Multiple sclerosis (MS) is a chronic inflammatory, demyelinating and neurodegenerative disease of the central nervous system clinically characterized by dissemination of the plaques in time and space. It causes a variety of neurological symptoms, including cranial nerve symptoms, pyramidal tract symptoms, cerebellar symptoms, and cognitive function. It is well-known that MS occurs mainly in young adults. However, in more than half of patients, cognitive decline is observed in the early phase from the onset, even when motor symptoms are minimal (Rocca et al., 2015). Especially, attention deficits and slowed information processing are characteristic and frequent (Chiaravalloti and Deluca, 2008; Rao et al., 1991a).

The brief repeatable battery of neuropsychological test (BRB-N) is widely used in Europe and the United States as a test battery to evaluate cognitive functions in MS patients. BRB-N consists of seven subtests, including the symbol digit modalities test (SDMT), which reflects visual attention and information processing speed, and the paced auditory serial addition test (PASAT), which reflects auditory attention and information processing speed. BRB-N can detect cognitive decline in MS patients with a sensitivity of 71% and a specificity of 94% (Rao et al., 1991b). A study conducted using the Japanese version of the BRB-N reported that MS patients scored significantly lower than healthy controls on all subtests, with the SDMT and the PASAT being particularly useful in assessing cognitive decline in MS (Niino et al., 2014). Another study reported that SDMT is the most sensitive method to assess cognitive decline in MS (Benedict and Zivadinov, 2011).

One of the most important issues in daily life is driving a car. Drivers require advanced cognitive functions such as processing various traffic information on the road and have to pay attention to pedestrians (Michon, 1985). Among neurological conditions, driving issues have been discussed in dementias and movement disorders (Grace et al., 2005). However, an association between cognitive function and driving issues still remains unclear in patients with MS. In a study, accidents and concentration problems during driving were more common in MS patients than in healthy controls (Kotterba et al., 2003). The number of motor vehicle accidents requiring emergency treatment was 3.4 times higher when MS patients drove a car (Lings, 2002). Furthermore, in the other study, comparing MS patients with cognitive decline with MS patients without cognitive decline, the incidence of motor vehicle crashes was higher in the group with cognitive decline (Schultheis et al., 2001). In the real world, 23% of patients diagnosed with MS have quit driving (Ryan et al., 2009).

A driving simulator is a useful tool for objectively and quantitatively evaluating automobile driving ability (Jacobs et al., 2017). However, very few studies have evaluated off-road driving ability using a driving simulator in MS, and almost no studies have examined the relationship between off-road driving ability and cognitive function (Harand et al., 2018). In addition, although some studies have shown a significant correlation between the PASAT and accident rates on driving simulators (Kotterba et al., 2003), the relationship between the SDMT and driving ability has not yet been investigated. The SDMT is reported as an optimal test battery to assess cognitive function in MS patients (Benedict and Zivadinov, 2011; Benedict et al., 2017). Therefore, we aimed to clarify the relationship between the SDMT and driving ability in MS patients in this study.