Psychometric properties of the PEDI-CAT for children and youth with spinal muscular atrophy

Abstract

PURPOSE:

The purpose of this study was to examine the psychometric properties of the Pediatric Evaluation of Disability Inventory-Computer Adaptive Test (PEDI-CAT) in children and youth with Spinal Muscular Atrophy (SMA).

METHODS:

In this prospective cross-sectional study, caregivers of children and youth with SMA completed the PEDI-CAT Daily Activities and Mobility domains. A subset of caregivers completed a questionnaire about the measure.

RESULTS:

Mean ranks of scaled scores for Daily Activities (n = 96) and Mobility (n = 95) domains were significantly different across the three SMA types and across the three motor classifications. Normative scores indicated that 85 participants (89.5%) had limitations in Mobility and 51 in Daily Activities (53.1%). Floor effects were observed in≤10.4% of the sample for Daily Activities and Mobility. On average, caregivers completed the Mobility domain in 5.4 minutes and the Daily Activities domain in 3.3 minutes. Most caregivers reported that they provided meaningful information (92.1%), were willing to use the PEDI-CAT format again (79%), and suggested adding content including power wheelchair mobility items.

CONCLUSION:

Convergent validity was demonstrated for the Daily Activities and Mobility domains. Normative scores detected limitations in Mobility and Daily Activity performance for most participants with SMA. The PEDI-CATwas feasible to administer and caregivers expressed willingness to complete the PEDI-CAT in the future.

Keywords

Spinal muscular atrophy caregiver-reported outcome measure Pediatric evaluation of Disability Inventory-Computer Adaptive Test functional performance measure psychometric properties

1 Introduction

Spinal Muscular Atrophy (SMA) is a neuromuscular disorder and is most often a result of a homozygous deletion in the SMN1 gene [1]. The SMN1 gene codes for the production of the SMN protein which maintains normal function of the motor neurons. Insufficient levels of the SMN protein leads to loss of motor neurons in the spinal cord and brainstem and produces muscle weakness and compromised respiration. The three most common types of SMA vary in presentation and severity. SMA type I is the most severe with onset of symptoms during the first six months of life [2, 3]. Typically, children with SMA type I have severe muscle weakness, hypotonia, difficulty with breathing and feeding, and do not develop motor skills such as sitting or crawling [4 –6]. Children with SMA type II are usually diagnosed by age 2 years and initially can maintain sitting independently and often use a wheelchair for functional mobility [3, 6]. SMA type III is a milder phenotype and is usually diagnosed by age 3 years. However, in some cases it doesn’t present until later in childhood. Children with SMA type III can walk independently although some may lose the ability to walk as they get older [3, 6]. Ultimately, all individuals with SMA will experience some level of difficulty with mobility and activities of daily living and require the support of a rehabilitation team [5 –7].

The natural history of disease progression in SMA has been documented at the capacity level [6 –12]. Capacity tests are commonly used to record what a child can do in a clinic or test setting [13]. Several capacity-based motor tests have been validated for individuals with SMA including the Children’s Hospital of Philadelphia Infant Test of Neuromuscular Disorders [14, 15], Test of Infant Motor Performance Screening Items [16], Hammersmith Functional Motor Scale-Expanded [17, 18], Revised Hammersmith Scale [19], Motor Function Measure [20], Revised Upper Limb Module [21], and 6-Minute Walk Test [22, 23]. These capacity-based measures are critical when evaluating natural history [13] but they do not describe typical motor function in natural settings such as in the home, school, and community.

Performance-based tests provide information about what an individual does in their natural settings [13]. Differences in motor capacity and motor performance have been documented for children with cerebral palsy. This information is useful when designing interventions [24]. Very little information is available on typical functional performance of individuals with SMA in their natural settings [25]. The Pediatric Evaluation of Disability Inventory-Computer Adaptive Test (PEDI-CAT) is a performance-based outcome measure completed by caregiver report. It is designed to identify functional delay or limitations and to measure change in function in children and youth with disabilities up to 21 years of age. The PEDI-CAT has been validated for children with developmental disabilities [26 –29]. The full item bank properties of the Mobility and Daily Activities domains of the PEDI-CAT were evaluated specifically for children and youth with SMA [30]. There appear to be no other studies that have been published on the psychometric properties or feasibility of using the PEDI-CAT in children and youth with SMA.

