Clients often ask me how does a pre-existing injury affect a personal injury claim. For example, what happens if a person with a previously herniated disc in their back is then rear-ended in a car accident. There is no simple answer to this problem, and it is imperative that you hire an experienced personal injury attorney to deal with the complications that arise from prior injuries. Do not think that because you have a pre-existing injury that you are not entitled to compensation for the pain caused by a new accident.
The short answer, and the one taught to us in law school, is "You take the plaintiff as you get them." In other words, the defendant is liable for the damages to a plaintiff who has a very bad back, even though the damages will be much higher than if he hit a person in normal health. A defendant can not choose his or her victim. In the real world, however, the insurance companies are going to argue that the injury was not caused by the auto accident or slip and fall, but was pre-existing and not their responsibility. This is where it can get tricky!
First and foremost, the defendant will always be responsible for any exacerbation of the injury. In other words, if you had a bad back but were not treating with an orthopedist and were relatively pain-free, the defendant is responsible if you suddenly require extensive treatment and medication. It is also very important that your accident attorney obtain all of your prior medical records (regardless of how long ago the accident occurred) and compare those records to the current records. The reason this is so important is because the new accident may have worsened the pre-existing condition. The defendant will be liable for the new WORSE condition.
For example, you were in an auto accident in 2006 and suffered muscle strains and a cervical disc impingement. These are serious injuries and may cause long-term pain. Then you are in another auto accident in 2010 that is not your fault. An MRI in 2010 shows that the previously impinged disc is now herniated. That is very serious and the defendant from the 2010 auto accident is responsible for your herniated disc.
Pre-existing injuries can make a personal injury case quite complicated and it a case for which you absolutely must have an experienced personal injury attorney.