Videostroboscopy

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. 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. This technology was subsequently applied to observing vocal fold vibration, first described by Oertel in 1895.

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. The image sequences are played in advancing order to produce a "slow-motion" video clip of the vocal folds during phonation. 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.

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. Stroboscopy may also not accurately represent the movements of a single glottal cycle if there is irregularity from cycle to cycle. In these cases, videokymography (high-speed single-line video scanning) or high-speed videolaryngoscopy may be more effective diagnostic tools.