How do you treat radiation myelopathy?

The short answer is "I don't really know what the optimal management is for radiation myelopathy" (fortunately I don't have much experience with managing this.)  Since it is very rare, I would first suggest that you be clear about the diagnosis.  It is a diagnosis of exclusion, but beware that other equally rare entities can mimic radiation myelopathy.  For example, a patient that we thought might be developing myelopathy actually ended up having an intramedullary metastasis.  Another case suspected of myelopathy ended up having a spinal cord stroke outside the radiation portals but exhibited cord signal change at the level of radiation as well.  Another case involved an intramedullary hemorrhage outside of the radiation field after excessive anti coagulation.  We have seen a few cases of cord signal change on T2 weighted MRI that have developed at the site of radiation that have resolved spontaneiously.  We have had two cases of myelopathy after spine radiosurgery in over 900 treated lesions at MSKCC.  These events are too few for meaningful evaluation.  Although there is no evidence for it, I think it would be resonable to consider bevacizumab and even hyperbaric oxygen.  There is a small randomized trial that suggests that bevacizumab is helpful in resolving brain radionecrosis.  Hyperbaric oxygen has been shown to be helpful in soft tissue radionecrosis such as rectal bleeding after prostate RT.   

It is so rare that I have not seen one case in my long career in radiation oncology. You would need a consult with a neurologist and an MRI. Maybe hyperbaric oxygen can help.