People with Parkinson’s disease often experience tremor in only one hand initially because the disease typically begins with asymmetric degeneration of specific brain regions that control movement. This uneven damage causes symptoms like tremor to appear on one side of the body before affecting the other side.
Parkinson’s disease is a neurological disorder marked by the progressive loss of dopamine-producing neurons in a part of the brain called the substantia nigra, which is located within the basal ganglia. The basal ganglia play a crucial role in regulating movement, including the smooth initiation and control of voluntary muscle activity. When dopamine levels drop due to neuron loss, the communication between the basal ganglia and other motor areas of the brain becomes disrupted. This disruption leads to the characteristic motor symptoms of Parkinson’s, including tremor, rigidity, bradykinesia (slowness of movement), and postural instability.
The reason tremor often starts in just one hand is that the degeneration of neurons in the substantia nigra and related motor circuits usually begins on one side of the brain. Because each hemisphere of the brain controls movement on the opposite side of the body, damage on one side results in symptoms appearing on the contralateral (opposite) hand or limb. For example, if the left side of the substantia nigra is more affected early on, the right hand will show tremor first.
This asymmetry is a hallmark of Parkinson’s disease and helps distinguish it from other movement disorders that tend to affect both sides of the body more symmetrically from the start. The tremor seen in Parkinson’s is typically a resting tremor, meaning it occurs when the affected hand is at rest and diminishes or disappears with voluntary movement. This “pill-rolling” tremor involves rhythmic, involuntary movements where the thumb and fingers appear to roll a small object.
The exact reason why Parkinson’s disease begins asymmetrically is not fully understood, but it may relate to how the disease process spreads in the brain or individual differences in vulnerability of neurons. Over time, as the disease progresses, the degeneration usually spreads to the other side of the brain, causing tremor and other symptoms to develop in the other hand and limbs as well.
In addition to the localized neuron loss, the basal ganglia’s impaired ability to regulate motor signals leads to abnormal firing patterns and oscillations in motor circuits. These abnormal rhythms contribute to the tremor’s characteristic frequency and pattern. The tremor frequency in Parkinson’s is generally slower (around 4 to 6 Hz) compared to other types of tremor, such as essential tremor.
Other factors that influence why tremor may remain more pronounced in one hand for some time include differences in muscle tone, use-dependent changes, and compensatory mechanisms in the brain and body. For example, the more affected hand may become weaker or stiffer, which can influence how the tremor manifests and how noticeable it is.
In summary, the unilateral tremor in Parkinson’s disease arises because the underlying neurodegeneration starts asymmetrically in the brain’s motor control centers, leading to symptoms on one side of the body first. This asymmetry is a defining feature of the disease’s early stages and reflects the specific pattern of neuronal loss and disrupted motor circuit function.