Objective: We diagnosed a rare pediatric case of genetic focal hand dystonia (FHD) with dystonic tremor and optimized Onabotulinum Toxin A injection treatment dosing.

Background: DYT1 is an autosomal dominant genetic dystonia with sustained or intermittent contractures of muscles of varying severity and duration. Oral medications are not very helpful in task specific dystonia [1]. Botulinum toxin (BT) injection can be tried, but there is no consensus on proper dosage, technique or injection sites in pediatric patients. Instead, published treatment approaches for adult dystonia and pediatric spasticity are often referenced[1][2]. But there is uncertainty about dosage and complications in this age group. Here, we present a 15-year-old male of DYT1 type upper extremity dystonia and tremor, who failed oral trihexyphenidyl therapy, but improved greatly with BT injections using minimal pediatric spasticity and adult dystonia dosage recommendations.

Method: A 15-year-old 72 kg male presented with 4 years of R hand extensor type writer’s cramp, dystonic tremor and L hand tremor. He was R-handed prior to symptom onset. Development of R hand writer’s cramp made him pathologically L-handed. Then, onset of a L hand dystonia and tremor made writing difficult bilaterally. His father had similar undiagnosed symptoms. Physical examination showed extensor FHD, dystonic tremor, and geste antagoniste (Video 1). Brain MRI was normal. Genetic testing showed a GAG deletion in exon 5 of TOR1A, which causes early onset primary focal dystonia[3].

Trihexyphenidyl was titrated for 8 months up to a dose of 20mg/day with no improvement. Thus, a trial of BT was injected intramuscularly under EMG guidance. Full consent was obtained. Table 1 shows dosage and muscle groups treated.

Results: Thirteen weeks after the first injection, there was some improvement in both hands. But, there was L third and fourth finger extensor muscle weakness. A second injection was given with modified dosages[table1]. There was significant improvement of his extensor writer’s cramp and tremor bilaterally 5 weeks later (Video 2).

Conclusion: Early diagnosis and optimum treatment with BT injection in Focal Dystonia is helpful and safe in children. Dosage should be calibrated based on muscle activity, distribution, and type, age, symptom severity, and therapy complications. EMG is helpful to guide injections. Addressing complications and educating family is also important.

References: [1] Luc, Q. N. and Querubin, J. Clinical management of dystonia in childhood. Pediatr Drugs. 2017;19:447-461. doi:10.1007/s40272-017-0243-3 [2] Pandey S. A practical approach to management of focal hand dystonia. Ann Indian Acad Neurol. 2015;18(2):146–153. doi:10.4103/0972-2327.156563 [3] Ozelius, L. J., Hewett, J. W., Page, C. E., Bressman, S. B., Kramer, P. L., Shalish, C., de Leon, D., Brin, M. F., Raymond, D., Corey, D. P., Fahn, S., Risch, N. J., Buckler, A. J., Gusella, J. F. & Breakefield, X. O. The early-onset torsion dystonia gene (DYT1) encodes an ATP-binding protein. Nat Genet. 1997;17(1):40-8. doi:10.1038/ng0997-40

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MDS Abstracts - https://www.mdsabstracts.org/abstract/botulinum-toxin-dose-optimization-in-pediatric-dyt1-type-focal-hand-dystonia-and-tremor/