Beyond The Obvious: Does The Failure To Rule Out Idiopathic Causes Survive A Rule 702 Challenge?

In forming an admissible opinion about whether exposure to or use of a defendant's product caused a plaintiff's disease or condition, the plaintiff's expert must factor into their differential diagnosis the role played by potential alternative causes. Yet in many instances, plaintiffs' experts do not rule out unknown or unexplained causes—also known as idiopathic causes—even when idiopathy is the most likely explanation for the disease or condition. In the Zostavax MDL in the Eastern District of Pennsylvania, the court in two cases addressed whether a proper differential diagnosis must include idiopathic causes and excluded the plaintiffs' specific causation experts under Federal Rule of Evidence 702 based in part on their failure to rule out idiopathy as a potential cause of the plaintiffs' alleged injury.