How to Measure Essential Tremors

I want to introduce this blog in an unusual way by starting with autism. A 2016 news item reported:

The genetic risk for autism exists in every person, scientists today revealed. As a result, the principal signs of autistic spectrum disorder (ASD) are seen in each individual – just to varying degrees.

Those with the most severe symptoms are the proportion of the population officially diagnosed with ASD, the scientists from the University of Bristol, Harvard and MIT and Massachusetts General Hospital found.

?At first glance, autism bears no resemblance to the most common movement disorder, essential tremor (ET) that affects millions of people. ET often brings feelings of isolation because those who have it usually don’t see anyone else around them with shaking hands, head or other part of the body.

A spectrum of tremors

However, just as each of us can manifest aspects of the autistic spectrum disorder – usually so slight that it seems within the range of normal interaction – each of us also has tremors. That’s right, everyone has tremors, but they exist on a spectrum just as autism does. As scholarpedia.org tells us, “Tremor is found in every person, typically a barely visible tremor that occurs when the arms are extended and that is also observed during activities that require great precision.” Such tremors are not a progressive condition, and generally not visible to the naked eye, but sensitive instruments used in studies can detect them. They are called physiologic tremors, and they represent one end of the spectrum.

In the middle range, there are other naturally occurring but not disabling or progressive tremors. They are called exaggerated physiological tremors and may be quite noticeable. For example, as many people become elderly, they develop occasional tremors that are not ET. Or they may occur under conditions of nervousness, stress, fatigue, too much caffeine, etc.

On the other end of the spectrum, there are conditions that can lead eventually to severe tremors, so they are often called pathological (I.e. caused by disease or abnormal condition) tremors. These include ET, Parkinson’s disease, parkinsonism and others related to substance abuse, exposure to toxic substances, etc.

Measuring essential tremors

What puts them on the far end of the spectrum is a matter of degree, and the severity of tremors is “measured” in terms of frequency and amplitude. Think of tremors as oscillations, or regular back-and-forth movements. In ET, these oscillations are involuntary, and they are characterized by how many times they occur within a given time period (frequency or vibration) and how wide the movement is (amplitude).

Measuring tremor frequency (low, medium or high) is one of the ways in which tremors are classified. ET is usually recorded as medium to high frequency. Amplitude, on the other hand, is not useful for classification since many different types of tremor may result in the similar amplitude.

For ET, in early stages the frequency may be high but the amplitude is not very wide. In fact, amplitude can be so small that tremor is experienced as fairly inconsequential, and not obvious to an observer. Since tremors due to ET are action tremors (that is, they may not occur during rest but intentional activity such as reaching for a glass of water triggers them), it’s common for amplitude to increase as the hand approaches the target.

In cases of ET progression, the frequency may actually slow down, while the amplitude is becoming more expanded. This is what leads to obvious tremors and ultimately can created impaired function in daily life.

MRI-guided Focused Ultrasound for control of hand tremors

If tremors become disabling, and no longer respond to medication (or the idea of medication is not acceptable), there is a noninvasive treatment called MRI-guided Focused Ultrasound (MRgFUS) that interrupts tremor signals to the hand by deadening a small area in the brain. This outpatient treatment is safe, effective and durable. What is amazing to patients – who are awake during the treatment – is the process of gradually but dramatically reducing the amplitude of their tremors as treatment takes effect. Even before they get off the table and get dressed, they are astounded at being able to sign their names, draw a spiral, or steadily holding a glass of water without creating a ripple. Another way of saying it: if there’s no amplitude to measure, there’s no tremor.

For more information, contact Sperling Neurosurgery Associates.

i. Lizzie Parry. “EVERY one of us is on the autistic spectrum.” Daily Mail, Mar. 21, 2016. http://www.dailymail.co.uk/health/article-3502928/EVERY-one-autistic-spectrum-experience-key-symptoms-just-varying-degrees.html

ii. http://www.scholarpedia.org/article/Tremor