The first thing to determine is whether someone exhibiting pathological tremor has Parkinson’s or Essential Tremor (ET). ET is much more common than Parkinson’s and in the United Kingdom, for example, it is estimated that in a population of 60 million, up to two million people will exhibit ET, whereas the number with Parkinson’s will be in the region of 80 - 120,000. However, it is not unusual for people with ET to be diagnosed as having Parkinson’s.
ET presents usually as a bilateral (i.e. both sides) tremor of hand and forearm and can occur at any age, though it usually manifests itself in the mid-teens and between 50 and 65. About 50% of those with ET will have a family history of the condition so it is considered to be dominantly inherited. The use of the word ‘essential’ implies that there is no known cause.
Parkinson’s tremor, on the other hand, is initially unilateral (i.e. one sided), not generally inherited, and the tremors are considered to be symptomatic, meaning they reflect known changes in the nervous system.
ET has to be distinguished from Parkinson’s, both for prognostic purposes and because the treatments of Parkinson’s are not effective for people with ET. Careful clinical assessment usually allows a confident diagnostic separation between the two. ET is primarily distinguished from Parkinson’s for the following reasons:
- ET is virtually always a postural or kinetic tremor coming on simultaneously in both upper limbs
- it is not accompanied by bradykinesia (slowness of movement), generalised rigidity or the gait disturbances that are commonly found in Parkinson’s
- head tremor is found in more than 30% of people with ET, but is extremely rare in Parkinson’s. (But if shaking in Parkinson’s becomes violent, the head can manifest a secondary tremor as a passive consequence of the movements)
- when walking, a tremor of an upper limb caused by Parkinson’s invariably results in the loss of normal arm swing. This does not occur in ET.
Despite these differences, it is sometimes very difficult to correctly diagnose. For example, cogwheel rigidity - the interruption of passive movement of a body part around a joint - is a feature of both ET and Parkinson’s. There have also been occasions when some doctors have diagnosed the postural tremor found in Parkinson’s as ET.
Furthermore, as ET is relatively common, the two conditions can of course occur in the same individual. But whilst some studies have shown a higher incidence of ET existing in families that have a history of Parkinson’s, the development of ET at an early age does not predispose someone to Parkinson’s in later life.
Diagnosis
Assessment by a specialist in the field of movement disorders, usually a neurologist, is essential in diagnosing both Parkinson’s and ET. Accurate diagnosis can only be made after considering both personal history and family history and undergoing careful clinical appraisal. Various disorders of the nervous system can produce tremors, and usually the diagnosis is reached by considering the associated symptoms and signs that occur with the tremor.
It is not usually necessary to carry out extensive investigations; normally a simple blood screen will be all that is required. There may, however, be occasions when further tests are deemed appropriate and sometimes brain scanning, or a dopa scan (DaTSCAN) may be necessary. Generally, though, it is argued that unless there are clinical indications, routine brain scanning is not required in either Parkinson’s and/or ET.
It is worth reiterating that tremulous disorders can be difficult to diagnose, and that anyone with tremor should be reviewed by an expert at least once. This is the recommendation incorporated into the World Charter on Parkinson’s, and is also the strong recommendation of the National Tremor Foundation (UK).
