Jo-1
positive myositis patients tend to have more severe muscle and lung involvement
than PM-Scl positive patients

Background/Purpose: Jo-1 and PM-Scl positive myositis
patients share many clinical features, however the differences between these
two subtypes is unknown. The purpose of this study was to determine the unique
phenotypes of these two subgroups.

Methods:
From 5/1/2002 to
4/15/2015, 1700 patients with suspected myositis were enrolled in a
longitudinal cohort study at our center. Patients were included in our analysis
if they met Bohan and Peter (B&P) criteria for definite/probable
polymyositis (PM) or dermatomyositis (DM) and were positive for Jo-1 antibody
or PM-Scl antibody. Only patients who had confirmed autoantibody testing at a
CLIA certified lab were included. Retrospective chart review was conducted from
the time of first clinic visit until 4/15/2015 or last clinical encounter. To
determine the severity of disease phenotype maximum creatine kinase (CK) and
aldolase, the lowest DLCO and forced vital capacity (FVC) was used to assess
the severity of lung function. Muscle strength at first visit was scored on a
modified 10-point scale adapted from the manual muscle strength testing scale
recommended by the International Myositis Assessment and Clinical Studies
Group. Right and left sided measurements were combined (range of 0-20). The
presence of edema or fatty replacement on MRI of the thighs was categorized as
a dichotomous variable. Pulmonary hypertension was defined by either
echocardiography or cardiac catheterization and ILD on imaging findings. Differences
in baseline characteristics of patients with Jo-1 vs. PM-Scl antibodies were
assessed by Fisher’s exact and student’s t tests.

Results:
A total of 91
patients were identified in our single-center database who met the B&P
criteria for PM or DM and were positive for either Jo-1 or PM-Scl autoantibodies.
Of the 91 patients, 60/91 (65.9%) were Jo-1 positive and 31/91 (34.1%) were
PM-Scl positive. The mean follow-up time was 50.8 months for both groups. The
mean age at diagnosis of the Jo-1 positive group was 45.8 ± 12.9 years
vs. 47.1 ±14.3 years in the PM-Scl group. At initial visit to our
center, muscle strength did not differ between the two groups. The mean maximum
CK was 2197 ± 2589 IU in Jo-1 positive patients vs. 610 ± 893 IU
in PM-Scl positive patients (p=0.002). ILD was found more commonly in Jo-1
positive patients (84.8% vs. 46.7%, p=<0.0001), as well as muscle edema on
MRI of the thighs (79.1% vs. 25%, p=0.001).  Raynaud’s phenomenon and deltoid
atrophy were more commonly found in PM-Scl patients and both associations were
statistically significant.

Conclusion: Jo-1 positive myositis patients
tended to have more severe muscle involvement with higher CK levels, more
muscle edema on MRI, and more ILD when compared to PM-Scl positive patients.

Table 1. 
Clinical characteristics of Jo-1 positive vs. PM-Scl positive myositis patients