Multiple sclerosis (MS) commonly attacks people in their 20s or 30s and progresses at intervals and at varying rates. It involves patchy loss of myelin with hardening (sclerosis) of tissue in the CNS. The symptoms include vision problems, tingling or numbness in the arms and legs, urinary incontinence, tremor, and stiff gait. MS is thought to be an autoimmune disorder, but the exact cause is not known.

Parkinson disease occurs when, for unknown reasons, certain neurons in the midbrain fail to secrete the neurotransmitter dopamine. This leads to tremors, muscle rigidity, flexion at the joints, akinesia (loss of movement), and emotional problems. Parkinson disease is treated with daily administration of the drug L-dopa (levodopa), a form of dopamine that can be carried by the blood into the brain.

Alzheimer disease (AD) results from unexplained degeneration of neurons and atrophy of the cerebral cortex. These changes cause progressive loss of recent memory, confusion, and mood changes. Dangers associated with AD are injury, infection, malnutrition, and aspiration of food or fluids into the lungs. Originally called presenile dementia and used only to describe cases in patients about 50 years of age, the term is now applied to these same changes when they occur in elderly patients. AD is diagnosed by CT or MRI scans and confirmed at autopsy. Histologic (tissue) studies show deposits of a substance called amyloid in the tissues. The disease may be hereditary. People with Down syndrome commonly develop AD after age 40, indicating that AD is associated with abnormality on chromosome 21, the same chromosome that is involved in Down syndrome.

Multi-infarct dementia (MID) resembles AD in that it is a progressive cognitive impairment associated with loss of memory, loss of judgment, aphasia, altered motor and sensory function, repetitive behavior, and loss of social skills. The disorder is caused by multiple small strokes that interrupt blood flow to brain tissue and deprive areas of oxygen.

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