Before the 25th Amendment, the rules surrounding presidential succession existed on a case-by-case basis.
When Woodrow Wilson had a stroke in 1919, then-Vice President Thomas Marshall did not assume presidential powers. In one case, Grover Cleveland had surgery for oral cancer, and his vice president was never informed.
It wasn't until the assassination of President John F. Kennedy that a process of presidential succession was formalized, enshrined in the 25th Amendment.
The amendment was ratified in 1967 but wasn't used until 1973. It has only been used six times in the 56 years it has existed.
Oddly enough, three times the amendment was used involved colonoscopies.