The effect of prone positioning on intraocular pressure in anesthetized patients

Background: Ocular perfusion pressure is commonly defined as mean arterial pressure minus intraocular pressure (IOP). Changes in mean arterial pressur e or IOP can affect ocular perfusion pressure. IOP has not been studied in this context in the prone anesthetized patient. Methods: After institutional human studies committee approval and informed consent, 20 patients (American Society of Anesthesiologists physical status I-III) without eye disease who were scheduled for spine surgery in the pro ne position were enrolled. IOP was measured with a Tono-pen (R) XL handheld tonometer at five time points: awake supine (baseline), anesthetized (supi ne 1), anesthetized prone (prone 1), anesthetized prone at conclusion of ca se (prone 2), and anesthetized supine before wake-up (supine 2). Anesthetic protocol was standardized. The head was positioned with a pinned head-hold er. Data were analyzed with repeated-measures analysis of variance and pair ed t test. Results: Supine 1 IOP (13 +/-1 mmHg) decreased from baseline (19 +/- 1 mmHg ) (P < 0.05). Prone 1 IOP (27 +/- 2 mmHg) increased in comparison with base line (P < 0.05) and supine I (P < 0.05). Prone 2 IOP (40 +/- 2 mmHg) was me asured after 320 +/- 107 min in the prone position and was significantly in creased in comparison with all previous measurements (P < 0.05). Supine 2 I OP (31 +/- 2 mmHg) decreased in comparison with prone 2 1OP (P < 0.05) but was relatively elevated in comparison with supine 1 and baseline (P < 0.05) . Hemodynamic and ventilatory parameters remained unchanged during the pron e period. Conclusions. Prone positioning increases IOP during anesthesia. Ocular perf usion pressure could therefore decrease, despite maintenance of normotensio n.