Strobovideolaryngoscopy

True vocal fold vibration is a complicated physiologic function, the observation of which far outreaches the visual capabilities of the human eye with a normal light source. The human adducted vocal folds cyclically open and close between 60 to 1500 times per second, depending on the phonatory pitch. Stroboscopic light visually makes the vocal fold vibrations appear to slow down so that the impression of vocal fold vibrations can be observed and processed. Stroboscopy capitalizes on the inherent optic properties of our visual organ and exploits the limitations of observation of the unaided eye. According to Talbot’s law, the human eye can perceive no more than five distinct images per second. Each image therefore lingers on the retina for approximately 0.2 seconds after exposure. Stroboscopic flashes make the vocal folds appear to slow down by advancing the light pulse through successive glottal cycles in percentage increments. Individual still images are recorded at selected points from sequential vibratory cycles and the human eye automatically fills in the missing pieces by fusing the images into what it sees as motion. This apparent motion is attributable to a phenomenon called persistence of vision. Additional instrumentation added to the stroboscopic light can facilitate the recording and documentation of the perceived vocal fold vibratory properties . Strobovideolaryngoscopy as a whole allows the physician to observe important vocal fold activities, which allows appropriate diagnostic decision making.

A brief historical overview allows full appreciation of the evolution of strobovideolaryngoscopy. Indirect laryngoscopy was first described, but not yet popularized, by Bozzini in 1806 when he constructed an angled speculum with a mirror insert that was meant to examine various body cavities, including the human larynx. It was not until 1854 that indirect laryngoscopy gained wider acceptance when Manuel Garcia, a Spanish-born voice teacher who had a limited gag reflex, first visualized his larynx with a small dental mirror using sunlight as a light source. In 1895, Oertel followed suit and was credited with creating the first laryngostroboscope. His device consisted of a variable-speed perforated disc that was interspersed between a light source and the practitioner’s head mirror . Since that time, strobovideolaryngoscopy has evolved into finely controlled, high-intensity light sources with fiberoptic endoscopes or distal camera scopes coupled with analog or digital recording devices.

Strobovideolaryngoscopy in current clinical practice relies on a combination of several instruments: a stroboscopic light source, an endoscope, a microphone, a video camera, a recording device, and a video monitor. Stroboscopy is best performed in conjunction with video recording and archiving for complete clinical review and documentation. The examination may be performed by transnasal flexible laryngoscopy with distal chip technology or perorally with a rigid angled telescope. Video cameras are now available in single-chip and three-chip versions. A single-chip camera uses a single array of light-sensing elements known as charge couple devices (CCDs). Three-chip cameras use a dichroic prism, which divides the incoming images into the three primary colors and offers more accurate color and higher resolution. Analog or digital recording technologies are then used for image capture, documentation, and reproduction .

The illusion of apparent slow motion of the vibrating vocal folds during strobovideolaryngoscopy evolves from the collection of several sequential still images of the folds at selected time intervals during repeated glottal cycles at a given vibratory frequency. This illusion is called the stroboscopic glottal cycle and can be of any desirable duration. In addition, the stroboscopic flashes can be emitted either at the same frequency as phonation, known as synchronization, or at a slight variation of the frequency, known as asynchronization. This feature of stroboscopy is producible through technological communication between the microphone and the strobe light source. By synchronizing the stroboscopic flashes to the fundamental frequency of the vibrating vocal folds, a perceptual stopped image or standstill of the vocal folds is produced. An asynchronized mode is generated by calibrating the stroboscopic flashes at a consistent frequency slightly different than the produced phonatory fundamental frequency. This variation allows successive light impulses to strike at different phases of the vibratory cycle and produce a video image of one apparent cycle of vibration actually obtained from different portions of several cycles. Another option, which allows the examiner to manipulate the apparent glottal cycle by operation of a rocking foot pedal, furthers the stop-action capability of the strobovideolaryngoscopy system. This feature is particularly useful when the exact location of the vocal fold lesion is being determined in relation to movement of the upper and lower lips during an approximation phase of the cycle .

The strobovideolaryngoscopic examination is most clinically useful to the practitioner when a standard protocol is used for the acquisition of the data and its interpretation. Phonatory tasks during the examination should be performed at low, normal, and high pitches and in the range of the speaking or singing problem area, if known. Once recorded, a standardized approach to the interpretation of the examination allows consistency in diagnosing and comparing laryngeal pathology. Once the initial examination is completed and recorded, additional repeat testing at predetermined time intervals allows for evaluation of response to treatment. Although there is arguably no one gold standard for the interpretation of a strobovideolaryngoscopic examination, several aspects of the examination are often rated. The specific features of the vibratory pattern of the true vocal folds often addressed include symmetry, periodicity, mucosal wave ratings, amplitude of vibration, shape and contour of the glottal margin, and glottic closure. Particular attention is also given to any adynamic segments and the presence or absence of vertical phase difference . Vocal fold symmetry remains intact in the absence of abnormalities along the glottal margin. Periodicity refers to the regularity of the vibratory cycles with the idea that normal vocal folds should vibrate in mirror image to each other and vibrate the same with successive cycles. Aperiodic vibrations may prohibit the synchronization of the strobe light. The mucosal wave is generally described as the traveling wave across the vocal fold superior surface from medial to lateral. Abnormalities of the mucosal cover, including the epithelial layer or superficial lamina propria, are the most common causes of mucosal wave reduction. The mucosal wave should be differentiated from the vertical phase difference, which is created normally by the presence of an upper lip and lower lip at the medial vibratory vertical closing surface. Amplitude of vibration is a relative feature of the mucosal wave judged by the trained observer as reduced, normal, or excessive. Normal variations in amplitude occur with changes in vocal intensity. Glottal closure is described as complete; incomplete with anterior, mid, or posterior glottal chinks; and hourglass, usually secondary to mid-vocal fold lesions.

