Transporter psychosis was an incurable medical condition. It was caused by a breakdown of neurochemicals during transport, affecting the body's motor functions, autonomic systems and the higher reasoning centers of the brain.
A victim of transporter psychosis suffered from paranoid delusions, multi-infarct dementia, hallucinations (somatic, tactile, and visual), and psychogenic hysteria. Peripheral symptoms included sleeplessness, accelerated heart rate, diminished eyesight leading to acute myopia, painful spasms in the extremities, and, in most cases, dehydration. (TNG: "Realm of Fear")
While cases of transporter psychosis were reported as early as the mid 22nd century, an official diagnosis didn't arrive until 2209 on Delinia II. Further cases were prevented by the perfection of the multiplex pattern buffer in the 2310s. (ENT: "Daedalus", TNG: "Realm of Fear")
In 2369, Lieutenant Reginald Barclay believed that he was suffering from transporter psychosis after he was infected by quasi-energy microbes while transporting from the disabled USS Yosemite to the USS Enterprise. This was significant as there had not been a fully diagnosed case in almost sixty years. (TNG: "Realm of Fear")