Background <p>Congenital cervical fusion is associated with adjacent segment degeneration (ASD). The debate continues over whether ASD is a result of cervical fusion or a natural process of cervical aging.</p> Objective <p>This study aims to determine if congenital cervical fusion is associated with an increased risk of ASD.</p> Methods <p>This research utilized a retrospective cohort study with a 1:2 case-control design to investigate 70 individuals diagnosed with Type I Klippel-Feil Syndrome (KFS) fused at C5-6. These subjects were compared against a control group comprising 140 age- and gender-matched individuals. Participants were stratified into three distinct age groups. T2-weighted MRI scans were utilized for measurement.</p> Results <p>KFS group shows significantly higher the Pfirrmann grade at C4-5 (younger group) and C6-7 (middle-age group). The severe degeneration proportion increases notably with age in both groups. The KFS group also presents with a higher biological age scale in the younger and middle-age groups. Dural sac compression index (DSCI) of KFS group is significant higher at C3-4, C4-5, and C6-7. Among the young group, the KFS group shows higher DSCI at C3-4 and C4-5. In the middle-age group, the KFS group shows higher DSCI at C2-3, C3-4, C4-5, and C6-7. In the older-age group, the KFS group has a higher DSCI at C6-7.</p> Conclusions <p>(1) Congenital cervical fusion not only accelerates degeneration in adjacent segments but also extends its impact to non-adjacent segments. (2) The fusion hastens the degeneration of the entire cervical spine. (3) The effects of these accelerated degeneration are especially pronounced during the early to mid-adulthood years.</p>