Synesthesia May be Prevalent in Multiple Sclerosis

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Background:
Synesthesia is a neurological condition in which information in one sensory domain stimulates another unrelated sensory domain. The etiology of synesthesia remains unclear. Both synesthesia and MS may share common features, including a possible etiology of immune system dysfunction, similar structural abnormalities in brain white matter tracts, and altered 5-HT activity. Due to these commonalities, it may be that synesthesia is a marker for subsequent development of MS or that brain alterations in MS may encourage synesthetic experiences as the disease progresses.
Objectives:
To examine the relationship between synesthesia and MS.
Methods:
After approval from the local IRB, individuals with self-reported clinically definite MS were recruited via the internet and social media, from November, 2019 to December, 2020. Advertisements were posted on social media sites relevant to MS for participation with a link included to a Qualtrics survey that asked demographic information and included The Patient Determined Disease Steps (Hohol, Orav, & Weiner, 1995), a survey adapted from Jonas and Hibbard (2015) and Eagleman et al. (2007) to explain synesthesia, and other questionnaires. Only participants who completed all questions, and reported initial diagnosis with Relapsing Remitting (RRMS) or Primary Progressive (PPMS) were included (N=144).
Results:
Of the 144 participants, 135 reported a diagnosis of RRMS (117 women) and 9 reported PPMS (5 women). Nine (64.29%) of those self-reporting synesthesia said they had it for as long as they could remember or since childhood. One (7.14%) reported it began in the last 3-5 years. Synesthesia was reported in 9.72% of individuals with MS (14/144), with approximately equal percentages in women (9.84%) and men (9.09%). Additionally, disease course was similar in the synesthetic population (RRMS: 92.86%; PPMS: 7.14%) as in the sample as a whole (RRMS: 93.75%; PPMS: 6.25%).
Conclusions:
Brang and Ramachandran (2008) found that between 2% – 4% of the general population experiences some form of synesthesia. Here, we find individuals with MS to be ~3 times more likely to report synesthesia than the general population. We also found that synesthesia in MS does not appear to differentially affect men vs. women, nor those with RRMS vs. PPMS. Our results suggest further work examining the relationship between synesthesia and MS, with a larger sample, would be valuable