Have had cervical dystonia for over a decade. can it cause muscle and joint damage or bring on other conditions like arthritis?

Potentially: If the dystonia leads to the neck being held in flexed or rotated position, then a contracture can occur which can lead to early degenerative changes as the discs are being loaded asymmetrically. Don't hesitate to seek treatment for this.

Certainly possible: Both agonist & antagonist muscles contract simultaneously during dystonic movement.Causes of the disorder are predominantly idiopathic.The most common treatment for spasmodic torticollis aka cervical dystonia is botulinum toxin type A.Additional treatment is a physiotherapy program with muscle stretching and relaxation, balance and coordination training and exercises for muscles to minimize damage