Botulinum toxin (BTX) is derived from the bacterium Clostridium botulinum. It is a nerve “blocker” that binds to the nerves that lead to the muscle and prevents the release of acetylcholine, a neurotransmitter that activates muscle contractions.
In spasmodic dysphonia, laryngeal muscles spasm because too many or the wrong type of signals travel from the brain through the nerves to the muscles. BTX is a biologic product and is injected directly into the affected muscles. It blocks these nerve signals, reducing the number and severity of the spasms.
Since 1984, BTX has been used to treat the symptoms of thousands of people with SD and its efficacy and safety have been documented in numerous medical publications. Furthermore, the American Academy of Otolaryngology & Head and Neck Surgery has documented the use of BTX for SD in a policy statement. Its use by physicians to treat SD is considered “off-label” by the FDA. However many other medicines are used successfully off-label for a number of conditions and this designation does not limit a physician’s use of a medicine.
BTX exists in a number of forms, but only types A and B are available commercially. They are administered similarly but have different dosing, onset of action, and length of activity. The majority of people with SD receive BTX type A; therefore, the remainder of this discussion focuses on this type.
How is BTX Injected for Spasmodic Dysphonia?
BTX must to be injected directly into the target muscle for effect. The higher the dose, the weaker the muscle becomes and less spasming occurs. However, as described below, BTX treatments have side effects, so it is important for patients to collaborate with their physician and work together to identify the optimal dosage.
Most physicians inject BTX through the skin of the neck, and many use an EMG (electromyography) machine to help with correct placement. Some physicians do not use EMG and others deliver the medicine using a special curved needle through the mouth, sometimes using an endoscope to help guide them.
The majority of physicians administer the medicine in an office setting without any special preparation. Physicians may inject the affected muscles on one side (unilateral injections), or both sides (bilateral injections), together at one or separately in two sittings.
Successful results have been documened with all injection methods. Ultimate success depends on the specifics of the nerve and muscle interaction, which may not be fully known until after several treatment cycles. During this period of trial and error, the physician chooses and modifies the route, dose, and side based on his or her clinical experience and discussions with the patient about the symptoms and the results from the previous injection.
In Adductor SD (AdSD) the target muscles are often the Thyroarytenoid (TA) muscle, although some practitioners may dose the Lateral cricoarytenoid (LCA) muscle as well.
These muscles are active in closing the larynx; weakening them prevents the voice breaks associated with this form.
People with Abductor SD (AbSD) typically receive injections in the Posterior cricoarytenoid (PCA) muscle. Weakening the muscles that open the larynx keeps the vocal cords closer together, thus preventing the breathy voice breaks associated with this form of SD.
Persons with tremor may have their laryngeal strap muscles injected. The strap muscles, located in the neck, support the voice box and are often affected in tremor. In certain circumstances, the other smaller muscles such as the Cricothyroid (CT) or the Interarytenoid (IA) may be injected as well.
What is the Dosage?
A wide variety of doses are used and it can vary based on the type of botulinum toxin used. With BOTOX, most people receive less than five units per muscle. In their review of 901 patients, Blitzer, et. al., found the average dose per muscle at about three units of BOTOX (type A). Some patients see results with less than one unit.
While about 90 percent of AdSD patients are successfully treated with BTX, only one-half to two-thirds with AbSD find BTX treatment helpful. This might be because physicians consciously limit the dose delivered to the PCA, as these muscles are also intricately involved in respiration. A large dose delivered to these muscles could cause a patient to experience difficulty with breathing. Many physicians stage the dose from side to side or only inject one side in patients with AbSD for this reason.
What Do I Expect After the Injection?
Following injection, the patient may experience some slight discomfort at the injection site, but this is typically quite minimal. Speech may be difficult for about an hour. Bruising is uncommon. The BTX does not begin to work for about two days when the targeted muscles weaken and the person may experience some changes.
In AdSD, patients often develop a weak voice with an airy or breathy quality. Some describe a voice that sounds like "Mickey" or "Minnie Mouse." Rarely, the patient may have some temporary swallowing difficulty, especially with liquids. This only occurs with higher dosing and the person can usually compensate by drinking carefully in small amounts and following simple instructions from the physician or speech pathologist. Once these effects pass, the voice strengthens and becomes more fluent, with fewer or no breaks. The weak voice generally lasts only for a few days but can be longer in some instances. Ideally, the stronger voice lasts at least three months.
Following this, the effect of the BTX gradually wears away and the spasming or halting voice symptoms recur. Most physicians continually adjust the BTX dosage in order to minimize the negative effects of weak voice and maximize the length of the good voice period. Again, this requires accurate feedback from a patient about the duration of symptoms and the onset of the strong voice. The NSDA offers a chart to help SD patients track their voice quality and symptoms between injections.
People with AbSD may experience a similar period of BTX effect, but typically have fewer of the ups and downs immediately after the treatment. Since injection of the PCA can lead to some restriction of breathing, it is important that these patients inform the doctor about these types of symptoms so that a good voice can be balanced with good breathing.
If a patient does not respond to BTX therapy, this could be due to antibody development, inappropriate dosage, inaccurate delivery/placement of the toxin (i.e., missing the spot), incorrect diagnosis, disease progression, or co-existence of some other neurological disorder. The best outcomes for BTX therapy involve trial and error along with a team effort between the patient and the doctor.