.99. Training in using the AIMS in this study improved the ICC of the supine subscore from 0.82 to 0.90.11 Although no specific training is required to use the AIMS, there is an assumption that the users of the AIMS should be familiar with evaluating motor development in infants and have specific knowledge of the essential components of movements used in the scoring criteria for each item on the AIMS.8,12 In this study,11 the assessors included 3 social workers who might not have had any formal training in movement analyses. This may explain why training in using the AIMS, particularly regarding normal motor development, resulted in an improvement in the ICC of the supine subscores. The authors also commented that the most likely explanation for the initial low ICC value for the supine subscore was that the examiners found it difficult to determine placement of the window recording current motor abilities of these infants, particularly for infants aged between 4 and 7 months. One of the authors of the AIMS, Dr Johanna Darrah, recommended that the infant should be allowed to become familiar with the testing environment before scoring is commenced, and that the window should not be started too far back.11 Dr Darrah agreed that experienced clinicians would have an advantage in deciding where the window of the motor repertoire should start and end.11
Uesugi and colleagues13 investigated reliability of the AIMS among 6 pediatric physical therapists in 40 infants who were healthy (aged from 22 days to 16 months 27 days) and found that the ICCs for interrater reliability for experienced therapists and less experienced therapists were 0.93 (SEM at or below 1.62) and 0.89 (SEM at or below 1.60), respectively. A group of researchers also investigated the interrater reliability of the AIMS between 2 evaluators in a cohort of 42 infants born preterm (mean gestation = 32 wk, SD = 1, and mean birth weight = 1472 g, SD = 324) from birth to 18 months CA.14 They found that the ICC was 0.99 for the whole cohort but lower ICC values were found for the stand subscores at 0 to 3 months and 4 to 7 months CA (0.76 and 0.86, respectively) and for the sit subscores at 12 to 18 months CA (0.78).
The AIMS appears to have very high reliability among both trained and untrained examiners as well as experienced and less experienced pediatric physical therapists in assessing infants developing typically and those developing atypically. However, concerns have been raised that the limited number of test items in standing at a very young age might not be sensitive enough to detect the subtle differences among infants. This argument has been supported by a Rasch analysis of the AIMS,15 which showed that the AIMS was a good discriminative tool for infants with a middle range of motor abilities, but not before 3 months or after 12 months of age where few test items are available.
Infants born preterm have been identified as having atypical postures and motor development when compared with infants born at term.16–22 As the normative sample for development of the AIMS was randomly selected from a population in Canada of infants born at term and developing typically,8 it would be clinically relevant to determine whether the AIMS can be reliably administered in identifying subtle differences in spontaneous movements in the preterm population. The reliability of the AIMS in infants born at or before 29 weeks gestation age (GA) has not previously been investigated. This study was designed to examine the intra- and interrater reliability of the AIMS when used with a group of infants born at or before 29 weeks GA from 4 to 18 months CA. The secondary objective was to examine how these findings might improve our clinical management of this at-risk group.
A group of infants born preterm was recruited from a special care nursery at a women's hospital between April 2006 and February 2007. All infants were born at or before 29 weeks of gestation, which was calculated by the best obstetric estimation taking the date of the last menstrual period and/or ultrasound before 20 weeks GA into account. A convenience sample of infants born at term, the term comparison group, was also recruited in the community; these infants were born at or after 37 weeks of gestation and were typically developing in terms of their motor development. Infants with known congenital abnormalities and syndromes were excluded from this study. Ethical approval was sought prior to the commencement of the study from the human research ethics committees of the hospital for the infants born preterm and the University of Melbourne, Victoria, Australia, for the infants in the term comparison group. Informed consent was obtained from a parent of each participant.
This reliability study was part of a longitudinal observational study investigating the motor development of infants born at or before 29 weeks GA. All the infants were assessed using the AIMS at 4, 8, 12, and 18 months CA. The infants' motor performance was videotaped at each session. The video recording of each infant was scored according to the AIMS manual.8 As the main interest in the present study was to investigate the reliability of the AIMS in the preterm population, only the video recordings of infants born preterm were used. (See Appendix 2 for a summary of characteristics of the infants in the preterm and term comparison groups.) Two examiners participated in the reliability study and both of them had extensive clinical experience in pediatric physical therapy. Examiner 1 had more than 7 years of clinical experience in using the AIMS. Examiner 2 was a qualified pediatric physical therapist who received intensive training from Dr Johanna Darrah.
