Medical testimony typically comes from many different persons, not just medical doctors, during a personal injury trial.
First, there is the injured’s testimony who can best describe the injury, and how it has affected her or him physically and psychologically.
There are also witnesses called “lay medical witnesses” who are family, fellow employees, and friends who have been in regular contact with the injured person and who can describe the physical and mental state they observed before and after the accident.
Of course, the testimony of the treating physician, who typically knows the injured person best, can be expected.
In addition, there are doctors with specialized medical knowledge who can testify to special issues of diagnosis and treatment.
Many injuries are hard to diagnose with even the most modern technology. Traumatic brain injuries, as we are all learning from the soldiers returning from Iraq, are often subtle, but devastating. Subtle in the sense they did not lose consciousness or get a direct blow to the head from the IED, and appears to be walking around “normally.” However, the injuries have proven to be real, permanent and often devastating. It is the same with traumatic brain injuries from car wrecks.
There is almost always a medical expert found by the defense that will attempt to minimize the injury and its impact or say it was pre-existing. In particular, for all people but the young victims of accidents, there is almost always some pre-existing damage to the spine that the accident has aggravated. It then becomes a question for the jury to determine how much, if any, of the injured’s present condition was pre-existing (existing maybe, but perhaps not affecting the daily life of the injured) and how much was aggravated by the accident and how it will impact the injured person’s future life.
In short, the basic question the doctors will address along with the victim and any medical witnesses is what was the injury and will the injured person ever be as good as they were prior to the accident.