Anatomy & Pathophysiology:
Spasmodic dysphonia (SD) is an idiopathic, focal dystonic laryngeal disorder. It is classified as a hyperfunctional neurologic condition, wherein sustained muscle contractions produce unwanted repetitive movements. Other focal dystonias include blepharospasm (spasm of the eyelid), torticollis, facial tics, and writer's cramp. Like these other disorders, SD is exacerbated by voluntary muscle use (i.e., speech) and worsens with prolonged muscle contraction (fatigue). There are two main classifications of spasmodic dysphonia: adductor SD and abductor SD. Adductor SD is the more common (>80% of cases) and involves excessive vocal fold adduction (closure) during attempted speech, resulting in a strained voice quality and breaks in the voice due to repeated glottic closure interrupting phonation. By contrast, patients with abductor SD experience sustained vocal fold abduction (opening) during speech, which produces a breathy, whispering voice quality. Patients with features of both adductor and abductor SD are referred to as having a mixed laryngeal dystonia. For a review of the physiology of voice production, see The Larynx.
The dystonic movements in SD are generally limited to speech production, whereas other laryngeal tasks, such as singing, swallowing, and breathing are not affected. The pathophysiology of spasmodic dysphonia is unknown, although the prevailing theory involves neurotransmitter abnormalities in the basal ganglia.
The true incidence of spasmodic dysphonia is unknown, as it is thought that many patients do not seek medical attention and are therefore never diagnosed. There is a 4:1 female-to-male preponderance, with most patients presenting during the fourth and fifth decades of life. SD is an adult onset disorder, although rare cases of patients affected during adolescence have been reported. A family history of dystonia or voice disorder is reported in 12% of patients with SD.
Patients with focal dystonias may have a gradual, slowly progressive disease course, or some may have no progression at all. However, spontaneous recovery is not generally observed. Many patients, particularly those with mild SD, learn adaptive speech strategies to cope with the disorder.
Up to half of patients with SD report sudden onset of their symptoms after a stressful life event such as an illness. Patients with adductor SD present with a strained or strangled voice quality, and exhibit breaks in the voice, particularly with words beginning with vowels. Patients with abductor SD present with a breathy voice quality and prolonged complete loss of voice (aphonia) following production of voiceless consonants such as /f/, /h/, /k/, /p/, and /t/. Essential tremor of the voice in the 6-8 Hz range may accompany SD, and, in some cases, the tremor may extend to muscles of the pharynx, face, head, or hands. Patients are typically able to voice normally when performing laryngeal tasks other than normal speech, such as singing, whispering, or speaking in a character voice (e.g., in falsetto).
Anecdotally, many patients report temporary symptomatic improvement with use of distracting mechanisms, such as pinching the nose, pressing on the abdomen, or pulling on the ear during speech. Symptoms may also improve with alcohol consumption, which has been demonstrated to have an inhibitory effect on essential tremor. In general, SD worsens with anxiety, stress, or prolonged voice use.
Differential Diagnosis of Neurologic Voice Disorders:
- Pseudobulbar palsy
- Tic disorders (e.g., Tourette syndrome)
- Cerebrovascular accident
- Cortical neoplasm
- Cranial trauma
- Multiple sclerosis
- Myopathies (e.g., muscular dystrophy)
- Neuromuscular junction disorders (e.g., myasthenia gravis)
- Parkinson’s disease
- Essential tremor
- Vocal fold paresis
- Muscle tension dysphonia
- Functional aphonia
- Paradoxical vocal fold motion disorder
History includes exacerbating and relieving factors, along with symptoms suggestive of a systemic etiology.
During the examination, the patient's speech should be observed under relaxed circumstances (casual conversation) as well as more formally tested by reading passages designed to elicit adductor or abductor breaks. Speech quality can be rated based on the provider's perception; elements such as roughness, breathiness, and strain are graded according to perceived degree of deviance from normal.
Videolaryngostroboscopy is the primary diagnostic modality for evaluating spasmodic dysphonia. Stroboscopy produces the illusion of slow motion due to the use of a strobe light that flashes at a rate slightly offset from that of vocal fold vibration. This enables visualization of vocal fold hyperadduction (closure) in adductor SD and sustained glottic opening in abductor SD.
Imaging is not indicated unless the patient has systemic or other neurologic symptoms beyond the scope of spasmodic dysphonia.
Objective measurements of voice quality include acoustic measures such as jitter (standard deviation of fundamental frequency) and shimmer (amplitude modulation), both of which are increased in spasmodic dysphonia. Phonatory airflow rate (air flow through the larynx during speech) can also be measured; rates tend to be decreased in adductor SD and increased in abductor SD. Although muscle spasms underlie the symptoms of SD, electromyography is not usually necessary for diagnosis, unless there is suspicion for other neuromuscular disorders.
No curative therapy exists for spasmodic dysphonia. The current first-line treatment is botulinum toxin injection into the laryngeal muscles. Botulinum toxin is a neuroparalytic agent that causes flaccid muscle paralysis due to inhibition of acetylcholine release. The modulation of central nervous system output via peripheral feedback from the paralyzed musculature is thought to produce long-term symptomatic improvement. In adductor SD, the thyroarytenoid-lateral cricoarytenoid (TA-LCA) muscle complex is usually injected, while the posterior cricoarytenoid (PCA) is injected in abductor SD. Injections are generally performed transcutaneously via the cricothyroid membrane under electromyographic guidance to enable specific muscle targeting. Bilateral injections are usually performed, although unilateral injections may be employed as booster doses for maintenance therapy. Toxin dosages may be titrated to achieve optimal results.
Other, less effective treatment modalities, such as voice therapy and sedative pharmacotherapy may be utilized in selective cases as adjunctive treatments. Several surgical procedures have also been reported for the treatment of spasmodic dysphonia with varying success rates. These include the Isshiki type 2 thyroplasty to lateralize the vocal folds in adductor SD, selective laryngeal adductor denervation and reinnervation with ansa cervicalis, and TA-LCA myectomy for adductor SD.
Complications, Prognosis & Follow-Up:
Botulinum injection is reported to have a 90% success rate in producing symptom improvement in spasmodic dysphonia. Toxin resistance is rare and treatment failure is usually due to technical or dosing issues. Effects of injection are generally noticeable within 2-3 days of treatment. Individual sensitivity to botulinum toxin varies, and thus optimal dosage and injection interval will be different for each patient. Complications of injection for adductor SD include development of breathy or hoarse voice quality and transient dysphagia. Complications of injection for abductor SD include stridor and dysphagia due to PCA paralysis.
- Spasmodic dysphonia (SD) is an idiopathic laryngeal dystonia.
- Adductor SD accounts for over 80% of SD and involves vocal fold hyperadduction during speech, resulting in a strained voice quality and breaks in the voice, particularly with words beginning in vowels.
- Abductor SD involves sustained glottic opening, resulting in a breathy voice quality and prolonged aphonia after certain consonants
- Videolaryngostroboscopy is the primary diagnostic modality and allows the visualization of vocal fold motion under the illusion of slow motion.
- Botulinum toxin injection has a 90% success rate in improving symptoms, but requires repeated treatment as the effects are not permanent.