Some proteins, called prions, suffer post-translational modification(s) that change their shape so that they no longer perform their cellular functions and instead trigger equivalent modifications in normal proteins, thus creating a cascade of damage that eventually results in significant neurodegeneration. In humans, this can cause Creutzfeldt-Jakob disease or variant CJD (Mad Cow Disease).
Normally, neurodegeneration begins long before the patient experiences any symptoms. It can be months or years before any effect is felt. Symptoms are noticed when many cells die or cease to function and a part of the brain begins to fail.
These diseases have their own characteristics which usually affect middle aged or older people. They usually worsen over time.
Initial treatment is dependent on diagnosis of underlying disorder. At present there are few therapies for the wide range of neurodegenerative diseases. Treatment with L-dopa can inhibit symptoms of Parkinson's Disease for a short time, but then causes acceleration of the symptoms. Efforts are being made to develop therapies for Alzheimer's Disease that will stabilize cognitive function at the level existing at time of diagnosis and treatment.
Research is underway into Bio-Markers as part of an attempt to understand the progression of certain types of neurodegenerative disease. In theory, if relevant bio-markers were identified, people could be treated for such diseases prior to onset of symptoms, thus resulting in a significant extension of their normal functional lifespan. As yet, however, the science of bio-markers is in its infancy and consequently diagnosis of neurodegenerative disease tends to occur after the patient has already suffered the majority of the neural damages.