Oedematous myositis: a clinical presentation first suggesting dermatomyositis diagnosis.
AIMS: Oedema of the limbs is uncommon in idiopathic inflammatory myopathies (IIM). The few reported cases have been associated with severe and refractory dermatomyositis (DM), sometimes in association with cancers. We aimed to determine if oedematous myositis is a homogeneous subtype based on clinical, serological and pathological features. METHODS: This is a retrospective observational study performed between 2008 and 2015 in the French national referral center for myositis. All adult patients with an inflammatory muscle biopsy and upper limbs oedema were included as well as IIM cases without limb oedema as controls. Clinical, biological and pathological features were collected. RESULTS: Seventeen oedematous myositis were included and compared to 174 IIM without oedema, including 50 DM controls. Oedema was the first manifestation in 23% of patients. Muscle weakness was severe and symmetric, 71% of patients presented dysphagia and a restrictive ventilatory pattern was found in 40%. Fifty-two percent of patients had a typical DM skin rash and 23% had cancer within 3 years of diagnosing myositis. Fifty-three percent of patients presented a myositis specific antibody and only DM-specific antibodies were detected. Classic pathological DM features (perifascicular atrophy, perifascicular/perimysial perivascular inflammation) were uncommon but capillary C5b-9 deposition and MxA expression were seen in 79% and 73% of cases, respectively. A perimysial oedema was found in 82% of cases. Seventeen percent of patients died (median follow up of 18 months). Oedematous myositis demonstrated more marked capillary C5b-9 deposition compared to IIM controls. There was no clinical, biological or pathological difference with DM controls except for limb oedema. CONCLUSION: Our study underlines that limb oedema could be a symptom of IIM and that oedematous myositis are mostly DM. The vasculopathy seems to play a key role in its pathophysiology. Limb oedema associated to muscle impairment should suggest the diagnosis of DM in clinical settings.