In December 2016, nusinersen (Spinraza®) was approved by the Food and Drug Administration (FDA) for the treatment of infantile-onset and childhood SMA [31]. In May 2019, onasemnogene abeparvovec-xioi (Zolgensma®) was approved to treat children less than two years of age with SMA [32]. In August 2020, the orally administered small molecule risdiplam (Evrysdi^TM) was approved by FDA for all patients with SMA older than 2 months of age [33]. Improvements in motor capacity and survival have been reported following these therapeutic interventions [33 –41]. It is the experience of these authors that parents have reported improvements in their child’s typical function following use of nusinersen, risdiplam, or onasemnogene abeparvovec-xioi; yet, some of these improvements are not captured by capacity measures.

With new pharmacological therapies and rehabilitation interventions in development, valid and clinically feasible performance-based measures are needed to document typical function in home, school, and other community settings in individuals with SMA. These measures can serve as clinical trial endpoints and provide justification to consumers and insurance companies for these potentially costly interventions. The purpose of this study is to examine the psychometric properties of the PEDI-CAT Daily Activities and Mobility domains in a sample of children and youth with SMA. Specifically, this study evaluated the: 1) convergent validity (ability of the two domains to measure the construct of motor severity), 2) discriminative properties (ability to detect limitations in motor skills compared to typically developing children and youth), 3) floor and ceiling effects, and 4) clinical utility of the PEDI-CAT.

2 Methods

2.1 Procedures

This natural history of SMA study used a prospective cross-sectional design and was conducted by three hospitals of the Pediatric Neuromuscular Clinical Research Network. Capacity-based motor measures, quality of life measures, and biomarker data were routinely collected for a natural history study. Collection of the PEDI-CAT data was added as an amendment to this natural history study in September 2012 and was approved by the Institutional Review Boards of the three participating hospitals: Boston Children’s Hospital Office of Clinical Investigations IRB reference 05-02-028, Columbia University Medical Center Human Research Protection Office IRB reference AAAE8252, and The Children’s Hospital of Philadelphia IRB reference 10–007816. Only PEDI-CAT data are reported in this paper.

Study participants were recruited via a convenience sample by clinic physical therapists or research coordinators. Caregivers (parents or legal guardians) of participants under 18 years of age and participants 18 years of age and older provided informed consent prior to data collection. Children 7 through 17 years of age provided informed assent as able. Demographic information was recorded for each participant. Physical therapists rated children and youth according to three functional mobility classifications: 1) “Non-sitter” - Unable to sit independently with or without the use of arms for propping; 2) “Sitter” - Able to sit independently for at least 3 seconds with or without arms for propping but unable to walk hands-free; and 3) “Walker”- Able to sit and walk independently (hands-free) without an assistive device for at least five steps. For this study, function was compared across the classic SMA types as well as the newer classifications by gross motor ability. This was done because with new pharmacological therapies and rehabilitation interventions, children and youth with SMA may gain more gross motor milestones. In addition, with newborn screening programs infants can receive treatment even before symptoms occur. Because of these factors, gross motor function classifications of non-sitter, sitter, and walker may be better at conveying functional classifications than SMA types [2 –5].

Prior to the start of the study, clinic physical therapists attended an online training session and were trained by a physical therapist on the PEDI-CAT administration procedures, administration types, domains, and scoring options. The PEDI-CAT was designed to be completed by a parent, caregiver, or therapist familiar with the child’s typical functional performance [42]. Scoring instructions are included in the computer software, requesting respondents to report their child’s typical performance at the time of testing. During the development of the PEDI-CAT, the items were reviewed by parents of children from birth to 21 years of age to ensure that they were clear and would be interpreted in the same way across respondents [43].

For this study, the PEDI-CAT was completed by parents, other caregivers, or by clinic physical therapists in conjunction with the child’s parents on a laptop computer/tablet using PEDI-CAT software version 1.3.6 as part of the natural history study during a clinic visit. The physical therapist entered the demographic information including ID, age, sex, and assistive device or wheelchair use into the software and then handed the computer/tablet to a parent or caregiver and instructed them to complete the measure by reporting the child’s current level of functioning. The physical therapist assisted parents or caregivers who requested assistance with filling out the PEDI-CAT.

A subset of parents also completed a one-page questionnaire to gather their perspectives on the PEDI-CAT computer format and item content. Survey questions included: 1) Do you feel you provided meaningful information about your child? 2) Are there items that could be added to better describe your child’s motor and daily activity abilities? and 3) Would you be more willing to do this computer adaptive test rather than a paper/pencil test for ongoing assessments of your child?

2.2 Study participants

Participants were children or youth who met the following inclusion criteria: birth through 21 years of age with a clinical and genetically confirmed diagnosis of SMA. They were excluded if they were enrolled in a clinical trial or their caregiver could not read English or Spanish as PEDI-CAT software version 1.3.6 is only available in those two languages.

