Only infrequently does arthritis lead to total incapacitation.
Only after entering the store did Arthur realize that there was danger.
What's happening here is that the subjects of the sentences ("arthritis" and "Arthur") and the auxiliary verb "do" are in reverse order to the way they would normally appear. It's called subject-auxiliary inversion.
This occurs in declarative clauses only when certain types of element are put in front position. Negatives are one very obvious type of element that trigger subject-auxiliary inversion when fronted:
Never had I seen such chaos.
At no stage were they in danger.
"Only" is not negative, but it is semantically close to a negative, in that Only John liked it, for example, entails No one other than John liked it. The inversion is also found with some items that are not similar to negatives:
John enjoyed it and so did Robert.