Assessment of Carotid Siphon Pulsatility Index via Transorbital Window, an Alternative for Intracranial Measurements

Objective: To analyze the correlation between the hemodynamic indices of the carotid siphon (CS), the middle cerebral artery (MCA) and the optic nerve sheath diameter (ONSD) among patients with acute brain injuries. Materials and methods: An observational, analytical, and cross-sectional study was conducted. Pulsatility indices (PI) of the CS and MCA were recorded using transcranial Doppler, and ONSD was measured using ocular ultrasound. Pearson’s and Spearman’s correlation coefficients were applied, considering statistical significance at p < 0.005. Results: A total of 75 neurocritical patients were enrolled during 2024. A strong positive correlation was observed between the CS PI and the MCA PI, in both the right (r = 0.643; p < 0.001) and left (r = 0.702; p < 0.001) hemispheres. A weak but significant positive correlation was found between the left CS PI and left ONSD (r = 0.242; p = 0.039). Patients with a PI greater than 1.2 had a larger ONSD than those with a PI of 1.2 or lower (t = −2.29; p = 0.025). The mean difference was approximately 0.44 mm, based on the 95% confidence interval from −0.830 to −0.057. This association was observed only in the left hemisphere and showed a moderate effect size (Cohen’s d = −0.61). Conclusions: In this cohort of neurocritical patients, the carotid siphon pulsatility index showed a strong correlation with the middle cerebral artery pulsatility index, suggesting concordant hemodynamic behavior between both sites of insonation and supporting the potential of the carotid siphon as an alternative for Doppler evaluation when the acoustic window is limited. An optic nerve sheath enlargement may be associated with higher pulsatility index; however, invasive ICP monitoring is needed to evaluate the reliability of this association.