What is the size cutoff of Multiple Sclerosis (MS) lesions?

From the Guidelines

The size cutoff for multiple sclerosis (MS) lesions is typically 3mm in diameter, as lesions smaller than 3mm are generally not considered significant for MS diagnosis, while those 3mm or larger are counted as potential MS lesions when evaluating MRI findings. This size threshold helps distinguish true demyelinating MS plaques from non-specific white matter changes that can occur due to aging, vascular disease, or other conditions. The 3mm cutoff is part of the diagnostic criteria used for MS diagnosis, which combines clinical and radiological findings, as noted in the study by 1. When radiologists evaluate brain and spinal cord MRIs for MS, they specifically look for these characteristic lesions that are at least 3mm in size, particularly in periventricular, juxtacortical, infratentorial, and spinal cord locations.

Some key points to consider when evaluating MS lesions include:

• Lesions should be visible on at least two consecutive slices to exclude artefacts or small hyperintensities, as mentioned in the study by 1.
• The pattern, location, and evolution of lesions over time, along with clinical symptoms, are crucial for accurate MS diagnosis, as highlighted in the study by 1.
• Lesions in the spinal cord are often small, covering less than two vertebral segments and usually less than half of the cord area, as noted in the study by 1.
• Cortical lesions are a distinctive feature of MS and facilitate identification of patients with clinically isolated syndromes who are at higher risk of developing a second clinical attack, as mentioned in the study by 1.

It's essential to note that lesion size alone is not diagnostic, and a comprehensive evaluation of MRI findings, clinical symptoms, and other factors is necessary for accurate MS diagnosis, as emphasized in the study by 1.

From the Research

MS Lesion Size Cutoff

The size cutoff of MS lesions is a crucial factor in evaluating treatment effects and diagnosing multiple sclerosis.

• Studies have shown that lesions smaller than 10 mm2 (equivalent diameter <3.5 mm) make up nearly 20% of all lesions, but their relative contribution to the total lesion load is relatively small, varying from 0.0-5.7% (mean=1.1%, median=0.65%) in individual patients 2.
• The size distribution of lesions was analyzed in 15 patients with secondary progressive multiple sclerosis (SPMS) and 13 with relapsing-remitting multiple sclerosis (RRMS), and the results suggest that it is prudent to identify and measure small lesions in evaluating treatment effects 2.
• A study published in 2019 found that the WML size threshold, which discriminated best between patients and control subjects, was estimated with receiver operating characteristic curve analysis to be around 3 mm in diameter 3.
• This threshold is consistent with the diagnostic criteria for MS, which stipulate a minimum detectable diameter of 3 mm per WML, although this threshold is not evidence-based 3.

Lesion Size Threshold

• The WML size threshold of 3 mm in diameter seems to be a reasonable choice for three-dimensional MRI sequences at 3 T, based on the analysis of two cohorts of patients with RRMS and control subjects 3.
• The odds ratios increased continuously with increasing WML volumes, and discriminative power was highest at a WML size threshold corresponding to a diameter of about 3 mm 3.
• The study suggests that using a WML size threshold of 3 mm in diameter could help to support the diagnosis of MS and monitor its course 3.

Implications for MS Diagnosis and Treatment

• The size cutoff of MS lesions has implications for MS diagnosis and treatment, as it can affect the evaluation of treatment effects and the diagnosis of MS 2, 3.
• Further studies are needed to validate and standardize the assessment of WML size thresholds and their role in MS diagnosis and treatment 3, 4.