If you fail to disclose a pre-existing injury while your case is ongoing, this may or may not make a difference in your overall claim. Failing to disclose the injury to your personal attorney could lead to setbacks later on if the other party discovers the pre-existing injury, but you are under no obligation to tell the other side that you have been injured in the past.
However, if your claim involves seeking compensation for the aggravation of a pre-existing injury, then you will indeed have to disclose the pre-existing injury in order to show the insurance carrier or personal injury jury how the current incident aggravated the pre-existing condition.