Incidence, Etiology, and Long-Term Outcome of Acute Myelitis in Stockholm County, Sweden

BACKGROUND AND OBJECTIVES: Myelitis is a relatively common clinical entity for neurologists, with diverse underlying causes. The aim of this study was to describe the incidence of myelitis, its causes, clinical presentation, and factors predicting functional outcomes and relapses. METHODS: Using the Swedish National Patient Registry, we identified all adult patients in Stockholm County between 2008 and 2018 using International Classification of Diseases, 10th Edition (ICD-10) codes likely to include myelitis. We collected medical records and classified patients using a modification of the 2002 Transverse Myelitis Consortium Group criteria. Long-term follow-up data were collected for patients not diagnosed with multiple sclerosis (MS) or neuromyelitis optica spectrum disorder as a result of the initial myelitis. RESULTS: We identified 2,321 individuals, of whom 461 were patients with myelitis. The crude mean incidence of all-cause myelitis was 24.9 (95% CI 16.7-33.9) cases per million person-years, of which idiopathic myelitis had an incidence of 8.0 (95% CI 3.8-12.1) cases per million person-years. Partial myelitis was found in 80% of patients. Poor functional outcome was found in 11% of the cohort and correlated, in a multivariate logistic model, with age older than 50 years (OR 4.26, 95% CI 1.75-10.40), transverse spinal cord lesions (odds ratio OR 6.85, 95% CI 2.68-17.52), elevated CSF count of polymorphonuclear cells (OR 6.09, 95% CI 1.56-23.72), and elevated CSF/serum albumin ratio (OR 3.17, 95% CI 1.23-8.17). The median follow-up time was 5.4 years. Relapses occurred in 27% of patients with idiopathic myelitis and 72% of patients with unspecified demyelinating disease of the CNS. An increased relapse rate after idiopathic myelitis was found to be associated, in a multivariate model, with the presence of oligoclonal bands (incidence rate ratio IRR 4.47, 95% CI 1.70-11.73), transverse spinal cord lesions (IRR 2.81, 95% CI 1.11-7.12), and multifocal spinal cord lesions (IRR 2.82, 95% CI 1.03-7.69). Around half (48%) of all patients with myelitis received MS diagnosis during the study period. DISCUSSION: This large population-wide study describes a relatively high incidence of myelitis and low risk of relapses after idiopathic myelitis. A complete diagnostic workup of myelitis, including MRI of the entire CNS and collection of CSF, is essential in evaluating underlying causes and prognosis.