Vanishing Vacuum Phenomenon: A Diagnostic Key in Spinal Infection

Objectives Differentiating between pyogenic spondylodiscitis (SD) and degenerative disc disease (DDD) can be challenging due to overlapping clinical and radiological features. However, accurate distinction is critical to ensure timely and appropriate treatment and to avoid serious complications. This study aims to identify imaging features that reliably distinguish pyogenic SD from DDD, including its erosive subtype, in clinical practice. Methods A total of 163 patients (mean age 62.43 years; 97 males) were evaluated between April 2007 and January 2024. 92 cases with SD (mean age: 67.03 years; 63 males) and 71 controls with DDD (mean age: 57.82 years; 34 males). The vanishing vacuum disc phenomenon (VVDP) was observed exclusively in pyogenic spondylodiscitis, offering a reliable imaging marker for its differentiation from degenerative disc disease. This retrospective diagnostic accuracy study was conducted at a single centre, using pathogen-confirmed SD as the reference standard. Participants were selected from radiological and clinical databases. Imaging modalities included CT and MRI, with VVDP assessed on CT-scans obtained prior to the final diagnosis. Statistical comparisons were performed using Chi-squared and Fisher's exact tests. Results VVDP was observed on repeat CT imaging in SD cases and was absent in DDD (P < 0.001). There were no significant differences in the frequency of intervertebral gas between SD and DDD with or without erosive changes (27% vs 39%, P = 0.137). 99% of SD cases had adjacent phlegmons or abscesses, but none of DDD cases (P < 0.001). Elevated C-reactive protein (CRP) and white blood cell (WBC) counts differentiated SD from DDD. The VVDP was observed exclusively in pyogenic spondylodiscitis, offering a reliable imaging marker for its differentiation from degenerative disc disease. Conclusions VVDP is a sensitive and specific radiological sign for distinguishing SD from DDD in longitudinal imaging. Its diagnostic value is comparable to, and may even outweigh, traditional factors such as soft tissue changes and blood-based inflammatory markers. Additionally, it may provide insights into preceding degenerative changes, as the vacuum phenomenon showed no statistically significant difference between SD and DDD, supporting its potential role in identifying infection superimposed on degeneration.