Multiple sclerosis (MS) is classically described as:
an immune-mediated inflammatory disease that attacks myelinated axons in the central nervous system (Luzzio, n.d.).
Since MS only involves the CNS and
cranial nerves I and II are part of the CNS, and the rest are considered part of the PNS (Hagan, et al. 2012)
it makes sense for MS to be associated with optic neuritis and other visual conditions.
Why then is MS also associated with trigeminal neuralgia (Fallata, et al. 2017), which involves CN V and is not part of the CNS?
Fallata, A., Salter, A., Tyry, T., Cutter, G. R., & Marrie, R. A. (2017). Trigeminal Neuralgia Commonly Precedes the Diagnosis of Multiple Sclerosis. International journal of MS care, 19(5), 240–246. https://doi.org/10.7224/1537-2073.2016-065
Hagan, C. E., Bolon, B., & Keene, C. D. (2012). Nervous system. In Comparative Anatomy and Histology (pp. 339-394). Academic Press. https://doi.org/10.1016/C2009-0-61166-1
Luzzio, C. (n.d.) Multiple Sclerosis (Updated 2020). Medscape https://emedicine.medscape.com/article/1146199-overview