Occupational cramps or spasms are included in this chapter because the prevailing opinion is that they are acquired forms of regional or focal “task-specific” dystonias. In the most common form, writer’s cramp, the patient experiences, upon attempting to write, that all the muscles of the thumb and fingers either go into spasm or are inhibited by a feeling of stiffness and pain or hampered in some other inexplicable way. The clinical descriptions of writer’s cramp by Sheehy and Marsden are worth consulting. Men and women are equally affected, most often between the ages of 20 and 50 years. The spasm may be painful and can spread into the forearm or even the upper arm and shoulder. Sometimes the spasm fragments into a tremor that interferes with the execution of fluid, cursive movements. Immediately upon cessation of writing, the spasm disappears. At all other times and in the execution of grosser movements, the hand is normal, and there are no other neurologic abnormalities. Many patients learn to write in new ways or to use the other hand, though that, too, may become involved.
Other highly skilled motor acts performed over long periods of time, such as playing the piano or fingering the violin, may induce a similar highly task-dependent spasm (“musician’s cramp,” “musician’s dystonia”) or in the past, telegrapher’s palsy. The “loss of lip” in trombonists and other brass and wind instrumentalists (embouchure dystonia) represents an analogous phenomenon, seen only in experienced musicians. In each case a delicate motor skill, perfected by years of practice and performed almost automatically, suddenly comes to require a conscious and labored effort for its execution. Discrete movements are impaired by a spreading recruitment of unneeded muscles (intention spasm). Once developed, the disability persists in varying degrees of severity, even after long periods of inactivity of the affected part.
Regarding pathogenesis, Byl and colleagues, found that sustained, rapid, and repetitive highly stereotypical movements of the hand in monkeys greatly expand the area of hand cortical representation. These authors have hypothesized that degradation of sensory feedback to the motor cortex was responsible for excessive and persistent motor activity, including dystonia. Many patients with focal acquired dystonia demonstrate minor sensory abnormalities by way of impaired temporal and spatial detection of stimuli on careful examination. A similar enlargement of the area of cortical response to magnetic stimulation has been found by a number of investigators in patients with writer’s cramp and the volume of gray matter was decreased in the sensorimotor cortex, thalamus, and cerebellum corresponding to the affected hand in the report by Delmaire and coworkers. There is a special category of dystonia following nerve injury, often with severe burning pain and autonomic changes that conforms to reflex sympathetic dystrophy. In these cases, it may be the injury that causes a reconfiguration of the sensory receptive fields. Berardelli et al have reviewed other theories pertaining to the physiology of the focal dystonias. More recent notions have included changes in synaptic plasticity as a result of overuse.
A high degree of success has been obtained by injections of botulinum toxin into specifically involved muscles, such as those of the hand and forearm in cases of writer’s cramp (Cohen et al; Rivest et al), and this is now widely used. The best results are obtained by guiding the injection from both palpation and EMG detection of the specific muscles that are active in the dystonic posture. Various forms of hand retraining are also said to be useful.
Transcutaneous electrical stimulation (TENS) of the forearm in 20-minute sessions has a modest effect according to a study by Tinazzi and colleagues. It had been claimed that the patient can be helped by a deconditioning procedure that delivers an electric shock whenever the spasm occurs or by biofeedback, but these forms of treatment have been largely abandoned in favor of botulinum treatments. There have been some preliminary investigations of thalamotomy and deep brain stimulation for resistant cases.