We present a quantitative method for determining the optimal degree of arytenoid rotation when performing AA using real-time voice measurements. This is a preliminary study establishing the validity of this method and the sensitivity of the chosen measures to small changes in arytenoid position. Ex vivo canine larynges have been used extensively to study vocal fold paralysis.10,12-14
There are several anatomical differences in the canine larynx relative to the human, including more angulated thyroid and cricoid cartilages, and the absence of a well-defined vocal ligament.12
These differences did not affect the procedures performed in this study.
This is our second study examining the added benefit of performing AA after ML in excised canine larynges.10
By optimizing the degree of arytenoid rotation, we observed even greater changes in all parameters of interest. While assessing the added benefit of AA following ML in patients can be difficult as the two procedures are often performed simultaneously, this can easily be done using the excised larynx setup. Our results provide additional support for the use of AA in patients who can tolerate it.
Aerodynamic parameters behaved as expected, with increases in PTF and PTP occurring in the simulated paralysis condition, and stepwise decreases occurring from paralysis to ML to AA. As observed previously,10,14
PTF varied significantly across treatments while PTP did not. PTF is directly related to the cube of neutral glottal half-width15
and is more sensitive than PTP to changes in glottal abduction.16
While the threshold parameters displayed differences across the three AA conditions, the differences were not as evident as for perturbation parameters or VE
. Accurate measurement of threshold aerodynamics, particularly PTF, also remains a challenge. PTP and PTF, therefore, are likely not suitable parameters for intraoperative voice assessment.
Perturbation parameters, however, may offer a more reliable and feasible alternative for determining optimal arytenoid rotation. Recording requires only a microphone and software equipped with the real-time measurement employed in this study. Percent jitter and percent shimmer were both sensitive to arytenoid position, with the optimal AA angle producing the lowest values. Fundamental frequency was significantly lower for the AA conditions relative to normal, but this may be a result of the experimental design. Phonation tokens were recorded at the phonation threshold. As PTP was lowest in the AA conditions and pressure has a direct relationship with frequency, fundamental frequency was lower. A decrease in F0
following AA has also been reported previously.17
Increases in SNR can be attributed to decreased flow (noise) as well as increased sound production (signal), with optimal AA producing the most dramatic improvement. Real-time intraoperative measurements of SNR may not be beneficial though, as an extremely hyperadducted larynx would have a high SNR due to minimal flow escaping the glottis.
Changes in mucosal wave amplitude were of particular interest, as dramatic changes occurred across the three AA conditions for both folds (). Real-time mucosal wave analysis is currently not feasible due to the laborious nature of extracting parameters from a segment of high speed video. As extraction and analysis techniques are improved, real-time mucosal wave amplitude analysis may offer another means of determining the optimal degree of arytenoid rotation. Intrafold phase difference reflects the presence or absence of a normal mucosal wave. Ideally, there should be a phase difference of pi between the upper and lower lips of the vocal fold. A phase difference similar to this was observed in both the normal and optimal AA conditions. Thus, the optimal degree of arytenoid rotation produced the greatest improvements in vocal fold vibratory characteristics. Intrafold phase difference was decreased in the other conditions. There did not appear to be a pattern for change in interfold phase difference across conditions.
Differences in VE and perturbation parameters across the three AA conditions were not significant; however, significant differences were not expected for the subtle changes in arytenoid position that were analyzed. The primary objective of this study was to demonstrate that VE and perturbation parameters were sensitive to changes in arytenoid rotation, increasing and decreasing, respectively, until the optimal degree of rotation was reached. As the differences did approach significance, particularly for VE, one could anticipate that significant differences may be found with a larger sample size.
VE was used with success in this study to distinguish among subtle differences in arytenoid rotation when performing AA. It may be potentially more useful than perturbation parameters as it evaluates both aerodynamics, the input to voice production, and acoustics, the output. For several other measured parameters such as percent jitter and threshold aerodynamics, hyperadducted AA performed rather similarly to optimal AA. However, there was over a four-fold increase in VE for optimal AA compared to hyperadducted AA (, ). As hyperadduction can lead to postoperative dyspnea, it is important to measure parameters which vary significantly with subtle changes in arytenoid rotation. VE, therefore, may represent the most useful parameter for determining the optimal degree of arytenoid rotation during AA.
and perturbation parameters can be measured intraoperatively. Measuring perturbation parameters would be easier, as it would require only a microphone and the software used in this study. Measurement of VE
could be done using the airflow interrupter,18
which uses a mechanical balloon valve to interrupt sustained phonation. Airflow interruption has been used with success to assess disordered subjects19
and could be applied to patients with VFP. This method would require more patient cooperation than measurement of perturbation parameters. Alternatively, aerodynamic measures could be obtained directly via a cricothyroid membrane puncture. This, coupled with acoustic measurements obtained with a microphone, could also be used to measure VE
. Patients would then need only to produce a sustained vowel for all measurements to be recorded. Evaluating both VE
and perturbation parameters clinically would be valuable to determine which can adequately distinguish among different degrees of arytenoid rotation while minimizing demands on patient vocal effort. The degree to which the implementation of each increases operative time must also be analyzed. Intraoperative edema resultant from increased operative time may confound measurement and lead to an arytenoid position which is not optimal. Real-time voice analysis also has the ability to reduce intraoperative edema, as it eliminates potentially time-consuming subjective voice assessment. A surgeon can rotate the arytenoid along an arc until VE
stops increasing and perturbation measures stop decreasing. This point would not be found as easily if using subjective voice assessment.
It is also important to evaluate the use of real-time voice measurements in patients to determine if optimal vocal outcomes are associated with optimal respiratory and swallowing outcomes. AA has been associated with improvement in voice, swallowing, and respiratory function;6
however, examining swallowing and respiratory function after performing AA with real-time voice measurements is necessary if this method is to be applied clinically. One limitation of the excised larynx setup is that only vocal function can be analyzed. The angle we found to be optimal may be slightly more acute than what would be optimal for the larynx of a human patient. Though the paralyzed vocal fold was adducted just past the midline (), this degree of adduction may be sufficient to cause episodic dyspnea. This concern must be considered when utilizing intraoperative voice analysis. Future investigations will focus on balancing the desire for a good vocal outcome with the need for a good respiratory outcome.