How do they know

If a Neurologist says to a patient that they have sustained demylination damage, how does he or she know, as that can only be assertained as a definite by autopsy, but the language a Neuro uses is always not 100% er, either using terms like i.e. probably, highly likely, it appears, etc etc always not 100%, thats very clever at covering themselves by leaving room for error, from a legal point of view, thats good, very good.