Idiopathic inflammatory myopathies (IIM) form part of a heterogeneous group of diseases of autoimmune origin characterised by non-suppurative inflammation of the skeletal muscles, progressive muscle weakness and variable expressivity in different organs and systems.1 IIM are rare, with a mean incidence ranging from 1 to 19 cases/million inhabitants/year,2,3 and vary according to geographical area, methodology and classification criteria. A study on the estimated incidence in Spain between 1997–2004 showed an incidence rate of 8.9 new cases/million inhabitants/year (95% confidence interval [CI] 8.6–9.2), with a higher incidence of dermatomyositis (DM) than polymyositis (PM).4 The aetiology and predisposing factors for developing IIM are still not entirely clear. They are currently considered immune-mediated disorders, possibly triggered by environmental factors in genetically predisposed individuals.5 IIM presents with a wide variety of clinical manifestations; muscle weakness secondary to chronic inflammation of striated muscle is characteristic. Other organs affected include the skin, joints, lung, gastrointestinal tract, and heart. Complications include infections and cancers.6–8 Although the prognosis has improved substantially in recent decades, mortality has increased with respect to the general population.9 Furthermore, at least one third of patients are moderately or severely disabled, mainly due to pulmonary involvement, the development of calcinosis and muscle weakness.10

The identification of specific and associated antibodies, access to imaging techniques such as magnetic resonance imaging and ultrasound, improvements in the interpretation of muscle biopsy and the development of new classification criteria in recent decades have given us a better understanding of this group of diseases.11

However, the heterogeneity of their clinical manifestations, their evolution and prognosis, along with their low prevalence and incidence, make study of these diseases difficult. In Spain, prospective studies are limited, especially those that include all IIM subtypes. No information is available on disease activity, severity, disability, quality of life, treatment management or prognostic factors. In addition, it has not been studied whether the disease behaves differently when comparing patients who meet the new American College of Rheumatology and European League Against Rheumatism (ACR/EULAR) classification criteria at diagnosis with those who meet them during follow-up, or those who never meet them.

Therefore, the Myo-Spain project (Spanish Registry of patients with idiopathic inflammatory myopathy (IIM)) was set up with the main objective of analysing the evolution and management of a cohort of patients with IIM attended in rheumatology departments in Spain. This paper describes the project’s methodology and potential strengths and limitations.

This paper describes the methodology of the Myo-Spain registry. The project represents the creation of a Spanish multicentre cohort of incident and prevalent patients with any type of IIM, with a prospective two-year follow-up.

Several registries have been established at the international level to investigate different aspects of IIM. The multicentre EuroMyositis registry was created in 1999 and currently involves more than 20 countries, primarily in Europe. The registry seeks to obtain uniform longitudinal data on juvenile and adult myositis to gain a better understanding of the course and prognosis of the disease. There is a mandatory data collection area containing basic information present at some time over the duration of the myositis, and another area, which is not mandatory to complete, corresponding to individual visits to monitor the course of the disease (activity and damage, muscle weakness, treatment, antibodies, and biopsy results). The registry has more than 3000 participants and publications have studied associations between clinical subtypes, extramuscular involvement, environmental exposure, antibodies, and drugs14,15. In addition, this cohort together with the UK and Ireland cohort, the Juvenile Dermatomyositis Research Group (JDRG), validated the new EULAR/ACR classification criteria11,16.

The American Myositis Patient Association launched the MYOVISION national registry in 2010. From December 2010 to July 2012, 1806 patients with IIM (PM, DM, IBM) were included in this registry and agreed to complete a paper or online survey with 83 questions on demographics, work or school, leisure activities, environmental exposure, disease exposure and health-related quality of life. Several cross-sectional studies on environmental exposure and quality of life have been published17,18. From the US multicentre Childhood Arthritis and Rheumatology Research Alliance (CARRA) registry, which included patients with childhood rheumatic diseases between 2010 and 2014, cross-sectional studies of the 688-patient juvenile DM cohort have been published on clinical characteristics and the association between exposure to ultraviolet radiation and disease severity19,20. Because of the limitations in the number of variables, in the information on treatments, and in the data collection process that was not systematic at each visit, the new CARRA registry, which was limited to juvenile idiopathic arthritis, was created in 201521.

