Essential Tremor and Parkinson’s Disease: A Closer Look at Non-Tremor Similarities

What do Essential Tremor (ET) and Parkinson’s Disease (PD) have in common? Well, the obvious answer is tremors. However, the tremors that characterize each condition differ. ET is characterized by action and postural tremors, while PD has rest tremors.

As it turns out, ET and PD have some non-tremor things in common, too. A recent analysis of both conditions reveals that they can both affect mental capabilities, or neuropsychological function, in similar ways. Unlike neurology studies that involve the physical structure, pathways, connectivity, etc. of the brain, neuropsychology is closer to a study of the mind. It studies the physical function of brain areas “… as they relate to specific psychological processes and behaviors. It is an experimental field of psychology that aims to understand how behavior and cognition are influenced by brain functioning and is concerned with the diagnosis and treatment of behavioral and cognitive effects of neurological disorders.”1

ET, PD, and mental function

In terms of tremor similarities, ET is often misdiagnosed as PD. For many decades, ET was called benign essential tremor. In medicine, benign means that a condition or disease is not harmful in its effect. While it was known that the tremors of ET could gradually become more severe to the point of impairing daily activities and intense embarrassment—and we may well ask, “How is this NOT harm?”—it was believed that the extent of harm stopped there.

More recently, the term benign has been dropped. ET appears to be a family of conditions that all have tremor, but there are also variances in brain processes underlying tremors. It is now recognized that many people with ET also experience a decline in mental functions like memory and language skills, though some clinicians debate this.

Since similar decline is also observed in PD (even in the early stages), a Spanish research team set out to analyze the common impact of both conditions on mental function.

Neuropsychological analysis

The study included both previously diagnosed ET and PD patients, who were re-evaluated for confirmatory diagnosis. In addition, control subjects who did not have ET or PD were recruited. The study goal was “to compare the cognitive profile of PD and ET using a healthy control group as a reference point.”2 Each group had 32 members, for a total of 96 subjects. ET and PD participants continued on their medication schedules, whereas controls were not taking medication. Since all were from the same clinic, researchers had access to their clinical history. Participants were told that “…the purpose of the study was to complete a test battery to assess neuropsychological and personality status.”3 The following functions were tested and evaluated:

  • Attention
  • Executive function (mental skills that help organize, manage and accomplish time and tasks)
  • Visuospatial ability (represent, analyze and mentally manipulate objects)
  • Verbal memory (recall words)
  • Visual memory (information that resembles objects, places, animals or people in a mental image)
  • Language
  • Depression
  • Psychopathology and personality symptoms (mental, behavioral or personality disorders)

Results

The researchers found that both the ET and PD groups performed worse on the tests than the control group, which has been observed in similar but smaller studies. There were no significant differences in scores between ET and PD participants, though in some tests the ET group did slightly better, and in others the PD group did slightly better—but such differences were negligible. More importantly, the kinds of neuropsychological decline were similar in both groups. The commonalities (e.g. worse performance in attention, executive function, memory and naming) imply that, in terms of neuropsychological function, both ET and PD affect the same brain circuits between the cerebellum and the prefrontal cortex.

The authors concluded, “Our findings suggest a similar cognitive profile for PD and ET groups in the absence of dementia and, interestingly, an overlap in the affected domains [brain areas].”4

While ET and PD are not the same condition, this published paper identifies non-motor similarities that may ultimately shed light on our understanding of the effect they have on brain anatomy, the mind, and how we use it.


1 https://en.wikipedia.org/wiki/Neuropsychology
2Puertas-Martín V, Villarejo-Galende A, Fernández-Guinea S, Romero JP et al. A Comparison Study of Cognitive and Neuropsychiatric Features of Essential Tremor and Parkinson’s Disease. Tremor Other Hyperkinet Mov (N Y). 2016 Dec 15;6:431.
3Ibid.
4Ibid.

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