fundamentals
What is Video Laryngeal Stroboscopy?
Laryngeal stroboscopy is a specialized form of video endoscopy with its main function to visualize vocal fold vibration as a means of evaluating voice function. Most often performed by an otolaryngologist or speech-language pathologist, this technique allows for diagnosis and management of a variety of conditions related to abnormalities in vibratory function.
Principles of Stroboscopy
Stroboscopic light uses the principle of the retention of light on the human retina. A single image will be retained on the retina for 200 milliseconds and when a series of individual images are presented in sequence to the eye, the brain will fuse them, leading to the perception of motion (Talbot’s Law). Modern stroboscopy machines are able to detect the fundamental frequency of vibration of a subject’s voice (via a contact microphone placed on the neck) and then turn the strobing light frequency several hertz (1-3) above it thus leading to the appearance of slow pseudo-motion where individual frames of the glottic cycle are perceived to be fused together in apparent motion.
Initially, rigid endoscopes with bulky cameras were used to perform laryngeal stroboscopy. Later developments in flexible fiberoptic endoscopy led to the ability to combine flexible technology with a strobe light. Exams could now be conducted transnasally with a patient’s connected speech. Although the fiberoptic technology provided ease of examination, there was a notable decline in image quality.
In the modern era of stroboscopy, charged coupled device (CCD) cameras are mounted onto the tip of endoscopes offering greater image resolution. This technology, initially used on the tips of gastroscopes, has been adopted for our indirect laryngoscopes by manufacturers such as Olympus and Pentax and has gained widespread use in the voice professional community.
Despite its clinical contributions, stroboscopy has some disadvantages. If the patient’s vocal folds vibrate in a non-quasiperiodic manner (such as in patients with intermittent vibratory abnormalities) stroboscopy can fail to adequately evaluate the patient. Consequently, high speed video imaging was developed for such scenarios. This equipment however can be cost prohibitive and currently remains largely utilized in the research arena
General Principles of Interpretation
Glottal pressure
Phonation is related to passing air through a relatively closed glottis. Factors such as lung capacity, chest wall compliance, play a role in generating pressure at the level of the glottis with inspiration. Once the pressure against a closed glottis is high enough, air escapes and leads to phonation.
Fundamental frequency
Also known as F0, this refers to an acoustic measure reflecting the rate of vibration of the vocal folds expressed in hertz (Hz). Specifically, it is the inverse of the time taken to complete a signal glottic vibratory cycle. This can be obtained from analysis of a acoustic signal or a electrolaryngograph. The F0 for men is typically 80 and 175 Hz compared to 160 and 270 Hz for women. F0 is by the structure and elasticity of the TVFs, with changes or disruptions to these resulting in alterations in the F0. In general, stiff, tense, and short vocal folds (e.g., post-radiation laryngitis) require a higher F0 to sustain vibratory oscillations, and vocal folds with higher mass and thickness have a lower F0 required (e.g., Reinke’s edema). F0 is related, but separate from pitch which is our perception of the F0.
Periodicity
Degree of similarity in time it takes to complete one phonatory cycle compared to the next cycle. This is dependent on the balance between subglottic pressure and vocal fold configuration, and is affected by neurologic asymmetry, lesions, flaccidity, unsteady tonus or pulmonary disease. If there is aperiodicity, other parameters measured by stroboscopy can be inaccurate. This is best assessed in stop or locked mode during the stroboscopic exam, represented by a static image if there is periodicity. If there is periodicity, there will be uniform amplitude of the vibration as well as time to complete the glottic cycle. Periodicity can be described as irregularly regular, regularly irregular, or irregularly irregular based on when the aperiodicity occurs.
Glottal closure configuration
Glottic closure is the distance between the medial edge of the right and left true vocal folds during phonation. Small anterior or posterior gaps in closure can be seen in normal phonation. However, pathologic gaps can also be noted during stroboscopy. Various scales exist from binary (present/not present) to scales with significant detail (0-100, where 0 means normal to 100 to significantly abnormal). The are several common configurations described, such as: complete, incomplete, mid-cord gap, anterior gap, posterior chink, hourglass pattern or midcord contact, irregular closure, spindle shaped closure.
