As mentioned earlier, the Superior Court recently clarified this whole issue in Barrick v. Holy Spirit Hospitals, 2011 PA Superior 251 (Docket No. 1856 MDA 2009, Nov. 23, 2011).
First, you have the right to make opposing counsel identify their expert witnesses and to discover everything those experts are using as the basis of expert testimony. Barrick, 2011 PA Superior 251, citing Pa.R.C.P. 4003.5(a)(1). If you want further information from the expert — such as correspondence between the expert and his client that is not the basis of an expert opinion — this is considered “further discovery” that requires you to show cause and get a court order. Barrick, supra, citing .” Cooper v. Schoffstall, 905 A.2d 482, 492 (Pa. 2006) and Pa.R.C.P. 4003.5(a)(2).
In other words, the expert correspondence is not sacred. But it is treated just like attorney work product, where you have to show that you have good cause for thinking that the correspondence will be required for the case.
OK, I hope this is helpful, or more helpful than my earlier statement that it’s not discoverable. Back to work!