Essential tremor

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Overview

Tremor is a neurological disorder characterized by involuntary and rhythmic movements of agonist and antagonist muscles it affects numerous body parts(1), including the hands, vocal cords, legs, arms, head, face, and tongue[1] . It is not a life-threatening disease but has a major influence on a person's everyday activities. Tremors are classified into two types: rest tremors', which occur while a person is at rest and activity tremors, which occur while moving. Action tremors are classified into numerous categories, including postural, kinetic, purposeful, task-specific, and isometric tremors[2]. In 1874 Burresi initially used the term Essential tremor (ET), or family tremor, to describe a condition marked by a “bilateral upper extremity action tremor.” Lasting for at least 3 years without any other neurological signs such as Parkinsonism. An essential tremor with neurological signs is referred as an “essential tremor plus” is one of the most prevalent hyper kinetic tremors in adults. Historical Perspective [3].

Historic Perspective

The term essentieller Tremor (essential tremor) was first coined by Raymond (France, 1892).This term gained widespread use among neurologists during the second half of the 20th century to describe the familial cases of tremors.[3].

Classification

There is no system to classify essential tremor.

Pathophysiology

The cause of tremor in ET is still unknown; several hypotheses have been made to understand the pathophysiology of the disease including the neurodegenerative hypothesis, central oscillatory network hypothesis, and GABAergic hypothesis. the neurodegenerative hypothesis is a matter of debate due to contradicting outcomes in the studies like degeneration or remodeling of Purkinje cells in ET other studies do not support the findings many studies report degeneration of the cerebellum but other studies do not support the finding, central oscillatory network hypothesis suggests that interconnected signals among brain region including cerebellum, thalamus, cortex, and inferior olive collaborate to generate tremors. The GABAergic hypothesis suggests that the irregular activity within a brain region cerebello-thalamo-cortical occurs due to disturbances in the transmission of the neurotransmitter GABA or alterations in a specific type of receptor for GABA known as GABA-A receptors[4].

Cause

The cause of the diseases is not clear but various studies suggest that genes play an important role in developing diseases (13), environmental factors are also considered as disease-causing factors.

Genetics:

ET is a hereditary disease and about 30-70% of individuals suffer from owing to past family history, the chances of ET rise 4.7 times if the first-degree relative has the disease many types of research have been done to identify the genetic marker for the diseases  .but the responsible gene is not clear yet. One study linked ET susceptibility to the DRD3 gene in Icelandic families. At the same time, another found the HS1BP3 gene in an American family[5]. ETM3 on chromosome 6p23 was identified as a locus . GWAS highlighted potential ET susceptibility genes like LINGO1 and SLC1A2 [6]. Exome sequencing revealed a pathogenic FUS variant in a French-Canadian family. Further research found HTRA2 in Turkish and pathogenic variants in the TENM4 gene in a Spanish population. These findings highlight the complex genetic basis of ET [7].

Acquired factors:

Neurological illnesses are strongly influenced by lifestyle and environmental factors. Recent study reveals that ethanol, which was formerly considered to ease tremors, may instead act as a cerebellar toxin, inadequate sleep has been linked to a higher incidence of tremors and Lead exposure has been connected with essential tremors (ET)[8]

Epidemilogy

ET is a genetic disease and it is more common in males than females initially its symptoms are mild but it worsens with age (26). ET has a worldwide incidence of 0.9%, which can rise to 4.0% and 22% in patients aged greater than 40 and 90 years respectively .

Risk Factors

Over time mild ET can progress and age-related factors can act as a bridge in the progression of the diseases resulting in the emergence of neurological symptoms.Late onset of the disease is associated with rapid progression, dementia, Parkinson's diseases, and death, caffeine, medication, low blood sugar, fatigue strong emotion, stress can trigger tremor related to ET.

Diagnosis

Initially, ET was considered a purely motor disease. Still, letter non-motor features are also recognized[9].ET symptoms include fatigue, personality changes, olfactory dysfunction, hearing impairment, and sleep disturbances. anxiety[10].Tremors observed in ET patients with the frequency of  4-8 Hz(1)[11], depression is considered as primary symptom and associated with disability in  ET patients, cognitive impairment is also reported in these patients and it is progressive from mild to dementia,  the affected cognitive domain is an executive function and memory. These patients feel difficulty in writing, drinking, eating, personal hygiene, and other routine task. In some situations, ET can constantly shake the hands or tongue, making it difficult to thread a needle, and using a ruler becomes difficult[10]. As it is suggested that it is a hereditary disease it is initially diagnosed by the medical history, family history, and physical examination which help to evaluate the tremor of the head, arm, voice, and leg. In 2018 Moment Disorder Society (MDS) classified ET into two categories including ET and ET plus through electromyography and accelerometry artificial intelligence to interpret accelerometry data and distinguish between ET and Parkinson's disease. It has been applied to the analysis of video recordings of tremors.

Treatment

Patients might seek medical help only when symptoms interfere with daily activities. To overcome the diseases, medication and surgical treatments are provided. Combined therapy can sometimes show better clinical outcomes and thus surgical intervention is considered in severe cases and disabling despite treatment[12]. The primary therapy for ET is Propranolol and primidone [13] and other medications are also used for the treatment including gabapentin, alprazolam, and topiramate. The second option to overcome the disease is surgery which includes deep brain stimulation (DBS), radio-frequency thalamotomy, and MRI-guided focused ultrasound thalamotomy. Some other Therapies are also introduced nowadays that help to overcome the disease burden including TAPS (FDA approved).

