The laryngeal dystonias include spasmodic dysphonia, tremor, and paradoxical breathing dystonia. All of these conditions are idiopathic and all have distinctive symptoms, which form the basis for diagnosis. In adductor spasmodic dysphonia (ADSD), voice breaks during vowels are associated with involuntary spasmodic muscle bursts in the thyroarytenoid and other adductor laryngeal muscles, although bursts can also occur in the cricothyroid muscle in some persons (Nash and Ludlow, 1996). When voice breaks are absent, however, muscle activation is normal in both adductor and abductor laryngeal muscles (Van Pelt, Ludlow, and Smith, 1994). In the abductor type of spasmodic dysphonia (ABSD), breathy breaks are due to prolonged vocal fold opening during voiceless consonants. The posterior cricoarytenoid muscle is often involved in ABSD, although not in all patients (Cyrus et al., 2001). In the 1980s, “spastic” dysphonia was renamed “spasmodic” dysphonia to denote the intermittent aspect of the voice breaks and was classified as a task-specific focal laryngeal dystonia (Blitzer and Brin, 1991). Abnormalities in laryngeal adductor responses to sensory stimulation are found in both ADSD and ABSD (Deleyiannis et al., 1999), indicating a reduction in the normal central suppression of laryngeal sensorimotor responses in these disorders. ADSD affects 85% of patients with spasmodic dysphonia; the other 15% have ABSD.

Vocal tremor is present in at least one-third of patients with ADSD or ABSD and can also occur in isolation. A 5-Hz tremor can be heard on prolonged vowels, owing to intensity and frequency modulation. Tremor can affect either or both the adductor or abductor muscles, producing voice breaks in vowels or breathy intervals in the abductor type. Voice tremor occurs more often in women, sometimes with an associated head tremor. A variety of muscles may be involved in voice tremor (Koda and Ludlow, 1992).

Intermittent voice breaks are specific to the spasmodic dysphonias, either prolonged glottal stops and intermittent intervals of a strained or strangled voice quality during vowels in ADSD or prolonged voiceless consonants (p, t, k, f, s, h), which are perceived as breathy breaks, in ABSD. Other idiopathic voice disorders, such as muscular tension dysphonia, do not involve intermittent spasmodic changes in the voice. Rather, consistent abnormal hypertense laryngeal postures are maintained during voice production. Such persons may respond to manual laryngeal manipulation (Roy, Ford, and Bless, 1996). Muscular tension dysphonia may be confused with spasmodic dysphonia when ADSD patients develop increased muscle tension in an effort to overcome vocal instability, resulting in symptoms of both disorders. Some patients with voice tremor may also develop muscular tension dysphonia in an effort to overcome vocal instability.

Paradoxical breathing dystonia is rare, with adductory movements of the vocal folds during inspiration that remit during sleep (Marion et al., 1992). It differs from vocal fold dysfunction, which is usually intermittent and often coincides with irritants affecting the upper airway (Christopher et al., 1983; Morrison, Rammage, and Emami, 1999).

Botulinum toxin type A (BTX-A) is effective in treating a myriad of hyperkinetic disorders by partially denervating the muscle. The toxin is injected into muscle, diffuses, and is endocytosed into nerve endings. The toxin cleaves SNAP 25, a vesicle-docking protein essential for acetylcholine release into the neuromuscular junction (Aoki, 2001). When acetylcholine release is blocked, the muscle fibers become temporarily denervated. The effect is reversible: within a few weeks new nerve endings sprout, which may provide synaptic transmission and some reduction in muscle weakness. These nerve endings are later replaced by restitution of the original end-plates (de Paiva et al., 1999). In ADSD, BTX-A injection, either small bilateral injections or a unilateral injection produces a partial chemodenervation of the thyroarytenoid muscle for up to 4 months. Reductions in spasmodic muscle bursts relate to voice improvement (Bielamowicz and Ludlow, 2000). This therapy is effective in least 90% of ADSD patients, as has been demonstrated in a small randomized controlled trial (Truong et al., 1991) and in multiple case series (Ludlow et al., 1988; Blitzer and Brin, 1991; Blitzer, Brin, and Stewart, 1998). BTX-A is less effective in ABSD. When only ABSD patients with cricothyroid muscle spasms are injected in that muscle, significant improvements occur in 60% of cases (Ludlow et al., 1991). Similarly, two-thirds of ABSD patients obtain some degree of benefit from posterior cricoarytenoid injections (Blitzer et al., 1992). When speech symptoms were measured in blinded fashion before and after teatment, BTX-A was less effective in ABSD (Bielamowicz et al., 2001) than in ADSD (Ludlow et al., 1988).