In testing intrarater reliability of the 2 examiners, a random sample of 30 videorecordings of the whole preterm cohort was used. Each video recording was coded. These codes were written on separate pieces of paper and placed in a hat. Thirty codes were randomly drawn. In order to include infants of a wide range of motor abilities, it was ensured that each infant was represented no more than once in these 30 test recordings. Drawing the codes from the hat continued until 30 test recordings from 30 different infants were obtained.
The first and second scorings were at least 4 weeks apart to minimize memory bias from the 2 examiners, and neither examiner had access to the previous scorings. The ICC values using a 2-way random effects model and SEM were calculated to assess the intra- and interrater reliability of the AIMS. As a general guideline, an ICC greater than 0.90 indicates high reliability, greater than 0.75 as good reliability, between 0.5 and 0.75 as moderate reliability, and less than 0.50 as poor reliability.23
Sixty-two infants born preterm were recruited by the end of the 10-month recruitment period. Two infants died before any assessment was undertaken and 1 infant died after his 4-month assessment. One infant was determined to have a diagnosis of pseudobulbar palsy at 18 months CA and her results were discarded, because, as outlined in the AIMS manual,8 the AIMS is inappropriate for use in infants with abnormal movement patterns. Four infants were lost to follow-up at different time points in the 18-month follow-up period. Hence, 225 assessments from the 59 infants (including the one who died after 4 months CA) were used in the interrater reliability study. From these 225 assessments, 30 assessments were randomly selected for the intrarater reliability study, and comprised assessments of 10 infants at 4 months CA, 6 infants at 8 months CA, 8 infants at 12 months CA, and 6 infants at 18 months CA.
The ICCs for the intra- and interrater reliability are summarized in Tables 1 and 2, respectively. The ICC for intrarater reliability was very high for both examiners (all ICCs greater than 0.98 for both examiners) with low SEM values (all less than 1). The ICC for interrater reliability of the total scores at the 4 age levels ranged from 0.85 to 0.97, with the SEM ranging from 0.81 to 1.69. The ICCs for the subscores at the 4 age levels varied substantially, ranging from 0 to 0.97 (SEM 0.21-1.15).
Figure 1 shows the percentage of infants who passed the test items in the 4 subscales from 4 to 18 months CA. At 4 months CA in the prone position, there was an unexpectedly large number of infants born preterm who were able to roll from prone to supine without rotation (Pr 8) but this was not found in the term comparison group. Similarly, at 8 months CA, fewer infants born preterm compared with the infants in the term comparison group were able to move from sitting to a 4-point or half-sitting position (Pr 16). At 8 months CA, almost all the infants in the term comparison group were able to sit unaided (Sit 2 and from Sit 4 onward) when placed, and maintained good head control when pulled to sit (Sit 3). Although most of the infants born preterm were able to pass item Sit 3, most infants did not successfully complete Sit 2 or from Sit 4 onward.
This study was designed to investigate intra- and interreliability of the AIMS in a cohort of infants born at or before 29 weeks GA. As a general rule of thumb, if the ICC of an assessment tool is greater than 0.75, it is considered to have good reliability.23 Based on a sample of 30 different assessments, the ICC values of intrarater reliability of the 2 examiners in the present study were greater than 0.96 with the SEM less than 0.9 (Table 1). This finding is consistent with the results of previous studies using the AIMS in various population groups.8,10,13 The high intrarater reliability between the 2 examiners was not surprising as both examiners were experienced pediatric physical therapists who have had years of training in analyzing movement in infants and very young children.
