Nathan Phillips’s review published on Letterboxd :
Alternately harrowing and mildly silly chronicle of the disorienting, often diabolical treatment endured by a woman (Olivia de Havilland) after she's committed for reasons she finds obscure. Using Mary Jane Ward's semi-memoir as an inspiration, Litvak's stroke of genius here is to drop us in the deep end with de Havilland (who's of course very capable of occupying every dark corner of this characterization, one of the most incredibly versatile of all Hollywood actors) without explanation; we piece the past together slowly along with her, so our identification is powerful. His surreal, almost horror-like interpretations of medical treatment and attendant fantasies and nightmares makes The Exorcist look dumber yet, and the scenes that place her with other institutionalized women are remarkably modern and hard-hitting in their realism -- far more convincingly chaotic than Cuckoo's Nest, for example. Of course it's all quite dated, like so many of the Fox social problem movies of the era, with Freudian therapy shoehorned in and unabashedly celebrated with all the awkwardness of Hitchcock-Selznick's Spellbound, but at its best moments it's a singular, shocking exploration of one of the darkest human fears, of totally losing touch with one's identity.
The title is stylized as the snake pit in the film (and it's not a font choice; all other words in the credits use caps) but even Maltin, who usually gets this right, fails to adjust it accordingly.