Botulinum toxin is a naturally occurring substance that weakens muscle. It exists naturally as poison in improperly canned foods. For medical use, it is injected directly into muscles rather than swallowed, so that the effect is limited and precise. It is best known as a cosmetic treatment to remove wrinkles, which it does by weakening forehead muscles, but botulinum toxin is also widely used as a treatment for disorders of muscular hyperfunction.
Botulinum toxin has been used to treat laryngeal disorders since 1984. The principle of treatment is to weaken muscles that are moving inappropriately, with an injection performed through the skin of the neck. The small needle is placed in the correct spot with the help of electromyography (EMG), a technique that reveals the electrical activity in muscle. Because botulinum toxin is targeted at muscle, EMG is in many ways tailor-made for this purpose. A laryngologist will usually ask a patient to activate the target muscle, for example by voicing or sniffing, and then look to the EMG's increased electrical activity to confirm placement. There may be more than one injection per session depending on the goal of the treatment. Afterwards, patients can usually go on with normal daily activities.
Potential side effects involve function of the muscles which are treated. When the muscles that bring the vocal folds together (thyroarytenoid, lateral cricoarytenoid muscles) are weakened with the injection, this is normally followed by a period of breathy, whispery voice, and sometimes coughing while drinking liquids. If side effects are severe, or last beyond 10 days, the dose of botulinum toxin may be adjusted. When the muscles that pull the vocal folds apart (posterior cricoarytenoid muscles) are treated, the situation is more complicated, as overweakening may result in difficulty breathing. For this reason, such treatment must be more cautious.