Let me- ask a bit...
Are you in Facebook? Typically- there are these expat groups, non locals chatting, asking questions..Merida likely has its own brigade...Anybody know a good doctor who is interested in (don't get too specific)....neurologist or autoimmune or something. One thing down there- if you could locate a bit more cash- you might be able to get some detailed blood work or testing done for way less than up here.
We did find a doctor (in oaxaca, sorry now forgetting)...a woman who was some type of autoimmune expert...and I had considered going to her next time..let me ask about that again..