For interrater reliability, the ICC values of the total AIMS scores for the whole cohort of infants born preterm between the 2 examiners were all greater than 0.75, with the lowest at 4 months CA (0.85) and the highest at 12 months CA (0.97), indicating good to high reliability (Table 2). This shows that the AIMS is a reliable assessment tool among experienced examiners in a population of infants born at or before 29 weeks GA. The SEM of all the ICCs was around 1.0. This finding is again consistent with the existing literature reporting use of the AIMS.8,10,11,13,14
Although there have already been 2 studies of the reliability of the AIMS in infants born preterm,10,14 our findings have filled a gap in the existing literature and completed evaluation of the reliability of the AIMS in infants born extremely preterm. The mean gestational age of our infants was 26.5 weeks (SD = 1.4, ranged from 23 to 29 wk) in contrast to the study by Jeng and colleagues,10 in which the infants were recruited on the basis of birth weight instead of GA, and hence, the range of their gestation varied from 26 to 36 weeks. In the study by Almeida and colleagues,14 the infants were recruited if born less than 37 weeks GA, and hence, the mean gestation was 32 weeks (SD = 1), which was higher than in our study.
A close examination of the individual subscores at each age level raises some interesting points for discussion. The ICC values between the 2 examiners for each individual subscore at each age level were greater than 0.75, except for the supine, sit, and stand subscores at 4 months CA, supine subscore at 12 months CA, and prone, supine, and sit subscores at 18 months CA (Table 2). As in previous studies, it appears that there were consistently lower ICC values of individual subscores at 4 months10,11 and 18 months CA. The AIMS has been shown to be a good discriminative tool for infants with midrange motor abilities, but not for very young infants or those who have started to walk independently at an early age. The reason for this is that there are limited test items at the extreme ends of the age range.15 Hence, the range of the subscores in supine, sitting, and standing positions was narrow at 4 months CA. At 18 months CA, most of the infants achieved almost full scores in the prone, supine, and sit subscales, and hence, the range of standard deviations of these subscores was very narrow. Because the ICC is defined as the ratio of the adjusted variance among the subjects to the sum of variance among the subjects and error variance,23 the small variability in the supine, sit, and stand subscores contributed to the low ICC values at 4 and 18 months CA.
In the AIMS, the assessment window is made up of items between the least and most mature skills observed, which represents the movement repertoire of the infant. Once the window has been determined, those test items below the window will be automatically credited, even though the items might not have been observed during the assessment. Items within the window would be credited if the skills were observed during the assessment. Hence, it is crucial to have an accurate placement of the window. The problem of determining the least and most mature skills, in other words the placement of the window on the movement repertoire in each subscale, has been raised in a previous study.11 In this study,11 the placement of the movement window was problematic because of the atypical postures in the infants born preterm. Twenty-six of 225 assessments in the present study had more than a 2-point difference in 1 subscale between the 2 examiners. The main reason for this discrepancy was the different placement of the window on the movement repertoire by the 2 examiners.
An example was the assessment of Infant 046 at 8 months CA (Figure 2). As shown in Figure 2, Infant 046 was able to creep reciprocally and reach from an extended arm in the 4-point position. Examiner 1 considered the transition between a 4-point kneeling position to sitting or half-sitting position an important motor skill at this age, which was not demonstrated by this infant, and hence, the window of the movement repertoire was pushed toward the lower level of skills in the subscale. In contrast, Examiner 2 set the window at a higher level of skills. As a result, the prone subscore of this infant was 15 as judged by Examiner 1 but 18 by Examiner 2.
Another example was the assessment of Infant 062 at 8 months CA (Figure 3). Infant 062 was able to move out of sitting to prone by pulling with his arms. According to the AIMS manual,8 “the infant must be able to maintain sitting with or without arm support” to pass this item. As Examiner 1 considered that this infant was only able to sit with propped arms briefly on his own, this infant did not fulfill the scoring criterion. Hence, the window was set between the first 2 items in the sit subscale. However, Examiner 2 considered that the infant was able to sit with arm support and so the infant was credited with being able to move out of sitting into prone. As a result, the sit subscore from Examiner 1 was 2, but 6 from Examiner 2.
