By Steven Bellows, MD1 and Joseph Jankovic, MD1
Parkinson’s Disease Center and Movement Disorders Clinic, Department of Neurology,
Baylor College of Medicine, Houston Texas (www.jankovic.org)
When people come to their doctor for evaluation of a tremor, two common possible diagnoses are essential tremor and Parkinson’s disease. Essential tremor is a common condition that may start at any age and often runs in families, even though no genetic cause has yet been found. Parkinson’s disease is neurodegenerative disease associated with loss of dopamine neurons in the brain, characterized by not only tremor but also a slowly progressive slowness of movement, stiffness of muscles, gait and balance problems and other, non-motor symptoms including cognitive decline. Patients with essential tremor usually have a predominant “action tremor”, meaning their tremor is most noticeable when they are doing something with their hands, like reaching for an object, holding a plate, drinking from a glass of water, or writing. Tremor in Parkinson’s disease is usually a “rest tremor”, meaning that it happens when an arm or leg is not doing anything or when a patient walks, and typically improves when performing an action. “Positional tremor”, or a tremor that occurs when holding a limb in the air, often starts immediately after raising an arm in patients with essential tremor. In contrast, in patients with Parkinson’s disease this postural tremor may only emerge after waiting several seconds, the so-called “re-emergent tremor”. Patients with essential tremor may have head tremor or tremulous voice, which is almost never present in patients with Parkinson’s disease. Essential tremor patients often have other family members with tremor, and frequently note improvement in their tremor with alcohol. Despite these characteristic differences in clinical presentation the distinction between the two diseases can be difficult in some cases, particularly when there are overlapping features. Thus, people with essential tremor can sometimes have mild parkinsonian features such as slight slowness, stiffness, and even a rest tremor, but these symptoms are not enough to make a diagnosis of Parkinson’s disease. A person with essential tremor is also not immune to Parkinson’s disease, and can develop the condition later in life much like anyone else.
What has been more controversial is whether people with essential tremor are at higher risk of developing Parkinson’s disease.1 Some studies have shown a higher proportion of patients with essential tremor developed Parkinson’s disease than would be expected, ranging from 3.1 to 20.8% of those included in these studies.2–5 In one study, which followed patients with and without essential tremor for several years, patients with essential tremor were four times as likely to develop Parkinson’s disease (3.0% of essential tremor patients versus 0.7% of patients without essential tremor).2 Several families have been noted to have members with both essential tremor and Parkinson’s disease.6 In one study, patients with Parkinson’s disease were more likely to have a family history of essential tremor.7 Another study compared patients with and without Parkinson’s disease, and those with Parkinson’s disease were over 7 times as likely to have a tremor 10 years before their diagnosis,8 although it’s unclear if this tremor was essential tremor or an early isolated tremor. Both conditions can have similar “non-motor” features, such as thinking and memory changes, mood issues, and dream-enactment behavior.1
The evidence is more complicated when researchers have looked at the brains of patients with essential tremor and Parkinson’s disease. “Lewy bodies”, a typical pathological finding noted in the brains of patients with Parkinson’s disease when examined at autopsy, have been also noted in the brains of patients of essential tremor,9 but this has not been a consistent finding.10 Some studies have used an imaging technique called a “DaTscan” to look for similarities between essential tremor and Parkinson’s disease.11 In a DaTscan, a radioactive tracer that attaches to dopamine-producing neurons is given to the patient. The DaTscan is able to see how much tracer is present in a person’s brain, and thus measure if there is any loss of dopamine-producing neurons (such as would be expected in Parkinson’s disease). One study found normal DaTscan results in healthy patients, abnormally low uptake of tracer in patients with Parkinson’s disease, and results in-between for patients with essential tremor.12 Another study showed lower tracer uptake in people with essential tremor versus people without essential tremor when using computer-aided analysis, suggesting some degree of dopamine-producing neuron loss.13
Clarifying the relationship between essential tremor and Parkinson’s disease is challenging as there is no specific diagnostic test or biomarker that reliably differentiates these two conditions. Many studies looking at the rates of Parkinson’s disease in patients with essential tremor are “retrospective”, meaning that they gather data from looking in the past through patients’ charts. Data that is more reliable comes from “prospective” studies that follow groups over time, but these studies are often difficult to undertake. Complicating matters further, as noted earlier, essential tremor patients can sometimes have parkinsonian symptoms such as rest tremor, slowness of movement and loss or balance.14 If someone develops a rest tremor, it can sometimes be difficult to say whether their essential tremor is getting worse or whether they are beginning to show signs of Parkinson’s disease. In a detailed review of 300 patients followed at the Parkinson’s Disease Center and Movement Disorders Clinic, Baylor College of Medicine in Houston, Texas, 26% had evidence of associated Parkinson’s disease.
If there is any concern for symptoms of Parkinson’s disease, the first step is to seek out evaluation from an experienced neurologist, and preferably one with additional training in movement disorders. The diagnosis of Parkinson’s disease can typically be made by listening to a patient’s history and performing a physical examination, where the neurologist will look closely at your type of tremor and see if you have any slowness, stiffness, changes in your walking, or other signs of Parkinson’s disease. However, if there is any uncertainty about the diagnosis, the neurologist may order a DaTscan to look for evidence of decreased dopamine neurons, which would be suggestive of Parkinson’s disease.
Patients with essential tremor and Parkinson’s disease do often need different treatment strategies. Medications for essential tremor, such as propranolol or primidone, are often aimed at improving action tremor and not rest tremor or other parkinsonian symptoms. These other symptoms require Parkinson’s disease medications, such as carbidopa/levodopa, dopamine agonists, or anticholinergic medications such as trihexyphenidyl or amantadine. Botulinum toxin injections can potentially be used for treatment of tremor in both conditions.15 Deep brain stimulation and focused ultrasound have been used for patients with disabling tremor due to either condition.16
- Essential tremor and Parkinson’s disease are two common causes of tremor, particularly in elderly patients.
- Essential tremor typically is an action tremor while Parkinson’s disease usually has a rest tremor.
- Patients with essential tremor can develop some signs and symptoms of Parkinson’s disease, such as rest tremor, but patients with Parkinson’s disease rarely have head or voice tremor.
- There is some epidemiologic evidence to suggest that essential tremor patients have a higher chance of developing Parkinson’s disease.
- A movement disorders neurologist is best equipped to distinguish between the two most disorders, but additional studies such as a DaTscan may be helpful.
- Making a proper diagnosis is important for future prognosis and treatment.
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