To assess whether multiple asymptomatic spinal cord lesions, in the absence of brain lesions, increase the risk of conversion to MS compared to a single lesion.

Spinal cord involvement is frequent, and up to 50% of patients presenting with myelitis eventually develop MS. The presence of multiple spinal cord lesions has been suggested as an additional risk factor for conversion. However, their prognostic significance in the absence of brain lesions remains uncertain.

A retrospective cohort study was conducted at a referral neurological center in Argentina between 2008 and 2024. Patients with a first episode of acute or subacute myelitis, without brain lesions at baseline MRI were included. Exclusion criteria were longitudinally extensive lesions, positive anti-AQP4 or anti-MOG antibodies, and alternative diagnoses. Clinical and paraclinical variables were analyzed. Statistical analysis included parametric and nonparametric tests, as well as Cox regression models to estimate the risk of conversion to MS.

We evaluated 112 patients, of whom 37 (33%) converted to MS during a median follow-up of 32.6 months. The presence of multiple asymptomatic spinal cord lesions was associated with a higher risk of conversion to MS (HR=2.04; 95%CI:1–3.9; p=0.032), even after adjusting for age (HR=0.976; 95%CI:0.94–1; p=0.143), female sex (HR=2.6; 95%CI:1.1–6.15; p=0.029), and the presence of gadolinium-enhancing lesions (HR=2; 95%CI:0.92–4.33; p=0.077). Patients who converted to MS more frequently had cervical lesions (70% vs. 44%; p=0.019) and were significantly younger at onset (median 34.8 vs. 42 years; p=0.02).

Multiple asymptomatic spinal cord lesions are associated with a higher risk of conversion to MS, supporting their value as a radiological marker of risk. Their identification may facilitate earlier diagnosis and timely initiation of disease-modifying therapies. Prospective studies are warranted to validate their inclusion in future diagnostic criteria.