Is it possible for large breasts (G cup) to cause cervical dystonia? I had D cup, 500cc saline sub muscle implants 8 yrs ago.

The answer is yes, large breasts can certainly cause your symptoms, whether naturally large or large due to an augmentation.  Discuss this situation with your plastic surgeon.  I have had a number of patients decide to have their implants removed to reduce the weight of their breasts to help alleviate symptoms of musculosketetal strain.  Such symptoms are often multifactorial in nature, but taking some of the weight off your shoulders may eliminate one exacerbating symptom.  

Breast Reduction is routinely done for patients who are double D cups or greater and have symptoms including neck pain, shoulder pain, upper back pain and sometimes upper extremity symptoms.  Your G cup breasts would suggest that are very heavy and could be causing your symptoms.   The fact that all of the other treatments failed supports this.  You could be a treated with a removal of your implants and a breast reduction.  

Large heavy breasts whether from your own breast tissue or from large implants can exacerbate your neck symptoms. You can certainly remove or replace your implants with smaller ones. A breast lift may also be helpful with either choice if your breast are sagging as well as too large.