Yes, you should absolutely disclose your pre-existing injuries to your attorney, and you should do so as soon as possible. The existence of a pre-existing injury does not mean that you will receive no compensation, and hiding any previous injuries from your attorney will only harm your case later on.
When an attorney can operate with full information of your situation, they will be entirely prepared to counter any tactics brought forward by the insurance carriers and legal teams for the at-fault party. First, your attorney will work diligently to ensure that the other party does not gain access to your complete medical history. All the other side needs is current medical records related to the current incident.
Even if a pre-existing injury is aggravated by a current incident caused by another party, you should still be able to recover compensation because the “aggravation” of the injury would not have occurred had it not been for their actions.