What type of tremor is associated with Parkinson's disease?

Parkinson's Disease Tremor Characteristics

Parkinson's disease is primarily characterized by resting tremor, which occurs when the affected limb is relaxed and supported against gravity, along with bradykinesia and rigidity. 1

Clinical Features of Parkinsonian Tremor

• Resting tremor is the most distinctive tremor type in Parkinson's disease, appearing when the affected limb is at rest and typically diminishing with voluntary movement 1
• The clinical presentation of Parkinson's disease includes the classic triad of resting tremor, bradykinesia, and rigidity 1
• Parkinsonian tremor typically begins unilaterally, often in the hands, with a characteristic "pill-rolling" quality (movement of thumb against fingers) 1
• The tremor in Parkinson's disease is related to progressive degeneration of dopaminergic neurons in the substantia nigra projecting to the striatum 1

Pathophysiology of Parkinsonian Tremor

• Parkinson's tremor develops after approximately 40-50% of dopaminergic neurons in the substantia nigra have been lost 1
• The tremor involves dysfunctional networks in two partially overlapping central motor circuitries: the cerebello-thalamo-cortical and basal ganglia-cortical loops 2
• Unlike other parkinsonian symptoms that directly correlate with dopamine deficiency, tremor pathophysiology is more complex and may involve additional neurotransmitter systems 2

Distinguishing Features from Other Tremors

• Unlike essential tremor, which is primarily a postural or action tremor, Parkinson's tremor is predominantly present at rest 3
• Parkinsonian tremor may be accompanied by other cardinal features including bradykinesia, rigidity, and postural instability, which help differentiate it from isolated tremor disorders 1
• Some patients may present with "tremor-predominant Parkinson's disease" where tremor is the most prominent symptom with relatively less significant rigidity and bradykinesia 4

Clinical Assessment and Measurement

• Parkinsonian tremor can be objectively measured using tri-axial accelerometers, which can detect frequency-domain features strongly correlated with resting tremor activity 5, 6
• The Unified Parkinson's Disease Rating Scale (UPDRS) is commonly used to clinically assess tremor severity in Parkinson's disease patients 5
• Tremor amplitude constitutes a significant portion of the UPDRS motor score, making it an important clinical marker 1

Treatment Implications

• Levodopa, the metabolic precursor of dopamine, crosses the blood-brain barrier and is converted to dopamine in the brain, which helps alleviate tremor and other parkinsonian symptoms 7
• First-line medications for Parkinson's disease tremor include levodopa, dopamine agonists, and anticholinergics, which are often effective in controlling tremor 4
• For patients with tremor refractory to medication, neurosurgical interventions targeting the thalamus, globus pallidus, or subthalamic nucleus may provide excellent tremor control 4

Clinical Pitfalls and Caveats

• Some patients may present with isolated resting tremor for years before developing other parkinsonian features, making early diagnosis challenging 3
• Imaging studies like MRI and PET can help differentiate isolated resting tremor as a variant of Parkinson's disease rather than essential tremor 3
• The severity of tremor does not always correlate with disease progression or response to dopaminergic therapy, unlike bradykinesia and rigidity 2
• Tremor may be absent in up to 25% of Parkinson's disease patients, so its absence does not exclude the diagnosis 6