From a clinical standpoint, strobovideolaryngoscopy has proved to be a valuable tool for the diagnosis of laryngeal pathology given the detailed physical examination it provides of the vocal tract and the vibratory margin of the vocal fold. Stroboscopic features of nodules, for example, often include symmetric but reduced amplitude of vibration, maintenance of periodicity, intact mucosal waves, and hourglass closure. Vocal fold polyps, which are frequently unilateral, have asymmetric vibration and variable periodicity depending on the size and shape of the polyp. Mucosal wave can be absent because of mass effect with large polyps or intact with broader-based polyps. The wave is generally intact on the contralateral side. Glottic closure is understandably asymmetric. Cysts within the vocal fold lamina propria can have the greatest adverse effect of the nonneoplastic lesions on the vibratory characteristics. Mucosal wave is frequently absent and aperiodic if present. A change in diagnosis and altered assessment of vocal pathology based on the strobovideolaryngoscopic findings can occur in 10% to 30% of cases . Furthermore, abnormal findings have been reported in up to 58% of healthy, asymptomatic professional singers stressing the importance of screening examinations for certain populations of patients .

Strobovideolaryngoscopy is not a test to be done in the absence of other clinical data. It is only a valuable complement to a thorough vocal history and physical examination. The technique inherently suffers from the limitation of being a composite recording made from several glottal cycles, in contrast to high-speed photography or high-speed digital video, which records an entire vibratory cycle and provides detailed cycle-to-cycle variations. Even with this limitation, strobovideolaryngoscopy remains an invaluable tool in the diagnostic armamentarium of the voice specialist.

Glottography

Glottography is a general technique that monitors the vibration of the vocal folds by the transmission of a probe signal from one side of the larynx to the other. The probe signal itself can be directed in either a vertical plane or horizontal plane. Current probing signals most commonly used in glottography include electrical current flow, light transmission, and ultrasonic waves. The time variation of the glottis combined with laryngeal tissues that are in constant partial stages of contact during phonation modulates the probe’s properties. This modulation is then detected and recorded supplying immediate objective data in the form of graphic displays that can be clinically interpreted. Glottography thus makes possible the physical measurement of acoustic parameters, such as pitch, jitter (frequency perturbations), shimmer (amplitude perturbations), or other perturbations. It also provides a possible objective method that can be used to evaluate and detect vocal fold pathology. Overall, glottography provides some clinical data about vocal fold vibration. This technique fails to determine the vibration capacity of an individual vocal fold or diagnose individual laryngeal lesions without an additional visual examination .

Glottography

Glottography is a general technique that monitors the vibration of the vocal folds by the transmission of a probe signal from one side of the larynx to the other. The probe signal itself can be directed in either a vertical plane or horizontal plane. Current probing signals most commonly used in glottography include electrical current flow, light transmission, and ultrasonic waves. The time variation of the glottis combined with laryngeal tissues that are in constant partial stages of contact during phonation modulates the probe’s properties. This modulation is then detected and recorded supplying immediate objective data in the form of graphic displays that can be clinically interpreted. Glottography thus makes possible the physical measurement of acoustic parameters, such as pitch, jitter (frequency perturbations), shimmer (amplitude perturbations), or other perturbations. It also provides a possible objective method that can be used to evaluate and detect vocal fold pathology. Overall, glottography provides some clinical data about vocal fold vibration. This technique fails to determine the vibration capacity of an individual vocal fold or diagnose individual laryngeal lesions without an additional visual examination .

Electroglottography

Electroglottography (EGG) is a technique based on the principle that human tissue can conduct an electrical current with laryngeal tissues being a moderately good conductor of electricity. It is performed by placing two electrodes above the thyroid laminae on the external neck and measuring the impedance between them with a high-frequency, low-current signal. Ohm’s law states that a current must flow through a system if its resistance is to be measured. Based on this law, when the vocal folds are touching a greater current flows through them compared with when they are open. The electroglottographic signal represents the contact area between the two vocal folds and can be used to determine when the vocal folds are closed and how fast they are closing . This characteristic contrasts with photoglottography (PGG), which gives information about the separation of the vocal folds and little information about the nature of vocal fold contact.

Various manufacturers provide instrumentation that produces, records, and displays the electroglottographic signal. Several authors over the past two decades have commented on the shape of the EGG waveform as it relates to the underlying physiology of vocal fold vibration. Interpretation of EGG waveforms remains controversial, however, especially as it relates to analyzing vocal fold pathology. When used in conjunction with other laboratory techniques, the interpretation of the EGG display becomes more reliable. For example, synchronized strobovideolaryngoscopy and EGG have been shown to be an effective tool for verifying information from the EGG waveform with stroboscopic images . Also, recent research is moving toward standardization of normal EGG measurements with the goal of allowing this test to serve as a reference for the diagnosis and follow-up of dysphonic patients .

There are limitations of EGG. The most obvious one for the voice specialist is that it cannot be used with all dysphonic subjects. Patients who have a unilateral vocal fold paralysis have a considerably diminished or absent signal because of lack of good contact of the vocal folds. Obese or thick necks may impede proper placement of the electrodes or hinder the electrical current resulting in a poor EGG tracing. Finally, severe hoarseness may render laryngeal tissue irritable and passing an electrical current through this environment may produce an undesirable physiologic response .