Strobokymographic and Videostroboscopic Analysis of Vocal Fold Motion in Unilateral Superior Laryngeal Nerve Paralysis

Abie H. Mendelsohn, MD; Myung-Whun Sung, MD, PhD; Gerald S. Berke, MD;Dinesh K. Chhetri, MD

The clinical diagnosis of superior laryngeal nerve paralysis (SLNp) is infrequently made, because of the heterogeneity of clinical presentations and laryngoscopic findings. Laryngeal electromyography (LEMG) can provide the definitive diag-nosis of this abnormality. With increasing use of LEMG in clinical practice, this condition is now more frequently appre-ciated by otolaryngologists. A characteristic, but infrequently reported, videostroboscopic vocal fold motion termed Ge-genschlagen (“dashing-against-each-other”) has previously been described to occur in unilateral SLNp. We encountered such motion in a clinical case, which we subsequently verified as unilateral SLNp by means of LEMG. This characteristic glottic motion was then verified in an in vivo canine model of phonation after unilateral SLNp. Videostrobokymography was performed to generate kymograms that illustrated this vocal fold motion clearly. Kymograms of both human and ca-nine subjects with SLNp demonstrated an undulating motion of the horizontally shifting glottic space as the medial edges of the vocal folds chased each other 90° out of phase. As one vocal fold mucosal edge was opening, the other was clos-ing, and this repeated motion appeared as vocal folds chasing or dashing against each other. Although not uniformly seen in all cases, this vocal fold motion appears to be unique to SLNp.Key Words: kymography, laryngeal electromyography, paralysis, paresis, superior laryngeal nerve.

Annals of Otology, Rhinology & Laryngology 116(2):85-91.© 2007 Annals Publishing Company. All rights reserved.

85

From the Division of Head and Neck Surgery, David Geffen School of Medicine at the University of California–Los Angeles, Los Angeles, California (Mendelsohn, Berke, Chhetri), and the Department of Head and Neck Surgery, Seoul National University, Seoul, South Korea (Sung). This study was performed in accordance with the PHS Policy on Humane Care and Use of Laboratory Animals, the NIH Guide for the Care and Use of Laboratory Animals, and the Animal Welfare Act (7 U.S.C. et seq.); the animal use protocol was approved by the Institutional Animal Care and Use Committee (IACUC) of the University of California–Los Angeles.Presented at the meeting of the American Laryngological Association, Chicago, Illinois, May 19-20, 2006.Correspondence: Abie H. Mendelsohn, MD, Division of Head and Neck Surgery, UCLA School of Medicine, 10833 Le Conte Ave, Room 62-132 CHS, Los Angeles, CA 90095-1624.

INTRODUCTION Almost 30 years ago, Ward et al1 stated that supe-

rior laryngeal nerve paralysis (SLNp) is a frequent-ly overlooked entity because of its complex clinical picture. Today, there continues to be a lack of con-sensus on the typical presentation of and laryngo-scopic findings in SLNp.

The function of the superior laryngeal nerve (SLN) can be divided into sensory and motor com-ponents. Its sensory function is to provide a variety of afferent signals from the supraglottic larynx.2 The motor function is innervation of the ipsilateral cri-cothyroid (CT) muscle. The CT muscle originates from the external surface of the arch of the cricoid cartilage and is composed of two portions. The pars recta portion is anterior and superior and inserts on the inferior border of the thyroid lamina, and the pars oblique portion inserts on the inferior horn and the inner surface of the thyroid lamina. Cricothyroid muscle contraction tilts the cricoid lamina backward at the cricothyroid joint, thus lengthening, tensing, and adducting the vocal folds.3

The canine model has long been used in elucidat-ing the characteristics of unilateral SLNp. However, experimental observations of the larynx have var-ied among the studies, and a consensus has not been generated. For example, Tanabe et al4 found ipsilat-eral vocal fold elongation, but Hanson et al5 found the fold to be shortened. A few studies have noted a higher vertical position of the paralyzed fold.5,6 Some studies have found horizontal laryngeal rota-tion with the posterior commissure pointing toward the affected side at rest,5-8 but others have noted this to occur only during phonation.9

Studies of glottic motion in unilateral SLNp, us-ing the canine model, have also resulted in incon-sistent reports. The current view is that equal vi-brating frequencies are maintained on both vocal folds. However, this vibratory frequency is 90° out of phase with the opposite vocal fold and appears asymmetric on stroboscopy.4,6 In addition, greater cycle-to-cycle variation as compared to normal pho-nation with constant horizontal shifting of the glot-tic space has also been reported.8

