Zhian'tek Syndrome is a rare condition in which the synaptic connections between a Trill host and symbiont become disrupted through the transporter process. About one in 10,000 joined Trills suffer from this condition.
When a joined host-symbiont pair is put through a transporter beam, the condition causes the isoboramine levels in the host to become abnormally lowered, which causes an involuntary separation between host and symbiont. This result, unless the effect can be reversed, is inevitably fatal for the host and on occasion the symbiont as well. (Starship Independence)
Captain Breawyn Lem of the USS Independence was known to have this condition. She was almost killed when she underwent an emergency transport in mid-2383, causing the synaptic bonds between host and symbiont to be separated. These connections were restored through an experimental genomic regeneration technique developed by the ship's chief medical officer, Doctor Orlando Sturges, and the captain made a full recovery. (Starship Independence: "War's Embers")