Surgical Treatment of Dystonia
Cervical dystonia (spasmodic torticollis-CD) went through a rocky history of being neglected, unknown, and attributed to psychogenic causes to being recognized as the most common type of focal dystonias. Anatomical substrate and pathophysiology of CD remain unknown. Treatment is limited to symptomatic intervention. Availability of botulinum toxin has dramatically improved management of the majority of patients with CD. However, a subset of patients with CD do not have sufficient response to pharmacological therapy, or become resistant to botulinum toxin. Surgery has been attempted for treatment of CD for decades. Surgical alternatives include peripheral denervation and intracranial ablative versus DBS procedures. The two major intracranial targets are globus pallidus and thalamus.
According to the model of basal ganglia motor circuitry, hyperkinetic movement disorders are believed to result from decreased inhibitory output from basal ganglia nuclei, known as globus pallidus (GPi). That leads to disinhibition of activity of another nucleus, VIM thalamus, which sends excitatory projections to the cortex, resulting in involuntary movement. This hypothesis is supported by data from electrophysiologic microelectrode recording on patients with dystonia that underwent pallidotomy: the GPi neuronal discharge rate is irregular and significantly lower than in normals. It is unclear what physiological mechanism defines the part of the body effected by dystonia but for the fact that thalamic recordings of dystonia patients who underwent thalamotomy revealed widening of the area of representation of the dystonic body part in VIM part of thalamus compared to patients who have other movement disorders. So, it is believed that dystonia results from the combination of widening of the receptive fields and altered pattern of spontaneous neuronal activity in the pallidothalamocortical motor circuit. That hypothesis provides the physiologic rationale for the choice of two surgical targets: thalamus and globus pallidus.
Recent introduction of deep brain stimulation (DBS) technology has revolutionarized surgical treatment of movement disorders. DBS achieves benefits comparable to the lesioning procedure with the additional benefist of reversibility and adjustability of stimulation. The result is a more favorable side-effect profile, specifically for the patients who require a bilateral procedure. The bulk of DBS experience has been with the treatment of tremor and Parkinson's. Limited data is available on the efficacy of the procedure in dystonia. Kumar et. al. reported beneficial effect of bilateral Gpi DBS in a 49 year old patient with primary generalized dystonia. Krauss et. al. reported benefit of bilateral Gpi DBS in 3 patients with cervical dystonia. Brin et. al. described two patients with primary dystonia who improved with the procedure without a negative impact on cognition. At this time, data is insufficient to draw conclusions on the efficacy of DBS versus ablation, as well as the preferable target for surgery in dystonia. Prospective long term outcome studies are necessary. However, initial experience is promising.
In conclusion, the majority of patients with CD can be effectively managed with a combination of oral medications and botulinum toxin injections. For the ones who have failed pharmacological management, novel surgical approaches offer promising but still experimental alternatives.