Idiopathic Inflammatory Myopathy: A Hospital-based Case Review in the State of Perak, Malaysia.

Background: Idiopathic inflammatory myopathy (IIM) is a rare condition characterized by proximal muscle weakness and myositis. It includes dermatomyositis (DM), polymyositis (PM), inclusion body myositis (IBM), and juvenile dermatomyositis (JDM). They may share clinical and histological features but differ in underlying pathophysiology. Objective: To assess the diagnostic probabilities, and clinical characteristics of IIM. Methods: Clinical data from 50 IIM patients were retrospectively reviewed from Rheumatology and Dermatology clinics across four centres in Perak, Malaysia (2013–2023) and were evaluated using the 2017 The European League Against Rheumatism/American College of Rheumatology (EULAR/ACR) classification criteria for adult and juvenile IIM. Results: According to the EULAR/ACR criteria, 98.2% of cases were classified as definite IIM (mean score 10.6), 74.1% as probable (mean score 6.6), and 47.8% as possible (mean score 6.4). IIM was excluded in 36% of patients (mean score 3.9; probability 20.8%). Eighty-four percent had DM, 8% had ADM or PM, with a mean age of 45.9±18.1 years. 95.6% were diagnosed within a year of symptom onset, and 72% showed proximal myopathy, significantly common in DM (P=0.001). Ethnic breakdown showed 46% Chinese, 42% Malay, and 8% Indian patients, with Malays developing DM at a younger age (39.8±16.9 years). The shawl sign and telangiectasia were more common in Chinese (P=0.018) and Malay (P=0.023) patients, respectively. Conclusion: The EULAR/ACR scoring system effectively identified IIM subgroups. Despite varying probabilities, cases of both DM and ADM were classified into definite, probable, and possible categories, with or without biopsy data. DM was more prevalent than ADM, primarily affecting Chinese patients, while Malays had earlier onset.