The three types of spasmodic dysphonia (SD) are adductor spasmodic dysphonia, abductor spasmodic dysphonia and mixed spasmodic dysphonia.
Adductor spasmodic dysphonia 
In adductor spasmodic dysphonia (ADSD), sudden involuntary muscle movements or spasms cause the vocal folds (or vocal cords) to slam together and stiffen. These spasms make it difficult for the vocal folds to vibrate and produce voice. Words are often cut off or are difficult to start because of the muscle spasms. Therefore, speech may be choppy but differs from stuttering. The voice of an individual with adductor spasmodic dysphonia is commonly described as strained or strangled and full of effort. Surprisingly, the spasms are usually absent while laughing, speaking at a high pitch, or speaking while singing, but singers can experience a loss of range or the inability to produce certain notes of a scale or with projection. Stress, however, often makes the muscle spasms more severe.
Abductor spasmodic dysphonia 
In abductor spasmodic dysphonia, sudden involuntary muscle movements or spasms cause the vocal folds to open. The vocal folds cannot vibrate when they are open. The open position of the vocal folds also allows air to escape from the lungs during speech. As a result, the voices of these individuals often sound weak, quiet and breathy or whispery. As with adductor spasmodic dysphonia, the spasms are often absent during activities such as laughing or singing, but singers can experience a loss of range or the inability to produce certain notes of a scale or with projection.
Mixed spasmodic dysphonia 
Mixed spasmodic dysphonia involves both muscles that open the vocal folds and those that close them and therefore has features of both adductor and abductor spasmodic dysphonia.
Whispering dysphonia 
A fourth type has also been described. This appears to be caused by mutations in the TUBB4 gene on the short arm of chromosome 19 (19p13.2-p13.3). This gene encodes a tubulin gene. The pathophysiology of this condition has yet to be determined.
The exact cause of spasmodic dysphonia (SD) is unknown. According to the National Institute on Deafness and Other Communication Disorders, "research has revealed increasing evidence that most cases of spasmodic dysphonia are in fact neurogenic or having to do with the nervous system (brain and nerves)."
SD is a neurological disorder rather than a disorder of the larynx, and as in other forms of dystonia, interventions at the end organ (i.e., larynx) have not offered a definitive cure, only symptomatic relief. The pathophysiology underlying dystonia is becoming better understood as a result of discoveries about genetically based forms of the disorder, and this approach is the most promising avenue to a long-term solution.
The National Institute of Neurological Disorders and Stroke (NINDS) and the American Academy of Neurology (AAN) classify SD as a neurological disorder. However, because the voice can sound normal or near normal at times, some practitioners believe it to be psychogenic, that is, originating in the affected person's mind rather than from a physical cause. No medical organizations or groups take this position. A comparison of SD patients compared with vocal fold paralysis (VFP) patients found that 41.7% of the SD patients met the DSM-IV criteria for psychiatric comorbidity compared with 19.5% of the VFP group. However, another study found the opposite, with SD patients having significantly less psychiatric comorbidity compared to VFP patients: "The prevalence of major psychiatric cases varied considerably among the groups, from a low of seven percent (1/14) for spasmodic dysphonia, to 29.4 percent (5/17) for functional dysphonia, to a high of 63.6 percent (7/11) for vocal cord paralysis." A review in the journal Swiss Medicine Weekly states that "Psychogenic causes, a 'psychological disequilibrium', and an increased tension of the laryngeal muscles are presumed to be one end of the spectrum of possible factors leading to the development of the disorder". Alternatively, many investigators into the condition feel that the psychiatric comorbidity associated with voice disorders is a result of the social isolation and anxiety that patients with these conditions feel as a consequence of their difficulty with speech, as opposed to the cause of their dysfluency. The opinion that SD is psychogenic is not upheld by experts in the scientific community.
Evidence for a neurological basis 
SD is formally classified as a movement disorder, one of the focal dystonias, and is also known as laryngeal dystonia.  Supporting evidence that SD is a neurological disorder includes:
- SD may co-occur with other neurological movement disorders such as blepharospasm (excessive eye blinking and involuntary forced eye closure), tardive dyskinesia (involuntary and repetitious movement of muscles of the face, tongue, body, arms and legs), oromandibular dystonia (involuntary movements of the jaw muscles, lips and tongue), torticollis (involuntary movements of the neck muscles), or tremor (rhythmic, quivering muscle movements).
- Spasmodic dysphonia runs in some families and is thought to be inherited. Research has identified a possible gene on chromosome 9 that may contribute to the spasmodic dysphonia that is common to certain families.
- Histological examination of the nerve to the vocal cords in patients with SD demonstrates that the percentage of abnormally thin nerve fibers was higher than in normal controls
- Functional MRI signal is reduced in sensorimotor cortices associated with movement of the affected body part in laryngeal dystonia, supporting a dystonic basis for this voice disorder.
Diagnosis of spasmodic dysphonia is often delayed due to lack of recognition of its symptoms by screening physicians. Most patients who are correctly diagnosed are evaluated by a team that usually includes an otolaryngologist, a speech-language pathologist and a neurologist. The otolaryngologist examines the vocal folds to look for other possible causes for the voice disorder. Fiberoptic laryngoscopy, a method whereby a small lighted flexible tube is passed through the nose and into the throat, is a helpful tool that allows the otolaryngologist to evaluate vocal cord movement during speech. Additional diagnostic testing may include stroboscopy, which allows the physician to view the vibrations of the vocal cords in slow motion. The speech-language pathologist evaluates the patient's voice and voice quality. The neurologist evaluates the patient for signs of other movement disorders.
