Hi,
Please go for MRI of brain and spine, blood work, and spinal tap or lumbar puncture. Besides nerve compression in the back, neuromuscular diseases like Multiple sclerosis and ALS may be suspected to be the cause of your symptoms. Please discuss these possibilities with your neurologist and take a second opinion after the test results. Hope this helps you. Take care and regards!
Why haven't they done an MRI of your cervical spine? Have you had a cardiac work-up?