Essential tremor (ET) is considered the most common movement disorder. No one knows the cause but scientific research has led to widely held theories about dysfunction in particular cells (Purkinje cells) or a neurotransmitter called GABA. Correct diagnosis is essential for proper planning, tremor management, and ongoing quality of life adaptations. But ET can mimic other conditions that cause tremors.
A condition such as heart disease can be accurately identified by physical examination (e.g. listening to a patient’s heart), asking questions (e.g. symptoms and history), heart-specific tests (e.g. electrocardiogram, blood tests), condition-specific imaging (e.g. cardiac CT scan) or a combination of these. For heart disease, diagnosis is usually straightforward and decisive.
With regard to ET, however, getting a definitive diagnosis is a little more complicated. Initially, the process parallels that of heart disease: physical exam, gathering information about family history, personal history and symptoms, and tremor-specific tests (e.g. drawing spirals, drinking from a glass). However, the only current use of blood tests or imaging is to help rule out other conditions that resemble ET. There is no brain scan that shows exactly what’s causing ET.
ET research uses brain imaging
On the other hand, scientific research is increasingly turning to neurological magnetic resonance imaging (MRI) of the brain in a concerted effort to identify the areas involved in tremor signals. According to Sharifi et al. (2014)i there are three main hypotheses about the possible mechanisms and structural changes in the brain that result in tremors:
- A progressive process in which certain brain cells are degenerating or dying off
- A disorder caused by an imbalance or dysfunction of the cells that produce the GABA molecule
- A problem connected with abnormal signals between nerve cells in the tremor network
In fact, there may be overlap among them. The authors note, “Neuroimaging techniques might give insight into these three and even other concepts concerning the pathophysiology of essential tremor. … A variety of imaging studies have been performed in essential tremor patients over the past decade.”
The Sharifi article was published in 2014, and research into ET neuroimaging continues to evolve and grow. Among the newest studies are two papers on a particular type of MRI that can show changes in the brain’s white matter.
What is white matter?
The two main tissue components of the brain are gray matter and white matter (WM). Most of us have heard about gray matter, the darker-colored outer part of the brain that are made up of the brain’s neurons (nerve cells). The gray matter includes areas that are active in muscle control, information from senses, verbal/speech function, executive function (planning/decision making), emotions, memory, and self-control. WM is more interior. It used to be thought of as less active but nothing could be less true.
WM has a lighter color than gray matter because many of its neurons are “myelinated.” Myelin serves as a sort of light-colored fatty sheath that insulates the length of each neuron’s axon (the long part of the nerve cell that carries electrical messages to other cells). This insulation speeds up transmission since it conserves the energy. WM is filled with such axons, and has factories that form myelin: “If the myelinated axons of an adult human brain were laid out end to end, the total length would reach approximately 160,000 km … WM also contains myelin-producing oligodendrocytes, astrocytes, microglia, and oligodendrocyte precursor cells (OPCs).”ii The properties of WM and its myelination process give it tremendous plasticity; it can learn from experience and literally teach itself new tricks.
Diffusion Tensor Imaging, white matter and ET
Diffusion Tensor Imaging (DTI) is a special type of MRI that reveals the brain’s white matter and locates abnormalities in its structure and how it’s functioning. Two studies used DTI to identify changes in white matter that correlate with ET symptoms. Benito-León et al (2017)iii found abnormalities in white matter that may be connected with cognitive dysfunction in ET. Nestrasil et al. (2018)iv discovered abnormalities that appeared linked with tremor severity.
DTI not yet diagnostic
On the one hand, we do not yet have imaging technologies available that can confirm a diagnosis of ET, which may seem disappointing. On the other hand, the way in which DTI and other emerging types of scanning are revealing information about ET-related brain dysfunction is bringing us closer to the day when a brain scan is able to confirm a diagnosis of essential tremor. Let’s hope the future is approaching faster than we imagine.
iSharifi S, Nedervven A, Booij J, van Rootselaar AF. Neuroimaging essentials in essential tremor: A systematic review. Neuroimage Clin. 2014; 5: 217–231.
iiSampaio-Baptista C, Johansen-Berg H. J Neuron. 2017 (Nov);
iiiBenito-León J, Mato-Abad V, Louis E, Hernández-Tamames JA et al. White matter microstructural changes are related to cognitive dysfunction in essential tremor. Sci Rep. 2017; 7: 2978.
ivNestrasil I, Svatkova A, Rudser K, Ravishankar C et al. White matter measures correlate with essential tremor severity—A pilot diffusion tensor imaging study. Brain Behav. 2018 Aug; 8(8): e01039.