This "Diagnosis" is so vague and nonspecific, that I don't think it can be coded. "Segmental Instability" of the Cervical Spine can be from several causes, i.e. post-traumatic, degenerative, congenital, or secondary to some other disease process. Your doctor needs to give you something more exact.
Respectfully submitted, Alan Pechacek, M.D.
I have given your query more thought, and the is one scenario that comes to mind, so I am posting this as a "hypothetical, possible" explanation. The patient in question may have had an Anterior Cervical Disc and Fusion at one, possibly 2 levels, in the past from which they recovered and did well for a period of time. We will assume a one level fusion of C4-C5. The patient may have returned now with "neck pain" (with or without radicular involvement) for evaluation and treatment. The current X-rays may show that the fusion looks solid/well healed, but the patient is now showing X-ray findings of disc degeneration/spondylosis at the levels at the upper &/or lower disc levels at either end of the fusion ("adjacent segments"), i.e., C3-4 &/or C5-6. It is reasonably well established that after fusion of two or more vertebrae in the cervical region, over time the "stresses and strains" applied to the cervical spine will tend to focus at either or both levels above &/or below the fused levels. These would be the "adjacent segments." However, the term "dysfunction" is not a "diagnosis", and as such has no code. Most likely this is degenerative spondylosis at one or both levels, with or without myelopathy, radiculopathy, or radiculitis. I hope this helps, but you still need your physicians help.