I have skimmed this paper about solitary confinement and the consequences seem disastrous
"The paradigmatic psychiatric disturbance was an agitated
confusional state which, in more severe cases, had the characteristics
of a florid delirium, characterized by severe confusional, paranoid,
and hallucinatory features, and also by intense agitation and random,
impulsive, often self-directed violence. Such disturbances were often observed in individuals who had no prior history of any mental
illness. In addition, solitary confinement often resulted in severe
exacerbation of a previously existing mental condition. Even among
inmates who did not develop overt psychiatric illness as a result of
solitary confinement, such confinement almost inevitably imposed
significant psychological pain during the period of isolated
confinement and often significantly impaired the inmate’s capacity to
adapt successfully to the broader prison environment. "
A statement from the US supreme court regarding the matter (in the same paper)
"[E]xperience [with the penitentiary system of solitary
confinement] demonstrated that there were serious objections
to it. A considerable number of the prisoners fell, after even a
short confinement, into a semi-fatuous condition, from which it
was next to impossible to arouse them, and others became
violently insane; others, still, committed suicide; while those
who stood the ordeal better were not generally reformed, and
in most cases did not recover sufficient mental activity to be of
any subsequent service to the community."
http://openscholarship.wustl.edu/cgi/viewcontent.cgi?article=1362&context=law_journal_law_policy&sei-redir=1&referer=https%3A%2F%2Fscholar.google.ch%2Fscholar%3Fhl%3Den%26q%3Dsolitary%2Bconfinement%26btnG%3D%26as_sdt%3D1%252C5%26as_sdtp%3D#search=%22solitary%20confinement%22
