A 68-year-old man presents for evaluation by a neurologist for an involuntary hand tremor that started approximately 3 months ago. The shaking improves when he reaches for objects but returns when he is not using his hand. His temperature is 98.6°F (37.0°C), blood pressure is 115/70 mmHg, pulse is 70/min, and respirations are 12/min. His left hand exhibits a 4-6 Hz tremor when resting on his lap. The tremor extinguishes with voluntary movement. In addition, his left upper extremity has increased tone with passive range of motion. Which of the following is the most likely diagnosis?
Cerebellar stroke
Essential tremor
Lacunar stroke
Parkinson disease
Parkinson disease
This patient who presents with a low amplitude, 4-6 Hz resting tremor that resolves with voluntary movement and associated rigidity most likely has Parkinson disease.

Parkinson disease is caused by degeneration of dopaminergic neurons in the substantia nigra pars compacta of the basal ganglia. This neurodegenerative process leads to movement abnormalities such as bradykinesia, “cogwheel” muscle rigidity (intermittently increased muscle tone with passive range of motion), shuffling gait, postural instability, and a resting tremor sometimes referred to as “pill-rolling” in quality. Parkinson disease usually presents with unilateral symptoms and subsequently progresses to bilateral involvement. First-line treatment involves dopaminergic drugs such as carbidopa-levodopa. Antimuscarinic agents can also be used for symptom control.

Zhong et al. review the role of the cerebellum in tremor in Parkinson disease. The authors find that the cerebellum may modulate tremor amplitude via cerebello-thalamo-cortical circuits. The authors recommend further study of the cerebellum as an alternative therapeutic target in Parkinson patients with dopamine-resistant tremors.

Incorrect Answers:
Answer A: Cerebellar stroke can manifest as an intention tremor, which presents as an oscillatory tremor that increases in amplitude at the endpoint of visually goal-directed movement. Intention tremors are best exhibited in the finger-nose-finger test in which the patient repeatedly touches their nose and then extends their arm to touch the examiner's finger. Patients would also have more severe symptoms including ataxia, nausea, vomiting, and nystagmus.

Answer B: Essential tremor is a condition that is often familial and presents as a high frequency (4-9 Hz) tremor that occurs with sustained posture and is worsened with movement. Treatment includes propranolol and primidone.

Answer C: Huntington disease is an autosomal dominant trinucleotide repeat disorder that leads to neurodegeneration of the caudate and putamen. Chorea is a hallmark movement abnormality associated with Huntington disease and presents with spontaneous, jerking, purposeless involuntary movements.

Answer D: Lacunar stroke can result in ischemia to the subthalamic nucleus and produce hemiballismus. This presents as spontaneous unilateral explosive movements of an upper extremity and occasionally the ipsilateral lower extremity. Other motor deficits are also expected when there is a lacunar infarct.

Bullet Summary:
Resting tremor is a hallmark of Parkinson disease and presents with low amplitude, 4-6 Hz oscillatory rhythmic movements that temporarily extinguish with voluntary movement.