This was something somehow forgoten until OCT appeared. Then it was shown that MS patients have a thinner retina layer than controls, and the retinal thinning seems to be related to the gray matter damage. Therefore it is expected that if retinal layer thinning process is understood, the gray matter damage process will also be understood.
This investigation has been retaken in the article "Retinal glymphatic system: an explanation for transient retinal layer volume changes?" (https://academic.oup.com/brain/article/ ... nation-for), and they explain several findings that I didn't know before. The most striking is that inflammation reflects in the INL layer of the retina, and treatment of MS also reduced the inflammation of the INL
"In patients in whom inflammatory disease activity completely ceased under treatment, a reduction of the INL volume could be detected within 6 months ... In contrast, no changes of the INL volume were observed in those patients in whom inflammatory disease activity could not be controlled. So is INL volume reduction a response marker for successful treatment of inflammation in multiple sclerosis?"
Some people think that all the problems in the eye are due to a retrograde degeneration of the neurons because of the damage in the optic nerve myelin. But this data shows that the two main characteristics of MS, inflammation and neurodegeneration, are present in the retina, and that the effect of any therapy can be therefore visualized.