The American and European NEtwork of Antisynthetase Syndrome (AENEAS) collaborative cohort is part of an international, multicentre, retrospective study, started in 2014, which includes patients with antisynthetase syndrome with at least six months’ follow-up. The publications in this cohort have focused on the form of presentation of the disease, progression, outcomes, and its specific behaviour, according to the presence of the different anti-synthetase antibodies or specific manifestations such as arthritis22–24.

We should highlight three initiatives at the national level. The first was internal medicine’s registry of patients with anti-synthetase syndrome of the Systemic Autoimmune Diseases Study Group (GEAS), which provided information on manifestations and long-term outcomes in patients with anti-jo1 antibodies25. The second was the registry of patients with inflammatory myopathy of the Madrid Rheumatology Society (REMICAM), with the participation of 12 rheumatology units of the Community of Madrid and retrospective collection of clinical and mortality data for patients under follow-up at some point between 1980 and December 2014. This registry has provided information on mortality, long-term pulmonary outcomes, overlap syndromes with myositis and juvenile forms26–29. Finally, the national registry of patients with MDA5 syndrome (MEDRA5), a multicentre and multidisciplinary registry that aims to understand the natural history of MDA5 syndrome, and has collected clinical data on these patients retrospectively until December 2019.

As a strength of the project, and unlike most of the abovementioned registries, Myo-Spain will carry out a prospective protocolised follow-up with annual visits of all patients. This was initially planned for two years, but is expected to be longer, given that IIM are infrequent and heterogeneous diseases and greater follow-up is required to determine their progression and management (the main objective of the project). All registry variables have been standardised, the data will be strictly monitored and patients who do not reach 80% of the required variables will be excluded from analysis. Although data from follow-up visits can also be collected prospectively in the EuroMyositis registry, this is not mandatory, and the incidence of new manifestations over time may be underestimated. In addition, in the early years of the registry, not all current subtypes of IIM, such as IMNM, had been established and some myositis-specific antibodies had not been discovered, and therefore a percentage of patients may be incorrectly classified. Another strength of Myo-Spain is that from the outset it was designed with two cohorts, one with incident patients (<12 months from diagnosis) and another with prevalent patients (>12 months from diagnosis), in which all patients must be alive and under active follow-up to be included. As this is a rare disease, we decided on an extended recruitment period, increasing the likelihood of including incident cases. At the same time, we aimed to determine disease activity, damage, disability, muscle strength and quality of life using validated indices and questionnaires to assess disease status and look for associations in a more objective way. Furthermore, by including all the subtypes of IIM, we will be able to make comparisons and identify differences in our population. We will also be able to study the diagnostic usefulness of the new classification criteria in this cohort of the future classification criteria for anti-synthetase syndrome and to determine the differences between patients with complete and incomplete forms at onset of the disease.

However, our registry also has a few limitations. First, it was not designed to be representative of the geographical distribution of IIM at the national level. This cohort does not include patients from all autonomous communities; to be specific, there are no data from centres in Aragón, Asturias, Cantabria, Castilla La Mancha, La Rioja, and Navarra. Another possible limitation is patient selection bias, including patients who may have an easier follow-up or who have fewer comorbidities. To avoid this bias, the need to recruit all patients who meet the inclusion criteria per centre was considered, which also becomes a necessity due to the prevalence and incidence data of IIM. Recruitment of all patients meeting the inclusion criteria has been reinforced through regular communication with participating investigators and at meetings at the start of the study and follow-up. Although this is a project of the Spanish Society of Rheumatology and the principal investigators at each centre are rheumatologists, to minimise possible recruitment bias, it was opened for the participation of collaborating investigators from other specialties such as internal medicine and neurology.

The Myo-Spain study represents a cohort of patients with any type of IIM recruited in rheumatology units throughout Spain. This is the first prospective study of these characteristics that the SER has promoted in a rare disease. The cohort will include a group of incident patients and a group of prevalent patients, which will allow detailed assessment of the clinical characteristics of the disease at different times. It is expected that the comprehensive information collected at the visits will provide a valuable source of data for future analysis and research.

Myo-Spain is funded by Kern Pharma (Spain), Nordic Pharma (Spain), Sandoz (Spain) and Bristol-Myers Squibb (Spain). None of these companies were involved in the study design, data collection or analysis, or in drafting this article.

The authors have no conflict of interests to declare.