Orthostatic Tremor

Orthostatic tremor is a rare disorder in which standing causes a rapid, rhythmic (13 to 18 Hz) tremor of the legs and trunk, with a feeling of unsteadiness and need to sit or walk. The tremor is often too fast to be seen, and is felt as shaking or fear of falling.

It is rare, with a prevalence of probably under 1 in 100,000, but it is under recognised. Onset is usually after 50 years and women are more affected (about 2 to 1).

Forms. Primary orthostatic tremor, and secondary orthostatic tremor with parkinsonism, cerebellar disease, or neuropathy. Slow orthostatic tremor (under 13 Hz) is distinguished from the classical form.

Course. Slowly progressive over years, with growing difficulty standing still and falls in some; a minority develop other neurological signs.

Pathophysiology.

A central oscillator in the brainstem and cerebellar circuits produces a rapid rhythmic output that is synchronous across muscles.

Weight bearing and proprioceptive input appear to trigger it, which explains relief on walking or sitting.

• Unsteadiness and shaking of the legs on standing still, relieved by sitting, walking or leaning on something.
• Palpable or auscultated fine, fast rippling in leg muscles (the "helicopter" sound on auscultation).
• Normal examination when sitting, and little tremor when walking.
• Fear of falling, anxiety, and avoidance of queuing and standing.
• Arm tremor, mild gait ataxia or parkinsonism in some.

Red flags.

• Progressive weakness, sensory loss or cerebellar signs.
• Rapid progression or parkinsonism.
• Onset after exposure to toxins or drugs.

• Surface electromyography of leg muscles while standing, showing a 13 to 18 Hz rhythmic burst, is diagnostic.
• Auscultation of the thigh or calf with a stethoscope shows a low pitched thumping sound on standing.
• MRI of the brain and spine in atypical or secondary cases.
• DAT SPECT when parkinsonism is suspected; nerve conduction studies for neuropathy.
• Review of drugs and a falls assessment.

Differential diagnosis. Functional tremor and functional gait disorder, essential tremor, Parkinson disease, orthostatic hypotension, cerebellar ataxia, peripheral neuropathy and anxiety.

• Reassure that the condition is not life threatening, give a clear explanation, and use walking, leaning and seating strategies.
• Clonazepam, first line: 0.25 mg orally at night, increased by 0.25 to 0.5 mg each week to a usual 1 to 3 mg daily in 2 to 3 divided doses. Maximum 4 mg daily. Monitor sedation, falls, cognition, dependence and respiratory depression. Avoid in severe respiratory disease, sleep apnoea, myasthenia gravis and severe hepatic impairment; use lower doses in older adults and renal impairment; avoid in pregnancy and breastfeeding unless essential; withdraw slowly.
• Gabapentin 300 mg orally at night on day 1, 300 mg twice daily on day 2, then 300 mg three times daily on day 3, increased by 300 mg every 3 to 7 days to a usual 1200 to 2400 mg daily in 3 divided doses. Maximum 3600 mg daily. Monitor sedation, dizziness, peripheral oedema, mood and misuse. Adjust for renal function: creatinine clearance 50 to 79 mL/min, 600 to 1800 mg daily; 30 to 49 mL/min, 300 to 900 mg daily; 15 to 29 mL/min, 150 to 600 mg daily; under 15 mL/min, 150 to 300 mg daily. Avoid in pregnancy unless benefit outweighs risk.
• Primidone 25 mg orally at night, increased by 25 mg each week to a usual 125 to 250 mg daily in 1 to 2 divided doses. Maximum 750 mg daily. Monitor sedation, ataxia and folate; avoid in porphyria; reduce dose in renal and hepatic impairment; avoid in pregnancy unless needed.
• Levodopa, pramipexole or other drugs may help in a minority, particularly with coexisting parkinsonism, through specialist advice.
• Spinal cord stimulation or deep brain stimulation of the thalamic ventral intermediate nucleus for refractory, disabling cases.
• Physiotherapy, a walking frame or stool, and falls prevention.

Monitoring. Review at 4 to 6 weeks, then every 6 months; track standing time, falls, sedation and mood.

Referral and escalation. Movement disorder specialist for diagnosis and drug choice; neurosurgery for stimulation.

Special groups. Older adults: increased falls and sedation risk with clonazepam, so start low and titrate slowly.