References

  1. ↑ Bhatia KP, Bain P, Bajaj N, Elble RJ, Hallett M, Louis ED; et al. (2018). "Consensus Statement on the classification of tremors. from the task force on tremors of the International Parkinson and Movement Disorder Society". Mov Disord. 33 (1): 75–87. doi:10.1002/mds.27121. PMC 6530552 Check |pmc= value (help). PMID 29193359.
  2. ↑ Grinnon ST, Miller K, Marler JR, Lu Y, Stout A, Odenkirchen J; et al. (2012). "National Institute of Neurological Disorders and Stroke Common Data Element Project - approach and methods". Clin Trials. 9 (3): 322–9. doi:10.1177/1740774512438980. PMC 3513359. PMID 22371630.
  3. ↑ 3.0 3.1 Louis ED, Broussolle E, Goetz CG, Krack P, Kaufmann P, Mazzoni P (2008). "Historical underpinnings of the term essential tremor in the late 19th century". Neurology. 71 (11): 856–9. doi:10.1212/01.wnl.0000325564.38165.d1. PMC 3461999. PMID 18779514.
  4. ↑ Kosmowska B, Wardas J (2021). "The Pathophysiology and Treatment of Essential Tremor: The Role of Adenosine and Dopamine Receptors in Animal Models". Biomolecules. 11 (12). doi:10.3390/biom11121813. PMC 8698799 Check |pmc= value (help). PMID 34944457 Check |pmid= value (help).
  5. ↑ Higgins JJ, Lombardi RQ, Pucilowska J, Jankovic J, Tan EK, Rooney JP (2005). "A variant in the HS1-BP3 gene is associated with familial essential tremor". Neurology. 64 (3): 417–21. doi:10.1212/01.WNL.0000153481.30222.38. PMC 1201396. PMID 15699368.
  6. ↑ Thier S, Lorenz D, Nothnagel M, Poremba C, Papengut F, Appenzeller S; et al. (2012). "Polymorphisms in the glial glutamate transporter SLC1A2 are associated with essential tremor". Neurology. 79 (3): 243–8. doi:10.1212/WNL.0b013e31825fdeed. PMC 3398434. PMID 22764253.
  7. ↑ Unal Gulsuner H, Gulsuner S, Mercan FN, Onat OE, Walsh T, Shahin H; et al. (2014). "Mitochondrial serine protease HTRA2 p.G399S in a kindred with essential tremor and Parkinson disease". Proc Natl Acad Sci U S A. 111 (51): 18285–90. doi:10.1073/pnas.1419581111. PMC 4280582. PMID 25422467.
  8. ↑ Louis ED, Jurewicz EC, Applegate L, Factor-Litvak P, Parides M, Andrews L; et al. (2003). "Association between essential tremor and blood lead concentration". Environ Health Perspect. 111 (14): 1707–11. doi:10.1289/ehp.6404. PMC 1241711. PMID 14594619.
  9. ↑ Shalash AS, Hamid E, Elrassas H, Bahbah EI, Mansour AH, Mohamed H; et al. (2021). "Non-motor symptoms in essential tremor, akinetic rigid and tremor-dominant subtypes of Parkinson's disease". PLoS One. 16 (1): e0245918. doi:10.1371/journal.pone.0245918. PMC 7840014 Check |pmc= value (help). PMID 33503062 Check |pmid= value (help).
  10. ↑ 10.0 10.1 Gerbasi ME, Elble RJ, Jones E, Gillespie A, Jarvis J, Chertavian E; et al. (2024). "Associations Among Tremor Amplitude, Activities of Daily Living, and Quality of Life in Patients with Essential Tremor". Tremor Other Hyperkinet Mov (N Y). 14: 22. doi:10.5334/tohm.877. PMC 11067966 Check |pmc= value (help). PMID 38708124 Check |pmid= value (help).
  11. ↑ Kim YW, Kim S, Jin MJ, Im CH, Lee SH (2024). "The Importance of Low-frequency Alpha (8-10 Hz) Waves and Default Mode Network in Behavioral Inhibition". Clin Psychopharmacol Neurosci. 22 (1): 53–66. doi:10.9758/cpn.22.1035. PMC 10811390 Check |pmc= value (help). PMID 38247412 Check |pmid= value (help).
  12. ↑ Antonazzo IC, Rozza D, Conti S, Fornari C, Cortesi PA, Eteve-Pitsaer C; et al. (2024). "Treatment patterns in essential tremor: Real-world evidence from the United Kingdom and France primary care database". Eur J Neurol. 31 (1): e16064. doi:10.1111/ene.16064. PMID 37738526 Check |pmid= value (help).
  13. ↑ Zhang M, Li W, Hu L, Chen L, Yang L, Zhang T; et al. (2020). "Oral propranolol for treatment of the subgroups of essential tremor: a systematic review and meta-analysis protocol". BMJ Open. 10 (1): e032096. doi:10.1136/bmjopen-2019-032096. PMC 7044890 Check |pmc= value (help). PMID 31948986.

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