86 Mendelsohn et al, Superior Laryngeal Nerve Paralysis 86

Clinical studies have also proposed a variable presentation in unilateral SLNp. For example, laryn-geal rotation has been seen at rest,10 only on phona-tion,1,7,9 or not at all.11,12 Other laryngoscopic ob-servations have been noted in only a few studies and have been left out of many others. Such sporadic observations include ipsilateral vocal fold shorten-ing1,10 and a height mismatch between the two vo-cal folds.1,13 During phonation, the traveling wave and vibratory frequency have been described as “ab-normal” and “asymmetric,”10,11 with “vocal cord lag.”12,13

Because of the complex clinicopathologic presen-tation of SLNp, many suggest basing the diagnosis largely on symptomatology and clinical suspicion. Common symptoms associated with SLNp include raspy vocal quality, vocal fatigue, volume deficit, and loss of singing range.1,9,12,13 The presence or ab-sence of any or all of these symptoms is not pathog-nomonic for SLNp. However, with increasing use of laryngeal electromyography (LEMG) in routine clinical practice, it has become possible to defini-tively diagnose SLNp and make objective note of the laryngoscopic findings.13

We encountered a clinical case with a character-istic videostroboscopic vocal fold vibratory pattern that has previously been reported to occur in unilat-eral SLNp in the canine model. This characteristic vocal fold vibration was first described in German by Dohne14 as “Gegenschlagen,” which is translated into English as (vocal folds) “dashing-against-each-other.”6 We definitively diagnosed SLNp in this pa-tient with LEMG. We then demonstrated similar laryngeal vibratory behavior in a canine model of SLNp. The objective of this study was to describe this unique videostroboscopic motion, and to further illustrate this motion by use of strobokymography.

MATERIALS AND METHODS

Clinical Case. A 57-year-old man presented to the UCLA Voice Center with a 1-month history of dys-phonia. He had no prior history of voice problems. He had a history of gastroesophageal reflux disease, but denied use of tobacco or alcohol or voice abuse. Two and a half months previously, he had under-gone a 2½-hour elective lumbar discectomy that required general endotracheal anesthesia. Video-stroboscopy was performed with a 70° rigid endo-scope attached to a charge-coupled device camera (Telecam 20210120, Karl Storz Endoscopy-Amer-ica, Inc, Culver City, California) with illumination from a stroboscopic light source (model RLS 9100, KayPENTAX, Lincoln Park, New Jersey). Video re-cordings were performed on a ¾-inch videocassette

recorder (model VO9850, Sony, Park Ridge, New Jersey).

For the LEMG examination, the patient was asked to lie supine on an examination table with a small shoulder roll to keep the head slightly extended. A Nikolet Viking IV Electromyography Machine (Ni-colet Biomedical, Madison, Wisconsin) was used to obtain the recordings. The LEMG was performed with a 37-mm monopolar electrode. A reference electrode was placed over the sternum, and a ground disk electrode was placed over the clavicle. The low-frequency setting was at 20 Hz, and the high-frequency setting was at 10 kHz. Motor unit recruit-ment tracings were recorded with sweep speeds at both 10 ms and 50 ms per division and a gain of 200 μV per division. Our protocol for LEMG is similar to that described by Munin et al.15 The identity of the strap muscles is confirmed by slight head eleva-tion without phonation. A brisk signal is seen during this maneuver from the strap muscles. There can be a mild increase in activity from the strap muscles during phonation; however, it is not the typical re-cruitment pattern seen from intrinsic laryngeal mus-cles during phonation. The CT muscles were located by inserting the needle 5 to 10 mm off midline at the CT membrane level and angling it laterally to-ward the cricoid cartilage. If abnormal signals were obtained from the CT muscle, we reconfirmed the CT muscle location by first using the needle to pal-pate the upper border of the cricoid cartilage at the origin of the CT muscle and then slowly advancing superolaterally at a slightly deeper level to sample the CT muscles at multiple locations. The thyroary-tenoid muscles were located by inserting the needle 3 to 5 mm off midline and angling it superolaterally and under the inferior border of the thyroid ala. If no signals or abnormal signals were obtained, the needle was inserted several more times into the re-spective muscles with slightly different angulations to resample and confirm the initial findings.

In Vivo Canine Model of Phonation. The in vivo canine model of phonation has been established in our laboratory for the study of laryngeal physiology and has been detailed previously.16 Briefly, general anesthesia is induced in the animal, and upper and lower tracheotomies are made. The upper tracheot-omy is used to provide rostral airflow via a cuffed endotracheal tube to drive phonation, and the lower tracheotomy is used to help ventilate the animal dur-ing the experiment. The proximal endotracheal tube is connected to a flowmeter (model 1600, Edwards Datametrics, Wilmington, Massachusetts), and the airflow rate is controlled with a wall-mounted con-trol. The inhaled air is humidified and heated by

bubbling it through 5 cm of heated water so that the temperature of the air is 37°C when measured at the glottic outlet.

