What causes progressive supranuclear palsy?
Progressive supranuclear palsy is a disorder characterized by symptoms similar to Parkinson's disease (including unsteady gait, stiff movements, and mild dementia). Other similar disorders include dysfunction of the cerebellum, multiple system atrophy, and senile dementia (Alzheimer's type). The cause of the damage to the brain cells in this condition is unknown, but the disease is degenerative (gets worse over time). Progressive supranuclear palsy involves
damage to multiple cells of the brain, with loss of the myelin sheath (the covering of the nerve cell that speeds nerve impulse conduction) in some nerves and destruction of the entire nerve in other areas. People with this condition have deposits in brain tissues that resemble the deposits found in Alzheimer's disease. There is atrophy (loss of tissue) in most areas of the brain. The disorder is most often seen in people over 60 years old, and is somewhat more common in men.
Progressive supranuclear palsy affects the brainstem, the basal ganglia, and the cerebellum. The brainstem is located at the top of the spinal cord. It controls the most basic functions needed for survival--the involuntary (unwilled) movements such as breathing, blood pressure, and heart rate. The brainstem has three parts: the medulla oblongata, the pons, and the midbrain. The parts affected by PSP are the pons, which controls facial nerves and the muscles that turn the eye outward, and the midbrain, the visual center. The basal ganglia are islands of nerve cells located deep within the brain. They are involved in the initiation of voluntary (willed) movement and control of emotion. Damage to the basal ganglia causes muscle stiffness (spasticity) and tremors. The cerebellum is located at the base of the skull. It controls balance and muscle coordination.
Vision is controlled by groups of cells called nuclei in the brainstem. In progressive supranuclear palsy, the nuclei continue to function, but the mechanisms that control the nuclei are destroyed. The term supranuclear means that the damage is done above (supra) the nuclei. Patients with PSP have difficulty with voluntary (willed) eye movement. At first, the difficulty only occurs in trying to look down. As the disease progresses, ability to move the eyes right and left is also affected. However, reflex or unwilled eye movements remain normal. Thus, when the patient's head is tilted upwards, the eyes move to look down. These reflex movements remain normal until late in the course of the disease. The upper eyelids may be pulled back, the eyebrows raised, and the brow wrinkled, causing a typical wide-eyed stare. Rate of blinking may decrease from the normal 20-30 per minute to three to five per minute. It becomes difficult to walk downstairs, to maintain eye contact during conversation, or to move the eyes up and down to read.