Table 3.

Multivariate analysis of the extent of the different thigh MRI features (percentage of muscles involved) among different clinical subsets using fractional probit regression

	Oedema dy/dx (95% CI)	Atrophy dy/dx (95% CI)	Fatty replacement dy/dx (95% CI)	Fascial oedema dy/dx (95% CI)
Clinical subset (referenced to IMNM)
IBM	−5.82 (−13.31 to 1.66)	0.92 (−4.23 to 6.06)	1.52 (−5.56 to 8.61)	1.68 (−3.87 to 7.23)
PM	−23.72 (−30.63 to −16.80)***	−10.92 (−16.38 to −5.46)***	−10.77 (−17.69 to −3.86)**	0.05 (−4.53 to 4.64)
DM	−22.14 (−29.36 to −14.93)***	−18.24 (−24.00 to −12.47)***	−17.39 (−24.21 to −10.57)***	8.82 (4.53 to 13.11)***
CADM	−61.46 (−85.87 to −37.05)***	−25.76 (−39.13 to −12.39)***	−31.18 (−46.55 to −15.80)***	1.85 (−6.86 to 10.57)
Age at onset (10 years)	0.73 (−1.10 to 2.56)	2.41 (0.84 to 3.97)**	2.05 (0.34 to 3.77)*	−0.88 (−1.76 to 0.00)
Time from onset to MRI (logarithm of months)	−8.83 (−14.34 to −3.33)**	10.44 (6.13 to 14.74)***	13.90 (8.90 to 18.89)***	−4.82 (−7.39 to −2.26)***
Sex (female)	−9.60 (−14.58 to −4.61)***	3.24 (−0.45 to 6.92)	−8.61 (−13.11 to −4.11)***	−2.92 (−5.72 to −0.11)*
Race (referenced to white patients)
Black	6.11 (−1.70 to 13.92)	0.84 (−5.26 to 6.95)	8.18 (1.59 to 14.77)*	−0.49 (−4.77 to 3.80)
Other races	2.26 (−5.86 to 10.37)	4.08 (−1.70 to 9.86)	−0.02 (−7.69 to 7.66)	−0.51 (−4.74 to 3.73)

dy/dx, marginal effect (% change of the dependent variable for each one point of the predictor variables).

^*<0.05;

^**<0.01;

^***<0.001.

CADM, clinically amyopathic dermatomyositis; DM, dermatomyositis; IBM, inclusion body myositis; IMNM, immune-mediated necrotising myopathy; PM, polymyositis.