Determining minimally important change thresholds for the Seizure Severity Questionnaire (SSQ)

The Seizure Severity Questionnaire (SSQ) was developed to evaluate changes in seizure severity and bothersomeness. Determination of a threshold value reflecting meaningful patient benefit on the SSQ would improve clinical interpretation of scale results. The objective of this analysis was to define a minimally important change (MIC) threshold for the SSQ, using data from patients with treatment-resistant partial-onset seizures from two clinical trials (N=776). Minimally important change thresholds were calculated using standard anchor-based methods, with the Patient Global Impression of Change (PGIC) score as the anchor with the categories of 'much improved,' 'minimally improved,' 'much worsened,' and 'minimally worsened' combined. The calculated MIC thresholds (range: 0.34 to 0.50) suggest that a 0.48-point change in the SSQ total score reflects a clinically meaningful change in seizure severity from the patients' perspective.