Tremor is an involuntary, rhythmic muscle contraction producing oscillating movements in one or more body parts. It is the most common movement disorder in adults. The two conditions most frequently confused are essential tremor and the resting tremor associated with Parkinson's disease, and the distinction matters clinically because the underlying neurology, prognosis, and treatment approach differ substantially.
The Fundamental Distinction
Essential tremor is an action tremor: it occurs when a limb is actively being held against gravity or during purposeful movement, and it largely disappears when the limb is fully at rest. Parkinson's tremor is predominantly a resting tremor: it appears when the affected limb is relaxed and unsupported, and tends to diminish during voluntary movement. A person with essential tremor will notice their hand shaking while holding a glass; a person with Parkinson's tremor will notice it shaking while their arm hangs by their side.
Key Clinical Features of Essential Tremor
- Frequency: 4 to 12 Hz oscillation, typically in the hands and forearms
- Head tremor: present in approximately 30% of ET cases; uncommon in Parkinson's
- Voice tremor: affects 20% of ET patients
- Bilateral: both hands typically affected from early in the condition
- Family history: present in approximately 50% of cases, suggesting a strong genetic component
- Response to alcohol: 50 to 70% of essential tremor patients report temporary improvement after small amounts of alcohol; this is not a treatment but a useful diagnostic indicator
Key Clinical Features of Parkinson's Tremor
- Pill-rolling tremor: the characteristic thumb-against-forefinger movement at rest
- Typically unilateral at onset: starts on one side and progresses to the other over years
- Associated features: bradykinesia, rigidity, postural instability
- Asymmetry: Parkinson's tremor remains asymmetrical in many patients even at later stages
- Jaw and lip tremor: more common in Parkinson's than ET; head tremor is less common
Does Essential Tremor Progress to Parkinson's?
This question has been debated in the literature for decades. A 2006 prospective study by Benito-León and Louis in Brain followed 4,411 adults over 65 and found that those with essential tremor had a 4-fold increased risk of developing Parkinson's disease compared to those without ET. However, the absolute risk remained small: approximately 4% of ET patients developed PD. The current consensus is that ET and PD are distinct conditions that occasionally co-occur, rather than ET being a precursor to PD in most patients.
Diagnosis and What to Do If You Are Concerned
Both conditions are diagnosed clinically, through history and examination rather than a single definitive test. DaTscan (dopamine transporter imaging) can be helpful in distinguishing Parkinson's tremor from ET in ambiguous cases. If you notice a progressive tremor, especially one that is interfering with daily activities such as writing, eating, or using a phone, a referral to a neurologist is appropriate. Early diagnosis improves management options for both conditions.
Sources
- Benito-León J, Louis ED. (2006). Essential tremor: emerging views of a common disorder. Nature Clinical Practice Neurology, 2(12), 666-678.
- Bhatia KP, et al. (2018). Consensus statement on the classification of tremors. Movement Disorders, 33(1), 75-87.
- Louis ED, et al. (2020). Essential tremor: evolving clinicopathological concepts in an era of intensive post-mortem enquiry. Lancet Neurology, 19(3), 211-221.