Validation and clinical significance of the Childhood Myositis Assessment Scale for assessment of muscle function in the juvenile idiopathic inflammatory myopathies.
OBJECTIVE
To examine the measurement characteristics of the Childhood Myositis Assessment Scale (CMAS) in children with juvenile idiopathic inflammatory myopathy (juvenile IIM), and to obtain preliminary data on the clinical significance of CMAS scores.


METHODS
One hundred eight children with juvenile IIM were evaluated on 2 occasions, 7-9 months apart, using various measures of physical function, strength, and disease activity. Interrater reliability, construct validity, and responsiveness of the CMAS were examined. The minimum clinically important difference (MID) and CMAS scores corresponding to various degrees of physical disability were estimated.


RESULTS
The intraclass correlation coefficient for 26 patients assessed by 2 examiners was 0.89, indicating very good interrater reliability. The CMAS score correlated highly with the Childhood Health Assessment Questionnaire (C-HAQ) score and with findings on manual muscle testing (MMT) (r(s) = -0.73 and 0.73, respectively) and moderately with physician-assessed global disease activity and skin activity, parent-assessed global disease severity, and muscle magnetic resonance imaging (r(s) = -0.44 to -0.61), thereby demonstrating good construct validity. The standardized response mean was 0.81 (95% confidence interval 0.53, 1.09) in patients with at least 0.8 cm improvement on a 10-cm visual analog scale for physician-assessed global disease activity, indicating strong responsiveness. In bivariate regression models predicting physician-assessed global disease activity, MMT remained significant in models containing the CMAS (P = 0.03) while the C-HAQ did not (P = 0.4). Estimates of the MID ranged from 1.5 to 3.0 points on a 0-52-point scale. CMAS scores corresponding to no, mild, mild-to-moderate, and moderate physical disability, respectively, were 48, 45, 39, and 30.


CONCLUSIONS
The CMAS exhibits good reliability, construct validity, and responsiveness, and is therefore a valid instrument for the assessment of physical function, muscle strength, and endurance in children with juvenile IIM. Preliminary data on MID and corresponding levels of disability should aid in the clinical interpretation of CMAS scores when assessing patients with juvenile IIM.
Huber
Adam M
AM
IWK Health Centre and Dalhousie University, 5850 University Avenue, Halifax, Nova Scotia B3J 3G9, Canada. adam.huber@iwk.nshealth.ca
Feldman
Brian M
BM
Rennebohm
Robert M
RM
Hicks
Jeanne E
JE
Lindsley
Carol B
CB
Perez
Maria D
MD
Zemel
Lawrence S
LS
Wallace
Carol A
CA
Ballinger
Susan H
SH
Passo
Murray H
MH
Reed
Ann M
AM
Summers
Ronald M
RM
White
Patience H
PH
Katona
Ildy M
IM
Miller
Frederick W
FW
Lachenbruch
Peter A
PA
Rider
Lisa G
LG
D000293:Adolescent; D002648:Child; D002675:Child, Preschool; D004185:Disability Evaluation; D006801:Humans; D009043:Motor Activity; D009220:Myositis / Q000175:diagnosis* / Q000503:physiopathology; D015588:Observer Variation; D017063:Outcome Assessment, Health Care; D011237:Predictive Value of Tests; D015203:Reproducibility of Results; D012720:Severity of Illness Index