You are in: eMedicine Specialties > Otolaryngology and Facial Plastic Surgery > LARYNGOLOGY StroboscopyArticle Last Updated: May 25, 2006AUTHOR AND EDITOR INFORMATIONSection 1 of 8
  Author: Robert A Buckmire, MD, Associate Professor,Department of Otolaryngology-Head and Neck Surgery, University of North Carolina; Chief, Divison of Voice and Swallowing Disorders, Director, University of North Carolina Voice Center Robert A Buckmire is a member of the following medical societies: American Academy of Otolaryngology-Head and Neck Surgery, American College of Surgeons, and National Medical Association Editors: Clark A Rosen, MD, Director, University of Pittsburgh Voice Center; Associate Professor, Department of Otolaryngology and Communication Science and Disorders, University of Pittsburgh School of Medicine; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Erik Kass, MD, Chief, Department of Clinical Otolaryngology, Associates in Otolaryngology of Northern VA; Christopher L Slack, MD, Otolaryngology-Facial Plastic Surgery, Private Practice, Associated Coastal ENT; Medical Director, Treasure Coast Sleep Disorders; Arlen D Meyers, MD, MBA, Professor, Department of Otolaryngology-Head and Neck Surgery, University of Colorado School of Medicine Author and Editor Disclosure Synonyms and related keywords: stroboscopy, videostrobe, videostroboscopy, stroboscopy, videostrobolaryngoscopy, strobolaryngoscopy, strobe, periodicity, amplitude, glottic closure, mucosal wave, vocal fold cysts, vocal fold polyps, vocal fold nodules, sulcus vocalis, endoscopy, vocal fold anatomy, flexible endoscopy, rigid transoral endoscopy, glottic area waveform, GAW, phonatory cycle, videokymography, glottography, kymography, videostrobokymography, stroboscope INTRODUCTIONSection 2 of 8
  In the modern laryngological practice, videostroboscopy is an essential diagnostic procedure for the detection of vocal pathology. Many voice clinicians believe that successful voice surgery depends as much on stroboscopy as otology depends on audiometry or microscopy. Laryngologists recognize the vibratory pattern of the vocal folds as one of the most crucial determinants of voice quality and production. As such, videostroboscopy is the most accessible and clinically relevant technique in the otolaryngologist's armamentarium to readily assess this important parameter. Results have important implications for both diagnosis and treatment of phonatory pathology. Over the past 3 decades, knowledge of vocal fold anatomy and physiology has revolutionized the clinical practice of laryngology. Since Hirano's original description of the layered microanatomy of the human vocal fold in the 1970s, increasingly sophisticated diagnostic and surgical techniques have evolved to more appropriately address this delicate and complex structure now than then. Innovative diagnostic modalities have grown out of an improved understanding of the critical importance of vocal-fold oscillation to voice production. Videostroboscopy has evolved as one of the most practical and useful techniques for the clinical evaluation of vocal-fold vibration. Videostroboscopy fulfills several important requirements of a complete voice examination. It provides useful, real-time information concerning the nature of vibration, an image to detect vocal pathology, and a permanent video record of the examination. As important as any of these aspects, stroboscopy substantially improves the sensitivity of subtle laryngeal diagnoses over techniques, such as rigid or flexible transnasal laryngoscopy, with continuous light sources. BACKGROUND AND SURGICAL PRINCIPLESection 3 of 8
The concept of stroboscopy is not new. For several centuries, stroboscopic images generated by using a flashing light source have been used to create the illusion of motion for entertainment. This practice is believed to date back to the ancient Greeks, who recreationally enjoyed these moving pictures. From the early 19th century, several examples illustrate the creation of moving-picture and optical toys, as well as scientific instruments. One device that used rotating disks to observe apparent motion, developed by a Viennese scientist named Stampfer, was called a stroboscope. This term is still used today to connote any pulsatile light-generating device designed to observe motion. The history of using a stroboscopic light source to examine the larynx is nearly as long as that of the continuous light source, dating back to the introduction of the laryngeal mirror by Manuel Garcia in 1855. In 1895, an internist named Oertel used a stroboscopic light source with a laryngeal mirror to investigate voice production in different registers. The application of the stroboscopic light source allowed the observer to view the vibrating vocal folds in arrested or apparent slow motion, permitting detailed observations of the structure in the open or