What are the requirements of an expert witness in a medical malpractice case?

and provide, among other things, "[a] brief narrative statement of the general substance of the testimony the expert is expected to give." (Code Civ. Proc., 2034.260, subd. (c)(2); see Bonds v. Roy (1999) 20 Cal.4th 140, 148.) If a party wishes to expand the scope of an expert's testimony beyond those topics identified in the declaration, it must successfully move for leave to amend its expert witness declaration. (Code Civ. Proc., 2034.610, subd. (a)(2); see Bonds, at p. 145.) Otherwise, expert opinion offered by any party who has "unreasonably failed" to comply with these requirements "shall be excluded." (Code Civ. Proc., 2034.300; see Bonds, at pp. 146-147.)