Eliot wrote Part V of The Waste Land while undergoing treatment at Lausanne (see more on this at the commentary for line 261). Relatively little editing was done on Part V.
Eliot's widow mentions in a note to the "The Facsimile" (p. 129) that Eliot was describing his own experience at Lausanne when in his 1931 essay "The Pensées of Pascal" he wrote of a mystical experience that Blaise Pascal had prior to writing his Lettres écrites à un provincial:
We know quite well that he was at the time when he received his illumination from God in extremely poor health; but it is a commonplace that some forms of illness are extremely favourable, not only to religious illumination, but to artistic and literary composition. A piece of writing meditated, apparently without progress, for months or years, may suddenly take shape and word; and in this state long passages may be produced which require little or no retouch. I have no good word to say for the cultivation of automatic writing as the model of literary composition; I doubt whether these moments can be cultivated by the writer; but he to whom this happens assuredly has the sensation of being a vehicle rather than a maker. ... You make call it communion with the Divine, or you may call it a temporary crystallization of the mind.
Source: Selected Essays (p. 358)