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Focal Laryngeal Dystonia: Two Case Reports Applying Navigated TMS over the Laryngeal Motor Cortex

Background: Laryngeal dystonia (LD) is a rare neurological movement disorder with an unknown cause affecting the intrinsic muscles of the larynx with a prevalence of 14-35 per 100 000. Vocal symptoms range from sporadic difficulty to sustained inability to phonate, with vocal tremors (voice breaks) or strained or choked speech. In severe cases, an affected subject may barely speak, which significantly impacts the quality of life and may cause psychiatric disturbances, including depression and anxiety. The diagnosis is challenging due to a lack of pathophysiological biomarkers. Alterations in the microstructural and functional integrity of the corticobulbar tract, descending from the primary motor cortex for representation of laryngeal musculature to the brain stem nuclei involved in voice/speech production might be implicated in the pathophysiology of LD [1, 2]. Neurophysiological studies indicate altered inhibitory mechanisms in LD, as with cervical dystonia and focal hand dystonia. More precisely, the cortical silent period (cSP), as a measure of intracortical inhibition in primary motor cortex (M1), has been reported to be shortened in laryngeal muscles in LD subjects [3]. Objective: To present two LD cases, abductor and adductor LD type, and the use of navigated TMS in mapping the laryngeal motor cortex by presenting results of MEP and cSP. Methods: Single magnetic pulse was applied over the M1 for laryngeal muscle representation during slight vocalization. Hook wire electrodes were used to record MEP and cSP from cricothyroid and vocal muscle. Results: In the subject with adductor LD, MEP was elicited from the left cricothyroid muscle (latency of 11.3 ms) with no cSP induced in the cricothyroid and vocal muscle at maximal stimulator output that subject could tolerate. In subject with abductor LD (left vocal muscle abduction), alterations in the duration of cSP were observed when comparing left and right cricothyroid muscles, and no cSP was elicited from the left vocal muscle. Conclusions: In LD, the cortical activation during phonation may not be efficiently or effectively associated with inhibitory processes, leading to muscular dysfunction. These cSP findings may give insight into the maladaptive cortical control during phonation in people with LD. Promising neuromodulatory techniques such as TMS might bring new light to the diagnosis and treatment of LD disorder. References [1] Simonyan, K. ; Tovar- Moll, F. ; Ostuni, J. ; Hallett, M. ; Kalasinsky, V.F. ; Lewin-Smith, M.R. ; Rushing, E.J. ; Vortmeyer, A.O. ; Ludlow, C.L. Focal white matter changes in spasmodic dysphonia: A combined diffusion tensor imaging and neuropathological study. Brain 2007, 131, 447–459. [2] Rogić Vidaković, M ; Gunjača, I ; Bukić, J. ; Košta, V. ; Šoda, J ; Konstantinović, I. ; Braco Bošković, B. ; Bilić, I. ; Režić Mužinić, N. Neurophysiological Basis and Treatment of Focal Laryngeal Dystonia: A Narrative Review and Two Case Reports Applying TMS over the Laryngeal Motor Cortex. J. Clin. Med. 2022, 15, 11(12), 3453. The Patho- [3] Chen, M. ; Summers, R. ; Goding, G.S. ; Samargia, S. ; Ludlow, C.L. ; Prudente, C.N. ; Kimberley, T.J. Evaluation of the Cortical Silent Period of the Laryngeal Motor Cortex in Healthy Individuals. Front. Neurosci. 2017, 11, 88.

dystonia ; laryngeal dystonia ; TMS ; cortical silent period ; motor evoked potential

First runner-up Best Abstract Award 2022. In the abstract book: page 47-48, A-115.