Continuing Education Activity

Videostroboscopy is the most commonly used method to visualize vocal fold vibration and is an essential tool for voice assessment. Recordings of the vocal fold vibration pattern are analyzed and shared with a speech-language pathologist to guide treatment. Videostroboscopy can also monitor disease processes and assess pre- and post-operative outcomes. This activity reviews the videostroboscopic evaluation of the larynx and highlights the role of the interprofessional team in evaluating patients with dysphonia.

Objectives:

• Apply the mechanism by which videostroboscopy provides improved dynamic visualization of the glottis.

• Identify the indications for videostroboscopy.

• Determine the components of the mucosal wave that can be visualized with videostroboscopy.

• Communicate interprofessional team strategies that can improve patient care and outcomes for individuals who undergo an assessment with videostroboscopy.

Introduction

Video endoscopy with stroboscopy (also known as "videostroboscopy" or "stroboscopy" for short) is the most common method of visualizing vocal fold vibration and is an essential tool for voice assessment.[1][2] It is used to evaluate the pliability of the vocal fold mucosa, which indicates the health and function of the mucosal tissue and the deeper layers. The term stroboscopy comes from the Greek words "whirling" and "to look at." The concept of stroboscopy was developed in the early 1800s using a rotating wheel with slits to view images on a separate rotating wheel, which produced the appearance of movement.[3] This technology was subsequently applied to observing vocal fold vibration, first described by Oertel in 1895.[4] 

During phonation, the vocal folds vibrate at high frequencies, too rapidly to be perceived by the naked eye. Stroboscopy is used to visually "slow down" the vibration to facilitate assessment. It uses a flexible or rigid endoscope, a microphone, and a flashing strobe light. The microphone is placed next to the larynx and is used to estimate the fundamental frequency of the voice. The strobe frequency is then synchronized at a rate slightly below the larynx's fundamental frequency, thus capturing successive phases of the glottic cycle.[3] The image sequences are played in advancing order to produce a "slow-motion" video clip of the vocal folds during phonation.[4][5][6] The effect is similar to that of a flipbook, in which a series of images produces the perception of motion. This sampling across successive glottic cycles displays an estimate of the underlying vibratory function.

Historically, the "slow-motion" effect of laryngeal stroboscopy has been mistakenly attributed to Talbot's law (which concerns the relationship between perceived brightness of an object and illumination exposure time) and the persistence of vision (which posits that images are retained upon the human retina for up to 0.2 ms) when, in fact, it is a result of the phenomenon of visual perception of flicker-free images when the frequency of the strobe lamp exceeds 50 Hz and the resulting appearance of motion due to the effect of visual sampling during sequential phases of vocal fold motion.[3]

Stroboscopy is a low-cost alternative to other technologies, such as high-speed video, requiring a more specialized camera and a large data storage volume. There are some drawbacks to stroboscopy due to the sampling technique required; as only periodic vocal fold movements from successive glottic cycles are captured, aperiodicity or fluctuating vocal fold movements cannot be tracked. Another situation in which stroboscopy performs poorly is diplophonia. This condition produces 2 separate pitches simultaneously, disrupting the stroboscopy microphone's fundamental frequency estimate and causing irregular flashing and inconsistent images.[7] Stroboscopy may also not accurately represent the movements of a single glottal cycle if there is irregularity from cycle to cycle.[8] In these cases, videokymography (high-speed single-line video scanning) or high-speed videolaryngoscopy may be more effective diagnostic tools.[9][10]

Anatomy and Physiology

Sound production requires 3 components: a power source, a vibratory source, and a resonator. Human vocal sound production utilizes expired air as the power source, driven from the lungs by the diaphragm and intercostal muscles. The vocal folds within the larynx are the vibratory source, and the upper airway and oral cavity constitute the resonator chamber.

