Section 42-9-35 - Evidence of preexisting injury or condition(A) The employee shall establish by a preponderance of the evidence, including medical evidence, that:(1) the subsequent injury aggravated the preexisting condition or permanent physical impairment; or(2) the preexisting condition or the permanent physical impairment aggravates the subsequent injury.(B) The commission may award compensation benefits to an employee who has a permanent physical impairment or preexisting condition and who incurs a subsequent disability from an injury arising out of and in the course of his employment for the resulting disability of the permanent physical impairment or preexisting condition and the subsequent injury. However, if the subsequent injury is limited to a single body part or member scheduled in Section 42-9-30, except for total disability to the back as provided in Section 42-9-30(21), the subsequent injury must impair or affect another body part or system in order to obtain benefits in addition to those provided for in Section 42-9-30.(C) As used in this section, "medical evidence" means expert opinion or testimony stated to a reasonable degree of medical certainty, documents, records, or other material that is offered by a licensed health care provider.(D) The provisions of this section apply whether or not the employer knows of the preexisting permanent disability.(E) On and after the effective date of this section, an employee who suffers a subsequent injury which affects a single body part or member injury set forth in Section 42-9-30 is limited to the recovery set forth in that section.2007 Act No. 111, Pt I, Section 19, eff 7/1/2007, applicable to injuries that occur on or after that date.