The phase II study at University College London involved 86 people with optic neuritis - a symptom associated with MS in which the covering of the optic nerve is damaged. Participants took either phenytoin or placebo for three months. Researchers measured the width of the retina - the layer of nerves at the back of the eyeball - at the start of the trial and after six months. Thinning of the retina is known to indicate damage to nerves elsewhere in the brain and spinal cord. Read on.