title: Normative intra-operative electrophysiologic waveform analysis of superior laryngeal nerve external branch and recurrent laryngeal nerve in patients undergoing thyroid surgery.
abstract: BACKGROUND
  Injury to the external branch of the superior laryngeal nerve (EBSLN) can occur during superior pole dissection in thyroid surgery; the EBSLN injury rate is reported as high as 28 % (Cernea et al., Head Neck 14:380-383, 1992). Injury to the EBSLN leads to variable symptoms that may be overlooked, but that can be significant, especially to professional speakers and singers. Intraoperative nerve monitoring (IONM) is employed widely to aid in nerve identification. We report on normative electroneuromyography (EMG) data on EBSLN-IONM and cricothyroid muscle (CTM) twitch response during stimulation as an aid to EBSLN identification.
  
  
  METHODS
  A prospective study of the SLN and the recurrent laryngeal nerve (RLN) IONM data in 72 consecutive thyroid surgeries was carried out. All patients underwent preoperative and postoperative laryngeal exams, and patients with abnormal preoperative laryngeal function were excluded. Normative EMG data and CTM twitch response during EBSLN stimulation were recorded and analyzed.
  
  
  RESULTS
  Stimulation of the EBSLN resulted in a positive CTM twitch response in 100 %, whereas EMG response was recordable in 80 %. Electromyographic amplitude was ~1/3 of ipsilateral RLN amplitude and did not change through the case with multiple stimulations. Stimulation of the EBSLN was similar for men and women and at 1 and 2 mA stimulation levels.
  
  
  CONCLUSIONS
  Intraoperative nerve monitoring of the EBSLN aids in EBSLN identification and provides electroneuromyographic information in 80 % of cases. The laryngeal head of the sternothyroid muscle is a useful landmark to locate EBSLN.
authors.affiliation: Division of Thyroid and Parathyroid Surgery, Department of Otology and Laryngology, Massachusetts Eye and Ear Infirmary, Harvard Medical School, 243 Charles Street, Boston, MA 02114, USA.
authors.forename: Andre S
authors.initials: AS
authors.lastname: Potenza
authors.forename: Eimear A
authors.initials: EA
authors.lastname: Phelan
authors.forename: Claudio R
authors.initials: CR
authors.lastname: Cernea
authors.forename: Cristian M
authors.initials: CM
authors.lastname: Slough
authors.forename: Dipti V
authors.initials: DV
authors.lastname: Kamani
authors.forename: Ashlie
authors.initials: A
authors.lastname: Darr
authors.forename: David
authors.initials: D
authors.lastname: Zurakowski
authors.forename: Gregory W
authors.initials: GW
authors.lastname: Randolph
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D059925:Anatomic Landmarks; D004568:Electrodiagnosis / Q000379:methods*; D005260:Female; D006801:Humans; D007431:Intraoperative Complications / Q000517:prevention & control*; D061224:Laryngeal Nerve Injuries / Q000209:etiology / Q000517:prevention & control*; D007823:Laryngeal Nerves / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D016343:Monitoring, Intraoperative / Q000379:methods*; D017063:Outcome Assessment, Health Care; D011446:Prospective Studies; D012009:Recurrent Laryngeal Nerve / Q000502:physiology; D013965:Thyroidectomy