Neck exploration is performed to locate both re-current and superior laryngeal nerves close to their entrance into the larynx. Custom-designed mono-polar electrodes with silicone insulation are applied to the isolated nerves. The electrodes are attached to a constant-current nerve stimulator (model 2SLH, WR Medical Electronics Co, St Paul, Minnesota). The nerves are typically stimulated at 80 Hz with 0 to 3.0 mA for a 1.5-ms pulse duration to achieve complete adduction of the vocal folds. Once the vo-cal folds are adducted, the airflow drives phonation and sound is generated. To simulate SLNp, we sever the right SLN completely and apply electrical stim-ulation to both recurrent laryngeal nerves and the contralateral SLN while videostroboscopy is per-formed.

Videostroboscopy was performed on a single dog with a rigid 0° endoscope attached to a charge-cou-pled device camera (Telecam 20210120, Karl Storz) with illumination from a stroboscopic light source (model RLS 9100, KayPENTAX). Recordings were performed on a ¾-inch videocassette recorder (mod-el VO9850, Sony).

Videostrobokymography. Laryngeal kymography is a technique used to visualize the vibratory pattern of the mucosal edges of the vocal folds at selected coronal sections along the longitudinal glottic axis. A single coronal section or “slice” of the vocal fold vibration is analyzed over time, and each section is then stacked sequentially over time to produce a 2-dimensional image of the vibrating larynx (Fig 1). For example, in analyzing a line through the mid-point of the vocal folds, one can see a progressive increase and then decrease of the glottic space as the glottis opens and closes symmetrically. Normal laryngeal motion creates a typical rhomboid kymo-graphic pattern, and any aberration from that pat-tern can be considered abnormal. Analysis of ky-mograms can be quantitative or qualitative. Quan-titative results include area formulations such as the open quotient (OQ) and the asymmetry index (AI). The OQ is the open phase divided by the to-tal phase. (The total phase is the open phase plus the closed phase.) The AI is calculated as (ALM − ARM)/(ALM + ARM) × 200 where ALM = area left of midline and ARM = area right of midline. These figures can be then studied for normal ranges, as well as the severity of abnormalities.

Laryngeal kymography was originally introduced in the 1970s as an analytic tool for examinations of

periodic or aperiodic vibration.17 Svec and Schutte18 further developed the technology by producing high-speed photographic kymography. Subsequent-ly, Sung et al19 described videostrobokymography (VSK), in which the limitations of high-speed pho-tography are bypassed. A hybridized system of ky-mography and videostroboscopy, VSK digitizes re-corded stroboscopic laryngeal examinations and an-alyzes them with proprietary software. Therefore, one advantage of VSK is that it does not require re-peated endoscopic examinations of the larynx, and instead allows the user to carefully analyze the vid-eo images in order to choose the optimal areas of in-terest. The program can make multiple kymograms from multiple lines of interest by use of the prere-corded video images. Videostrobokymography has been used in quantitative documentation of abnor-mal vocal fold vibration in various benign lesions such as nodules, polyps, cysts, Reinke’s edema, and unilateral vocal fold paralysis.20,21

The VSK technique has been described before.21

We converted laryngeal videostroboscopic images of the patient and the dog to a digital format (.avi) and analyzed them on a computer monitor. The most stable image segments with periodic vibration of sufficient duration were selected for kymographic analysis. For this study, the mid-fold was selected as the most representative section for VSK analy-sis. The same lines of consecutive images were re-arranged in a new output window from top to bot-tom along a vertical time axis to construct a kymo-gram. Before constructing a kymogram, we aligned the longitudinal axis of glottic area with the y-axis of the window to compensate for obliqueness of the glottic areas.

87 Mendelsohn et al, Superior Laryngeal Nerve Paralysis 87

Fig 1. Consecutive stroboscopic images and selected line (upper stack of video frames) compose videostroboky-mogram (lower 2-dimensional output).

RESULTS

Clinical Examination. The patient’s dysphonia and videostroboscopic findings were suspicious for laryngeal nerve neuropathy. The voice was some-what raspy and showed a moderate decrease in pitch range. The patient underwent LEMG, which showed a normal recruitment pattern in both thyroarytenoid muscles and the right CT muscle. The strap muscles had normal activity. The left CT muscle displayed severely reduced recruitment on phonation (Fig 2).

