Here's a case report and brief review.
Barney Marriott used to cite these red flags for the condition:
Abrupt, unexplaind (non-physiologic) lengthening in the PR interval.
Apparent “Mobitz II” AV block but with a normal conducted QRS (but intrahisian AV block can cause this too).
Manifest junctional extrasystoles seen elsewhere.
Electrophysiology at the bedside. Fascinating stuff.