Background/Purpose: We examined demographic, clinical and laboratory features of juvenile polymyositis (JPM) and muscular dystrophy in children to improve classification of these two conditions.
Methods: Thirty-nine patients with probable or definite JPM (31 JPM and 8 JPM overlapping with another connective tissue disease) by Bohan and Peter criteria and 9 patients with various muscular dystrophies (1 Duchenne’s carrier, 8 limb girdle dystrophies) were examined. Differences in demographic and clinical features (including musculoskeletal, cutaneous, gastrointestinal, pulmonary, cardiac, constitutional signs and symptoms), muscle enzyme levels, Magnetic Resonance Imaging (MRI), electromyography (EMG), and muscle biopsy features between patients with JPM and dystrophies were evaluated by Fisher’s exact and Rank sum tests. Backwards step-wise logistic regression modeling, followed by exact logistic regression, was performed to examine significant univariable differences between JPM and dystrophies in a multivariable model.       
Results: Delay to diagnosis was longer in patients with dystrophies (median 12.0 months) compared with JPM (median 4.0 months, respectively, p=0.03). In addition, the following demographic, clinical and laboratory features distinguished JPM and dystrophies:

Features	JPM n/N (%)	Muscular dystrophies n/N (%)	P value
African-American Race	17/39 (43.6)	0/9 (0.0)	0.018
Insidious illness onset (› 6 mo from first symptom to diagnosis)	11/31 (35.5)	7/9 (77.8)	0.05
Muscle atrophy on exam	17/37 (45.9)	8/9 (88.9)	0.027
Muscle atrophy on MRI	4/21 (19.0)	5/6 (83.3)	0.008
Increased insertional and spontaneous activity/fibrillation potentials on EMG	17/18 (94.4)	5/9 (55.6)	0.03
Complex repetitive discharge on EMG	16/23 (69.6)	2/9 (22.2)	0.02
Diffuse variation of myofiber size on muscle biopsy	20/38 (52.6)	9/9 (100.0)	0.008
Fiber hypertrophy on muscle biopsy	3/38 (7.9)	4/9 (44.4)	0.018
Myofiber fibrosis on muscle biopsy	4/38 (10.5)	5/9 (55.6)	0.007
Absence of response to treatment with prednisone	2/39 (5.1)	3/9 (33.3)	0.039
Absence of response to treatment with other immunosuppressive agents	2/39 (5.1)	4/9 (44.4)	0.008

Other features did not differ between JPM and dystrophies, including frequency of abnormal values and levels of serum muscle enzymes (CK, LDH, Aldolase, AST, and ALT); muscle edema and fatty replacement of muscle on MRI; inflammatory features on muscle biopsy; presence of short duration, small amplitude polyphasic motor units action potentials (MUAPS) and positive sharp waves on EMG; and demographic features (age at diagnosis, family history of autoimmune disease). Examining the 8 variables that were significant on univariate analysis in backwards selection stepwise logistic regression, followed by exact logistic regression, revealed less frequent myofiber fibrosis on muscle biopsy (OR = 0.1) and onset speed (OR = 0.16) to be significant predictors of JPM vs. muscular dystrophy. Because there were no African-American patients with dystrophies, a point estimate for race could not be obtained. The final multivariable model, including myofiber fibrosis, onset speed and race, provided a sensitivity of 55.6%, specificity of 97.2%, positive predictive value of 83.3%, and negative predictive value of 89.7%.

Conclusion: Muscular dystrophy can present similarly to patients with idiopathic inflammatory myopathy, particularly polymyositis.  Selected demographic, clinical, and laboratory features may be helpful in distinguishing these patients, particularly the presence of myofiber fibrosis on muscle biopsy and a slower illness onset.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/clinical-and-laboratory-features-distinguishing-juvenile-polymyositis-and-muscular-dystrophy-in-children/