People need to be understood as wholes, as their brains are only part of them. The brain mediates cognition, emotions and behaviour but it is not the brain that perceives, thinks, feels and acts. It is people as a whole that do that (see eg. yet another previous post).
Brain disease or abnormality can cause mental disorder. Such organic mental disorders can be due to a primary brain disorder or secondary to a systemic illness, or result from an exogenous toxic agent, or be due to physical withdrawal of an addictive substance. But not all mental disorder is due to brain disease or abnormality. Mental disorder shows through the brain but not necessarily in it.
Brain disease can be detected when assessing for mental disorder by disturbances of sensorium and cognitive functioning (see eg. previous post). In particular, level of consciousness, orientation, attention and memory are affected. Jaspers in his General Psychopathology referred to the "state of consciousness" as the "momentary whole" of the "psychic state". Brain disease may be detected by its effects on consciousness and that sense of wholeness.
Depression and schizophrenia, as examples of functional mental disorders, need to be understood in meaningful context. But the meaning of dementia, as an example of an organic mental disorder, could be said to be more to do with brain abnormality. It's people that become demented rather than their brains, as such, but the brain abnormality of dementia, whether Alzheimer’s or whatever, affects the wholeness of people's experience of themselves.