Videostroboscopy. The laryngovideostroboscopic findings in the patient included complete glottic clo-sure without horizontal laryngeal rotation. The la-ryngeal vibration was periodic, with occasional sud-den shifts into aperiodicity. In the dog, there was also complete glottic closure, with most of the glot-tic cycles displaying periodicity. However, unlike in the human, a horizontal laryngeal rotation was pres-ent in the dog during phonation, with the posterior commissure pointing toward the side of SLNp. In both human and canine stroboscopic examinations, there was asymmetry of the mucosal waves, with the wave velocity traveling faster in the normal fold than in the paralyzed folds as assessed by frame-by-frame analysis of the images.

In the canine experiment, the SLNp was induced on the right vocal fold. The phonation produced by simulated SLNp was of normal intensity. The undu-lating motion proceeded as follows. At the onset of phonation, the right vocal process slightly overrode the left vocal process (Fig 3, canine SLNp, frame 1). As phonation began, the right vocal process low-

ered to the same level as the normal vocal fold. The glottic space first appeared left of midline, toward the normal side. On axial view, the paralyzed fold took a convex shape, whereas the normal fold was concave (Fig 3, canine SLNp, frame 2). The slit-like glottic space took a crescent shape and shifted with progression of the glottic cycle toward the midline, at which point the formation reversed. The para-lyzed fold now appeared concave, and the normal appeared convex (Fig 3, canine SLNp, frame 4). The glottic space width remained fairly constant while its position moved toward the paralyzed side. This mo-tion created a stroboscopic undulating movement of the medial edges of the vocal folds, described previ-ously as “dashing-against-each-other.” The motion is observed as if the normal vocal fold is chasing the paralyzed fold. As the glottis closes, the folds re-lax back down to their starting levels (Fig 3, canine SLNp, frame 6).

The vocal fold vibratory pattern in the patient, who had a left SLNp, was undulating as in the canine study, but there was less acute horizontal shift of the glottic space. The periodic phonation was modal, with a fundamental frequency of 180 Hz. The open glottic cycle began with the glottic space offset to the normal right side. A high-velocity mucosal wave began to propagate on the right side as the paralyzed fold remained in a stationary position (Fig 3, human SLNp, frame 1). The right fold’s traveling wave continued as the glottic space reached a relatively constant width (Fig 3, human SLNp, frame 2). Then, as the right normal fold began its return toward mid-line, the left fold began its lateral propagation (Fig

88 Mendelsohn et al, Superior Laryngeal Nerve Paralysis 88

Fig 2. Laryngeal electromyography recordings from right cricothyroid muscle (CT.R; top) and left cricothyroid muscle (CT.L; bottom). Note se-verely decreased recruitment pattern with occa-sional motor unit potentials on left cricothyroid muscle. (Gain and sweep settings are identical.)

3, human SLNp, frame 3). The glottic space width remained constant as the normal fold traveled medi-ally at the same rate that the opposite fold traveled laterally. The glottic space continued to move until the left fold reached its apex, at which point the right fold finally caught up and glottic closure occurred left of midline (Fig 3, human SLNp, frame 6). As the glottic cycle repeated, this unique undulating mo-

tion of the horizontally shifting glottic space from right to left was appreciated.

Kymography. The kymogram of normal human phonation displays a characteristic rhomboid shape with particular symmetry (Fig 4). The glottic excur-sions on both sides begin at midline, travel equal distances at equal rates, and return back to midline.

89 Mendelsohn et al, Superior Laryngeal Nerve Paralysis 89

Fig 3. Stroboscopic images (circles) and videostrobokymograms (rectangles) from normal human subject, human subject with left superior laryngeal nerve paralysis (SLNp), and canine subject with right SLNp. Arrows display selected image line with its corresponding position on kymogram.

In contrast, the kymogram of the patient with left SLNp displays the opening of the glottic cycle be-ginning on the right of the midline glottic longitu-dinal axis, opposite the paralyzed side. As the glot-tic space enlarges and reaches its maximal width, it begins to shift across the glottic midline, toward the paralyzed left side. The kymogram therefore has a skewed orientation, appearing as a diagonal line (Fig 4). The calculated OQ for the patient was 63, which is above the normal range21 (normal mean ± SD, 45.69 ± 4.8). The AI was −73 (normal mean ± SD, 2.49 ± 2.5), confirming that the open phase is skewed severely to the normal right side. The ky-mogram of the canine SLNp represents an exaggera-tion of the human motion. Just as the glottic open-ing in the human SLNp is skewed toward normal, the same occurred in the dog (Fig 4). However, the glottic opening is so severely skewed that it appears as if it were almost horizontal. The kymogram is se-verely skewed because of a faster shift of the glot-tic space from one side to the other compared to the clinical case and the very small OQ. This severely altered kymogram made it impossible for the soft-ware to compute the quantitative measures. The OQ was therefore calculated manually to be 12.5, well below normal, and the AI could not be calculated.

