ITM is considered a distinct, homogeneous subgroup of inflammatory TM of unclear etiology. Prior studies have variably estimated ITM incidence between 1 and 6.2/million but they precede AQP4-IgG and MOG-IgG discovery.

To determine the epidemiology and characteristics of idiopathic transverse myelitis (ITM) in the era of aquaporin-4 (AQP4)-IgG and myelin-oligodendrocyte-glycoprotein (MOG)-IgG disease-associated TM.

We performed a population-based study of ITM incidence (2003-2016), prevalence (December 31, 2016) and characteristics in Olmsted County, Minnesota. ITM was defined by 2002 TM consortium diagnostic criteria. Serum was available for testing AQP4-IgG and MOG-IgG in 28/30 (93%) and 26/30 (87%) patients initially meeting ITM criteria: 4 incident (17%: AQP4-IgG, 2; MOG-IgG, 2) and 3 prevalent (16%; AQP4-IgG, 2; MOG-IgG, 1) ITM patients were consequently excluded and reclassified as disease-associated TM. All had TM characteristics consistent with AQP4-/MOG-IgG associated disorders including: LETM, 4/4 (100%); CSF pleocytosis, 4/4 (100%); and absence of oligoclonal bands, 3/4 (75%).

The ITM incidence was 9.5/1,000,000 and prevalence was 10.4/100,000. Median age of ITM onset was 40.5 years (range, 9-72) and 16 (62%) were female. Median EDSS at nadir was 2.5 (range, 0-8). Cerebrospinal fluid (CSF) findings included elevated white blood cell count, 7/22 (32%); and positive oligoclonal bands, 4/19 (21%). MRI spine revealed LETM in 4/25 (16%), gadolinium enhancement in 20/25 (80%) and ≥2 spinal cord lesions in 6/25 (24%). Five ITM patients (19%) converted to MS during a median follow-up of 53 months (range, 0.5-400), three with subsequent relapses and 2 with development of brain MRI lesions.

ITM incidence and prevalence were modestly reduced by the availability of AQP4-IgG and MOG-IgG testing, but remained higher than prior reports.  Future updates of ITM diagnostic criteria are needed to account for AQP4-IgG and MOG-IgG discovery and for the clinical and MRI heterogeneity which characterize ITM cases.