20 CFR § 404.1579
(1) The rules for determining whether disability continues for widow's or widower's monthly benefits for months after December 1990 are discussed in §§ 404.1594 through 404.1598. The rules for determining whether disability continues for monthly benefits for months prior to January 1991 are discussed in paragraph (a)(2) of this section and paragraphs (b) through (h) of this section.
(2) If you are entitled to disability benefits as a disabled widow, widower, or surviving divorced spouse, and we must decide whether your disability continued or ended for monthly benefits for months prior to January 1991, there are a number of factors we consider in deciding whether your disability continued. We must determine if there has been any medical improvement in your impairment(s) and, if so, whether this medical improvement is related to your ability to work. If your impairment(s) has not so medically improved, we must address whether one or more exceptions applies. If medical improvement related to your ability to work has not occurred and no exception applies, your benefits will continue. Even where medical improvement related to your ability to work has occurred or an exception applies, in most cases (see paragraph (e) of this section for exceptions) before we can find that you are no longer disabled, we must also show that your impairment(s), as shown by current medical evidence, is no longer deemed, under appendix 1 of this subpart, sufficient to preclude you from engaging in gainful activity.