An Introduction to Essential Tremor

Chapter 2 CLINICAL FEATURES OF ESSENTIAL TREMOR EPIDEMIOLOGY: Essential tremor affects about 5 to 6 percent of the patients over the age of 65. About 5 to 15 percent of essential tremor cases occur during childhood. Essential tremor may be familial in cases that begin before the age of 20. Essential tremor is common in all races across the world. The prevalence of essential tremor is significantly higher in individuals above the age of 40. In some studies the prevalence of essential tremor in patients above the age of 40 has been reported to be as high as 10 percent, however, the peak age of onset for essential tremor is 70 to 79 years. The prevalence of essential tremor is 10 times greater in 70 to 79 years old individuals as compared to 40 to 69 years old individuals. Some studies have reported a slightly higher prevalence in men, but other studies could not find any difference between men and women. There is no effect of essential tremor on life expectancy. A similar survival rate has been reported among patients with essential tremor and their age and sex matched controls. The likelihood of patients with essential tremor having first degree relatives with essential tremor is five times greater than the normal population. There seems to be an increase in prevalence of essential tremor with age. It is estimated that almost 5 million people in the United States, over the age of 40, are affected with essential tremor. Essential tremor is more common than Parkinson's disease. ONSET: Essential tremor usually affects both sides of the body, although initially it may only be noticed on one side. It can occur at any age. Although, it may be seen in the early twenties, late onset, after the age of 55 years is more common. Essential tremor may begin in early childhood but its prevalence and intensity increase with advancing age and eventually, it may interfere with writing, eating and other activities of daily life. In familial cases the onset of essential tremor may be much earlier than sporadic cases. COURSE: Essential tremor is a slowly progressive condition in which the amplitude of tremor usually increases with time. In some cases there may be no change noted in the tremor for several years, and then in advanced age, the tremor may get worse relatively quickly. In addition, as the amplitude of tremor increases, the frequency of tremor may decrease. EXACERBATING FACTORS: Fatigue, central nervous system stimulation, sexual arousal, emotional excitement and temperature extremes can exacerbate the tremor. Alcohol may dampen the tremor significantly. The effect of alcohol seems to be centrally mediated. Caffeine on the other hand, seems to precipitate essential tremor. Essential tremor, like most other movement disorders, disappears in sleep. ETIOLOGY AND PATHOGENSIS: The exact cause of essential tremor is unknown. Some patients may have a family history of tremor in their parents, siblings or close relatives. However, sporadic cases are seen quite frequently. The exact mechanism of inheritance is unclear. The terms like "Familial Essential Tremor" and “Benign Essential Tremor" have also been used for essential tremor in literature. The term “Benign Essential Tremor" is misleading as the essential tremor may be quite disabling. In a significant number of cases, essential tremor is hereditary and is transmitted in an autosomal dominant pattern. Chromosome 3q13 and chromosome 2p22-p25 have been suggested to be the disease loci in many reports. Environmental factors may also play a role in the causation of essential tremor. This is supported by the lack of a complete concordance of essential tremor in monozygotic twins. There is a lack of clear understanding of the pathophysiological mechanisms of essential tremor. The central nervous system pathology is supported by the observation of response of tremor to thalamotomy and centrally acting drugs. Cerebellum may play an important role in pathophysiology of essential tremor. It is believed that essential tremor may emerge from abnormal oscillations within thalamocortical and cerebello-olivary loops in the brain. This theory is supported by the findings that the lesions or injury of the cerebellar and thalamic regions reduces the intensity of essential tremor. Neuronal discharges correlated to tremor have been observed to occur in the ventrolateral thalamus, particularly in the ventralis intermedius nucleus. Contralateral limb tremor can be suppressed by the ablation or high frequency stimulation of ventralis intermedius nucleus of thalamus. Essential tremor may be the result of abnormal oscillations of a central nervous system pacemaker. This central oscillator could be enhanced or suppressed, however the exact location of this oscillator is unknown. GENERAL PRESENTATION: The patients usually complain of their handwriting becoming sloppy and large, trouble holding objects like a cup of coffee, using fork, spoon, keys, screwdriver and pouring liquids. They may spill liquids and writing a cheque may be a challenge. In severe cases essential tremor may interfere with dressing, preparing meals and other activities of daily living. Essential tremor is temporarily dampened by intake of alcohol. The history of response to alcohol is helpful diagnostically.