​What is tremor? 
Tremor is an involuntary, rhythmic muscle movement involving back-and-forth oscillations of one or more parts of the body. It is the most common type of involuntary movement and can affect the hands, arms, head, face, voice, trunk, and legs. Most tremors occur in the hands.

What are the characteristics of tremor? Characteristics may include a rhythmic shaking in the hands, arms, head, legs, or trunk; a shaky voice; difficulty writing or drawing; or trouble holding and controlling utensils, such as a fork. Some tremors may be triggered by or worsen during times of stress or strong emotion, when the person is physically exhausted, or during certain postures or movements.

Tremor can occur at any age but is most common in middle-aged and older adults. It may be occasional, temporary, or intermittent. Tremor affects men and women equally.​
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​Tremor can be classified into two main categories: resting and action.
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Resting tremor occurs when the muscle is relaxed, such as when your hands are resting on your lap. Your hands, arms, or legs may shake even when they are at rest. Often, the tremor only affects the hand or fingers. This type of tremor is often seen in people with Parkinson's disease.  

Action tremor occurs with the voluntary movement of a muscle.  Most types of tremor are considered action tremors. There are several sub-classifications of action tremor, many of which overlap.

• Postural tremor occurs when you are holding a position against gravity, such as keeping your arms outstretched.
• Kinetic tremor is associated with any voluntary movement, such as moving your wrists up and down or closing and opening your eyes.
• Intention tremor starts when you make an intended movement toward a target, such as lifting a finger to touch your nose.  
• Task-specific tremor only appears when you are performing highly skilled, goal-oriented tasks such as handwriting or speaking.
• Isometric tremor occurs during a voluntary muscle contraction that is not accompanied by any movement, such as when you are holding a heavy book in the same position.​​

Head tremors

Essential tremor is by far the most common cause of head tremor. 
Essential, in medicine, means “of unknown cause,” like Essential Tremor.​
 
Are head tremors serious?
In addition to your hands, tremors can affect other parts of your body such as your head, voice, face, and trunk. The tremor itself isn't dangerous, but it can cause problems with everyday activities as the condition worsens.
 
To help control head tremor, turn your head to the side. To reduce tremors when you're using your hands, hold your elbows close to your body. When you apply makeup, rest your elbows on a table or countertop.
 
The cause of Essential tremor is unknown.
However, one theory suggests that your cerebellum and other parts of your brain are not communicating correctly. The cerebellum controls muscle coordination.

Triggers
Some factors can make essential tremor more pronounced. These include:

• fatigue
• stress and anxiety
• hunger or low blood sugar
• caffeine consumption
• hot or cold temperatures
• tobacco use

 
Vitamin D has been associated with many distinct neurological functions, particularly neuroprotection. Insufficient levels have consequently been linked to an increased likelihood of developing neurological diseases such as movement disorders, neuromuscular disorders, multiple sclerosis, and dementia Contrary to Parkinson’s disease, vitamin D deficiency, and bone abnormalities are under-investigated in hyperkinetic movement disorders (HKMDs). 

Internal Tremors

Internal Tremors are like tremors that happen inside your body. You can’t see internal Tremors, but you can feel them. They produce a quivering sensation inside your arms, legs, chest, or abdomen.

Internal Tremors aren’t as life-altering as external tremors. For example, you won’t physically shake while trying to pour a cup of tea or write a letter. Internal Tremors also aren’t the same as vertigo, which is another symptom of some neurological conditions. Vertigo feels like the world is spinning around you.

Still, internal tremors can feel unpleasant. 

Tremors are caused by damage in your brain affecting the nerves that control your muscles. Internal Tremors are thought to stem from the same causes as tremors. The shaking may simply be too subtle to see.

Conditions such as  Essential Tremor can cause these tremors. 54.5% of people with essential tremor reported feeling Internal Tremors
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​A tremor that no one can see can be hard to describe to your doctor. To help you explain this symptom, start keeping a diary of your tremors. Write down:

• at what time of the day they happen
• what you were doing when they started
• what they feel like
• how long they last
• what other symptoms you have with them, such as dizziness or weakness

Bring this diary with you to your appointments. Use it as a guide during conversations with your doctor.
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Internal tremor (IT) is a poorly recognized symptom that has been described in ET. Described as a feeling of tremor in the extremities or trunk without actual movement, ITs are not debilitating but can be bothersome to patients. The origin of the sensation is unknown.. The present study sought to expand knowledge about IT by confirming their presence and determining their prevalence in  Essential Tremor
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There are more than 20 types of tremor.  Some of the most common are:

Essential tremor--Essential tremor (previously also called benign essential tremor or familial tremor) is one of the most common movement disorders. Its key feature is a tremor in both hands and arms during action and when standing still. It also may affect your head and voice and how you walk. Although the tremor can start at any age it most often initially appears during adolescence or in middle age (between ages 40 and 50). It can be mild and stay the same, or slowly get worse. 
The exact cause of ET is unknown. Studies show essential tremor is accompanied by a mild degeneration of the cerebellum, which is the part of your brain that controls movement. About 50 percent of the cases of essential tremor are thought to be caused by a genetic risk factor (referred to as familial tremor). Children of a parent who has familial tremor have a greater risk of inheriting the condition. Familial forms of essential tremor often appear early in life.

