cricket,

They can give a clinical diagnosis in someone who has no lesions on MRI. I have been told that I fulfill the clinical criteria for definite MS.

A clinical diagnosis can be established if their is evidence for lesions disseminated in space and time. They tend to rely heavilly on things that aren't fakeable. They worry way too much about people faking symptoms.

My eyes, for example, show clinical evidence of lesions disseminated in space and time. I developed optic disc pallor a number of years after I developed an eye-traction problem (bilateral INO). The optic disc pallor indicates an optic nerve lesion. The eye traction indicates a brainstem lesion. So, there are lesions in two different locations, and they happened at different times.

There is a lot of caution out there against giving a definite diagnosis to someone in whom all of the tests have been negative. It's really up to the doctor what tests they need to show positive for a definite diagnosis if the person has shown clinical evidence of lesions disseminated in space and time.