From the Guidelines
Dermatomyositis diagnosis is typically based on a combination of clinical, laboratory, and diagnostic test findings, with the most recent and highest quality study suggesting the use of the 2017 European League Against Rheumatism/American College of Rheumatology (EULAR/ACR) classification criteria 1. The main criteria for diagnosing dermatomyositis include:
- Symmetric proximal muscle weakness
- Characteristic skin rashes, including:
- Gottron's papules (over knuckles, elbows, or knees)
- Heliotrope rash (purplish discoloration around eyes)
- Elevated muscle enzymes (e.g., creatine kinase, aldolase)
- Abnormal electromyography (EMG) showing myopathic changes
- Muscle biopsy showing inflammatory changes According to the EULAR/ACR criteria, a diagnosis of dermatomyositis can be made with a high probability based on a combination of these clinical and laboratory features, with a typical DM skin rash contributing significantly to the probability score 1. It's essential to note that the EULAR/ACR criteria have been validated and tested for sensitivity and specificity, demonstrating high accuracy in classifying patients with dermatomyositis, including those with overlap diagnoses such as mixed connective tissue disease or systemic lupus erythematosus with myositis 1. The use of the EULAR/ACR classification criteria is recommended for diagnosing dermatomyositis, as they offer advantages over previous criteria, including high sensitivity and specificity, and can be applied to patients with overlap diagnoses 1. In clinical practice, the diagnosis of dermatomyositis should be based on a comprehensive evaluation of the patient's symptoms, laboratory results, and diagnostic test findings, with the EULAR/ACR criteria providing a useful framework for classification and diagnosis 1.
From the Research
Diagnostic Criteria for Dermatomyositis
The diagnostic criteria for Dermatomyositis (DM) include:
- Characteristic skin abnormalities, such as Gottron papules and heliotrope rash 2
- Perifascicular atrophy on muscle biopsy 2, 3
- Presence of myositis-specific antibodies, such as anti-Mi-2, anti-MJ, and anti-p155 2
- Muscle weakness, particularly proximal muscle weakness 4, 3
- Histological findings, including vacuolar interface alterations, epidermal atrophy, and sparse perivascular lymphocytic infiltrate 5
Clinical Subsets of Dermatomyositis
There are two distinct subsets of DM:
- Pure DM: characterized by a dominant DM rash, high cutaneous score, and chronicity, with a positive predictive value (PPV) of 91% for concurrent heliotrope rash and Gottron papules 2
- Overlap myositis with DM features (OMDM): characterized by a low cutaneous extent score, transient DM rash, and presence of overlap autoantibodies, with a PPV of 100% for adermatopathic DM 2
Importance of Muscle Histology
Muscle histology is important for the diagnosis of DM, with perifascicular atrophy being a key feature 3 However, relying solely on perifascicular atrophy may underestimate the true frequency of DM 3 Other histological features, such as perivascular/endomysial inflammation, can also be present in DM 3
Autoantibodies and Their Associated Phenotypes
Myositis-specific antibodies, such as anti-Mi-2, anti-MJ, and anti-p155, are associated with pure DM 2 Overlap autoantibodies, such as anti-Jo-1, anti-PL-7, anti-PM-Scl, anti-U1RNP, and/or anti-U5-RNP, are associated with OMDM 2