2.3 Outcome measure

The PEDI-CAT is a newer version of the Pediatric Evaluation of Disability Inventory (PEDI) that encompasses a larger item bank and was developed using item response theory and computer adaptive testing methods to improve administration feasibility and item scaling. The larger item bank allows for application to a broader range of functional abilities and ages. In a previous study, the full item bank properties of the Mobility and Daily Activities domains of the PEDI-CAT were evaluated specifically for children and youth with SMA [30]. Each domain is unidimensional suggesting that items define these two constructs and overall item fit was good. The item level of difficulty for both domains was high for participants with SMA type I. The Mobility domain best represented participants with SMA type III whereas the Daily Activities domain best represented participants with SMA type II and III [30].

Table 1 describes the PEDI-CAT administration type, content, item examples, response scale, and scores. The PEDI-CAT has an item bank of 276 items distributed across four domains: 1) Daily Activities, 2) Mobility, 3) Social/Cognitive, and 4) Responsibility [41]. The domains are self-contained and can be used separately or in combination with other domains. The focus of this study is on the Daily Activities and Mobility content since SMA is primarily a motor disorder. The Content-Balanced administration type was used for most participants because an item map was only available with the Content-Balanced and not the Speedy option for software version 1.3.6. Item maps provide a visual representation of items and their level of difficulty in a specific domain.

Table 1
Description of the PEDI-CAT Daily Activities and Mobility domains

Domains Daily Activities Mobility

Description - Basic self-care and instrumental activities of daily living including eating, grooming, dressing or basic home maintenance - Basic motor skills of rolling, sitting, or walking and more difficult gross motor skills content

- Device item content: Walking items specifying use of a walking device (cane, crutches, walker)

- Wheelchair subdomain: Transfers and wheelchair mobility over different surfaces, distances, and environments; no power wheelchair items at this time

Number of Items 68 85 (75 items with an additional 10 items for youth who use a device)

Wheelchair subdomain: 12 items

Content Areas - Getting dressed - Basic movement and transfers

- Keeping clean - Standing and walking

- Eating and mealtime - Steps and inclines

- Home tasks - Running and playing

- Standing and walking with device (administered if child uses a device)

- Wheelchair mobility (administered if child propels manual wheelchair; items are scored separately and not combined with mobility items)

Item Examples - Feeds self with spoon - Rolls over in bed or crib

- Ties shoelaces - Walks from room to room in home

- Trims fingernails on both hands - Climbs a stepladder to put a heavy box on a shelf

- Uses a computer keyboard to type - Hikes 2–3 miles

Scores Available - Scaled scores: Interval level scores by domain and for the Wheelchair subdomain. Not age-related scores.

Represent a child’s abilities along a continuum of function [41].

- Normative scores: Describe a child’s functional abilities in comparison to typically developing children of the same age. Provided in T scores and percentiles by domain but not for the Wheelchair subdomain. T scores have a mean of 50 and a standard deviation of 10.[41]

Response Scale 4-point response scale:

- Easy (child does without help, extra time or effort)

- A little hard (does with a little help, extra time or effort)

- Hard (does with a lot of help, extra time or effort)

- Unable (can’t do, doesn’t know how or is too young)

Administration Type - Speedy: 5–15 items are administered per domain so it is an efficient CAT while still providing precise score estimates

- Content-Balanced: approximately 30 items administered per domain; includes a balance of items from each of the content areas within each domain; recommended for children who function in the middle to high end of the scale since content areas are hierarchical (e.g., basic movement and transfers vs running and playing)

- Two types can be used interchangeably since scores do not vary significantly between the two [41]

