What is the typical cause of a resting tremor in an older adult?

Resting Tremor: Cause and Diagnostic Approach

Primary Cause

Parkinson's disease is the typical cause of resting tremor in older adults, as resting tremor is the hallmark motor feature of PD and typically begins asymmetrically in one hand. 1, 2

Pathophysiology

• Resting tremor in PD results from loss of dopaminergic neurons in the substantia nigra, with symptoms appearing after approximately 40-50% of these neurons have degenerated. 2
• The tremor is present when the hand is fully supported against gravity and characteristically disappears with voluntary movement. 1
• Patients with isolated resting tremor for many years demonstrate the same striatal dopamine transporter deficits and MRI abnormalities (smudging or decreased distance between substantia nigra and red nucleus) as typical PD patients, confirming this represents a variant of Parkinson's disease rather than essential tremor. 3

Clinical Features That Confirm PD

• Bradykinesia (slowness of movement) is the essential diagnostic feature and must be present alongside the resting tremor for PD diagnosis. 2, 4
• Rigidity with cogwheel quality when combined with tremor is a cardinal sign. 2, 4
• The tremor typically exhibits a rhythmic, pill-rolling pattern at 4-6 Hz and begins unilaterally before potentially spreading to the other side. 1, 4
• Postural instability occurs later in the disease course. 2, 5

Critical Differential Diagnoses

Essential tremor must be distinguished from PD, as it presents primarily with postural and action tremor (not resting tremor), rarely involves isolated hand tremor at rest, and predominantly affects hands bilaterally, head, and voice. 4, 6, 7

Other causes of resting tremor to consider include:

• Drug-induced parkinsonism from dopamine antagonists (antipsychotics), SSRIs, TCAs, or lithium. 8, 1
• Dystonic tremor, which can have a rest component but is associated with dystonic postures. 9
• Holmes tremor (midbrain tremor) from brainstem lesions, which combines rest, postural, and intention components. 6, 9
• Functional/psychogenic tremor, characterized by variable frequency, entrainability with voluntary movements, and sudden onset in context of stress. 1

Diagnostic Confirmation

Obtain I-123 ioflupane SPECT/CT (DaTscan) when the clinical diagnosis is uncertain, as this shows decreased striatal uptake in PD but is normal in essential tremor and drug-induced tremor. 2, 4

• A normal DaTscan essentially excludes parkinsonian syndromes. 2, 4
• MRI brain without contrast should be obtained first to exclude structural lesions, vascular disease, or atypical parkinsonian syndromes (progressive supranuclear palsy, multiple system atrophy, corticobasal degeneration). 1, 2
• Referral to a neurologist or movement disorder specialist is essential for diagnostic confirmation, as correctly diagnosing parkinsonian syndromes on clinical features alone is challenging. 2

Red Flags for Alternative Diagnoses

• Early severe autonomic dysfunction, cerebellar signs, or pyramidal signs suggest multiple system atrophy rather than PD. 2
• Vertical gaze palsy (especially downward) suggests progressive supranuclear palsy. 2
• Asymmetric rigidity with alien hand phenomenon suggests corticobasal syndrome. 2
• Tremor that worsens with posture or action rather than at rest points toward essential tremor. 1, 4

Medication Review

Review and discontinue medications that can cause or exacerbate tremor, including antipsychotics, metoclopramide, SSRIs, TCAs, MAOIs, lithium, and stimulants. 8, 1

• Calcium levels should be monitored, as hypocalcemia can induce or aggravate tremors. 8

Common Pitfalls

• Do not diagnose essential tremor based solely on unilateral resting tremor—essential tremor is primarily postural/action tremor and typically bilateral. 1, 4
• Do not assume isolated resting tremor is benign; it represents early or monosymptomatic PD with the same underlying pathology. 3, 9
• Do not start tremor medications before completing the diagnostic workup and excluding secondary causes. 1
• Do not use propranolol for parkinsonian resting tremor—beta-blockers are only effective for essential tremor and enhanced physiologic tremor. 1, 4