Dipping and rotating: two maneuvers to achieve maximum magnification during indirect transnasal laryngoscopy

One important feature in office-based endoscopy is the fact that when the lens gets closer to the target, the image resolution is enhanced. This is even more true for small endoscopes with tiny lenses. Here, millimeters in distance can make a big difference. We describe two easily performed clinical handling maneuvers in flexible endoscopy which enable a much better magnification of the mucosal surfaces of the endolarynx. These maneuvers allow a close-to-target imaging, thus gaining an extraordinary magnification and quite astonishing 'insights' within office-based endoscopy.

We demonstrate two techniques we routinely use for highest image resolution in flexible transnasal laryngoscopy.

Office-based flexible transnasal laryngoscopy is a common routine procedure since many years. The development of new technical equipment such as flexible high-definition endoscopes allows for very precise examination [1‐9]. The basic principle for getting optimal magnification in laryngoscopy is to decrease the distance of the lens to the target, i.e., the nearer one gets to the vocal fold, the more details one can see. This is especially important for transnasal flexible laryngoscopy because of the small lenses with wide aperture angles. Very small lesions and fine structures of the vocal folds can be evaluated best when the tip of the endoscope is advanced very close-to-target and almost touching the vocal folds. Tiny but diagnostically significant lesions can be missed with insufficient magnification.

For assessing the resolution of the imaging system one is working with, it is helpful to use a device that can demonstrate its possibilities and limitations. For this purpose, we use a special endoscope calibration cube (“Eppendorf cube” provided by Olympus, [10]), but a graphic with mm measures would help as well.

The advantage of flexible endoscopy, compared with rigid endoscopy, is the possibility to move the tip of the endoscope more easily into the endolarynx. Especially, the dipping maneuver with positioning the tip of the endoscope very close to the vocal folds is an easily performed maneuver in routine flexible endoscopy [10]. Our experience shows that this close-up maneuver is most easily performed during long nasal inspiration—here, the vocal folds normally abduct maximally. By increasing the nasal airway resistance by closing one nasal inlet, inspiration can be intentionally prolonged with high inspiratory airflow resistance due to an overall small nasal lumen. For successful performance of the dipping maneuver, one should avoid eliciting a gag response by touching sensitive laryngeal structures. Mostly it is advisable for patient’s compliance to first do some trial maneuvers without endoscope dipping to get the patient used to the procedure [11].

Still, even with best compliance during the dipping maneuver, some structures cannot be seen when the tip of the endoscope advances towards the vocal folds from this vertical direction. Here, another helpful maneuver in flexible endoscopy can be applied, the ‘rotation laryngoscopy’. It is a well-established, however, rarely used technique. Although we could not find any literature describing its first publication, we acknowledge the colleagues who invented this technique. The rotation laryngoscopy allows examination of some endolaryngeal regions of interest one would otherwise not see, especially the anterior commissure with its inferior part, the inferior aspect of the vocal folds, Morgagni’s ventricles, and the subglottic region. Here, in our experience, even slight changes of the angle of the tip of the endoscope can be extremely helpful. When performing office-based surgery in local anesthesia with a flexible channelled endoscope, e.g., for injections or laser surgery, access to these regions is often more easy with rotation laryngoscopy. The rotation laryngoscopy can also be combined with the dipping maneuver for optimal laryngeal exposure of the region of interest.

To avoid camera image overmodulation and “washout” under zoom conditions, sometimes it is advisable to switch between camera settings (automatic gain control (AGC) versus manual gain control), or to use dimmed light, or to change to another illumination-like (mostly less bright) stroboscopy light or narrow band imaging (NBI).

In rigid laryngoscopy, resolution is often better due to better optical systems, but further enhancement by decreasing the distance between lens and target for getting optimal magnification might be a problem. In some cases, but not in all, this can be achieved by lowering the tip of the rigid endoscope as near to the laryngeal inlet as possible by pressing down the base of the tongue.

The described maneuvers can be performed in nearly all patients—in some cases, additional anesthesia might be helpful. We use these maneuvers routinely and never had any complications such as severe vasovagal reactions or laryngospasm.

For achieving an optimal image magnification (and, therefore, resolution) in flexible laryngoscopy, one has to decrease the distance of the lens to the target, due to the fact that the closer one gets to the vocal fold, the more details one can see. The combination of dipping maneuver and rotation laryngoscopy enables the examiner to accomplish two targets: enhanced image resolution and visualisation of otherwise not accessible parts of the endolarynx such as the anterior commissure with its inferior part, the medial and inferior aspect of the vocal folds with the subglottic region, Morgagni’s ventricles and sometimes the trachea as well. In our clinical practice, both maneuvers are routine procedures since many years.