Background: Vocal cord paresis (VCP) may occur following high intensity focused ultrasound (HIFU) of thyroid nodules. We hypothesised its occurrence relates to the distance of the focus point (FP) of the HIFU beams from the recurrent laryngeal nerve (RLN) and the thermal power that this point received. Their relationships were examined.
Methods: One hundred and three patients who underwent HIFU for symptomatic benign thyroid nodule from October 2015 to March 2017 were analysed. All treatment images were captured and were later watched by 2 reviewers to identify three FPs closest to the tracheoesophageal groove (TEG) on transverse sonographic view. TEG was taken as the RLN position. After identifying these FPs, their distance (mm) from the TEG, thermal power (W) used and depth from skin (mm) were recorded. These parameters were compared between those with and without VCP. VCP was defined as a cord with reduced or no movement.
Results: Four (3.9%) patients suffered from a unilateral VCP afterwards but they all recovered fully within 6 weeks. There were no significant differences in baseline characteristics and treatment efficacy between the two groups. The distance from TEG (OR = 1.706, 95%CI = 1.001 to 2.915, p = 0.050) was the only significant factor for VCP. None of the other variables including thermal power were significant.
Conclusions: The incidence of VCP was 3.9% (4/103) and they completely recovered within 6 weeks. The distance between the FP and the TEG was the only related factor for VCP. The safe distance between FP and TEG should be ≥1.1 cm.
Keywords: Focused ultrasound; benign thyroid nodule; recurrent laryngeal nerve; thermal ablation; vocal cord palsy.