Design: Systematic review and meta-analysis
Population: 20 studies, 4,786 patients with multiple sclerosis
Primary outcome: Proportion of T2 lesions that are slowly expanding; proportion of patients with at least one
Effect: 14% of lesions (10-21%); 78% of patients (67-85%)

Slowly expanding lesions are a sign of chronic active inflammation, seen on serial conventional MRI as lesions that grow steadily outward. This meta-analysis pooled 20 studies with 4,786 patients.

Slowly expanding lesions made up 14% of all T2 lesions (95% CI 10% to 21%), but 78% of patients had at least one (67% to 85%). Only 11% overlapped with paramagnetic rim lesions, a different marker of chronic activity seen on susceptibility-weighted imaging. The proportion of patients with one was higher in progressive disease. Heterogeneity between studies was high.

These lesions are linked to progression independent of relapses, which current drugs address poorly. For now they are a research and trial tool, and detection methods are not standardised.

Expect chronic active lesions in most patients, including those with relapsing disease.
Recognise that slowly expanding and rim lesions capture different, only partly overlapping, activity.
Keep serial MRI protocols consistent so that lesion change can be compared.
Look for disability progression between relapses, which these lesions may reflect.

Why it matters

It suggests smouldering disease is present in most patients, not only those labelled progressive.

The statistics, in plain English

High heterogeneity means the studies gave quite different answers, likely because they detected lesions in different ways. The pooled figures are averages across varied methods rather than a single precise estimate.