Supreme Court Justice Potter Stewart's memorable quote “I cannot define it, but I know it when I see it” was initially used in reference to obscenity; however, it is equally applicable to identifying treatment failure in multiple sclerosis (MS).
Defining treatment failure in MS is notoriously difficult. The available disease-modifying therapies are only partially effective in suppressing disease activity, and the disease itself can go into periods of remission spontaneously. The thresholds used for defining treatment failure, or “suboptimal response” as it is often called, vary considerably between clinicians. Some clinicians ignore worsening magnetic resonance imaging (MRI) activity in the context of a clinically “stable” patient with no new symptoms of a relapse. Some would argue that a single new lesion is evidence of disease activity and therefore a suboptimal response. In the middle ground, there is much uncertainty and few data about how much disease activity should be tolerated before deciding that partially effective therapies are failing.
Giacomini PS, Arnold DL, Bar-Or A, Antel JP. Defining Multiple Sclerosis Treatment Response With Magnetic Resonance ImagingHow Much Activity Is Too Much?. Arch Neurol. 2009;66(1):19-20. doi:10.1001/archneurol.2008.529