Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with a doctor.
Without the ability to examine and obtain a history, I can not tell you what the exact cause of the symptoms is. However I will try to provide you with some useful information.
I can certainly understand that you are worried given the extension of the syrinx. This question would be better addressed on the neurosurgery forum since neurosurgeons have more experience with these.
From what you have told me, there does not seem to be any significant clinical deterioration associated with this extension of the syrinx. Has his limping and numbness gotten progressively worse in the 2 years? If not, then this may be the reason they are not doing a surgical intervention.
The surgical interventions are quite invasive, options are decompressive surgery or even a shunt. However, these are associated with significant risks themselves like bleeding, infection, CSF leak, paralysis, etc.
I would advise maintaining a followup with your current doctors or seeking a second opinion with another neurosurgeon if you are very worried about clinical deterioration
Thank you for this opportunity to answer your questions, I hope you find the information I have provided useful, good luck
My daughter is going through same thing. She has had no treatment as of yet, but has MRI on yearly basis, but she has developed kyphosis which is a forward bend in the spine and I have since been told this condition goes hand in hand with their diagnosis. X