Background: Prone positioning and positive end expiratory pressure (PEEP) can improve pulmonary gas exchange and respiratory mechanics. However, they may be associated with the development of intracranial hypertension. Intracranial Pressure (ICP) can be non-invasively estimated from the sonographic measurement of the optic nerve sheath diameter (ONSD) and from the transcranial Doppler analysis of the pulsatility (ICPPI) and the diastolic component (ICPFVd) of the velocity waveform. 

Methods: The effect of the prone positioning and PEEP on ONSD, ICPFVd, and ICPPI was assessed in a prospective study of 30 patients undergoing spine surgery. One-way repeated measures analysis of variance, fixed-effect multivariate regression models and receiver operating characteristic analyses were used to analyse numerical data. 
Results: The mean values of ONSD, ICPFVd and ICPPI significantly increased after change from supine to prone position, but not after 8 cmH2O PEEP application. Receiver operating characteristic analyses demonstrated that, among the non-invasive methods, the mean ONSD measure had the greatest area under the curve signifying it is the most effective in distinguishing a change in ICP between supine and prone positioning (0.86±0.034 [0.79 to 0.92]). A cutoff of 0.43 cm was found to be a best separator of ONSD value between supine and prone with a specificity of 75.0 and a sensitivity of 86.7.
Conclusions: Non-invasive ICP estimation may be useful in patients at risk of developing intracranial hypertension who require prone positioning.

Effects of prone position and positive end expiratory pressure on intracranial pressure (ICP) in patients undergoing spinal surgery using different surrogate estimations of ICP