Preexisting impairments not considered in fixing amount of compensation

Where an employee has a definitely ascertainable impairment resulting from an occupational or a nonoccupational injury, disease or any other cause, whether or not disabling, and the employee thereafter receives an injury in the course of and resulting from his or her employment, unless the subsequent injury results in total permanent disability within the meaning of section one, article three of this chapter, the prior injury, and the effect of the prior injury, and an aggravation, shall not be taken into consideration in fixing the amount of compensation allowed by reason of the subsequent injury. Compensation shall be awarded only in the amount that would have been allowable had the employee not had the preexisting impairment. Nothing in this section requires that the degree of the preexisting impairment be definitely ascertained or rated prior to the injury received in the course of and resulting from the employee's employment or that benefits must have been granted or paid for the preexisting impairment. The degree of the preexisting impairment may be established at any time by competent medical or other evidence. Notwithstanding the foregoing provisions of this section, if the definitely ascertainable preexisting impairment resulted from an injury or disease previously held compensable and the impairment had not been rated, benefits for the impairment shall be payable to the claimant by or charged to the employer in whose employ the injury or disease occurred. The employee shall also receive the difference, if any, in the benefit rate applicable in the more recent claim and the prior claim.