By Alexander Shtilbans, MD, PhD

Assistant Professor of Neurology, Weill Cornell Medical College

Dr. Shtilbans reports no financial relationships relevant to this field of study.

SYNOPSIS: Rest tremor is seen in patients with more advanced essential tremor, yet its prevalence varies significantly between patient groups.

SOURCE: Louis ED, et al. Prevalence and correlates of rest tremor in essential tremor: Cross-sectional survey of 831 patients across four distinct cohorts. Europ J Neurol 2015, 22:927-932.

Essential tremor (ET) is a common neurological disorder affecting approximately 4% of people age 40 years and older. It manifests as kinetic and postural tremor, but occasionally resting tremor is seen as well, contributing to diagnostic confusion with Parkinson’s disease (PD). Few studies have looked at the prevalence of rest tremor in patients with ET, and the authors of this study aimed to estimate the prevalence of rest tremor in patients with ET and to assess the clinical correlations of that tremor. They used four different patient groups: a population-based study in northern Manhattan, a genetics study of movement disorders, a study of the environmental epidemiology of essential tremor, and a brain repository group of patients who were examined and signed consent for postmortem evaluation of their brains. The authors hypothesized that the prevalence of rest tremor could be related to the severity of the disease (ET). In total, 831 patients were analyzed from all four patient groups. Experienced movement disorder neurologists performed examinations, which included assessments of postural and kinetic tremors in the arms, as well as head, voice, and jaw tremors. Rest tremor was evaluated in seated and standing positions and while walking.

The results showed that the severity of arm tremor, duration of tremor, and the prevalence of head tremor were lowest in the population-based study and highest in the brain repository group. Rest tremor occurred in 1.9% of the population-based study, 9.6% of the family study group, 14.7% of the study of the environmental epidemiology of essential tremor, and 46.4% of patients in the brain repository group. No patient had tremor in the legs or feet. The patients with rest tremor were found to be significantly older than those without and they had their tremor for a longer period of time. Interestingly, patients with rest tremor also were more likely to have voice or head tremor. Rest tremor appeared to be asymmetric in approximately half of the patients who had it, and in those patients, action and postural tremor also were more pronounced in the same arm. The authors concluded that the prevalence of the rest tremor in ET patients varied greatly among the examined groups, ranging from 1.9% in the population-based setting to 50% in a brain repository group.

COMMENTARY

The authors of this study evaluated the prevalence of resting tremor in patients with essential tremor in four different patient groups and attempted to find clinical correlates of that tremor. The study was well-designed and the patients had similar demographic characteristics, except for the ET brain repository group, in which the patients were considerably older. It was astutely observed that the rest tremor was only noted in the arms, not the legs, which was a fundamental difference from what we see in PD, where leg tremor at rest can be common. The other observation was that the rest tremor emerged as the ET progressed later in life, and is consistent with what is usually seen in movement disorder clinics. The mechanisms underlying the resting tremor in ET remain unclear, but the authors suggest cerebellar pathology as the potential cause based on the animal models with cerebellar lesions.

The results of this study were logically interpreted, but the certainty of the diagnosis is based on the clinical examination. It is well-known that patients with ET are at five-fold increased risk for developing PD later in life. In fact, it is quite common to see people with both conditions. The authors stated that none of the patients with ET had any parkinsonian features except for the rest tremor. It is possible, however, that some of the patients with longstanding ET developed Lewy body pathology in the brain, which did not yet result in any rigidity, bradykinesia, or reduced rapid alternating movements, suggestive of PD. DaT scans were not performed in this study to rule out dopamine deficiency, so we cannot exclude some patients with very early-stage PD who developed parkinsonian tremor superimposed on the existing action and postural tremor, except for some deceased patients from the brain repository group whose brains were examined pathologically. More prospective studies would be needed to follow the progression of rest tremor and find anatomical correlations that can help understand the pathophysiology of the disease and lead to more effective treatment.