Primary Writing Tremor
The unilateral tremor that occurs only while writing is called Primary Writing Tremor (PWT). This is a task-specific tremor that predominantly affects the dominant hand during the act of writing and significantly interferes with handwriting ability 1, 2, 3.
Classification of Primary Writing Tremor
PWT has been subdivided into two distinct types based on when the tremor appears 4:
- Type A (Task-Induced Tremor): Tremor occurs only during the actual act of writing 1, 4
- Type B (Positionally Sensitive Tremor): Tremor appears both during writing and when the hand assumes a writing posture, even without actual writing 1, 2, 4
Clinical Characteristics
PWT typically presents with a 5-7 Hz frequency tremor that predominantly affects the dominant hand 1, 3, 4. Key distinguishing features include:
- Mean age of onset around 50 years, with male predominance 4
- Significantly reduced writing speeds (approximately 73 letters per minute compared to 128 in healthy controls) 4
- Temporary suppression by alcohol in approximately one-third of cases 1
- Worsening with anxiety 3
- Family history present in about one-third of patients 4
Distinguishing PWT from Writer's Cramp (Dystonia)
PWT can be differentiated from writer's cramp through specific neurophysiological findings 4, 5:
- Normal reciprocal inhibition of the H-reflex in PWT (abnormal in writer's cramp) 1, 4
- No excessive EMG "overflow" activity to proximal muscles (present in dystonia) 3, 4
- Normal intracortical excitability 1
- Hyperactivity in cerebellar hemispheres on PET imaging 1
- Underactivation of cingulum and overactivation of primary motor and supplementary areas on fMRI 3
The shortening of silent period during near-maximal voluntary contraction indicates that PWT is a separate nosological entity rather than a variant of focal task-specific dystonia 5.
Treatment Approach
There are no randomized controlled trials for PWT treatment, but anecdotal evidence supports several therapeutic options 1, 3:
Pharmacological Management
- Propranolol has shown benefit in some patients 1, 4
- Primidone has demonstrated efficacy in select cases 1, 4
- Approximately 10 out of 21 patients in one series obtained benefit from drug treatment, primarily with propranolol or primidone 4
Other Treatment Modalities
- Anticholinergics have shown anecdotal benefit 1
- Botulinum toxin injection may be considered 1, 2, 3
- Deep brain stimulation (DBS) has been reported as beneficial in refractory cases 1, 3
- Occupational therapy and adaptive strategies should be incorporated into the treatment plan 3
Critical Diagnostic Pitfall
Do not confuse PWT with writer's cramp based solely on the task-specific nature of the tremor. The key differentiating feature is that PWT shows normal reciprocal inhibition of the H-reflex, whereas writer's cramp demonstrates decreased presynaptic inhibition 4. Additionally, PWT lacks the excessive proximal muscle EMG activity characteristic of dystonic writer's cramp 3, 4.