Distinguishing Essential Tremor From Parkinson's Disease

Essential tremor (ET) is at least eight times more common than Parkinson’s disease (PD). There are many differences in the presentation, course and treatment of ET and PD; however, these differences are not always recognized by healthcare professionals and misdiagnoses are common (Table 1). It has been suggested that up to 20 percent of patients with ET may develop PD, but whether ET is a risk factor for PD remains a controversial issue.¹

Signs and Symptoms

Usually ET starts as a low- amplitude tremor but can gradually increase to a coarse, disabling tremor. As ET progresses, tremor frequency (number of repetitions per second) may decrease; however, tremor amplitude (magnitude/strength) may increase. Increased amplitude is associated with a decreased ability to manage fine motor tasks. The amplitude varies up to 23 percent throughout the day, but the frequency usually does not change during the course of the disease.²

ET generally presents bilaterally and is primarily seen during action, such as when writing or eating, or when holding a posture, for example, when holding an object against gravity. On the other hand, parkinsonian tremor most often presents unilaterally and later progresses to include both sides of the body. PD tremor most commonly occurs at rest, when the body part is relaxed and not in use, but can also be seen in the postural position, often referred to as reemergent tremor.³

ET most commonly affects the hands, legs, head, and voice, and tremor is the primary symptom of ET. In PD, the cardinal symptoms include bradykinesia, rigidity, tremor, and gait/balance issues. It is important to note that although it occurs in the majority, tremor does not have to be present to make a diagnosis of PD. The tremor of PD most generally occurs in the upper/lower extremities and the chin/jaw and generally does not affect the head or voice.

ET most commonly onsets during middle age, but can occur at any time in the lifespan, even in childhood. The progression of ET can be variable. In some patients ET remains mild throughout the lifespan and does not result in significant disability; however, in others ET progresses and can cause significant disability, making many daily activities very difficult or impossible to complete. On the other hand, the average age of onset of PD is 60 years although it may be much later and about 10 percent present prior to the age of 40 years. PD is a progressive disorder with virtually all patients having increased disability over time.

Although the gene accounting for the majority of ET patients has not been identified, ET is an autosomal dominant disorder with greater than 50 percent of patients reporting a family history of tremor. It is not uncommon to have a patient report multiple family members from several generations that have been affected by tremor. PD is the result of a loss of dopamine in the substantia nigra; however, the cause of PD is currently unknown and it is suspected that it may be a combination of a genetic predisposition and environmental influences. A family history of PD is reported in less than 20 percent of patients.

Both ET and PD are affected by stress, anxiety and emotion and it is not uncommon to see an increase in tremor under stressful conditions.

Diagnosis

At this time, there are no tests that can definitively diagnose either ET or PD and it is not uncommon to have the two mistaken for each other. The diagnosis is based on a complete medical/symptom, family and medication history and an examination by a physician, preferably a neurologist who specializes in movement disorders. Obtaining a handwriting sample may be helpful in making an accurate diagnosis; in ET handwriting is generally large and tremulous, whereas in PD, micrographia (very small handwriting) is common. DaTscan, a single photon emission computed tomography (SPECT) scan, which measures dopamine uptake, has been approved as a diagnostic aid to help physicians differentiate between ET and parkinsonian tremor.⁴ Other imaging techniques are under investigation to determine their utility in differentiating ET and PD. An accurate diagnosis is critical as the treatment regimen is very different for the two disorders.

Medications

Alcohol has been known for decades to improve ET; however, it generally has little to no effect on PD.⁵ Excessive use of alcohol is not recommended due to the potential for abuse; however, judicial use of alcohol prior to a social or stressful event can be helpful. Currently, treatments are under investigation for ET based on the dramatic effect often seen with ethanol.

The first-line pharmacological treatments for essential tremor include propranolol and primidone. They can be used individually or in combination if needed. Other treatments include gabapentin, topiramate and benzodiazepines, which can be particularly helpful in ET patients with significant anxiety.^6,7 Botulinum toxin injections may be helpful in ET but they are used less frequently and can result in weakness.

On the other hand, pharmacological treatments for Parkinson’s disease to date have focused on dopamine depletion and include carbidopa/levodopa, dopamine agonists such as ropinirole, pramipexole and rotigotine monoamine oxidase type B (MAO-B) inhibitors such as rasagiline and selegiline, and less commonly amantadine and anticholinergics.⁸

For both ET and PD, deep brain stimulation (DBS) can be an effective treatment. In ET, DBS is an option if adequate control of symptoms cannot be obtained with medications. The most common site for DBS is the ventral intermediate nucleus of the thalamus. In PD, DBS is an option when medications are not consistently controlling symptoms throughout the day. For PD, DBS is generally done in the subthalamic nucleus or the globus pallidus interna.

Conclusion

ET and PD are both movement disorders for which tremor is a primary symptom and therefore, the two are often mistaken for each other. However, there are many differences between the signs and symptoms of the two disorders and it is critical that patients receive an accurate diagnosis early in the disease process such that they can receive the appropriate treatment, education and support.

Dr. Jankovic is Professor of Neurology, Distinguished Chair in Movement Disorders, Director, Parkinson’s Disease Center and Movement Disorders Clinic, Co-Director, Parkinson’s Disease Research Laboratory, Baylor College of Medicine, Department of Neurology, Houston, Texas; and IETF Medical Advisory Board.

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