Essential tremor asymmetry is not inherited.
Essential tremor consists of involuntary rhythmic shaking that most often appears when a person is making a deliberate action, such as reaching for a glass of water. This is called action tremor, whereas rest tremors may happen while a person is sitting still but they are so slight that many people are not aware of them. At least 50% of essential tremor (ET) cases are genetically passed on from one generation to another; when there is a family history of ET, it is also called familial tremor. Familial tremor is considered a dominant trait, meaning the tremors can be inherited from only one parent.
To date, science has not been able to identify a specific gene mutation that accounts for the malfunction of nerves that send messages to the muscles affected by ET. However, efforts are being made to isolate patterns of inheritance, since that might provide research clues.
One such pattern is asymmetry, meaning not symmetric on both sides of the body. A different way of saying this is that patients almost always have one side more affected than the other. For instance, the left hand might have a more pronounced tremor than the right when performing the same task. This is because most neurodegenerative disorders such as ET or Parkinson’s disease affect one side of the brain more than the other. With regard to ET, what would you expect: would several generations in an affected family line all have the same side of the body more affected? A multi-institution team lead by neurologist Elan Louis, M.D. set out to determine if such an asymmetry pattern ran in each family.i
When a medical or psychiatric study of an inherited trait is conducted, the person who is the starting point is called the proband and all other affected family members (parents, siblings, children, etc.) are simply called the relatives. Probands had to meet three criteria to enroll in the study:
- A doctor had diagnosed them with ET
- Age at ET onset was less than or equal to 50 years
- At least 2 living relatives in the U.S. had been diagnosed with ET and no other condition that might manifest tremors.
Other screening included interviews with probands and their affected relatives, a drawing test (tracing a spiral twice in two different directions), etc. The final study group was composed of 59 probands and 128 affected relatives for a total of 187 participants. After numerous evaluations and analytics, the authors found no familial pattern with regard to tremor asymmetry.
The importance of this study lies in two features. First, to the best of the authors’ knowledge, no other study has explored the hypothesis that tremor asymmetry runs in families; while more research along these lines is needed, this study concluded that asymmetry does not run along family lines. The second aspect of important lies in the implications for counseling ET patients who look to affected relatives in order to know what to expect might happen to them:
ET patients frequently search for predictors of the course their disease will take and in that search often draw comparisons with other affected relatives. Some tremor features aggregate in families, providing predictive information, as is the case with rate of progression of tremor… while others do not … However, with regards to asymmetry profile of tremor, there seems to be no familial pattern, and ET cases should not look toward their relatives for predictive information.
ET is not a life-threatening condition, but doctors and patients alike are frustrated that research has not unlocked many of its secrets. However, hope exists in the form of medications and interventions that can help control tremor as it becomes more pronounced and bothersome. Many patients who start with one treatment such as an anticonvulsant or beta blocker drug find that they have to switch around if the first medication is not as successful as expected. If ET progresses to where it no longer responds to medical treatment, a more aggressive approach such as deep brain stimulation may be offered. The Sperling Medical Group offers a noninvasive, image guided treatment called Neuravive that uses MRI-guided Focused Ultrasound, or MRgFUS. This approach sends sound waves into the very small area of the brain from which tremors originate. Where the sound waves meet, they are able to destroy that area which lessens or completely stops the tremor on the most affected side. This does not affect other parts of the body.
If you or a loved one is struggling to cope with ET that is not responsive to other therapies and that is lessening quality of life, contact the Sperling Medical Group for more information.
iLouis ED, Hernandez N, Chen KP, Naranjo KV et al. Action tremor asymmetry profile does not aggregate in families with essential tremor. Front Neurol. 2017 Apr;19(8): 148.
- Essential Tremor