Tremor is defined as a rhythmical, involuntary oscillation of a part of the body, due to alternating contractions of a pair of agonist-antagonist muscles. The cause of the tremor is usually diagnosed by taking a careful history of the symptoms and previous illnesses, current medications, and habits, followed by a neurological examination targeted at excluding or eliciting certain significant findings.
There are various types of tremor. A dystonic tremor is one which is irregular in amplitude and arrhythmic. The frequency is typically less than 7 Hz and there is no tremor at rest. Instead, the tremor comes on when posture maintained against gravity, or with the initiation of a movement i.e., focal postural or kinetic tremor.
The tremor and the dystonia may affect the same or different body parts. For instance, the head may shake while the neck muscles are spastic and holding the neck in a twisted position, because of dystonia. Accompanying symptoms and signs of dystonia may include:
- Mild blepharospasm
- Alterations in phonation due to spasmodic dysphonia
- Unnoticed torticollis
- Family history of dystonia because of its often familial nature
- The tremor rapidly reduces or disappears in response to sensory tricks (gestes antagonistiques)
- Task-specific tremor; for example, it may occur only when one is doing a task such as writing, and be mistaken to be an action tremor, but may actually represent dystonic tremor
- It may be a position-specific tremor
- It may persist at rest
- It may affect nearby parts of the body
- Dystonic tremor may often disappear in certain positions, called null points
Dystonia produces at least two forms of tremor:
- Dystonic tremor due to contraction of dystonic muscle contractions
- Tremor associated with, but not caused by, dystonia in another body part
Dystonic contractions causing tremor are distinguished from tremor that occurs in opposition to the major position caused by the dystonic contraction, and is not seen when the posture accompanies the direction of pull of the contraction.
Dystonic tremor may be overlooked because it is so mild or appears in response to a compensatory change in posture. It may also be misdiagnosed as another type of tremor such as essential.
Based on available research, several explanations have been offered for dystonic tremor. For instance, tremor in all muscle contractions may be induced by dystonic muscle contraction. Again, the attempt to establish a normal body position against the dystonic pull or posture may result in a tremor. Finally, pathology of the cerebral cortex, or of the lentiform nucleus, may be responsible for dystonic tremor.
The medical management of dystonic tremor is not uniformly successful. The most commonly used drugs include:
- Anticholinergic drugs reduce acetylcholine activity and are therefore especially useful when tremor is troublesome
- Anticonvulsant which may help to inhibit excitability and reduce tremor
- Beta-blocking agents reduce general autonomic excitability and may help to reduce the tremor intensity
Botulinum toxin injections
These improve tremors of the head and vocal cords, also called axial tremors. The safety and effectiveness of this procedure has been established.
Deep brain stimulation
Deep brain stimulation (DBS) has been tried in several cases when the tremor failed to respond to other treatment. Common sites of stimulation include the globus pallidus internus, thalamus, or subthalamic nuclei. This is most successful when the tremors are axial or appendicular.
Specific ablation of certain parts of the brain may improve dystonic tremor.
Reviewed by Afsaneh Khetrapal BSc (Hons)