Closure Phase
During phonation the open and closed phase of vocal fold vibration should be present 50:50 of the time. However, patients can also have open phase dominant or closed phased dominant vibration.
Phase Symmetry
This describes whether the vibration of the right and left fold mirror each other by moving towards each other together and away from each other together. When this occurs, the phase is considered to be “symmetric”. Phase asymmetry occurs when the vibration of one vocal fold is “out of sync” with the other. This can vary in severity, resulting in mild asymmetry or, in its most severe form, a “chasing wave” occurs.
Amplitude
Refers to the amplitude of the mucosal wave seen on stroboscopy (the degree of lateral excursion during vibratory motion). There is a wide variety in the scales to assess this. Often, this can be described as zero, normal, decreased, or increased. Reduced amplitude can be seen in vocal fold scar or nodules from resulting stiffness. Increased amplitude can be seen in paralyzed vocal fold with the atrophic fold appearing floppy and with greater vibratory excursion.
Mucosal Wave
The mucosal wave is different from amplitude, it is a marker of the pliability of the superficial cover layer of entire vocal fold (Reinke’s space) from the undersurface of the fold to superior surface where it transitions into the ventricle. Whereas amplitude is a measure that incorporates deeper layers such as the vocalis muscle layer and focuses just on lateral to medial excursion. Mucosal waves can be normal, increased, absent, or reduced. The stiffer the cover layer, the more reduced the wave is. For example, conditions such as Reinke edema have increased mucosal waves.
Vocal Fold Edge
The medial aspect of the vocal fold can either be smooth, or rough and irregular. Irregularity can be seen in a variety of pathologies from polyps to malignant lesions.
Non-vibrating Segments
These are parts of the vocal fold where the amplitude and mucosal wave are absent. Can be focal (e.g. post laryngeal microsurgery) or involve the entire fold (e.g. infiltrative mass).
Equipment and Clinical Operation
The main aim of this website is in teaching laryngeal stroboscopy analysis, however, below is a brief introduction to the typical set up of a clinical stroboscopy room.
Examination Chair
A wide chair with a foot rest, moveable headrest and the ability to raise and lower the height will facilitate patient and provider comfort during examination.
Topical Anesthetic
Nasal decongestant spray and topical anesthesia can be used to improve patient comfort during stroboscopy. Topical anesthesia can be applied to the nasal cavity, nasopharynx, and oropharynx. Nasal decongestants are applied to the nasal cavity. After application, wait for adequate anesthesia/vasoconstriction to take effect before proceeding with flexible laryngoscopy. Medication selection for laryngeal anesthesia (lidocaine, pontocaine, cetacaine) or decongestant (neosynephrine, oxymetazoline) is up to the endoscopist. Excessive use of topical anesthesia can lead to increased secretions and can cause systemic toxicity.
Patient and Stroboscopist Positioning
The patient should be in an upright forward position, with the neck slightly flexed and head slightly extended with feet firmly placed flat on the footrest. The examination chair should be raised or depressed based on the height of the examiner to avoid strain and fatigue.
Stroboscope light source
this provides the light output. Many modern machines have a combination of halogen and xenon light for non-strobe and strobe type examinations. Sound is then transmitted from the patient to the stroboscope via a microphone or contact stethoscope. Circuitry within the strobe machine then derives the fundamental frequency from the voice. A foot pedal is helpful for the endoscopist to control various aspects of the strobe light and trigger a digital recording when ready.
Recording device and Video reviewing software
CCD camera technology on distal tip chip scopes provide excellent image quality. The ability to record and easily access stroboscopy videos is paramount and can be saved either directly on a local processing hard drive or uploaded to a local server or cloud technology.
Laryngoscope
Various types of flexible laryngoscopes can be used for laryngeal stroboscopy, however in recent years, distal tip chip scopes have replaced fiberoptic technology. Many clinicians prefer rigid telescopes with angled tips (70 or 90 degree) for transoral visualization which can provide sharper image quality and greater magnification of the laryngeal structures.
Output monitor
Many offices have several high-definition output monitors, one for the endoscopist to view during the exam and one for the patient to view directly in front of the patient. The important factor is that the exam is properly recorded digitally and able to be reviewed with the patient, including frame by frame analysis.
