Furthermore, according to ASHA, organic dysphonia can be subdivided into structural and neurogenic; neurogenic dysphonia is defined as impaired functioning of the vocal structure due to a neurological problem (in the central nervous system or peripheral nervous system); in contrast, structural dysphonia is defined as impaired functioning of the vocal mechanism that is caused by some sort of physical change (e.g. a lesion on the vocal folds). Notably, an additional subcategory of functional dysphonia recognized by professionals is psychogenic dysphonia, which can be defined as a type of voice disorder that has no known cause and can be presumed to be a product of some sort of psychological stressors in ones environment. It is important to note that these types are not mutually exclusive and much overlap occurs. For example, Muscle Tension Dysphonia (MTD) has been found to be a result of many different causes including the following: MTD in the presence of an organic pathology (i.e. organic type), MTD stemming from vocal use (i.e. functional type), and MTD as a result of personality and/or psychological factors (i.e. psychogenic type). Causes
The most common causes of hoarseness is laryngitis (acute 42.1%; chronic 9.7%) and functional dysphonia (30%). Hoarseness can also be caused by laryngeal tumours (benign 10.7 - 31%; malignant 2.2 - 3.0%). Causes that are overall less common include neurogenic conditions (2.8 - 8.0%), psychogenic conditions (2.0 - 2.2%), and aging (2%).A variety of different causes, which result in abnormal vibrations of the vocal folds, can cause dysphonia.
Laryngeal stroboscopy uses a synchronized flashing light passed through either a rigid or flexible laryngoscope to provide an image of the vocal fold motion; the image is created by averaging over several vibratory cycles and is thus not provided in real-time. As this technique relies on periodic vocal fold vibration, it cannot be used in patients with moderate to severe dysphonia. High speed digital imaging of the vocal folds (videokymography), another imaging technique, is not subject to the same limitations as laryngeal stroboscopy. A rigid endoscope is used to take images at a rate of 8000 frames per second, and the image is displayed in real time. As well, this technique allows imaging of aperiodic vibrations and can thus be used with patients presenting with all severities of dysphonia. Acoustic measures
Acoustic measures can be used to provide objective measures of vocal function. Signal processing algorithms are applied to voice recordings made during sustained phonation or during spontaneous speech. The acoustic parameters which can then be examined include fundamental frequency, signal amplitude, jitter, shimmer, and noise-to-harmonic ratios. However, due to limitations imposed by the algorithms employed, these measures cannot be used with patients who exhibit severe dysphonia. Aerodynamic measures
Aerodynamic measures of voice include measures of air volume, air flow and sub glottal air pressure. The normal aerodynamic parameters of voice vary considerably among individuals, which leads to a large overlapping range of values between dysphonic and non-dysphonic patients. This limits the use of these measures as a diagnostic tool.