Essential tremor (ET) is the most common movement disorder. While ET can occur in the absence of a family history of tremors, anywhere from 17% to upwards of 50% of cases appear to run in families. According to the National Organization for Rare Disorders (NORD), first degree relatives (parent, sibling, child) of some with ET is almost five times as likely to develop the condition as the general population.i
In familial cases, a specific gene (or genes) has not yet been discovered to account for hereditary tremors. Researchers are hard at work in the search to identify the source of inheritance. While that remains a genetic mystery, there is also something puzzling among family members who develop ET: why do differences or variations in tremors occur within the same family? NORD says, “This suggests that additional factors, most likely environmental or additional genetic (e.g. modifier genes) ones are necessary for the development [of] the disorder in an individual (multifactorial development).” In other words, a combination of factors may determine each individual’s tremor “fingerprint.”
Not just a single type of ET
In a 2014 article, tremor specialist Elan D. Louis, MD, MS (Chief, Division of Movement Disorders, Yale University Medicine) explains that scientists increasingly accept the hypothesis that multiple genes could be a source of ET, and that exposure to environmental toxins such as lead, ethanol and harmane may cause sporadic cases of ET (non-familial) or trigger activation of a genetic vulnerability in familial cases.ii
Since the underlying gene(s) or exposure to toxins is still unknown, the variety of tremor characteristics may be the most compelling clue that there is more than one type of tremor, even in the same family. Here are some tremor characteristics that frequently differ from one person to the next:
- Age at onset (can be as early as infancy, but most prevalent in adults middle age and older)
- Affected part of body (hand, head, voice or other body part)
- Tremor amplitude (range of motion) and frequency (speed per unit of time)
- Diverse response to medication
- Non-motor features (cognitive difficulties, psychiatric problems)
- Pattern of progression
- Minority of ET cases linked with eventual Parkinson’s disease
- Evidence of different damaged areas in the brains of those with ET (based on autopsy)
A family of ET’s
In the opinion of Dr. Louis and others, ET is not a single universal condition. “Rather, there are likely to be several ETs. ET is thus likely to be a family of diseases for which the term ‘the ETs’ is more suitable and a more useful construct for future basic and pharmacotherapeutic investigations.” If this is true—and it seems most likely that it is—it has implications that will guide ongoing and future research into causes and treatments.
A universal treatment that works for every ET type?
Knowing that there is more than one type of ET may be small consolation for a person whose tremors don’t respond to medication. However, there is increasing evidence that a single, noninvasive outpatient treatment may control most if not all types of tremor by cutting off tremor signals from the brain. That treatment is called Focused Ultrasound, and so far, it appears effective for those whose tremors don’t respond to drugs (or who don’t want to take drugs).
The principle is simple: Magnetic resonance imaging (MRI) is used to identify the tiny area of the thalamus in the brain that acts as a relay station for tremor signals. Once the area is targeted and the treatment is planned, over a thousand “beams” of ultrasound are simultaneously aimed at the problem area. When the beams meet, they create enough heat to ablate (destroy) the target. During the treatment, special software monitors the temperature so only the desired target is ablated, ensuring that no collateral damage is done. By the time the treatment ends, tremors have stopped, with minimal-to-no risks of side effects.
Sperling Neurosurgery Associates offers this treatment, called Neuravive. If you or a loved one has ET that is resistant to pharmaceuticals (or drugs are not desired), visit our website for more information.
iiLouis ED. ‘Essential tremor’ or ‘the essential tremors’: is this one disease or a family of diseases? Neuropeptide. 2014; 42:81-89