Recent examination reveals no knee jerk?

I have already been diagnosed with peripheral neuropathy and use a cane. I also sometimes stutter and have problems speaking. My condition has worsened since a fall some years ago. Today my new doctor is urging me to be checked for brain damage. Why do they suspect this? I have already had two surgeries, lumbar and ulnar nerve. I am very reluctant to have further tests because I have had to pay for them but she thinks I should. Why? I thought the neuropathy caused me to walk awkwardly and I thought spasms caused my speech problem(not always). It takes me awhile to pay for these things so want to know more before I give in to further tests.