The vocal folds' vibratory pattern is referred to as the "mucosal wave" because the movements of the vocal folds resemble waves in a fluid medium.[11] The complex 3-dimensional sinusoidal vibration of the vocal folds is a multi-step process that results from the interplay between air pressure and anatomical structures. It begins with exhaled air pressing upwards against adducted vocal folds; once the subglottic pressure has increased to a threshold level, air can escape between the vocal folds. The folds then separate in an inferior to superior direction, creating a dynamic gap between them that moves upwards until it opens completely at the superior margin of the vocal folds.

As the mucosal wave travels upwards and laterally across the vocal folds, there is a dynamic relationship between the 2 main functional parts of the vocal folds, the cover, and body, each composed of 2 or more layers, according to Hirano's classic 1974 description.[12] The vocal fold cover comprises the squamous epithelium and the superficial lamina propria (also known as Reinke's space). The vocal fold body beneath it comprises the intermediate lamina propria, the deep lamina propria, and the vocalis muscle. Subsequent rapid airflow between the open vocal folds creates negative pressure (Bernoulli's effect), and closure of the glottal gap from inferior to superior ensues. Therefore, the lower edge of the vocal fold begins to close as the upper edge is opening.[13]

Indications

Patients with voice problems such as hoarseness, breathiness, vocal fatigue, loss of vocal pitch range, laryngeal discomfort, tightness, irritation with frequent coughing or throat clearance, or altered sensation are candidates for videostroboscopy. Patients who recently underwent laryngeal or neck surgery, are undergoing voice therapy, or require oncological surveillance may also be candidates. Videostroboscopy is usually performed after visualizing the pharyngeal and laryngeal anatomy using flexible nasolaryngoscopy.[14] The use of videostroboscopy provides real-time evaluation and recording of the appearance of the patient's vocal fold vibration.[15]

Contraindications

There are few contraindications to videostroboscopy. The procedure is minimally invasive to the patient and requires the passage of a flexible laryngoscope through the oral or nasal airway or a rigid endoscopy into the mouth. The procedure is usually well tolerated with appropriate patient preparation and coaching by the examiner. As the "slow-motion" effect can only be produced with prolonged phonation that creates periodic vocal cord vibration, cooperation from the patient is essential. The effectiveness of the examination may be limited by anxiety and discomfort, however, as not all patients can comply with instructions calmly during the process. Patients with strong gag reflexes, histories of vasovagal episodes, anxiety regarding medical procedures, or airborne communicable infections may not be the best candidates for videostroboscopy or nasolaryngoscopy. Bleeding disorders and anticoagulation are relative contradictions because the passage of the scope through the nasal cavity may cause mucosal damage and epistaxis; in clinical practice, however, bleeding is rare and tends to resolve spontaneously.

Equipment

Videostroboscopy requires the same equipment and supplies that are used for flexible nasolaryngoscopy, with the addition of the videostroboscopic unit:

• Flexible fiberoptic or digital chip-on-the-tip (distal chip) nasolaryngoscope or a rigid 70-degree endoscope 
• Specialized laryngeal or throat microphone to be placed on the neck
• Stroboscopy apparatus that analyzes input from the microphone to synchronize the strobe frequency of the light source
• Recording apparatus to document the exam and review in slow motion or image-by-image [14]

Personnel

Videostroboscopy is usually performed and interpreted by an otolaryngologist, laryngologist, or speech-language pathologist.

Preparation

The appropriate steps to prepare for videostroboscopy are:

1. This procedure is performed in the outpatient clinic setting.
2. The procedure is approximately 2 to 3 minutes in duration.
3. The throat microphone is placed externally on the neck, overlying the thyroid cartilage, to assess vocal pitch and intensity.
4. Topical 1% lidocaine and a decongestant, usually with 0.05% oxymetazoline hydrochloride, is sprayed or nebulized into the nasal cavity (for transasal laryngoscopy) or the tongue and oropharynx (for a transoral approach).
5. The patient is instructed to lean forward with the lower neck flexed forward and the upper neck flexed in a "sniffing" position.
6. The laryngoscope is warmed or defogged.
7. The scope is then advanced into the nasal cavity past the nasopharynx to provide a view of the laryngeal structures.
8. The patient is then instructed to sustain an "ee" (/i/) sound at different pitches and volumes.