Domains	Daily Activities	Mobility
Description	- Basic self-care and instrumental activities of daily living including eating, grooming, dressing or basic home maintenance	- Basic motor skills of rolling, sitting, or walking and more difficult gross motor skills content
		- Device item content: Walking items specifying use of a walking device (cane, crutches, walker)
		- Wheelchair subdomain: Transfers and wheelchair mobility over different surfaces, distances, and environments; no power wheelchair items at this time
Number of Items	68	85 (75 items with an additional 10 items for youth who use a device)
		Wheelchair subdomain: 12 items
Content Areas	- Getting dressed	- Basic movement and transfers
	- Keeping clean	- Standing and walking
	- Eating and mealtime	- Steps and inclines
	- Home tasks	- Running and playing
		- Standing and walking with device (administered if child uses a device)
		- Wheelchair mobility (administered if child propels manual wheelchair; items are scored separately and not combined with mobility items)
Item Examples	- Feeds self with spoon	- Rolls over in bed or crib
	- Ties shoelaces	- Walks from room to room in home
	- Trims fingernails on both hands	- Climbs a stepladder to put a heavy box on a shelf
	- Uses a computer keyboard to type	- Hikes 2–3 miles
Scores Available	- Scaled scores: Interval level scores by domain and for the Wheelchair subdomain. Not age-related scores.
	Represent a child’s abilities along a continuum of function [41].
	- Normative scores: Describe a child’s functional abilities in comparison to typically developing children of the same age. Provided in T scores and percentiles by domain but not for the Wheelchair subdomain. T scores have a mean of 50 and a standard deviation of 10.[41]
Response Scale	4-point response scale:
	- Easy (child does without help, extra time or effort)
	- A little hard (does with a little help, extra time or effort)
	- Hard (does with a lot of help, extra time or effort)
	- Unable (can’t do, doesn’t know how or is too young)
Administration Type	- Speedy: 5–15 items are administered per domain so it is an efficient CAT while still providing precise score estimates
	- Content-Balanced: approximately 30 items administered per domain; includes a balance of items from each of the content areas within each domain; recommended for children who function in the middle to high end of the scale since content areas are hierarchical (e.g., basic movement and transfers vs running and playing)
	- Two types can be used interchangeably since scores do not vary significantly between the two [41]

Caregivers with a primary language of Spanish completed the PEDI-CAT in Spanish. The stringent translation process is described in the manual 1.3.6 by PEDI-CAT developers. The Spanish version can be used interchangeably with the English version for Spanish-speaking caregivers living in the US [42].

One of the purposes of the PEDI-CAT is to differentiate functional abilities between patient groups, specifically those with and without disabilities [27, 42]. The PEDI-CAT software produces normative scores that describe an individual’s performance in comparison to other individuals of the same age [42]. Normative scores are used by therapists to discriminate functional abilities of children with and without disabilities. The PEDI-CAT software also produces scaled interval level scores (criterion scores) as Item Response Theory methods were used to place test items on a hierarchical scale of easiest to hardest [26, 42]. Therefore, higher scaled scores indicate higher levels of functional ability. Scaled scores can be used to evaluate change in function over time and are especially helpful in evaluating change in those who are not expected to attain age appropriate gross motor skills.

2.4 Statistical analysis

Descriptive statistics were used to depict the demographic characteristics of the sample. Convergent validity for this study was defined as the ability of a domain to measure the construct of motor severity. Specifically, what was evaluated was whether the Daily Activities and Mobility domains could detect differences in daily activity function across SMA types and motor function classification groups. Since the Daily Activities and Mobility scores for some SMA types or functional classification groups were not normally distributed, the nonparametric Kruskal-Wallis test was used to determine if there were differences in the mean ranks of PEDI-CAT scores by SMA type and motor group classification. Pairwise comparisons were performed using Dunn’s procedure with a Bonferroni correction for multiple comparisons and adjusted p-values are presented.

Normative scores (T scores) were examined to determine the ability of the PEDI-CAT to identify decreased Mobility or Daily Activity skills compared to same-aged peers without disabilities. The percentages of children and youth with decreased functional skills (> 2 standard deviations) or slightly decreased skills (≥1 and≤2 standard deviations below normal) were calculated. For the floor and ceiling analysis, the percentages of the sample with floor (lowest score possible) and ceiling (highest score possible) scores were calculated for each domain.

Clinical utility of the PEDI-CAT was quantitatively evaluated using administration time and qualitatively evaluated using questionnaire data which was coded by the first two authors. Start and stop times generated by the PEDI-CAT program’s internal clock were used to determine the mean and standard deviation of the administration times for each domain. The percentage of tests that were administered in≤2 minutes,≤5 minutes, and≤10 minutes were also calculated.

3 Results

Demographic information for the sample is provided in Table 2. This data set included 96 participants for the Daily Activities domain and 95 for the Mobility domain. The PEDI-CAT was completed by parents (n = 85), other caregivers (n = 2), or by clinicians in conjunction with the child’s parents (n = 9). Most tests (96.9%) were administered in English with the rest administered in Spanish. Descriptive statistics for PEDI-CAT scores for the Daily Activities and Mobility domains and the Wheelchair subdomain are provided in Table 3 by SMA type and motor classification. Significant differences in the ages of participants were found by SMA type, with SMA type III participants being significantly older than SMA type I or SMA type II participants (p = 0.001). No statistically significant age differences were found among the three motor classification groups.

