Stroboscopy
Stroboscopy is a way of viewing the vibration of the vocal folds in apparent slow motion. The vocal folds vibrate at 100 to 250 times a second during speech, far too fast to be seen by the eye in continuous light. A flashing light synchronised to the voice pitch is used to show a series of images from different moments of successive vibrations, which the eye blends into a slow-motion picture of one cycle.
A rigid 70 or 90 degree telescope is passed through the mouth, or a flexible endoscope is passed through the nose. A microphone or a contact sensor on the neck picks up the voice frequency, and the light source flashes at a slightly different rate from the voice, producing an apparent slow motion of the vibration.
The patient is asked to say a long "ee" at a comfortable pitch and loudness, and then at higher and lower pitches and loudness. The test takes 5 to 10 minutes and is recorded for review and comparison over time. A related method, high-speed videoendoscopy, records thousands of frames per second and shows true vibration.
The test shows the mucosal wave (the ripple of the surface layer of the vocal fold over the deeper layers), which depends on the flexible layer of the fold, and detects scars, stiffness, small cysts, and early cancer that normal light examination can miss. It changes the diagnosis in a large share of patients with voice complaints, and it is needed in voice disorders.
Indications.
- Persistent hoarseness or voice change with an apparently normal larynx in continuous light.
- Suspected vocal fold scar, sulcus, cyst, or small mucosal lesion.
- Voice assessment in singers, teachers, and voice professionals.
- Vocal fold nodules, polyps, and cysts, before and after treatment or microsurgery.
- Suspected early laryngeal cancer or premalignant change (leukoplakia), to judge the depth of the lesion by the loss of the mucosal wave.
- Vocal fold paralysis or paresis, to show reduced vibration or asymmetry.
- Spasmodic dysphonia, vocal tremor, and muscle tension voice disorders.
- Assessment after laryngeal injury, radiotherapy, or surgery, and monitoring of treatment results. Not a replacement for general laryngoscopy of the airway, and cannot be done in patients who cannot phonate.
Contraindications and precautions.
- Strong gag reflex: topical anaesthesia of the throat and a flexible scope are used.
- Severe airway obstruction, stridor, or epiglottitis: the test is not done, and the airway is secured first.
- Allergy to local anaesthetic.
- Aphonia or inability to produce voice: vibration cannot be seen, and the test gives limited information.
- Patients who cannot cooperate, such as young children.
- Infection control: scopes must be decontaminated properly.
- Anaesthetic sprays on the larynx can change voice and vibration, so the rigid scope is used without anaesthesia when possible, or with a minimal amount.
- Excess light exposure with a rigid scope for a long time heats the tip and tissue, and the tip is kept clear of the mucosa.
Preparation and technique.
The patient sits upright, leaning slightly forward, with the chin lifted. The voice sensor is placed on the neck, and the scope is placed. For the rigid scope, the tongue is held gently with gauze, and the scope is placed at the back of the mouth. The patient is asked to say "ee" at a comfortable pitch for several seconds, then to glide from low to high pitch, then to say it softly and loudly.
The examiner watches for the symmetry of vibration, the amplitude, the mucosal wave, the closure pattern of the folds, the regularity of vibration, and the non-vibrating segments. Images and video are saved, and the examination can be repeated after treatment.
