This prospective study included consecutive patients who underwent transorbital endoscopic operations from March 2019 to December 2019. Cadaver studies were performed in the Surgical Neuroanatomy Laboratory.
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 The blade was withdrawn from 0 to 2.5 cm in 0.5 cm increments for orbital retraction from orbital rim to coronal plane, the average value of three IOP and IORP measurements were obtained.
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) was used to measure IOP. Tono-Pen AVIA® was utilized cautiously without applying orbital pressure, and calibrated according to the manufacturer’s manual. IOP was measured using right-angle contact with corneal surface and average value was recorded thrice.
Cannulation method involved insertion of a 26-gauge needle into the anterior chamber through the limbus (superomedial position) for each eye.[10, 11] Cannulation was done based on manufacturer’s instruction and as previously described.
The IORP was measured using a cuff manometer (Cufflator, JT Posey Company, Arcadia, CA, USA) and 3-mm tracheal tube, by placing a pressure measuring cuff on the tracheal tube between retractor blade and periorbit.
The inclusion criteria were: (1) TOA surgery to treat intracranial pathology, (2) written informed consent by patient, (3) no CSF leakage prior to IOP and IORP measurement. The exclusion criteria were: (1) preoperative visual field defect and visual acuity less than 0.05, (2) IOP more than 20 mmHg preoperatively, (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 intraoperative supine state with reverse Trendelenburg at 30 degrees. An oculoplastic surgeon performed the TOA protocol similar to previous cadaver studies. An ophthalmologist measured IOP at the beginning of surgery. Pressure measurement was taken at intervals of 10 minutes. A skilled inspector of each measuring instrument, blinded to the results of other tools measured the parameters.
IOP and IORP were measured individually according to the degree of orbital retraction during surgery. During orbital retraction from the orbital rim to the coronal plane, the blade were withdrawn from 0 to 2.5 cm in 0.5-cm increments, average value was obtained after measuring the IOP and IORP thrice.
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 analyzed (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 periorbit and retraction blade.
A paired Student t-test was used for statistical analysis of IOP and IORP changes. Pearson correlation coefficient was calculated with linear regression model to test the correlation between IOP and IORP. SPSS version 22.0 (Statistical Package for Social Sciences, SPSS Inc., Chicago, IL, USA) was employed for statistical analysis, with signiﬁcance level at p<0.05.