There may be an electrical shock when the unit fires. The degree of pain may be associated with the individual's perception, anxiety, etc. If the same pain has been ongoing for 7 years it is difficult to believe there is any infection. If the pain has increased (electical shocks) the unit may need an evaluation to insure that it is firing appropriately. You may want your husband to see a doctor to rule out any defects, etc.
Thanks for sharing, Ken