DISCUSSIONIn 1987, Moore et al6 observed vocal fold motion

in a dog with unilaterally induced SLNp. This move-ment was termed “dashing-against-each-other” and was described as one vocal fold’s chasing the other across the glottic gap. As one fold advanced later-ally, the other returned medially. The resulting glot-tic space moved from one side of the midline glottic axis to the other. This was in contrast to normal vocal fold motion, in which the glottic space remains cen-tered within its axis as the mucosal edges advance and return.6 The terms “dashing-against-each-other” and “undulating” therefore describe a shifting glot-

tic space, not the mode of vocal fold vibration.We now describe similar undulating vocal fold

movement in a patient with SLNp. The cause of SLNp in this patient is unclear and is most likely id-iopathic. Both human and canine larynges with uni-lateral SLNp share a subtle similarity of vocal fold vibration. In both the human and the dog, the glot-tic space was seen skewed toward the normal side at the onset of glottic opening, then skewed toward the paralyzed side at glottic closing. This pattern re-peated throughout each glottic cycle in modal pho-nation. This was corroborated and illustrated by ky-mographic analysis.

We believe, on the basis of past reports,4,14 this study, and our clinical experience, that this out-of-phase vibration may be pathognomonic for unilat-eral SLNp, but is not found in every case. For us to appreciate this motion stroboscopically, a periodic vibration is necessary, which may not be present in every case of unilateral SLNp. Other physiologic parameters may also play a role in generating such characteristic vibratory motion.

The consistent directionality of the shifting glot-tic space allows one to hypothesize on the laryngeal mechanics of this undulating motion. As has been reported previously in both clinical1,13 and canine studies,5,6 the vocal process on the side of SLNp is at a slightly higher vertical position relative to the normal fold. This height mismatch may play a role in directing the glottic airflow. As the airflow enters the glottis, it is deflected obliquely toward the nor-mal fold by the paralyzed fold (Fig 5). This altered aerodynamic environment may be primarily respon-sible for the undulating vocal fold motion, and may explain why this undulating motion is not seen in ev-ery patient with SLNp. If a vertical height mismatch were a necessary prerequisite, then patients without a significant mismatch would not display the undu-lating motion. In fact, some studies have not report-ed vertical height differences in SLNp.7,9,10 These

90 Mendelsohn et al, Superior Laryngeal Nerve Paralysis 90

Fig 4. Kymograms from normal human subject (left), hu-man subject with unilateral SLNp (center), and canine subject with unilateral SLNp (right).

Fig 5. Depiction of theoretical aerodynamic mechanism for undulating motion of unilateral SLNp as compared to normal phonation airflow.

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91 Mendelsohn et al, Superior Laryngeal Nerve Paralysis 91

omissions can also be due to clinical heterogeneity such as variable paresis of the CT muscle.

The variable observation of horizontal laryngeal rotation in unilateral SLNp may also be explained similarly. In our study, glottic rotation was only seen in the dog and not the human. The glottic space in the dog also shifted at a faster rate than was seen in the human. These differences are likely due to full experimental stimulation of the remaining canine la-ryngeal nerves, as opposed to physiologic levels of muscle activation in the clinical case.

The canine kymogram displays an exaggerated version of the undulating glottic motion, as seen by the almost horizontal orientation of the glottic space. The short open phase along the time axis represents the rapid rate of the glottic motion in the dog. Quan-titatively, the OQ for the dog was 12.5. This is in contrast to the human kymogram, which displays an

oblique orientation. Further scientific investigations are needed to un-

derstand the physiologic basis of the undulating vo-cal fold motion in unilateral SLNp. Although the motion we observed was periodic and was therefore well visualized, concerns that the motion is merely an illusion of stroboscopy could be examined with high-speed photography. Physical modeling incor-porating differences in cover-body tension and ver-tical height mismatch may provide further insight into possible contributing factors such as pressure gradients and entrainment. In this study we have further described the undulating “dashing-against-each-other” motion of the vocal folds in unilateral SLNp, and proposed a theory of its mechanics. This unique motion has so far been reported only in as-sociation with unilateral SLNp and perhaps will as-sist the clinician in recognizing the presence of this disorder.