A release validly extinguishes a person's claim for additional money damages if both sides intended the release to be final and to encompass injuries known and unknown at the time of the signing.
In limited situations, a release may be avoided if the injured person can establish that the release was executed by mutual mistake. A mutual mistake may occur when both sides believe that there is no injury and then an injury later develops. However, if the release covered all injuries, those known and unknown, mutual mistake will not be established.
A release can also be cancelled if the release was obtained by fraud in the inducement. For example, if the insurance company misrepresented that the release was final or stated that it would continue to pay medical bills, this may constitute fraud and void the release.
If you have any doubt as to the effect of a legal document that an insurance company is asking you to sign, you should go to an attorney and have the document reviewed.