Table 2
Sample demographics

Total Sample (n = 96) SMA Type I (n = 17) SMA Type II (n = 45) SMA Type III (n = 34)

Age, years; Mean (SD) 8.9 (6.3) 5.7 (5.9) 7.6 (5.5) 12.3 (6.0)

Range: years(y), months(m) 2m –21y, 11m 2m –21y, 11m 1y, 4m –20y, 1m 3y, 7m –21y, 8m

Sex Frequency (percent)

Male 56 (58.3) 12 (70.6) 23 (51.1) 21 (61.8)

Female 40 (41.7) 5 (29.4) 22 (48.9) 13 (38.2)

Race Frequency (percent)

White 79 (82.3) 15 (88.2) 36 (80.0) 28 (82.4)

Asian 14 (14.6) 1 (5.9) 8 (17.8) 5 (14.7)

Black 1 (1.0) 1 (5.9) 0 0

Multi-racial 1 (1.0) 0 1 (2.2) 0

Not reported 1 (1.0) 0 0 1(2.9)

Ethnicity Frequency (percent)

Hispanic 8 (8.3) 0 4 (8.9) 4 (11.8)

Non-Hispanic 86 (89.6) 16 (94.1) 41 (91.1) 29 (85.3)

Not reported 2 (2.1) 1 (5.9) 0 1 (2.9)

Motor Classification Frequency (percent)

Non-sitter 17(100) 9 (20) 0

Sitter 0 36 (80) 6 (17.6)

Walker 0 0 28 (82.4)

Wheelchair/Device Use Frequency (percent)

Manual wheelchair user 31 (32.3) 8 (47.1) 18 (40) 5 (14.7)

Propels manual wheelchair 15 (15.6) 0 12 (26.7) 3 (8.8)

Power wheelchair user 39 (40.6) 4 (23.5) 28 (62.2) 7 (20.6)

Device user (walker/crutches) 6 (6.3) 0 2 (4.4) 4 (11.8)

	Total Sample (n = 96)	SMA Type I (n = 17)	SMA Type II (n = 45)	SMA Type III (n = 34)
Age, years; Mean (SD)	8.9 (6.3)	5.7 (5.9)	7.6 (5.5)	12.3 (6.0)
Range: years(y), months(m)	2m –21y, 11m	2m –21y, 11m	1y, 4m –20y, 1m	3y, 7m –21y, 8m
Sex	Frequency (percent)
Male	56 (58.3)	12 (70.6)	23 (51.1)	21 (61.8)
Female	40 (41.7)	5 (29.4)	22 (48.9)	13 (38.2)
Race	Frequency (percent)
White	79 (82.3)	15 (88.2)	36 (80.0)	28 (82.4)
Asian	14 (14.6)	1 (5.9)	8 (17.8)	5 (14.7)
Black	1 (1.0)	1 (5.9)	0	0
Multi-racial	1 (1.0)	0	1 (2.2)	0
Not reported	1 (1.0)	0	0	1(2.9)
Ethnicity	Frequency (percent)
Hispanic	8 (8.3)	0	4 (8.9)	4 (11.8)
Non-Hispanic	86 (89.6)	16 (94.1)	41 (91.1)	29 (85.3)
Not reported	2 (2.1)	1 (5.9)	0	1 (2.9)
Motor Classification	Frequency (percent)
Non-sitter		17(100)	9 (20)	0
Sitter		0	36 (80)	6 (17.6)
Walker		0	0	28 (82.4)
Wheelchair/Device Use	Frequency (percent)
Manual wheelchair user	31 (32.3)	8 (47.1)	18 (40)	5 (14.7)
Propels manual wheelchair	15 (15.6)	0	12 (26.7)	3 (8.8)
Power wheelchair user	39 (40.6)	4 (23.5)	28 (62.2)	7 (20.6)
Device user (walker/crutches)	6 (6.3)	0	2 (4.4)	4 (11.8)

