1 Answers
Current standard for diagnosing multiple sclerosis is based in the 2018 revision of McDonald criteria. They rely in MRI detection of demyelinating lesions in the CNS, which are distributed in space and in time. It is also a requirement that any possible known disease that produces demyelinating lesions is ruled out before applying McDonald's criteria.
This last requirement makes MS an ill-defined entity, whose borders change every time that a new disease is set apart. Some previous cases considered MS are now considered apart, like Neuromyelitis optica or antiMOG associated encephalomyelitis. Besides, because of the requirement of distributed lesions, a single lesion is not considered MS. For the same reason, the prodromal stage of MS would not be considered as MS if it could be found.
Sometimes the diagnosis must be retrospective, relying on gradual worsening of neurological signs/symptoms, due to the lack of understanding of the pathogenicity driving disease progression. However, the only definite diagnosis of MS is post-mortem autopsy, where lesions typical of MS can be detected through histopathological techniques.