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Dysphonia is the medical term for disorders of the voice: an impairment in the ability to produce voice sounds using the vocal organs (it is distinct from dysarthria which signifies dysfunction in the muscles needed to produce speech). Thus, dysphonia is a phonation disorder. The dysphonic voice can be hoarse or excessively breathy, harsh, or rough, but some kind of phonation is still possible (contrasted with the more severe aphonia where phonation is impossible).
Dysphonia has either organic or functional causes due to impairment of any one of the vocal organs. However, typically it is caused by some kind of interruption of the ability of the vocal folds to vibrate normally during exhalation. Thus, it is most often observed in the production of vowel sounds. For example, during typical normal phonation, the vocal folds come together to vibrate in a simple open/closed cycle modulating the airflow from the lungs. Weakness (paresis) of one side of the larynx can prevent simple cyclic vibration and lead to irregular movement in one or both sides of the glottis. This irregular motion is heard as roughness. This is quite common in vocal fold paresis.
Dysphonia is measured using a variety of examination tools that allow the clinician to see the pattern of vibration of the vocal folds, principally laryngeal videostroboscopy. Acoustic examination is also common, obtained by recording the sounds made during sustained phonation or whilst speaking. Another tool is electroglottography.
Subjective measurement of the severity of dysphonia is carried out by trained clinical staff. The GRBAS (Grade, Roughness, Breathiness, Asthenia, Strain) scale or the Oates Perceptual Profile are widely used for this purpose. Objective measurement of the severity of dysphonia typically requires signal processing algorithms applied to acoustic or electroglottographic recordings. These include algorithms such as jitter, shimmer and noise-to-harmonics ratios, but these have been shown to have some critical limitations, particularly for severe dysphonia. Recent advances in signal processing theory have led to more robust algorithms.