A one-year follow-up study of the Symbol Digit Modalities Test (SDMT) and the Paced Auditory Serial Addition Test (PASAT) in relapsing-remitting multiple sclerosis: an appraisal of comparative longitudinal sensitivity

Background: Neuropsychological batteries are infrequently used to assess cognitive impairment in multiple sclerosis because they are time-consuming and require trained personnel. The Symbol Digit Modalities Test (SDMT) is suggested to be a useful screening tool to measure cognitive impairment in multiple sclerosis patients and is more valid and reliable over time than the Paced Auditory Serial Addition Test (PASAT). The purpose of this study was to evaluate which of these tests was more sensitive to cognitive impairment at one-year follow-up. Methods: A total of 237 patients with relapsing-remitting multiple sclerosis and 57 healthy controls underwent a complete neuropsychological assessment. One year later, we assessed 196 patients using the Brief Repeatable Battery of Neuropsychological Tests. We also administered other executive function and prospective memory tests, together with fatigue and depression questionnaires. Results: A total of 33.8% of patients were classified as cognitively impaired. The SDMT and the PASAT 3 seconds test (PASAT3) had a sensitivity of 0.809 and 0.783, respectively, thereby classifying patients as cognitively impaired. Analysis of 196 patients one year later showed 31.6% had cognitive impairment compared with 27.6% at the first assessment. The sensitivity to detect cognitive impairment after one year was 0.824 for SDMT and 0.796 for PASAT3. When the predictors were removed from the comparative standard battery, SDMT still showed a slightly higher sensitivity. Both SDMT and PASAT3 correlated significantly with all tests, but SDMT showed higher correlation values. Furthermore, SDMT was completed by all subjects while PASAT3 was completed by 86.9% of patients and 94.7% of controls. Conclusions: SDMT is simpler to administer than PASAT3 and may be slightly more sensitive to MS cognitive impairment. It could thus be a suitable test to assess cognitive impairment routinely in people with relapsing-remitting multiple sclerosis.