Flexible fiberoptic laryngoscopy

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Flexible fiberoptic endoscopes in 2018 are an older technology for viewing the larynx. They have an eyepiece that can be used to view the vocal folds directly but is most often attached to a separate camera via a C-mount. 

Benefits of fiberoptic technology 

  • The flexible aspect of this technology allows the endoscope to be passed through the nose, typically reducing the gag reflex during the examination (relative to a rigid endoscope in the mouth). 
  • Most laryngeal function is retained during a flexible examination. Palate function, pharyngeal function and laryngeal function may all be observed at different points in the examination.
  • Flexible endoscopes have a wider angle lens than rigid endoscopes and a wider angle perspective than the human eye. Consequently close objects appear relatively larger than objects farther away. There is a relatively high depth of field compared to rigid scopes so more of the image is in focus at a given time. 
  • The same camera used on the rigid transoral endoscope can be attached to a flexible fiberoptic endoscope. One camera attaching to two different endoscopes yielding relatively lower initial capital cost.
  • Endoscopes have an articulation for bending close to the tip of the endoscope. When there is an even curve near the tip, they are easy to maneuver, especially within the confines of the false cords or the trachea.
  • The angle of view of the structures of the larynx can be modified to provide various perspectives on the same pathology. For example vocal cords may be viewed from above (the same perspective as the rigid), or the flexible tip may be advanced close to the larynx and bent so that the camera lies parallel to the vocal cords, looking along their length.
  • With a wide angle lens, much of the field of view will be in focus. Most of the pharynx can be viewed at once, yet the endoscope can also be positioned close to the vocal cords and even passed beyond the structures of the larynx. The esophagus and trachea may be entered.
  • Fiber-optic technology is relatively inexpensive compared to chip-on-tip technology.

Flexible fiberoptic view of vocal cord leukoplakia. ‚Äč 

Example of a flexible fiberoptic view of leukoplakia on the left vocal cord during adduction.

Disadvantages

  • On their downside, although an attached camera can be updated from standard definition to high definition, the images provided by this technology are inherently quality-limited by the flexible glass fibers carrying both light to the interior of the larynx as well as transmitting the image of the larynx back to the camera to which it is attached. These relatively large glass fibers are visible in any image produced.
  • When a fiberoptic image is recorded, the pixelation of the glass fibers interact with the pixels on the recording device and create a moiré effect. Attempts to diminish the honeycomb image and moiré effect produced by this technology involve a reduction in resolution by blurring the image either physically or electronically.
  • Fiberoptic glass fibers absorb light reducing image resolution.
  • When fibers break over time, light absorption becomes complete and leads to a black spot in the image. 
  • Loss of light through the glass fibers is compensated by the auto gain features of the cameras which leads to artifact. Digital noise produced by increasing video gain alters both the color and clarity of the images.
  • Reflected light diminishes greatly with distance from a structure.

Comparison of flexible fiberoptic images obtained from different distances

The same individual viewed during breathing, with abduction of the vocal cords. Digital noise is particularly evident when the endoscope is relatively far away from the pathology (left). With the endoscope at the level of the tip of the epiglottis, there is significant loss of light at the level of the vocal cords several centimeters away. Since most users have the auto-gain function turned on, electronic amplification of the image occurs without the examiner even sensing the degradation of the image. Electronic gain increases to maintain even lighting and many pixels in the image are assigned colors that don’t actually exist. The image becomes is pixelated when viewed closely. Additionally, as capillaries become blurred, the image appears more red.

For comparison the endoscope has been advanced (right image) and the pathology (leukoplakia) appears brighter as more reflected light reaches the camera on the endoscope. Pixelation is reduced. There are fewer artifical colors so the larynx overall appears less red. There are a few black dots visible in both images from broken internal glass fibers. This same person will be viewed with a chip-on-tip scope for comparison.

While flexible fiberoptic laryngoscopy provides less clarity than a chip-on-tip scope, in many cases a flexible fiberoptic endoscope can be thought of as complementary to the rigid exam, providing additional information if an examiner can afford both the flexible fiberoptic and rigid endoscopes. I think of the combined exam of flexible fiberoptic followed by a rigid exam as one version of a high definition examination. Using two endoscopes on a single individual adds time to an examination, but together it can be a very valuable exam.

 

 

Tip: If false folds are obstructing the view, have the patient perform inspiratory phonation - then it is almost impossible to constrict the supraglottis.

In 2012, I evaluated a new Olympus endoscope as well as the new Pentax HD videoendoscope.