Table 3

PEDI-CAT scores by domain, SMA type, and motor type

PEDI -CAT Domain &Scores	SMA I	SMA II	SMA III	Non-sitter	Sitter	Walker	Full Sample
Daily Activities	n = 17	n = 45	n = 34	n = 26	n = 42	n = 28	n = 96
Scaled score mean (SD)	34.8 (6.5)	48.5 (4.5)	59.7 (5.8)	39.3 (8.8)	50.1 (6.2)	59.9 (5.2)	50.0 (10.2)
Scaled score range	29.8 –47.2	36.2 –61.0	48.8 –71.3	29.8 –61.0	36.2 –71.3	48.8 –69.5	29.8 –71.3
n (%) with T score < 30^*	11(64.7%)	28(62.2%)	12(45.3%)	19(79.2%)	24(55.8%)	8(27.6%)	51(53.1%)
n (%) with T score 30–40^∧	4(23.5%)	8(17.8%)	8(23.5%)	3(12.5%)	8(18.6%)	9(31.0%)	20(20.8%)
Mobility	n = 16	n = 45	n = 34	n = 25	n = 42	n = 28	n = 95
Scaled score; mean (SD)	28.5 (5.7)	44.3 (6.7)	57.8 (7.2)	32.7 (8.9)	45.5 (5.9)	60.5 (2.6)	46.5 (12.1)
Scaled score range	23.7 –40.2	23.7 –57.9	34.3 –68.1	23.7 –53.5	31.6 –59.7	56.0 –68.1	23.7 –68.1
n (%) with T score < 30^*	13(81.3%)	43(95.6%)	29(85.3%)	22(88%)	39(92.9%)	24(85.7%)	85(89.5%)
n (%) with T score 30–40^∧	3(18.7%)	1(2.2%)	4(11.8%)	3(12%)	1 (2.4%)	4(14.3%)	8(8.4%)
Wheelchair subdomain	n = 0	n = 12	n = 3	n = 1	n = 13	n = 1	n = 15
Scaled score mean (SD)		46.6 (4.7)	54.5 (3.6)	48.5	47.5 (5.3)	56.6	48.2 (5.5)
Scaled score range		38.9 –58.0	50.4 –56.6		38.9 –58.0		38.9 –58

^*T score has a mean of 50 and a standard deviation of 10, so this score is more than 2 standard deviations below the mean. ^∧This score is between 1 and 2 standard deviations below the mean.

3.1 Convergent validity

The mean ranks of the PEDI-CAT Daily Activities and Mobility scores were different among the SMA type and motor classification groups. Mean ranks of Daily Activities and Mobility scaled scores were lowest for participants with SMA type I and for those with a motor classification of Non-sitter and highest for participants with SMA type III and Walkers.

The mean ranks of Daily Activities scores were significantly different between the SMA types (χ² = 69.0, df = 2, p < 0.001). Pairwise comparisons revealed statistically significant differences in the mean ranks of Daily Activities scores among all three SMA types (p < 0.001 for all pairwise comparisons). The mean ranks of Daily Activities scores were also significantly different among the motor classifications (χ² = 54.0, df = 2, p < 0.001). Pairwise comparisons revealed significant differences in the mean ranks of Daily Activities scores between Non-sitters and Sitters (p = 0.002), Sitters and Walkers (p < 0.001), and Non-sitters and Walkers (p < 0.001).

The mean ranks of Mobility scores were significantly different among the SMA types (χ² = 62.9, df = 2, p < 0.001). Pairwise comparisons revealed statistically significant differences in the mean ranks of Mobility scores among all three SMA types (p < 0.001 for all pairwise comparisons). The mean ranks of Mobility scores were significantly different among the motor classifications (χ² = 67.5, df = 2, p < 0.001). Pairwise comparisons revealed significant differences in Mobility scores between Non-sitters and Sitters (p = 0.004), Sitters and Walkers (p < 0.001), and Non-sitters and Walkers (p < 0.001).

3.2 Discriminative properties

The PEDI-CAT detected decreased motor function in the majority of the participants (Table 3). Most of the sample (89.5%) demonstrated markedly decreased scores (T score < 30) on the Mobility domain and 53.1% demonstrated markedly decreased scores (T score < 30) on the Daily Activities domain.

3.3 Floor and ceiling scores

Floor level scaled scores were noted for 10 children (10.4% of the sample) for the Daily Activities domain, all with SMA type I (10 Non-sitters). Nine children (9.5% of the sample) had floor level scores on the Mobility domain (8 SMA type I and 1 type II; 9 Non-sitters). One ceiling level score was observed for the Daily Activities domain (SMA type III; Sitter). No ceiling level scores were observed for the Mobility domain.

3.4 Clinical utility

The majority of PEDI-CAT tests administered were Content-Balanced (95.8%). Most of the time (94.5%), respondents completed a domain in 10 minutes or less (Table 4). Thirty-eight caregivers completed a survey on the clinical utility of the PEDI-CAT. Most (n = 35; 92.1%) reported that they provided meaningful information about their child when using the PEDI-CAT to report functional abilities. The majority of caregivers (n = 30; 79%) said that they would be more willing to do the computer adaptive test than a paper and pencil test for future assessments. Three did not have a preference and five preferred paper and pencil tests. Nearly all (n = 36; 94.7%) reported that they would be willing to complete an assessment like the PEDI-CAT online in the future. Three caregivers who felt that they did not provide meaningful information reported that the items were too hard or that additional content was needed in order to describe their child’s abilities. Their children were classified as Non-sitter (n = 2; 5.3%) or Sitter and 21 years of age (n = 1; 2.6%). They suggested adding items involving the use of a touch screen and items related to work. Half (n = 19; 50%) also reported that additional items could be included to better describe their child’s abilities such as: 1) amount of time chewing/eating meals; 2) ability to use a stander and amount of time able to tolerate standing; 3) eye gaze items; 4) touch screen use; 5) ability to use a motion sensor sink; and 6) power wheelchair mobility.