closed positions. Because of the limitations in illumination, precise control of the flashing frequency, poor image quality, and patient discomfort, members of the scientific community did not embrace this technique. In the early to mid 1900s, nearly 100 years after Plateau first suggested the use of an intermittent spark to illuminate moving objects to produce a stationary pattern for the purpose of study, H.E. Edgerton and associates developed gas discharge tubes for stroboscopy. They used an oscillator to control the frequency of the discharge and the flashing rate. Many of the principles of modern stroboscopic devices evolved from this early instrumentation. Pioneers of modern strobolaryngoscopy include Dr J.W. van den Berg at the University of Groningen, Dr Rolf Timke at the University of Hamburg, Dr Hans von Leden at the University of California, and Dr Elimar Schonharl in Erlanger, who wrote the first definitive book on stroboscopic examination of the larynx in 1960. With the subsequent improvements in audio- and video-recording technology and with the ongoing advancements in optical image resolution and fiberoptic light-source intensity, the modern videostroboscopic unit can now produce a crisp, brightly illuminated, magnified image. The Talbot law takes into account the physical reality that images on the human retina linger for 0.2 seconds after exposure (persistence of vision). Therefore, sequential images produced at intervals less than 0.2 seconds produce the illusion of a continuous image. This understanding, along with the concept of correspondence (interpretation of a corresponding portion of sequential images representing an object in motion), allows for the illusion of motion when rapidly produced still images are presented. Finally, a characteristic of the visual system permits interpretation of a series of slightly altered still images by filling in the gaps between frames and completing the illusion of continuous motion. Strobolaryngoscopy takes advantage of these principles by producing intermittent light flashes in close relation to the frequency of the vocal-fold vibration. A microphone picks up the frequency of the examinee's sustained voice, which triggers the stroboscopic light source. With the provision that the vocal vibrations are periodic, a frequency of light flashes equal to the vocal frequency produces a clear, still image of the same portion to the vibratory cycle. When the frequency of the flashes is slightly less than the vibration of the vocal fold, it causes a delay in the portion of each vibratory cycle illuminated, and the illusion of slow motion is obtained. However, in all healthy humans, vocal-fold vibrations are aperiodic to a greater or lesser degree. Therefore, strobolaryngoscopy does not demonstrate fine detail of each individual vibratory cycle; rather, it shows a pattern averaged over many successive nonidentical cycles. In this sense, it is a less-than-perfect illustration of the true vibratory nature. INSTRUMENTATIONSection 4 of 8
A videostrobe unit consists of a stroboscopic unit (light source and microphone), a video camera, an endoscope, and a video recorder. Stroboscopy can be performed by using either rigid or flexible endoscopes; each has its own benefits and drawbacks. Although flexible endoscopy is ideal for observing unaltered laryngeal behavior from various angles and for viewing the glottis through a narrow supraglottic aperture, it suffers from the low intensity of light carried through the long fiberoptic bundle to the tip of the narrow endoscope. With standard endoscopes, the light bouncing off objects being observed must then travel the length of the endoscope back to a camera or the operator's eye to be detected. The introduction of distal-chip technology to flexible endoscopes, in which the camera is placed at the distal end of the scope, effectively lessened the drawback profile of flexible laryngoscopes. The enhanced digital picture quality with improved illumination has greatly improved the quality and resolution of transnasal laryngeal stroboscopy. Rigid transoral endoscopy produces a magnified bright image ideal for stroboscopy but requires holding of the patient's tongue throughout the examination, which distorts the natural phonatory posture of the pharynx and larynx. Moreover, the patient must have suitable anatomy and the physical tolerance to allow the clinician to visualize the entire glottis. Rigid endoscopy additionally requires increased patient cooperation and amenable patient anatomy for successful visualization of the larynx. Recent research has suggested that the application of the Mallampati classification system is useful for predicting the adequacy of transoral rigid laryngoscopic exposure for stroboscopy. CLINICAL APPLICATIONSection 5 of 8
Several parameters may be evaluated during the course of the stroboscopic examination.