Patients with adductor tremor confined to the vocal folds often receive some benefit from thyroarytenoid muscle BTX-A injections. When objective measures were used (Warrick et al., 2000), BTX-A injection was beneficial in 50% of patients with voice tremors. Either unilateral or bilateral thyroarytenoid injections can be used, although larger doses are sometimes more effective. BTX-A is much less effective, however, for treating tremor than it is in ADSD (Warrick et al., 2000), and it is rarely helpful in patients with abductor tremor.

BTX-A administered as either unilateral or bilateral injections into the thyroarytenoid muscle has been used successfully to treat paradoxical breathing dystonia (Marion et al., 1992; Grillone et al., 1994).

Changes in laryngeal function following BTX-A injection in persons with ADSD are similar, whether the injection was unilateral or bilateral. A few persons report a sense of reduction in laryngeal tension within 8 hours following injection, although voice loudness is not yet reduced. Voice loudness and breaks gradually diminish as BTX-A diffuses through the muscle, causing progressive denervation. Most people report that benefits become apparent the second day, while the side effects of progressive breathiness and swallowing difficulties increase over the 3–5 days after injection. Difficulty swallowing liquids may occur and occasionally results in aspiration. Patients are advised to ingest liquids slowly and in small volumes, by sipping through a straw. The difficulties with swallowing gradually subside between the first and second weeks after an injection (Ludlow, Rhew, and Nash, 1994), possibly as patients learn to compensate. The breathiness resolves somewhat later, reaching normal loudness levels as late as 3–4 weeks after injection, during the period when axonal sprouting may occur (de Paiva et al., 1999). Because improvements in voice volume seem independent of recovery of swallowing, different mechanisms may underlie recovery of these functions.

The benefit is greatest between 1 and 3 months after injection, when the patient's voice is close to normal volume, voice breaks are significantly reduced, and speech is more fluent, with reduced hoarseness (Ludlow et al., 1988). This benefit period differs among the disorders, lasting from 3 to 5 months in ADSD but from 1 to 3 months in other disorders such as ABSD and tremor.

The return of symptoms in ADSD is gradual, usually occurring over a period of about 2 months during end-plate reinnervation. To maintain symptom control, most patients return for injection about 3 months before the full return of symptoms. Some individuals, however, maintain benefit for more than a year following injection, returning 2 or more years later for reinjection.

The mechanisms responsible for benefit from BTX-A in laryngeal dystonia likely differ with the different pathophysiologies: although BTX-A is beneficial in many hyperkinetic disorders, it is more effective in some than in others. In all cases BTX-A causes partial denervation, which reduces the force that can be exerted by a muscle following injection. In ADSD, then, the forcefulness of vocal fold hyperadduction is reduced and patients are less able to produce voice breaks even if muscle spasms continue to occur. Central control changes also appear to occur, however, in persons with ADSD following BTX-A injection. When thyroarytenoid muscle injections were unilateral, spasmodic bursts were significantly reduced on both sides of the larynx, and there were also reductions in overall levels of muscle tone (measured in microvolts) and maximum activity levels (Bielamowicz and Ludlow, 2000). Such reductions in muscle activity and spasms may be the result of reductions in muscle spindle and mechanoreceptor feedback, resulting in lower motor neuron pool activity for all the laryngeal muscles. The physiological effects of BTX-A may be greater on the fusimotor system than on muscle fiber innervation (On et al., 1999). Although only one portion of the human thyroarytenoid muscle may contain muscle spindles (Sanders et al., 1998), mucosal mechanoreceptor feedback will also change with reductions in adductory force between the vocal folds following BTX-A injection. Perhaps changes in sensory feedback account for the longer period of benefit in ADSD than in other laryngeal movement disorders, although the duration of side effects is similar in all disorders. Future approaches to altering sensory feedback may also have a role in the treatment of laryngeal dystonia, in addition to efferent denervation by BTX-A.