Introduction: The vocal folds vibrate so fast during voice production (over one hundred times a second in men and double that in women) that these vibrations are impossible to see clearly with the naked eye.
Laryngeal Stroboscopy is the state-of-the-art, painless office based procedure of human voice disorders. Videostroboscopy has evolved as the most practical and useful technique for the clinical evaluation of the visco-elastic properties of the phonatory mucosa.
Principle: The basic principle of stroboscopy is based on Talbot law. It takes into account the physical reality that images on the human retina linger for 0.2 seconds after exposure. Therefore, sequential images produced at interval less than 0.2 seconds produce the illusion of continuous images. It is a special method of examination of a vibrating or fast-moving object, such as the vocal folds. A bright flashing light lasting a fraction of a second (10µs) is used to illuminate the vocal folds. This flash ‘freezes’ the movement of the vibrating vocal folds. By taking multiple snapshots at different phases of the vibratory cycle it is possible to see details of the change in shape of pliable surface of the vocal folds (i.e. the mucosa) with time.
Videostroboscopy, Laryngostroboscopy, Laryngo-videostroboscopy, Stroboscopic laryngoscopy, Strobolaryngoscopy.
Aims of Stroboscopy:
- Video documentation of laryngeal anatomy along with its mechanical function.
- Videolaryngoscopy with Stroboscopy is the essential diagnostic procedure for the evaluation of laryngeal mucosa, vocal fold motion biomechanics and mucosal vibration pattern.
Physiology of vocal folds:
Vocal folds vibrate 100-400 times in a second. The fundamental frequency for male ranges from 100-150 Hz and for females is 200-250 Hz. One vibration (1hz) is equal to one open and one close phase. Vocal folds open and close from bottom to top. The air pressure from subglottic area which is generated in the lungs, initiates the open phase of cycle. The low pressure created behind the fast-moving air column produces a “Bernoulli effect” which causes the bottom to close, followed by the top. Closure of the vocal folds cuts off the air pressure and releases the pulse of air.
In stroboscopy, we use strobe light, which is a very bright light, at short bursts which is synchronized with vocal fold vibration. It helps us to capture individual still images at selected point from sequential vibratory cycle. The images are then merged to create a likeliness of a vocal fold vibration.
- The flashing light from the stroboscope is passed through a flexible nasoendoscope or rigid endoscope to illuminate the vocal folds. The flashing of the stroboscope needs to be synchronized with vocal fold vibration to be able to see them ‘move in slow motion’. To do this the rate of vibration of the vocal folds (or fundamental frequency) needs to be measured. This can be done by analyzing one three types of signals:
- The acoustic signal of the voice from a microphone.
- The vibration of the vocal folds transmitted through the skin from a contact microphone placed over the thyroid cartilage of the larynx.
- The Laryngograph signal by measuring changes in electrical conductance between two electrodes placed over the thyroid cartilage of the larynx.
- Laryngograph signal used to trigger the stroboscope provides information about the change in vocal fold contact during each cycle.
- The flashes of light from the stroboscope are synchronized to the frequency of vocal fold vibrations with a slight time delay added for each flash. If the flash takes place in time with the fundamental frequency the vocal folds would appear not to move. The slight time delay means that the vocal folds are illuminated at a slightly later point in the vibratory cycle.
4. The brain interprets this series of ‘snapshots’ as movement and gives the illusion that the vocal folds are vibrating in slow motion.
5. It is an illusion because it takes time to build up the energy for each flash which means that the vocal folds have vibrated several times before the stroboscope is ready to flash again and so the ‘slow motion movie’ is actually made up of a series of images taken from different vibratory cycles.
- The slow motion allows details of the mucosal wave of the vocal folds to be seen. This is a complex vibratory or oscillatory movement of the mucosa of the vocal fold that takes place in order to produce voice. It can be seen by observing the changing reflection of light during stroboscopy or high-speed digital imaging.
- It is particularly helpful to clinicians who want to check that there is no subtle structural abnormality of the vocal folds that could be contributing a voice problem.
- It contributes significantly to the diagnosis in almost 30% of cases compared to examination with normal (non-stroboscopic) light.
- Normal vocal fold vibrations are usually associated with the following key observations:
- regular vocal fold vibrations.
- symmetrical or balanced vocal fold vibrations.
- a mucosal wave that becomes more prominent as the fundamental frequency becomes lower.
- the vocal folds should be pliable with no localized areas of stiffness.
- the gap between the vocal folds should only be visible for just over half the time of the vibratory cycle.
- These technologies provide valuable patient information.
- They allow images to be recorded on video or other media formats, permitting examiners to review the images of the voice box frame by frame.
- Capture still and close-up images.
- Patients can also view the recorded images and see the reason(s) for their voice problems.