Introduction
Botulinum Neurotoxin (BoNT) injection has been the gold-standard treatment for the neurological disorder known as Spasmodic Dysphonia (also known as Laryngeal Dystonia) since the first injection by Blitzer in 1984. BoNT is injected directly into the laryngeal muscles, inducing dose-dependent muscular weakness that reduces the vocal fold spasms. In cases of Adductor Spasmodic Dysphonia (ADSD), botulinum denervation of the thyroarytenoid-lateral cricoarytenoid muscle complex can create temporary debilitating weakness of the voice that is the most prominent side effect of this treatment. This prolonged breathiness has spurred the identification of alternative injection techniques that minimize this side-effect.
Two injection techniques target either the true vocal folds (TVF) or the false vocal folds (FVF). , TVF Botox injections performed under electromyogram (EMG) guidance typically utilize the transcricothyroid (TCT) approach, entering the endolarynx through the cricothyroid (CT) membrane. FVF Botox injections are performed via a transthyrohyoid (TTH) approach, passing the needle through the thyrohyoid (TH) membrane under nasolaryngoscopic guidance.
Due to the complicated nature of this neurogenic disorder, there is no known cause or established long-term treatment. The demand for injection frequency creates an increased necessity for informed decision-making and expectations. Preliminary data suggest that the TTH approach, targeting the FVF, results in a decreased period of breathy dysphonia for patients compared to the TCT approach; however, analysis is limited by the availability of anecdotal data and analysis of Voice Handicap Index (VHI-10) scores.
TECHNIQUE
Transcricothyroid
The patient is lying in a supine position on a flat bed. Botox is reconstituted using the standard dilution of OnabotulinumtoxinA, 2.5 mouse-units per 0.1 cc. The Botox is then drawn up in a 1 cc syringe and attached to a 25 mm 27-gauge hypodermic needle electrode (Bo-ject DHN Disposable Natus, Middleton, WI, US). The needle and 2 electrodes are attached to an EMG (TECA Synergy, Viasys Healthcare, US). The needle is bent at the hub to a 30 degrees angle. The bent needle is passed through the CT membrane to enter the endolarynx. Botox is injected bilaterally into each thyroarytenoid/lateral cricoarytenoid muscle complex.
For patients who struggle with comfort during this approach, a transtracheal injection of 2 cc 4% Lidocaine-HCl may be utilized. The patient should then be instructed to remain NPO for 90 minutes.
Transthyrohyoid
While the patient is seated in the upright position, the nasal cavities are prepped for laryngoscopy with a 2% lidocaine hydrochloride (HCl) and 0.025% oxymetazoline HCl spray. The laryngotracheal complex is anesthetized using a transtracheal injection of 2 cc of 4% lidocaine HCl.
Onabotulinumtoxin A (Botox) is reconstituted using 4 cc of preservative-free saline into the 100 mouse unit vial, resulting in a final concentration of 2.5 mouse-units per 0.1 cc. The dosage (ranging from 5 to 15 mouse units per side) is drawn up in a 1 cc syringe and a 1.5-inch 25-gauge needle, prepared with the double-bend technique as described by Achkar et al., is attached.
Visualization is provided by a nasolaryngoscope (Olympus ENF-VH, Olympus US) held in place by an assistant, while the 1.5-inch 25-gauge needle, prepared with the double-bend technique, is passed through the TH membrane to enter the endolarynx. The needle is directed in a caudal direction and enters the larynx from the petiole of the epiglottis. The needle is directed into each FVF, making sure to inject in such a way as to create a superficial wheal of BoNT. After the injection, the patient is observed for 30 minutes in the clinic and instructed to remain NPO for 90 minutes. There is no need for voice rest.
For patients who do not tolerate the TTH approach or have anxiety that precludes the use of the external approach, a transnasal approach may be considered. Lidocaine is sprayed through the working channel of a nasolaryngoscope, and medication is delivered to the FVF utilizing a 25-gauge interject needle (Boston Scientific, Marlborough, MA, US).
OUTCOMES
TTH and TCT options have both been established to be effective and safe methods of delivering BoNT for the treatment of ADSD. Blitzer et al. introduced the TCT technique in 1984 utilizing EMG for guidance. Since the introduction in 1997 as a treatment for ADSD, supraglottic BoNT injections (TTH injections) have provided relief for patients, specifically those with high voice demands. Simpson et al. report that 74% of patients did not experience a decline in voice quality postinjection, and those who did experienced a minor decline. Patients, typically professional voice users and singers, prefer this approach due to the decreased chance of a breathy voice and smoother onset of benefit.
A study in progress by authors LAS and RAF reports preliminarily that both TTH and TCT injections were clinically effective, with the period of benefit longer for the TCT approach than the TTH (17 weeks vs. 7.5 weeks; P <.01). The TCT approach also resulted in significantly longer duration of breathy dysphonia (4 weeks vs. 1 week for TTH; P <.01). The average dose for the TTH injection was 9.12 units per side vs 2.32 units per side for the TCT approach.
In this analysis, patients benefited from BoNT injection with both TTH and TCT approaches. While the TCT injection resulted in a benefit that lasted an average of 17 weeks, ∼2 times longer than the TTH approach (7.5 weeks), the TCT approach also resulted in a greater period of debilitating breathiness. The TCT approach may be better suited for patients who desire to undergo injections less frequently, who can accept the possibility of prolonged breathy dysphonia (4 weeks). The TTH approach may optimize quality of life (QoL) for patients whose professional demands require full strength of the voice even after a BoNT injection. The average of 1 week of breathy dysphonia is significantly less than what is seen after the TCT technique. Patients should be informed that they will require more frequent injections when switching from the TCT to the TTH approach.
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