DIAGNOSTIC FINDINGSSection 6 of 8
By increasing the illumination and evaluation of vibratory patterns, videostroboscopy has vastly increased the sensitivity of laryngologic diagnoses. Despite the variations attributable to lesion size, concurrent vocal pathologies, and compensatory phonatory behaviors, generalizations can be made about stroboscopic findings accompanying specific true vocal-fold pathology. The most common benign laryngeal lesions and their typical stroboscopic findings are described below. Vocal-fold cysts Vocal-fold cysts are encapsulated, benign lesions located in the superficial lamina propria of the vocal fold. They are generally unilateral, though several may be present at the time of diagnosis. On stroboscopy, the vocal fold generally has a rounded, full appearance on the affected side. Gaps anterior and posterior to the lesion may compromise glottic closure, depending on their size. The vibratory patterns of the 2 vocal folds are asymmetrical, with diminution of amplitude on the affected side. A substantially decreased mucosal wave is present throughout the affected side and absent over the cyst itself, an important finding for differentiating a cyst from a nodule or polyp (which may have similar appearances on continuous-light examination). An altered vibratory pattern is often the only physical finding in the presence of a small cyst that does not alter the contour of the vocal fold. Vocal-fold polyps Vocal-fold polyps may be unilateral or bilateral. These lesions represent pathology in the superficial lamina propria. They may be of any consistency, ranging from gelatinous to hyalinized. Glottic closure may be compromised, leaving gaps anterior and posterior to the lesion in maximal closure. The vibratory patterns of the 2 vocal folds are asymmetrical, with diminution of vibration near the lesion. Mucosal wave may be present over the polyp itself, depending on its physical consistency. Vocal-fold nodules Vocal-fold nodules are bilateral lesions that are roughly symmetrical. The relevant pathology occurs at the basement membrane zone between the overlying epithelium and the underlying lamina propria. The vocal folds have the appearance of medial fullness, typically at the junction of the anterior and middle third of the fold. Glottic closure is compromised. Mucosal wave is present bilaterally, though both it and the vibratory amplitude may be decreased. Sulcus vocalis Sulcus vocalis is a focal furrow in the covering of the vocal fold. These lesions most often are unilateral and represent a tethering of the overlying epithelium to the deeper connective tissue. Glottic closure is incomplete. Stroboscopy demonstrates a focal interruption in the mucosal wave at the site of the sulcus, and vibratory patterns are asymmetrical between the true vocal folds. EXTENDED APPLICATIONSSection 7 of 8
Although videostroboscopy greatly expands the diagnostic sensitivity of laryngoscopy, its interpretation depends on the skill and experience of the clinician performing the study, and specifically, the skill and experience of the diagnostic interpreter. The quality of the images collected is directly related to the skill of the operator performing the procedure. In addition, recent research suggests that even, among the most experienced interpreters, interrater correlations for judging specific parameters is moderate (kappa = 0.61-0.81) at best. Although increased experience in reviewing stroboscopic results appeared to have a modest positive effect on a clinician's intrarater reliability, it did not necessarily improve interrater correlation i a group of similarly experienced examiners. Several technologies have been developed to improve objective measurements of the amplitude of vibration and mucosal wave. Software was developed to measure the glottic-area waveform (GAW), a plot of the glottic area against the time of opening and closing of the glottis during a representative vibratory cycle (taken from of the stroboscopic image). From this information, glottal opening and closing rates are calculated. These measurements are purported to be correlates of vocal-fold pliability and statistically differ in preoperative and postoperative states for benign vocal-fold lesions. An admitted limitation of the stroboscopic image is that vocal-fold vibration must be relatively periodic to visualize a slow-motion representation of the phonatory cycle. Efforts to extend the sensitivity of laryngoscopy, to incorporate variations of wave characteristic across the glottis and in aperiodic patterns of vibration have yielded new techniques. Videokymography is a recent technique based on the principles of high-speed glottography. This method allows for the isolation of specific portions on the glottic image (obtained at rates of up to 7812 images per second), which are analyzed for closure. The suggested advantage over stroboscopy alone is the ability to evaluate 1 portion of the glottis in comparison to another while avoiding the limitation of vibratory periodicity. This technique also allows for accurate objective measurements of the glottal gap without the inherent error produced by stroboscopic averaging of many individual glottic cycles. An additional effort to extend the diagnostic range by combining the videostroboscopic image with kymography (line drawings representing the distance between defined points on successive images), resulted in the creation of videostrobokymography. This technique uses the standard image generated during videostrobolaryngoscopy and digitalizes individual frames. Kymograms of multiple segments along the glottis are then produced. This marriage of the stroboscopic image and the objective measure of vocal-fold vibration as it varies across the surface of the glottis is a potentially powerful tool, without the need for additional camera technology, as is required for videokymography. On the contrary, this newer technique is subject to all of the technical limitations of the original stroboscopic imaging process. REFERENCESSection 8 of 8
Article Last Updated: May 25, 2006 |
