(1) In occupational disease or injury claims other than those involving a preexisting mental disorder, work conditions or events constitute the major contributing cause of a pathological worsening of the preexisting condition.
(2) In occupational disease or injury claims involving a preexisting mental disorder, work conditions or events constitute the major contributing cause of an actual worsening of the preexisting condition and not just of its symptoms.
(3) In medical service claims, the medical service is prescribed to treat a change in the preexisting condition as specified in subsection (1) or (2) of this section, and not merely as an incident to the treatment of a compensable injury or occupational disease. [1995 c.332 §3]