These examples raise 2 important points about the clinical use of the AIMS. First, the description in the AIMS manual is sometimes unclear, especially in quantifying the quality of movement, for example, how long is “briefly”? Inadequate details in the descriptions were also found in other test items that are a progression of previous items, such as “unsustained sitting,” “sitting with arm support,” and “unsustained sitting without arm support” in the sit subscale, and “forearm support (1)” and “forearm support (2)” in the prone subscale. (See Appendix 1 for detailed descriptions of these items.) Second, natural variations have not been considered in some test items, for example, “pivoting” and “reciprocal crawling” in the prone subscale. Infants developing typically do not often pivot or crawl on their stomachs using reciprocal patterns of both arms and legs.24 In situation like this, therapists would rely on their clinical experience in determining whether the infant should be credited with the test item. This may put novice therapists at a disadvantage when using the AIMS.
Infants born extremely preterm have a different quality of movement from infants born at term who are healthy, such as a strongly extended posture,22,25–27 which makes it more difficult to determine the least and most mature skills in each subscale, and hence, may affect the subscores and total AIMS score at this young age. In the present study, the infants born preterm were often found to have “gaps” in their motor skills, particularly in the prone and sitting positions, that is, their motor skills did not follow the expected developmental sequence as laid out in the AIMS manual, because of their strong extensor strength.22 The infants born preterm sometimes achieved motor skills in a different trajectory when compared with the infants in the term comparison group. For example, at 4 months CA in the prone position, unlike the infants in the term comparison group, before they could support themselves with forearms or extended arms (Pr 6 or Pr 7), the infants born preterm rolled from prone to supine without rotation (Pr 8), largely because of their imbalanced extensor strength22 (Figure 1), and hence, their rolling from prone to supine was uncontrolled. Similarly, at 8 months CA, while most of the infants in the term comparison group could sit unaided (Sit 2 and Sit 4 onwards) and move from sitting to a 4-point position (Pr 16), the infants born preterm were lagging behind in these motor skills (Figure 1).
Possible solutions to overcome these pitfalls of the AIMS are that the window of movement repertoire should not be set too far back to start, and adequate time needs to be allowed for the infant to demonstrate his/her motor skills in an unfamiliar situation.8,11 Serial assessments of infants at risk are strongly recommended, as variations in motor development occur even in infants developing typically.28 Motor performance of infants should be investigated longitudinally to allow for the observation of young children over time, rather than on one occasion, if only to avoid misdiagnosis of these infants.29 Furthermore, the personnel using the AIMS need to have knowledge of infant motor development and skills in movement analysis. Training in using the AIMS is essential, especially for novice therapists, and should include awareness of variations in motor development and subtle differences in movement, such as rotation in the trunk during rolling and creeping in infants developing atypically.
Lastly, on the basis of the findings of the present study and as suggested by others,15 we recommend that the AIMS should not be used for infants younger than 4 months or after the infant has achieved independent walking, because of the limited test items at these time points. Other motor assessment scales that could be used are the General Movements Assessment30 or the Test of Infant Motor Performance31 before or at 4 months, and the Peabody Developmental Motor Scales, 2nd edition,32 or the Toddler and Infant Motor Evaluation33 at 18 months and onward.
An intrarater reliability study on a sample of 30 infants and an interrater reliability study on a sample of 59 infants between 2 experienced pediatric physical therapists showed that the AIMS in general has high intra- and interrater reliability in infants born at or before 29 weeks GA from 4 to 18 months CA. All the ICC values were greater than 0.75 and the SEM less than 1.2. The present results indicate that the AIMS is a reliable measurement tool to be used for the evaluation of the motor function of infants born at or before 29 weeks GA. As the number of test items at 4 and 18 months CA is limited in the AIMS, the ICC values of various subscores were low at these 2 extreme ages due to the limited variance in the scores. Clinicians need to be cautious in using the AIMS at a very early age and also after the infant can walk independently. It is crucial to place the window on the movement repertoire accurately to obtain a clear picture of the motor skills of the infants. Extra care is required when using the AIMS in infants developing atypically. Longitudinal serial assessments are recommended for infants at risk of motor delay.
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