There are a number of potential treatments for spasmodic dysphonia, including botox, surgery and voice therapy. A number of medications have also been tried including anticholinergics (such as benztropine) which have been found to be effective in 40-50% of people, but which are associated with a number of side effects.
Voice therapy 
Voice therapy appears to be ineffective in cases of true spasmodic dysphonia, however as it is difficult to distinguish between spasmodic dysphonia and functional dysphonias and misdiagnosis is relatively common, a trial of voice therapy is often recommended before more invasive procedures are tried. Some also state that it is useful in mild symptoms and as an add on to botox therapy and others report success in more severe cases.
A number of operations that cut one of the nerves of the vocal folds (the recurrent laryngeal nerve) has improved the voice of many for several months to several years but the improvement may be temporary.
An operation called "selective laryngeal adductor denervation-rennervation (SLAD-R)" is effective specifically for adductor spasmodic dysphonia which has shown good outcomes in about 80% of people at 4 years. Post-surgery voices can be imperfect and about 15% of people have significant difficulties. If symptoms do recur this is typically in the first 12 months. Another operation called "recurrent laryngeal nerve avulsion" has positive outcomes of 80% at three years.
Botulinum toxin (Botox) 
Botulinum toxin (Botox) is often used to improve some symptoms of spasmodic dysphonia. Whilst the level I evidence for its use is limited, it remains a popular choice for many patients due to the predictability and low chance of long term side effects. It results in periods of some improvement. The duration of benefit averages 10-12 weeks before the patient returns to baseline. Repeat injection is required to sustain good voice.
Notable cases 
- Scott Adams, the creator of the comic strip Dilbert.
- Frank Allison, musician, leader of Frank Allison and the Odd Sox
- Johnny Bush, country & western musician and songwriter
- Keath Fraser, Canadian author who has documented the challenges and treatment of his condition in the book The Voice Gallery: Travels With a Glass Throat (2002).
- Sjors Fröhlich, former Dutch radio presenter, who had to give up his job as a presenter due to this disease.
- Chip Hanauer, American hydroplane racing driver
- Robert F. Kennedy, Jr., son of United States Senator and presidential candidate Robert F. Kennedy, political and environmental activist
- Fred Lavery, a music producer, writer, musician, and recording studio co-owner from Cape Breton, Nova Scotia. Was lead singer of recording group Road and was later a solo artist, but developed the condition in the 1980s, and was forced to quit singing. Receives injections to keep voice fairly normal.
- Emily Lim, award-winning author of children's picture books.
- Mary Lou Lord, indie folk musician.
- Andy MacWilliams, former radio broadcaster for the Cincinnati Stingers, Chicago Blackhawks and Cincinnati Cyclones.
- Darryl McDaniels of the rap group Run DMC
- Lianne Morgan (singer/songwriter, in the first line up of the Spice Girls)
- Diane Rehm, host of the The Diane Rehm Show on National Public Radio (NPR).
- Mark Stuart, American Christian rock musician (Audio Adrenaline)
- Linda Thompson, British folk-rock musician.
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- Bressman, edited by Thomas T. Warner, Susan B. (2007). Clinical diagnosis and management of dystonia. London: Informa Healthcare. p. 111. ISBN 978-1-84184-317-9.
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- "'Dilbert' creator recovers from rare disorder". Associated Press. 2006-10-27. Retrieved 2007-07-30.
- Frank Allison
- Beal Jr., Jim (1998-04-29). "Golden country Johnny Bush hasn't run dry after almost 50 years of heartaches and honky-tonks". San Antonio Express-News.
- The Voice Gallery - Thomas Allen Publishers - Thomas Allen & Son Limited
- McHale, Laurie; Degginger, Craig (May 14, 1999). "Chip Hanauer's restored voice gives him a new lease on life". University of Washington News and Information. Retrieved September 22, 2012.
- Liebovich, Mark (2006-06-25). "Another Kennedy Living Dangerously". The New York Times. Retrieved 2007-07-30.
- "Author Interview: Emily Lim". Asia in the Heart, World on the Mind. Tarie at Blogger. Retrieved 7 April 2012.
- Sullivan, Jim (2004-02-19). "Hello, Mary Lou – Goodbye, Heartache". Boston Globe.
- Heffernan, Virginia (2006-02-25). "After the Hit Records, A Search for His Roots". The New York Times.
- "About Diane Rehm". WAMU 88.5 American University Radio. Retrieved 2007-07-30.
- "Mark Stuart informs his Facebook followers that he has Spasmodic Dyphonia".
- "Thompson's Trials: UK Folk Rock Great Makes Triumphant Return to the Stage". Tim Perlich, NOW Toronto. 2002-10-17. Retrieved 2007-08-23.
- Spasmodic Disphonia Online Community (VoiceMatters.net)
- Vocal Disorders
- American Academy of Otolaryngology-Head and Neck Surgery (AAO-HNS)
- American Speech-Language-Hearing Association (ASHA)
- National Spasmodic Dysphonia Association, Inc. (NSDA)
- American Academy of Neurology
- Worldwide Education and Awareness for Movement Disorders (WE MOVE)
- Spasmodic Dysphonia section of VoiceInfo.org
- Dystonia Society Network Group – for sufferers and carers to share experiences
- Society for the Management of Spasmodic Dysphonia