Writer’s cramp is also known as mogigraphia and scrivener’s palsy. It causes a cramp or spasm affecting certain muscles of the hand and/or fingers. Writer’s cramp is a task-specific focal dystonia of the hand. ‘Focal’ refers to the symptoms being limited to one location (the hand in this case), and ‘task-specific’ means that symptoms first occur only when the individual engages in a particular activity. Writer’s cramp first affects an individual by inhibiting their ability to write. Musician’s cramp (a similar focal dystonia which affects some 1% of instrumentalists) has historically been grouped together with writer’s cramp because of this and their common task-specificity.
69 cases per 100,000 populations; male-to-female ratio is 1.3:1
Typically, patients present in the third to fifth decades, and women usually present earlier than men. In the beginning, the prevalence of Writer’s cramp has been reported as being between 16.3 and 68.9 per million in various reported series.
Most cases are idiopathic. Writer’s cramp frequently affects persons who write a great deal or perform other repetitive hand movements such as typing. Approximately 5% of patients have a positive family history of a similar condition. Five to ten percent report an accident or injury to the hand or arm immediately preceding the onset of symptoms. Rare causes include C6 ruptured disk, lithium use, basal ganglia or cortical tumors, arteriovenous malformations (AVMs), and stroke.
Normally, an antagonist muscle relaxes when an agonist muscle is contracted. Patients with dystonia have simultaneous contraction of both groups of muscles. Spinal reciprocal inhibition, a process that inhibits the antagonist muscles when the agonist muscles are active, is reduced in patients with writer’s cramp. This is most probably due to aberrant descending commands. Abnormalities in the basal ganglia lead to abnormalities of sensory processing and motor output. The normal increase in cerebral blood flow in the supplementary motor area is reduced in response to vibration and abnormal somatosensory evoked potentials. These provide evidence for the abnormal sensory processing in patients with dystonia. Increased motor cortex excitability along with decreased cortical inhibition causes abnormal motor output.