This prospective study included consecutive patients who underwent transorbital endoscopic operations from January 2017 to December 2019. Cadaver studies were performed in the Surgical Neuroanatomy Laboratory. The study was approved by the Institutional Review Board and conducted in accordance with the ethical guidelines of the Declaration of Helsinki. Written informed consent was provided by all patients.
Five cadaveric specimens (10 eyeballs) without eye damage, including orbital disease, periorbital disease, and head trauma, were examined. Cadavers with enophthalmos due to premortem or postmortem dehydration were excluded. Cadaveric specimens were processed based on previously described methods. The retractor blade was withdrawn from 0.0 to 2.5 cm in 0.5 cm increments for orbital retraction from the orbital rim to the coronal plane. The average values of three IOP and IORP measurements were obtained.
IOP was measured by a handheld tonometer (Tono-Pen AVIA® Handheld Tonometer, Reichert Inc., USA) and direct cannulation to the eyeball using a Philips IntelliVue MP30 monitor (Philips Medical Systems, Eindhoven, The Netherlands).
This cannulation method consisted of the insertion of a 26-gauge needle into the anterior chamber through the limbus (superomedial position) of each eye.[10, 11] The cannulation was performed based on manufacturer’s instructions and as previously described.
The Tono-Pen AVIA® was used very carefully, without applying any orbital pressure, and calibrated according to the manufacturer’s manual. The IOP was effectively measured once a right-angle contact with the corneal surface was established. The average value was recorded three times.
The IORP was measured using a cuff manometer (Cufflator, JT Posey Company, Arcadia, CA, USA) and a 3-mm tracheal tube by placing a pressure measuring cuff on the tracheal tube between the retractor blade and the periorbit.
The inclusion criteria were as follows: (1) TOA surgery to treat intracranial pathology, (2) written informed consent from the patient, and (3) no CSF leakage prior to IOP and IORP measurements. The exclusion criteria were as follows: (1) preoperative visual field defect and visual acuity less than 0.05, (2) IOP greater than 20 mmHg preoperatively, and (3) any ophthalmological disease, particularly glaucoma. All patients were tested for IOP, visual acuity, and visual field before and after surgery by the ophthalmologist.
After induction of general anesthesia and intubation, the patient was positioned in an intraoperative supine state with the reverse Trendelenburg at 30 degrees. An oculoplastic surgeon performed the TOA protocol, as per previous cadaver studies. An ophthalmologist measured the IOP at the beginning of the surgery. Pressure measurements were taken at 10-minute intervals. A skilled inspector of each measuring instrument, blinded to the results of other tools, measured the parameters.
IOP and IORP were individually measured according to the degree of orbital retraction during surgery. During orbital retraction from the orbital rim to the coronal plane, the retractor blade (DORO® Blades for DORO LUNA® Retractor System) was withdrawn from 0.0 to 2.5 cm in 0.5-cm increments, and an average value was obtained after measuring the IOP and IORP three times.
The iCare pro (iCare Finland Oy, Helsinki, Finland) tonometer was used to measure the IOP. Similar to the cadaver studies, the average of three final measurements was acquired (Fig. 1A).
A micro strain gauge monitor system (CODMAN MICROSENSOR Transducer & CODMAN® MICROSENSOR® Basic Kit, Codman, Raynham, MA, USA) was used for measuring IORP in clinical cases based on the manufacturer’s instructions (Fig. 1B). The distal catheter tip was placed between the periorbit and the retraction blade.
A paired Student’s t-test was used for the statistical analysis of IOP and IORP changes. A Pearson correlation coefficient was calculated with a linear regression model to assess the correlation between IOP and IORP. SPSS version 22.0 (Statistical Package for Social Sciences, SPSS Inc., Chicago, IL, USA) was employed for statistical analyses, with a signiﬁcance level set to p<0.05.