How are the vocal cords and larynx examined?
An examination of the internal structures of the larynx, including the vocal folds, is called laryngoscopy. There are three principal ways to perform laryngoscopy, reviewed below. Each of these may be appropriate in certain circumstances, but none of these methods alone can evaluate the rapid vibration of the mucosa that serves to produce voice. To do this requires a technique called stroboscopy.
Three techniques of laryngoscopy
The first and most time-honored (having been available for over a century) method of examining the larynx involves inserting a small angled mirror, such as a dentist might use, into the back of the mouth. This mirror deflects a beam of light down onto the vocal folds and reflects the image of the vocal cords back up to the examiner.
Advantages: This method provides a good view of the vocal folds, and probably the most accurate picture with respect to color.
Disadvantages: It is not possible to see the vocal folds vibrating using a mirror, nor is it possible to magnify the view, nor is it possible to make a record of the examination. For these reasons, and because it requires a certain skill and lightness of touch, it is not often used today.
Flexible Fiberoptic Laryngoscopy
This is the most widely available method of examining the vocal folds. An endoscope measuring less than 4mm in diameter is inserted through one nostril and guided through the nose to the back of the throat, until it lies just above the larynx. The flexible fiberoptic scope is the workhorse endoscope in ear, nose and throat medicine – no otolaryngologist’s office is without one.
Advantages: Its advantages include ease of use and general availability, and the capability to examine the larynx during a variety of tasks, such as swallowing, connected speech, and singing. This is very important in the evaluation of many neurologic disorders such as spasmodic dysphonia and vocal fold paralysis.
Disadvantages: A flexible endoscope offers optics that are generally inferior to that of a rigid endoscope, for the image is carried by fiberoptic filaments rather than rigid glass rods. Because of this the view can appear grainy or pixilated, and may not permit precise differentiation among masses of the vocal cords, or easy identification of subtle abnormalities. Stroboscopy (see below) performed via a flexible endoscope is generally not as reliable as that performed via a rigid scope.
Rigid Transoral Laryngoscopy
This method operates along the same principles as a mirror, except that a rigid endoscope is inserted into the mouth in its place. This carries with it its own light source, so it allows the examiner to combine the best elements of the mirror – optical fidelity – with the ability to record the examination, and the ability to perform stroboscopy.
Advantages: A rigid endoscope offers the best optics, often combined with magnification, for laryngeal examination. This makes it the best means to perform stroboscopy and to evaluate subtle problems.
Disadvantages: Insertion of the rigid transoral scope requires that the examiner to hold the tongue. This creates a somewhat abnormal posture for the larynx, and makes connected speech, singing and certain other routine vocal tasks impossible. It also requires more technical expertise than the flexible scope.
Figure 1: A flexible fiberoptic view of the vocal folds.
Figure 2: The same vocal folds seen through a rigid transoral endoscope.
In stroboscopy, a microphone, usually applied to the skin of the neck overlying the larynx, registers the frequency of voicing. This is connected to a strobe light, which flashes just slightly out of synch with this frequency, offering a video image of the vibration of the covering of the vocal cord, known as the mucosal wave. Since this vibration is the source of sound, stroboscopy is the best method to evaluate masses or irregularities of the vocal fold (as opposed to neurological dysfunction). Stroboscopy is the best way, by far, to evaluate irregularities such as polyps, nodules, cysts, scar, and other lesions that affect vibration. However, stroboscopy is technically more difficult than simple endoscopy, and interpreting the examination is not always straightforward. A practiced eye is necessary to spot subtle abnormalities or small lesions.