Causes of Resting Tremors in Parkinson's Disease

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Resting tremors are among the most noticeable features of Parkinson’s disease (PD). The tremors are believed to be caused by complex interactions between a number of factors. Alterations in the activity of several areas of the brain including the substantia nigra, the basal ganglia, and the thalamus, as well as changes in the level and action of the neurotransmitter dopamine, are all related to each other and to the production of the tremors.

Characteristics of PD Tremors

The tremors of PD characteristically occur at rest, stop with voluntary action, and recur again after you hold your new position for a few minutes. PD tremors can affect the hands, arms, face, jaw, legs, and/or feet, and are often slightly more prominent on one side than the other.

The tremor almost always begins in the hand before affecting other parts of the body, and it usually looks like you are rolling a pill between your thumb and index finger. That’s why it’s called a pill-rolling tremor.

A pill-rolling tremor is the most common type of PD tremor, but shaking tremors—which may involve the hands or other areas of the body—can also occur.

Why They Occur

Research studies have identified a number of defects in the brain associated with the tremor of PD. The interactions between these structures are complex, and even though we have a great deal of information about the anatomical and biochemical causes of PD tremors, there are many unanswered questions.

One of the fundamental causes of Parkinson’s disease is a decrease in dopamine, an important neurotransmitter, in the areas of the brain that support movements. The substantia nigra, a region of the brainstem that produces dopamine, does not function as it should in PD, resulting in low levels of active dopamine. Studies have shown that the fall in overall dopamine levels begins years before the onset of PD symptoms.

This affects various aspects of movement:

  • Voluntary movement: The globus pallidus is the part of the basal ganglia that helps regulate voluntary movement, such as holding a cup of tea. It normally receives and responds to dopamine, and, when there is altered dopamine production due to PD, the globus pallidus does not function as it should.
  • Complex movement: The thalamus and the subthalamic nucleus in the brain are primarily involved in sensation. They receive information about your body's position from several regions in the brain, including the globus pallidus. The brain uses this sensory feedback from the thalamus and subthalamic nucleus to control complex movements, such as stirring sugar in that cup of tea. When the levels of dopamine reach a critically low level, the thalamus loses its normal regulatory input.
  • Coordinated movement: The cerebellum controls coordination, while the motor portion of your cerebral cortex controls voluntary movement. Both of these areas receive information about your body's position from the thalamus and then work together to carry out coordinated physical movements. These final steps are disrupted when the basal ganglia are at rest, which is why the tremor is prominent during rest and not during an action.

    When the brain cannot get accurate sensory feedback about how well movements are proceeding, it can no longer effectively adjust physical movements. In PD, the most complex movements of the body, which involve the fingers and hands, are the first and most severely affected.

    More Than Just Dopamine

    While it may seem that the whole problem of tremors in PD is completely caused by deficient dopamine production in the substantia nigra, that is not the case. There are several reasons that we know there is more to a resting tremor than just a dopamine deficiency.

    • The most effective treatment for symptoms of PD are medications that increase dopamine or prolong the action of dopamine in the brain. Even when dopamine is adequately replaced, a person with advanced PD may still experience tremors.
    • The regions of the brain that are involved in PD, including the thalamus, globus pallidus, cerebral cortex, and the cerebellum, often show structural and metabolic deficits in PD, suggesting that deficits in these structures are involved in causing the condition.
    • Surgical treatment that is effective for PD symptoms is targeted toward a number of regions in the brain, including the globus pallidus and subthalamic nucleus.

    A Word From Verywell

    PD is characterized by a number of symptoms, including muscle stiffness, rigidity, trouble walking, and tremors. Interestingly, some studies suggest that the presence of a resting tremor in the early stages of PD is associated with milder disease and better response to therapy, although the reasons for this are not clear.

    Treatments for PD include medications that increase and improve dopamine action, as well as surgical interventions. If you have PD, you should know that there is a great deal of research and progress being made in discovering the causes and developing new treatment options to help manage the symptoms of your condition.

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