(a) An award of an impairment income benefit, whether by the commissioner or a court, must be based on an impairment rating determined using the impairment rating guidelines described by this section.
(b) For determining the existence and degree of an employee’s impairment, the division shall use “Guides to the Evaluation of Permanent Impairment,” third edition, second printing, dated February 1989, published by the American Medical Association.

(c) Notwithstanding Subsection (b), the commissioner by rule may adopt the fourth edition of the “Guides to the Evaluation of Permanent Impairment,” published by the American Medical Association, or a subsequent edition of those guides, for determining the existence and degree of an employee’s impairment.