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There are no nerves in the brain—yes nerves originate from it (cranial nerves) but brain is essentially made up of grey and white matter.
Any pathology of brain, its extent etc can be detected by MRI. The extent of damage that is caused is assessed by the size of the lesion and its location as different parts of the brain perform different functions.
Apart form this some brain pathologies are detected by examination of cerebro-spinal fluid through lumbar puncture. EEG is done is epilepsy is suspected.
Progression of a disease depends on type, cause and stage of diagnosis. A neurologist determines that based on clinical and laboratory evaluation.
Hope this helps. Take care!
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