In Parkinson’s tremor arises, directly or indirectly, as a result of degeneration of the nigrostriatal dopaminergic pathway. This damage to the dopamine system produces a loss of inhibition in certain brain structures, allowing parts of the nervous system to fire rhythmically and thus set up a pacemaker for tremor. Such rhythmical impulses can be measured, for instance, in the thalamus (a part of the brain which relays information).
About 70% of people with Parkinson’s have tremor when first diagnosed, with rest tremor and action tremor being equally common.
Rest tremor in the hand is often described as ‘pill rolling’, as noted by the famous physician, Sir William Osler: “the tremor is usually marked in the hands, and the thumb and forefinger display motion made in the act of rolling a pill” . Rest tremor often affects legs and may also be seen in the lips and tongue. Head tremor, which is relatively common in Essential Tremor [see Diagnosis ], is extremely rare in Parkinson’s.
Postural tremor is also very common and is often seen when the hand is held in a particular position and during movement. As it manifests during voluntary activities, for example holding a cup or writing, it is usually more disabling or intrusive in its effects on function.
Many people with Parkinson’s will exhibit two separable tremors: a lower frequency rest tremor and a higher frequency postural action tremor. Special scientific instruments can detect this ‘double tremor’ and, in 1981, Findley and Gresty put forward this unusual pattern of tremors as a diagnostic marker for Parkinson’s 1.
In the Consensus Statement of the Movement Disorder Society on Tremor 2, four types of Parkinsonian tremor are described:
- type I – classical Parkinsonian tremor, that is a rest tremor and postural/kinetic tremor at the same frequency, i.e. the rest tremor in this group persisting as a postural tremor
- type II - rest, postural and kinetic tremors at different frequency
- type III - pure postural kinetic tremor. This is seen particularly where there is rigidity. This tremor is very similar to the more common ET (See also: Diagnosis)
- type IV - the monosymptomatic rest tremor, that is a pure tremor at rest which reduces with movement.
Due to the fact that almost any tremor phenomenon can be seen in someone with Parkinson’s, it can be difficult for doctors to distinguish between Parkinson’s tremors and tremors arising from other conditions.
How is tremor measured?
A tremor is measured by the frequency of its beats per second. Physiological or normal tremor usually measures between eight and 11 cycles per second. Rest tremor in Parkinson’s usually measures between 4.5 and 5.5 cycles per second. Pathological tremors, compared with physiological tremors, usually have a very fixed frequency that does not vary, even over long periods of time.
The severity of tremor is further measured in terms of its amplitude or size. Pathological or abnormal tremors are invariably of greater amplitude than physiological tremors. The size of the tremor will vary from time to time, and all tremors increase in amplitude with stress and anxiety. For example, those with Parkinson’s rest tremors will appreciate that their tremor can go from virtually zero amplitude to a very large tremor in a matter of seconds in certain social, or other stimulating situations.
