Establishing Minimal Clinically Important Difference in Sleep Outcomes after Spinal Cord Stimulation in Patients with Chronic Pain Disorders.

INTRODUCTION: As one of the most common medical conditions for which patients seek medical care, chronic pain can be debilitating. The relationship between chronic pain and sleep is thought to be bidirectional, suggesting that treatment of one can be beneficial to the other. There is mounting evidence that spinal cord stimulation (SCS) improves aspects of sleep. How meaningful that is to patients' lives has not been ascertained.

OBJECTIVE: The aim of the current study was to further elucidate the effect of SCS on sleep by examining the relationship between pain outcome measures with the insomnia severity index (ISI) and to establish the minimally clinical important difference (MCID), which is defined as the smallest noticeable change that an individual perceives as clinically significant.

MATERIALS AND METHODS: We prospectively collected ISI, Epworth sleepiness scale (ESS), Numerical Rating Scale, McGill Pain Questionnaire-Short Form, Oswestry Disability Index, Beck Depression Inventory, and Pain Catastrophizing Scale data both pre- and postoperatively for chronic pain patients who underwent SCS placement and had long-term outcomes. The ISI is a well-studied questionnaire used to assess an individual's level of insomnia.

RESULTS: We correlated the ESS and ISI with pain outcome measures in sixty-four patients at a mean follow-up of 9.8 ± 2.9 months. The ISI showed correlations with disability as measured through the Oswestry Disability Index (p = 0.014) and depression as measured through the Beck Depression Inventory (p = 0.024). MCID values for the ISI were calculated using both anchor- and distribution-based methods. The minimal detectable change method resulted in an MCID of 2.4 points, standard error of measurement resulted in an MCID of 2.6 points, and the change difference resulted in an MCID of 2.45. The receiver operating characteristic method yielded an MCID of 0.5-point change with an area under the curve of 0.61.

CONCLUSION: This study successfully established MCID ranges for the ISI outcome measure to help gauge improvement in insomnia after SCS. The ISI has ample evidence of its validity in assessment of insomnia, and MCID values of 2.4-2.6 correlate with improvement in disability and depression in our patients.