Mental ilness, causes and apaya realms

Mental illness is not uncommon in modern society, from mild to severe illness. Depression, (generalized) anxiety disorder, panic disorder and schizophrenia, to name a few. Do people with (long-term) mental illness are "destined" to be reborn in one of lower realms or they're seen just illnesses rather than unwholesome states of mind? What is the cause of mental illness from Dhamma point of view?