Table 4
Time to complete content-balanced PEDI-CAT tests by domain

PEDI-CAT domain Daily Activities Mobility

Content-balanced tests (n) 92 91

Administration time in minutes; Mean (SD) 5.4 (3.3) 3.3 (3.2)

Range in minutes 1–22 1–17

n (%) tests completed in≤2 minutes 11 (12) 52 (57.1)

n (%) tests completed in≤5 minutes 62 (67.4) 75 (82.4)

n (%) tests completed in≤10 minutes 86 (93.5) 87 (95.6)

n (%) total tests completed in ≤10 minutes 173 (94.5)

PEDI-CAT domain	Daily Activities	Mobility
Content-balanced tests (n)	92	91
Administration time in minutes; Mean (SD)	5.4 (3.3)	3.3 (3.2)
Range in minutes	1–22	1–17
n (%) tests completed in≤2 minutes	11 (12)	52 (57.1)
n (%) tests completed in≤5 minutes	62 (67.4)	75 (82.4)
n (%) tests completed in≤10 minutes	86 (93.5)	87 (95.6)
n (%) total tests completed in ≤10 minutes	173 (94.5)

4 Discussion

Rehabilitation professionals need valid and clinically feasible outcome measures that can be used to identify delays or limitations in functional performance and to document the function of children and youth with SMA of varying ages and abilities. This study demonstrates that the PEDI-CAT has good psychometric properties to support its application as a performance-based caregiver-reported outcome measure in children and youth with SMA [37].

The PEDI-CAT Mobility and Daily Activities domains were able to identify differences in functional skills for participants with different SMA types and motor classifications, demonstrating convergent validity. This is clinically important as children and youth with SMA differ in functional abilities across types and motor classifications. In addition to SMA disease factors, age can impact scaled scores since older children without disabilities have higher levels of function and therefore have higher scaled scores than younger children. Participants with SMA type III were significantly older than participants with SMA type I or II. However, no statistically significant age differences were found among the three motor classification groups which supports the ability of the PEDI-CAT to measure differences in Daily Activities and Mobility function according to SMA motor severity.

The PEDI-CAT was able to detect limitations in mobility function (more than 2 standard deviations below the mean) in 89.5% of the sample and mild limitations (1-2 standard deviations below the mean) in 8.4% of the sample. Therefore, 97.9% of the sample of children and youth with SMA had some limitations in mobility function. This result is consistent with the natural history of motor limitations in SMA [7 –11], demonstrating good discriminative properties of the Mobility domain of the PEDI-CAT. Limitations in function were also identified in the Daily Activities domain. Just over half of the sample (53.1%) had a limitation in daily activities function and 20.8% had mild limitations. This is consistent with the typical pattern of weakness found in SMA with more involvement in the lower extremities than in the upper extremities [12]. Preservation of wrist and finger flexors and extensors and biceps muscle function [12] may allow children and youth to have the ability to perform daily activities skills such as brushing teeth, cutting fingernails, and using a computer keyboard; but these children or youth may still experience limitations in mobility activities of walking up stairs or running.

Floor effects were present for a small group of participants (≤10.4%) in this sample. A few participants with SMA type I or Non-sitters received the lowest possible score on the Daily Activities and Mobility domains. This was due to the content and level of difficulty of the items. The easiest items in the Daily Activities domain are related to swallowing liquids and chewing food [42]; however, these items typically are difficult or impossible for children with SMA type I and Non-sitters [6] The Mobility domain addresses basic movement skills such as turning head in supine, lifting head in prone, and rolling. These activities are difficult for children with SMA type I or Non-sitters [4]. These findings are similar to those of a study that used Rasch methods to calibrate the PEDI-CAT Daily Activity and Mobility full item banks [30]. A lack of easier items for individuals with minimal motor function could potentially limit test sensitivity to small yet important changes after interventions. To improve responsiveness of the PEDI-CAT, items which are meaningful to children and caregivers could be added.

