Dystonic Tremor

Dystonic tremor is a tremor occurring in a body part affected by dystonia. It is irregular, jerky and position dependent, and often improves with a sensory trick or by relaxing into a null position of the dystonic posture.

It is probably under diagnosed and is often mistaken for essential tremor. Tremor occurs in about 40 to 50% of patients with focal dystonia, such as cervical dystonia and writer's cramp. Onset is usually in adult life and the sexes are affected about equally.

Forms. Dystonic tremor of the head, neck or arm, associated with a dystonic posture; and tremor associated with dystonia, which occurs in a different body part from the dystonic one.

Course. Slowly progressive, tending to remain focal or segmental. Family history of dystonia or tremor is common.

Pathophysiology.

Abnormal cerebello-thalamo-cortical and basal ganglia network activity produces a dystonic movement pattern with oscillation.

Impaired sensorimotor integration and loss of inhibition explain the sensory trick and task specific tremor.

• Irregular, jerky tremor of low to medium frequency (usually under 7 Hz) that varies with posture and position and may be absent at rest.
• Dystonic posture in the same body part: head tilt, shoulder elevation, or arm posturing.
• Sensory trick (touching the chin or face) reduces the tremor; a null point may reduce it markedly.
• Task specific worsening, such as writing; head tremor may be directional (a "no no" or "yes yes" tremor).
• Normal strength and gait, unless there is another cause.

Red flags.

• Rapid onset, onset in childhood or progressive generalisation.
• Rigidity, bradykinesia or other neurological signs.
• Psychiatric symptoms or a recent drug exposure (dopamine blockers).

• Clinical diagnosis: examine at rest, with posture, during action and in the null position, and look for dystonia in all body parts.
• Neurophysiology (surface electromyography and accelerometry) can show irregular tremor and dystonic bursts.
• MRI of the brain for early onset, atypical features or asymmetry.
• DAT SPECT when parkinsonism is suspected; copper studies and ceruloplasmin under age 40; genetic testing in young onset or family history.
• Review of drug history and thyroid function.

Differential diagnosis. Essential tremor, Parkinson disease, functional tremor, enhanced physiological tremor, orthostatic tremor, cerebellar tremor, Wilson disease and drug induced tremor.

• Explain that this is a dystonic disorder, which affects the choice of treatment and differs from essential tremor.
• OnabotulinumtoxinA by a specialist: 20 to 100 units intramuscularly, divided across the forearm, neck or other affected muscles under electromyographic or ultrasound localisation, repeated no sooner than every 12 weeks. Maximum 400 units in total each session. Assess response at 4 weeks and 3 months. Monitor for weakness, dysphagia (neck injections) and injection site reactions. Avoid with infection at injection sites and neuromuscular disease; avoid in pregnancy unless benefit outweighs risk.
• Trihexyphenidyl: 1 mg orally once daily, increased by 1 to 2 mg every 1 to 2 weeks as tolerated to a usual 6 to 15 mg daily in 3 divided doses. Maximum 30 mg daily under specialist supervision. Monitor confusion, memory, dry mouth, constipation, blurred vision and urinary retention. Avoid in angle closure glaucoma, gastrointestinal obstruction, prostatic hypertrophy and dementia; use lower doses in older adults and in renal or hepatic impairment.
• Propranolol 40 mg orally twice daily, increased every week to a usual 120 to 240 mg daily in 2 to 3 divided doses. Maximum 320 mg daily. Monitor heart rate and blood pressure. Avoid in asthma, bradycardia, heart block, hypotension and peripheral vascular disease; reduce dose in hepatic impairment; use caution in pregnancy. Benefit is often partial.
• 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.
• Deep brain stimulation of the globus pallidus internus or thalamus for severe, refractory tremor, assessed at a specialist centre.
• Physiotherapy, sensory trick training and occupational therapy; ergonomic aids.

Monitoring. Review at 3 months after each injection or drug change, then every 6 to 12 months.

Referral and escalation. Movement disorder specialist for diagnosis and botulinum toxin; neurosurgery for deep brain stimulation.

Special groups. Older adults: anticholinergics are poorly tolerated, so prefer botulinum toxin. Women of childbearing potential: avoid primidone where possible.