Is there a good exercise that helps syringomyelia?

No answer possible: What sort of exercises you can do, whether they will help or make things worse, will be determined by where the cyst is, how badly it impinges on the cord, how large it is, etc.. You need to discuss this with your neurosurgeon. You can not get a good answer from someone who is not familiar with you and your specific condition.

Perhaps...: While there is no exercise that specifically addresses syringomyelia, keeping yourself in good health is helpful. You probably want to avoid most exercises that increase cerebrospinal pressure. I would recommend yoga and general aerobic conditioning. Yoda in particular can be great for strength, flexibility, conditioning and mental centering. Just follow with your personal doctors.