Many people with Parkinson’s disease ask ‘why do I have a tremor?’
Tremor is one of the three primary features of Parkinson’s disease (along with rigidity and slowness of movement). But why do the limbs begin to tremble with this condition?
‘Study of Right Hand of Erasmus of Rotterdam and Portrait Study’ by Hans Holbein the Younger (1498–1543). Source: Oxford University
Different types of tremor
Parkinson’s disease is actually a combination of different types of tremors. And those tremors can vary according to different circumstances (resting or during movement), different body parts involved (hands, feet, head, etc), and different frequency of the tremor (from low (4–5 times per second) to high (8–10 times per second)).
There are many different types of tremor but only a few of them apply to Parkinson’s disease. In 1998, the Movement Disorder Society published a classification scheme that categorizes three types of tremor associated with Parkinson’s disease:
Title: Consensus statement of the Movement Disorder Society on Tremor. Ad Hoc Scientific Committee.
Authors: Deuschl G, Bain P, Brin M.
Journal: Mov Disord 1998;13 Suppl 3:2-23.
Those three types of tremor are:
1. A resting tremor with the same frequency – This tremor is the most common and it is usually inhibited during movement.
2. A resting tremor of different frequencies – this form of tremor occurs in less than 10% of people with Parkinson’s disease.
3. A postural tremor with a frequency varying between 4 and 9 times per second. These types of tremor are rare in the overall Parkinson’s population, but are common in the very rigid version of Parkinson’s disease.
Tremor is an independent feature of Parkinson’s disease
Tremor is considered a separate aspect of Parkinson’s disease from the rigidity and slowness of movement, as tremor does not progress at the same rate as these other features, or gait and balance problems. Tremor can also occur on the opposite side of the body to the side affected more by the slowness of movement and rigidity. This occurs in approximately 5% of people with Parkinson’s disease.
So what causes tremor in Parkinson’s disease?
The exact mechanism underlying tremor is the source of much debate, but here is a little bit of what we know:
The brain is causing the tremor:
We know that the central nervous system is responsible for tremor and not problems in the periphery because anaesthesia of tremulous muscles has little effect them.
The role of dopamine in tremor is unclear:
Autopsy studies suggest that people with Parkinson’s disease who are tremor-dominant (meaning that they suffer tremor more than rigidity or slowness of movement) generally have less loss of dopamine neurons in the substantia nigra pars compacta and in the locus coeruleus than people with non-tremor Parkinson’s disease (Click here and here for more on this).
Similarly, brain imaging studies have found that people with tremor-dominant Parkinson’s disease had less dopamine depletion in the striatum (the target region for the dopamine neurons in the substantia nigra) than those with non-tremor Parkinson’s disease. There is a pattern in the research suggesting that resting tremor does not correlate with dopamine depletion.
There is evidence suggesting that serotonin (another chemical in the brain that is affected in Parkinson’s disease) may be involved in resting tremor (Click here for more on this).