Just to correct some of the voice questions raised here (from someone recovering from dysphonia)

Central Dysphonia itself is loss of control of volume. That is caused partially by loss of control of support, and partially by loss of control of the resonance.

Voice is produced by vibrating the vocal chords using air from the lungs, controlled by the diaphragm and interstitial muscles between the ribs, which open and close the chest cavity. Support techniques allow greater movement in the diaphragm, but does not greatly change what I had to say in the last sentence. The sound produced is given tone by the space in the larynx, pharynx and sinuses, and to some extent other resonant spaces in the head and body. It is brought into speech by the tongue, lips and teeth. The volume, oddly enough, does not necessarily come from the amount of air put across the folds, but from how it is used.

I have three different tonal ranges to choose from, bass, baritone and tenor. The key determinant in this is actually the amount of space I use in the neck, and where I produce the resonance: bass is from below the diaphragm, baritone on it, tenor in the upper chest, resonating the body (a shape roughly like one of Dr Who's Sontarans), the lungs and my head space respectively. My clearest and loudest tone in all three is produced by directing the sound through the sinuses, without closing the soft palette (the cause of nasal tone).

My particular problem is that part of my larynx, the vestibular folds (also known as the false vocal folds) interfere with the vocal chords themselves, which lie immediately beneath. Their main function is to stop ingested substances from going down the windpipe, closing it while the glottis immediately above opens the oesophagus for swallowing, in the reverse mechanism, stopping us continuously swallowing air. I'm curing it by getting into the habit of keeping the folds open, but they do close during stress, which is a normal instinctual psycho-physiological reaction. Stage nerves can have a similar effect, making it difficult if not impossible to produce the sound you want, particularly when working in a tone different from that you habitually use.

Other problems can be caused by forcing the vocal chords continuously, causing callouses on the folds which can become pre-cancerous polyps. Rest can allow these to recover, but polyps usually need simple surgery, such as that which changed the tone of Frank Sinatra's voice between the start and core of his career.