Avitus of Vienne demands that all bishops should attend the councils personally.
However, if by chance, God forbid, someone is struck by such harsh corporal illness, that the spiritual desire would be conquered by carnal detriment, he should present to the brothers two presbyters in his stead of long and commendable life supported with the instruction of their mandate. It is proper to choose those, whom not only their knowledge, but also their reverence predispose to take part in the council of bishops; with whom it would be pleasing to the supreme pontiffs to have a conversation. They should be chosen with skill, as the authority of the law will stand behind them when they consent and subscribe in their bishop's stead. Nevertheless, it should not come to such a thing unless there is an utmost necessity.
(trans. J. Szafranowski)