Some of the parent-suggested items address functional capacity but could be represented at a performance level of measurement. Examples of performance level items include maintaining their head in the upright position in supported sitting in the classroom while listening to the teacher, operating a computer using eye gaze or touch screen to complete homework tasks, or moving around in the community with friends using a powered wheelchair.

A ceiling level score on the Daily Activities domain was only observed in one 21-year-old participant with SMA non-ambulatory type III (classified as a Sitter) who was independent with activities of daily living including dressing, eating, and bathing. He presented with proximal upper extremity weakness but had better distal strength. He compensated for his weakness by using adaptive equipment or modifying tasks. Also, he demonstrated good functional performance in his home environment. Even though there was only one participant who had a ceiling level score, it suggests that the Daily Activities domain may not capture disease burden for older individuals with SMA Type III. Therefore, harder items could be added to improve responsiveness.

Thirty-nine children in this sample were power wheelchair users and 31 were manual wheelchair users. However, the wheelchair items were only administered for children who had some ability to self-propel (n = 15). The Wheelchair subdomain only contains 12 items and excludes power wheelchair tasks, limiting its ability to record a wide range of function and potentially limiting its ability to measure changes in wheelchair function. Physical therapists frequently evaluate equipment needs, consult with families about equipment use, and teach children how to use power wheelchairs. Therefore, items on positioning and sitting tolerance as well as independence with power mobility are missing content that could be added to the wheelchair subdomain to improve its ability to document function related to wheelchair use.

Overall, the clinical utility of the PEDI-CAT was quite good as evidenced by the short administration times. The PEDI-CAT was completed by caregivers during clinic visits while children and youth were participating in capacity measures or being seen by specialists. Clinic physical therapists reported that in some cases, caregivers stopped answering questions on the PEDI-CAT to observe their child or to discuss rehabilitation needs with the team. These times might be higher than if the caregiver completed the test without any distractions.

Caregivers mostly provided positive feedback as to the meaningfulness of the items on the PEDI-CAT and their willingness to complete additional tests. Switching from the Content-Balanced to the Speedy administration option would improve administration time without sacrificing score precision. Also, it might improve parent feedback on item relevance since a different algorithm is used to efficiently select 15 or fewer items. Anecdotally, therapists reported that they selected the Content-Balanced option because they wanted to refer to the item map. Toward the end of the study, therapists determined that the Content-Balanced option was not helpful for children and youth who were non-ambulatory or diagnosed with SMA I or II since the content areas are hierarchical and many of the items were too hard for participants and did not need to be administered. Use of the Speedy option prevents or decreases the number of irrelevant items especially for children and youth with lower function since item selection is not made according to content areas and item level of difficulty, but strictly by item level of difficulty. The newer versions (version 1.3.9 and later) of the PEDI-CAT software include an item map for both the Speedy and Content-Balanced options [42].

In summary, the PEDI-CAT is a valid, discriminative, and feasible performance-based caregiver-reported outcome measure for children and youth with SMA. This study supports the clinical application and use of this outcome measure for research purposes. Future work should include prospective collection of longitudinal data on the PEDI-CAT in all SMA types to study its sensitivity to change and potential responsiveness to effective interventions. Adding easier but functionally relevant items to the PEDI-CAT would reduce the floor effect for children and youth with SMA type I. Also, it would be helpful to explore the addition of power wheelchair and positioning items to the wheelchair subdomain to improve its applicability to a wider range of wheelchair users.

Footnotes

Acknowledgments

The authors thank the families, children and young adults who participated in the study. The authors would like to acknowledge the following clinical trial coordinators: Heather (Szelag) Fournier, Lavanya Madabusi, Timothy Harrington, Sofia Georghiou, and Grace Ordonez from Boston Children’s Hospital; Nancy Videon from Children’s Hospital of Philadelphia; Rosangel E. Cruz and Nicole M. LaMarca from Columbia University Irving Medical Center. The authors would also like to acknowledge the Muscle Study Group and the Pediatric Neuromuscular Clinical Research Network for Spinal Muscular Atrophy.

Funding for this study was provided by the SMA Foundation.

Conflict of interest

Maria Fragala-Pinkham is one of the senior authors of the PEDI-CAT. She has a professional, non-financial relationship with CRECare, LLC which distributes the PEDI-CAT. She does not receive royalties from the PEDI-CAT; however, Franciscan Children’s Hospital has received payment for her work and consultation on the PEDI-CAT from CRECare.

Allan Glanzman receives consulting fees from Biogen and Atom International as well as fees to his institution from Roche Holding AG.

Basil T. Darras has received research support from Scholar Rock for the SRK-015 Phase 1/2 clinical trial.

The other authors have no conflicts of interest to report.

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