Spasmodic Dysphonia/Spastic Dystonia

  I was diagnosed with the abductor form of spasmodic dysphonia in November of "97" and have been reading any information I can find about it.  The letters "T", "P" and "C/K" are the most difficult to voice.  I understand this difficulty as being a symptom of SD.  I would think if the spasms are random than any letter could be difficult to voice.  Why are some letters consistently vulnerable to SD and some are not?  I appreciate any information.
Dear Bernadette,
The letters when produced in sounds as pa, ta, ka  tests  laryngeal phonation.  Spasmodic dysphonia of the abductor type is characterized by a breathy, often effortful, voice quaility with sudden cessation of voicing, resulting in whispered speech segments. The changes are caused by  LARYNGEAL dysfunction, hence, your difficulty with these sounds during speech.