MTD is the acronym used in a voice context for 'muscle tension dysphonia'. It is more correctly referred to as 'hyperfunctional voice'.Sometimes it is difficult to distinguish between MTD and SD, but putting it oversimply:
SD is a neurological movement disorder - the voice that results from focal dystonia affecting the larynx. It is typified by breaks in voicing.
MTD is typically a constant strained, tight voice. It can respond to appropriate voice therapy.
Sometimes, a patient with SD will develop compensatory voicing techniques to attempt to work around the SD, and this can introduce another component to the SD.
We have discussed how to differentiate between MTD and ADductor SD on the bulletin before. See the link below.
David Barton (AD/SD, North Shore, NZ)