See this article from Law.com:
"In a move that has irked plaintiffs attorneys, the American Academy of Neurology recently revamped its 16-year-old guidelines regarding expert witness testimony by neurologists. The new guidelines [PDF] were formally adopted earlier this year, but will not go into effect until Jan. 10, 2006.
"The AAN maintains that the guidelines, which call for tougher expert credentials and warn against doctors advocating for lawyers, are a response to several complaints by physicians about unqualified witnesses.
"But plaintiffs lawyers allege that the new rules are scare tactics designed to strip credentials from plaintiffs' experts and keep doctors from testifying in medical malpractice cases."