• Postural tremor of moderate amplitude is present in at least one arm
• Tremor of moderate amplitude is present in at least one arm during at least four tasks, such as pouring water, using a spoon to drink water, drinking water, finger-to-nose maneuver, and drawing a spiral.
• Tremor must interfere with at least one activity of daily living.
• Medications, hypothyroidism, alcohol, and other neurological conditions are not the cause of tremor.

Probable essential tremor

• Tremor of moderate amplitude is present in at least one arm during at least four tasks, or head tremor is present.
• Medications, hyperthyroidism, alcohol, and other neurological conditions are not the cause of tremor.

Dystonic tremor--Dystonic tremor occurs in people who are affected by dystonia—a movement disorder where incorrect messages from the brain cause muscles to be overactive, resulting in abnormal postures or sustained, unwanted movements. Dystonic tremor usually appears in young or middle-aged adults and can affect any muscle in the body. Symptoms may sometimes be relieved by complete relaxation or the severity may be reduced by touching the affected body part or muscle.

Cerebellar tremor--Cerebellar tremor is typically a slow, easily visible tremor of the arms, legs, hands, or feet that occurs at the end of a purposeful movement such as pressing a button. It is caused by damage to the cerebellum and its pathways to other brain regions, often from a stroke or tumor, injury from a disease or an inherited disorder, or from chronic damage due to alcoholism.
 
Psychogenic tremor--Psychogenic tremor (also called functional tremor) can appear as any form of tremor. Its symptoms may vary but often start and stop suddenly and may affect all body parts. The tremor increases in times of stress and decreases or disappears when you are distracted.  
Physiologic tremor--Everyone has physiologic tremor. It is rarely visible to the eye and typically involves a fine shaking of both of the hands and also the fingers. It is not considered a disease but is a normal human phenomenon that is the result of physical properties in the body (for example, rhythmical activities such as heartbeat and muscle activation). Enhanced physiologic tremor is a more noticeable case of physiologic tremor that can be easily seen. It is generally not caused by a neurological disease but by reaction to certain drugs, alcohol withdrawal, or medical conditions including an overactive thyroid and hypoglycemia. It is usually reversible once the cause is corrected.

Parkinsonian tremor--Parkinsonian tremor is common and one of the first signs of Parkinson's disease, although not all people with Parkinson's disease have tremor. Its shaking is most noticeable when the hands are at rest and may look as if someone's trying to roll a pill between the thumb and a finger. Parkinson's tremor may also affect the chin, lips, face, and legs. The tremor may initially appear in only one limb or on just one side of the body but may spread to both sides as the disease progresses. The tremor is often made worse by stress or strong emotions.
 
Orthostatic tremor--Orthostatic tremor is a rare disorder characterized by rapid muscle contractions in the legs that occur when you stand up. The tremor is usually relieved when you sit or walk. Standing may make you feel unsteady or unbalanced, causing you to immediately try to sit or walk. Because the tremor has such a high frequency (very fast shaking) it may not be visible to the naked eye but can be felt by touching the thighs or calves or can be detected by a doctor examining the muscles with a stethoscope. In some cases, the tremor can become more severe over time. The cause of orthostatic tremor is unknown, and many doctors don't understand the condition.

• Essential tremor usually appears on both sides of the body and is often noticed more in the dominant hand
• Dystonic tremor can affect any muscle in the body and usually is associated with abnormal body postures due to forceful muscle spasms or cramps
• Parkinsonian tremor, with shaking of one or both hands at rest, and may also affect the lips, face, and legs

​Generally, tremor is caused by a problem in the deep parts of the brain that control movements. Some forms of tremor are inherited and run in families, while others have no known cause. Sometimes tremor is a symptom of another neurological disorder or a side effect of certain drugs, but the most common form occurs in otherwise healthy people. Other causes of tremor can include the use of certain medicines, alcohol use disorder or withdrawal, and an overactive thyroid. Tremor may occur at any age but is most common in middle-aged and older adults.

Tremors can also be described based on their properties of amplitude and frequency.

• Amplitude means the range of motion. Think of a playground swing. If no one is in it and a wind comes up, it will be begin to sway as it dangles. In a gentle breeze it will act like a pendulum with a limited range of motion. But if a passing child walks up to it and gives it a hard push, the arc of its motion will have greater width back and forth. That’s the amplitude.
• Frequency is now many times something occurs within a specified amount of time. The faster the motion of the swing, the more times per minute it will swing from one end to another.

Thus, there are tremors that are low amplitude/low frequency (short range and slow speed); low amplitude/high frequency (short range and fast speed); high amplitude/low frequency (wide range and slow speed); and high amplitude/high frequency (wide range and fast speed).
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When you think about the variations that can occur with tremors, it’s clear that diagnosing the underlying condition or disease may be somewhere between a science and an art. Even when a neurologist has diagnosed a person as having ET, one person’s ET is not identical with another’s. There is much room for variation.
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​https://www.ninds.nih.gov/health-information/disorders/tremor
​https://essentialtremortreatment.com/six-types-of-tremors/?utm_source=monthly-newsletter&utm_medium=email