Temporal Evolution of Optic Nerve Sheath Diameter/Eyeball Ratio on CT and MRI for Neurological Prognostication After Cardiac Arrest

<b>Background:</b> Optic nerve sheath diameter (ONSD) and its ratio to eyeball transverse diameter (ETD; ONSD/ETD) are potential markers for elevated intracranial pressure in comatose survivors of out-of-hospital cardiac arrest (OHCA). However, their prognostic accuracy remains uncertain. We compared their predictive value via compted tomography (CT)and magnetic resonance imaging (MRI) before and after targeted temperature management (TTM) in OHCA survivors. <b>Methods:</b> This retrospective study included adult comatose OHCA survivors who underwent TTM and serial brain imaging. ONSD and ONSD/ETD ratios were measured on brain CT and MRI at two predefined time-points: within 6 h (pre-TTM) and at 72-96 h (post-TTM) after return of spontaneous circulation. Intra-rater reliability was assessed using intraclass correlation coefficients (ICC). Poor neurological outcome was defined as a Cerebral Performance Category score of 3-5 at 6 months. Prognostic performance was evaluated using area under the receiver operating characteristic curve (AUC). <b>Results:</b> Among 136 patients, 78 (57%) had poor neurological outcomes. Only ONSD (5.12 vs. 5.37 mm) and ONSD/ETD ratio (0.22 vs. 0.23) measured on post-TTM MRI were significantly higher in the poor outcome group. These results depicted modest predictive performance (AUC, 0.67 and 0.65, respectively), whereas all CT-based and early MRI measurements had AUC < 0.60. Intra-rater reliability for ONSD and ETD was higher on CT (ICC: up to 0.93) than on MRI (ICC: 0.73-0.80). <b>Conclusions:</b> Delayed MRI-based ONSD and ONSD/ETD showed statistically significant but modest prognostic value, with limited clinical applicability as a stand-alone tool. These findings underscore the relevance of measurement timing, supporting ONSD as an adjunctive, rather than definitive, tool in multimodal prognostication.