Which symptoms of Marfan syndrome can be treated with cannabis?

Not aware of any. I do not believe that marijuana is really medicine, no matter what a number of states have decided. The only proven effects of cannabis (which comes in a pill, by the way) are in increasing appetite, reducing nausea and (maybe) reducing intraocular pressure (glaucoma). Every other use - for arthritis pain etc. - is unproven. Medicine is not smoked, nor is it impure or has unknown amount of drug.