It all flows from the patient-doctor relationship. The essence of practising medicine is understanding your patient's condition, seeing it through your patient's eyes, and physicians make determinations of decisional capacity or competence every day. The backbone of the Carter decision supports that, that this is the foundational brick upon which our system is built.
This is new territory. Whereas making a determination of competence to achieve an informed consent is fundamental to medicine, I would anticipate that physicians in this situation will often get another opinion, as we always do. Ours is a collaborative profession. When faced with complex, novel questions like the one you describe, it's second nature for physicians to get collaborative opinions, whether that be from a psychiatrist or, in certain settings, as I think was referred to by Chair Oliphant's question, from ethics boards and health authorities. It's natural for physicians to seek comfort with opinions from others.