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The successful application of botulinum toxin (BTX) injections in the treatment of focal hand dystonia is largely dependent on careful evaluation and selection of muscles to be injected.1 It has been suggested that patients should be examined for abnormal postures at rest and while carrying out the affected task in question as well as other tasks (such as using a cup or a comb).2 Simple techniques such as the localisation of subjective pain and fatigue accompanied by palpation of the area of discomfort can also be used.2
Mirror dystonia consists of dystonic postures and movements of the dominant hand while writing or performing other tasks with the non-dominant hand.3 We present a series of six patients who were successfully injected with BTX using mirror dystonia as an additional tool for muscle evaluation.
We carried out a retrospective review of the case records of consecutive patients with writing dysfunction (writer’s cramp and writing tremor) who had been injected with BTX type A (Botox®, Allergan Pharmaceuticals, Irvine, California, USA) between November 2000 and October 2002 at our movement disorders clinic. Our study was limited to patients with writing dysfunction who displayed mirror dystonia while writing with their non-dominant hand. We specifically examined patients for mirror dystonia by asking them to write with their non-dominant hand while resting the dominant hand on the ulnar side of the forearm (unaware of our focus on the detection of mirror dystonia of the resting limb). Patients were injected under EMG guidance, using an Allergan® EMG needle. We recorded the muscles injected and the dose each muscle received. Peak effect was defined as the maximum benefit obtained from the injection. It was rated on a 0 to 3 global impression scale (0 = no effect; 1 = mild improvement; 2 = moderate improvement; 3 = marked improvement). The presence and severity of adverse events was also recorded. We also looked at the concordance between observation of the dominant limb in the action of writing and observation of the mirror dystonia movements of the same limb while writing with the non-dominant hand. The formulation and preparation of BTX was carried out using standard methods. Overviews of the demographic, clinical, and treatment variables are presented in table 1. We identified six patients with writing dysfunction (M/F:1/5, mean age 46 years (range 30 to 75), mean duration of disease 7.16 years (range 2 to 13)). Four patients (cases 1, 2, 5, and 6) had writer’s cramp and two (cases 3 and 4) had features overlapping writing tremor with writer’s cramp. Three patients reported marked (cases 1, 2, and 5), two moderate (cases 3 and 4), and one mild improvement (cases 6). We further identified two subgroups: four patients where mirror dystonia consisted of any combination of extension/abduction of the thumb, fingers, and wrist (subgroup A: cases 2, 3, 4, and 5) and two patients where mirror dystonia consisted of hyperflexion of wrist or thumb and fingers (subgroup B: cases 1 and 6). In subgroup A two patients experienced marked improvement and two had moderate improvement following injections. Two of these four patients did not show mirror dystonia movements when seen 15–17 weeks later at the time they were due for repeat injections. In subgroup B one patient experienced marked improvement and one mild improvement. None of our patients had evidence of mirror dystonia in the non-dominant hand when writing with the dominant hand. Regarding adverse events, transient weakness was experienced by four patients (for one to four weeks). No other adverse events were reported.
Concordance in the action of writing and observation of the mirror dystonia movements in the same limb was found in four patients. In three there was enrichment of the observation in that additional muscles could be shown to be active. The two discordant patients had writing tremor with no overt deviation noted when the dominant hand wrote.
The importance of recognition of mirror dystonia in patients with writing dysfunction has been previously highlighted by Jedynak et al.3 He reported that 29 of 65 patients with writer’s cramp had evidence of mirror dystonia and suggested that mirror dystonia may be useful in muscle selection (it may help in the differentiation between primary and compensatory movements). Borgohain et al also reported on the subject; however, that work has only been published in abstract form.4 The investigators proposed that mirror dystonia used as a guide for muscle selection for BTX injections may reduce the difference in outcome between extensor and flexor writer’s cramp and suggested that mirror dystonia was a superior method for muscle selection compared with compensatory movements.
Although the mechanism of mirror dystonia remains unclear it has been suggested that it is likely to be related to the metabolic abnormalities shown to involve the primary sensorimotor and supplementary motor cortices in patients with focal hand dystonia.5 Magnetic cortical stimulation has confirmed that corticocortical inhibition is reduced over both hemispheres.6 Jedynak et al suggested that mirror dystonia is the consequence of abnormal cortical inhibition and decreased selectivity of muscle patterns for highly skilled manual tasks.3
We conclude that analysis of the pattern of dystonic posturing displayed in mirror dystonia when examining patients with writing dysfunction is a useful guide for selection of muscles to be injected with BTX. A prospective trial of BTX injections in muscles selected through analysis of mirror dystonia could provide further information about the therapeutic results of this method.
This work was presented in part at the American Academy of Neurology 55th Annual Meeting in Honolulu, Hawaii, 31 March to 4 April 2003.
Competing interests: none declared