Technique or Treatment

A full view of the larynx extending from the anterior commissure (if possible without limitation from the epiglottis) posteriorly to the posterior commissure and arytenoid towers is optimal for assessing the mucosal wave. The patient is instructed to produce an /i/ sound at varying pitches and volumes. Sustained phonation is necessary to synchronize the stroboscopic light source. Once the necessary vocal tasks have been performed, the laryngoscope is removed, and the patient can rest.

Frequently employed maneuvers include asking the patient to sniff rapidly or for a prolonged period or say /i/ between breaths to evaluate vocal fold mobility and symmetry; say /i-i-i/; produce an /i/ sound at a comfortable pitch, normally, loudly and quietly; produce an /i/ sound in a falsetto voice, normally, loudly and quietly; produce an /i/ sound at a low pitch, normally, loudly and quietly; produce an /i/ sound gliding from low to high, produce an /i/ sound gliding from high to low, count from 1-10, and repeat other specific phrases, such as the days of the week or the months of the year.

The clinician then reviews the video recording. A Stroboscopy Evaluation Rating Form is commonly used to examine different portions of the vocal apparatus and rate the vocal fold movements.[16]

1. Amplitude: the lateral extent of vocal fold displacement during the opening phase
2. Mucosal wave: pattern of vocal fold tissue deformation
3. Vibratory behavior: the presence of any non-vibrating portions of the vocal fold
4. Supraglottic activity: extent of laryngeal compression
5. Edge: the rating of smoothness/roughness and straightness/irregularities of the vocal fold edges
6. Vertical level: on-plane versus off-plane vocal fold contact during the closing phase
7. Phase closure: the rating of open vs. closed phase duration
8. Phase symmetry: rating the percentage of time with symmetry between left and right vibratory phases 
9. Regularity: rating the percentage of time with regular vibration 
10. Glottal closure: describing the shape of the glottis at closure

Complications

Complications of videostroboscopy parallel those of any form of nasolaryngoscopy. Epistaxis, discomfort, gagging, vomiting, sneezing, coughing, and vagal episodes are the most common. There are no additional risks conferred to nasolaryngoscopy by adding stroboscopy; videostroboscopy is a safe procedure overall.[14]

Clinical Significance

Videostroboscopy is currently the gold standard for laryngeal imaging.[17] Its cost-effectiveness, ease of use, and real-time audio and visual feedback make videostroboscopy an effective means of diagnosing voice disorders. Interpretations of the different parameters of stroboscopy are highly subjective but have varied inter- and intra-rater reliability.[18][19] Advanced laryngeal visualization technology can provide further functional data regarding the vibration and detailed appearance of the vocal folds.[20][21] Laser-based high-speed videoendoscopy and videokymography are other modalities that can be used to analyze the vibration patterns of the vocal folds. Still, they are employed more frequently in research settings and large voice centers.[13][17] 

In addition to evaluating voice disorders, videostroboscopy plays a role in oncologic surveillance. A reduction or absence of the mucosal wave suggests abnormal structural health and pliability of the vocal cord, which may indicate dysplasia, early glottic carcinoma, scarring, marked hyperkeratosis, or inflammation.[6][22] Videostroboscopy can also be used to assess perioperative functional status, document the effectiveness of voice therapies, and monitor surgical outcomes.[17][23]

Enhancing Healthcare Team Outcomes

The use of videostroboscopy in interprofessional voice clinics, usually composed of laryngologists and speech-language pathologists, changes the diagnosis of the underlying etiology of dysphonia in 45% to 70% of cases that are referred to these clinics, typically resulting in modification or refinement of the treatment plan and improvement of patient outcomes.[19][24]