Background Clinically amyopathic dermatomyositis (CADM) is a subset of dermatomyositis (DM) presenting with the characteristic DM rash (es) without objective proximal muscle weakness. Asian studies report that anti-MDA5 auto-antibody (autoAb) in CADM is associated with a unique clinical phenotype, which has not been well established in US.
Objectives The goals of our study were to determine the clinical features associated with anti-MDA5 autoAb in CADM as compared to classic DM in US.
Methods CADM patients were selected consecutively from the patients enrolled in the University of Pittsburgh Myositis Database from January 1985 to July 2013 (n=450). CADM was defined a typical DM rash without objective muscle weakness for at least 6 months after rash onset and no or minimal abnormalities of serum muscle enzymes [<3 x ULN], electromyography or muscle biopsy. The classic DM was defined as per probable and definite criteria of Bohan and Peter. DM patients were selected similarly and 1:1 matched (gender and age ±5 years) to CADM patients. The clinical features were assessed using our prospective myositis database, along with electronic medical record review when necessary.
Results We identified 61 CADM patients and 61 matching DM controls. Demographics were similar in both groups. Anti-MDA5 frequency was similar in two cohorts (13.1% [8/61] vs. 13.1% [8/61]). Anti-MDA5 positivity was associated with a higher likelihood of abnormal capillary microscopy, vasculitic rash and digital tip ulceration (18.7% vs. 2.8%) and a lower frequency of Raynaud's as compared to anti-MDA5 negativity. The frequency of DM rashes (Gottron sign or papule, heliotrope, mechanics hands, etc were similar in both groups. However, puffy finger frequency was higher in the anti-MDA5+ patients as compared to anti-MDA5- patients (20% vs. 4.7%). Anti-MDA5 positivity was significantly associated with ILD (p=0.043), RPILD (p<0.001), and poor survival (p=0.007). Multivariate analysis suggested that anti-MDA5 positivity was predictive of survival even after controlling for diagnosis, age at diagnosis, gender, ethnicity, smoking, and ILD (p=0.001). ILD frequency was similar in CADM and DM (31.1% vs. 26.2%) as was RPILD (8.2% vs. 5%). CADM patients were more likely to have puffy fingers (9.8% vs. 4.9%) but less likely to have mechanic's hands (3.2% 9.8%) and heliotrope rash (36.1 vs. 19.7%) than DM patients. Rest of the clinical features were similar for both CADM and DM cohorts.
Conclusions Anti-MDA-5 autoAb is seen in similar frequency in CADM and DM patients in US. Anti-MDA-5 autoAb is associated with a unique clinical phenotype consisting of ILD, RPILD, abnormal nailfold capillary, vasculitic rash, and digital tip ulceration in U.S. patients. CADM patients were similar to DM apart from obvious disease defining features of muscle weakness and muscle enzyme. In addition there was more puffy fingers, less mechanics hands and heliotrope rash in CADM.
Disclosure of Interest None declared