(mf)ERG was recorded in both eyes after maximum pupil dilation with topical 10% phenylephrine hydrochloride and topical 1% tropicamide. After topical anesthesia with 0.4% oxybuprocaine hydrochloride, a Burian-Allen bipolar contact lens electrode (Veris IR Illuminating Electrode; EDI Inc., San Mateo, CA) with built-in infrared light sources for fundus illumination was placed on the cornea using 1% carboxymethylcellulose to provide full electrical contract, together with a ground electrode attached to the forehead. The fellow eye was occluded during the procedure. Visual stimuli were displayed on a 1.5-in. white light stimulator/infrared fundus camera (Veris; EDI Inc.), permitting optimal correction of refraction without changing the size of the stimulus elements and ensuring fixation by providing the operator with a live screen display of the patient's fundus. An array of 103 eccentricity-scaled hexagons was displayed at a frame rate of 75 Hz. Responses were band-pass filtered outside of 10 to 300 Hz, amplified at gain 10
5, and sampled every 0.833 ms. A standard
m-sequence length was used with
m = 15, resulting in a total recording time of 7.17 minutes, divided into eight short segments for patient comfort. If loss of fixation or an artifact was observed, the affected segment was discarded and rerecorded. The luminance of the hexagonal stimulus fields was 200 cd/m
2 when on (white) versus 2 cd/m
2 or less when off (black). The surround luminance was set to 50% of the bright (white) test luminance (i.e., 100 cd/m
2). The examination was made in ambient room light. The recording protocol agreed with the International Society for Clinical Electrophysiology of Vision (ISCEV) guidelines for basic mfERG.
9 A single iteration of artifact rejection was applied to the raw data, whereas no spatial smoothing was applied before derivation of first- and second-order kernels, implicit times, and amplitudes of the mfERG components N1 (first negative), P1 (first positive), and N2 (second negative). The N1 response amplitude was measured from the starting baseline to the base of the N1 trough, the P1 response amplitude was measured from the N1 trough to the P1 peak, and the N2 response amplitude was measured from the P1 peak to the N2 trough. For data analysis, the hexagonal stimulus fields were grouped by eccentricity.
Ocular pneumoplethysmography (OPG) can be used to assess the ophthalmic systolic pressure (OSP) which is the systolic arterial blood pressure of the ophthalmic artery.
10 After topical anesthesia with 0.4% oxybuprocaine hydrochloride, eye cups were placed bilaterally on the inferolateral quadrant of the eyeball, directly behind the corneal limbus. Vacuum suction at 300 mm Hg was applied via tubing connected to the eye cups and then gradually released over a period of 20 seconds with simultaneous measurement of pressure oscillations by a side port of the tube. The pressure at which the first pulse wave was noted was used in calculating the ophthalmic systolic blood pressure. The recording was done twice in each eye. If a pulse wave was noted immediately after vacuum induction, the suction pressure was raised to 500 mm Hg in a new recording.
10
Ultrasonographic internal carotid artery studies were performed by experienced examiners. Stenosis was classified into intervals: 0% to 14%, 15% to 49%, 50% to 69%, 70% to 79%, 80% to 99%, or occlusion.
11 Furthermore, the examiner estimated the exact grade of stenosis with attempted double-digit accuracy.
Additional investigations included best corrected visual acuity (BCVA) according to the Early Treatment Diabetic Retinopathy Study (ETDRS) protocol, intraocular pressure (IOP), slit lamp biomicroscopy, ophthalmoscopy, fundus photography, fluorescein angiography, and optic coherence tomography (OCT). Mean arterial blood pressure (MABP) was calculated as Pressurediastolic + ⅓(Pressuresystolic − Pressurediastolic) and the OPG-based ocular perfusion pressure (OPPOPG) as OSP − IOP.
Data were analyzed by using the two-sided paired t-test for analyses between OIS eyes and fellow eyes and the two-sided Student's t-test for analyses between OIS eyes and eyes in the control group without eye disease. For all parameters, the observed differences between eyes were in agreement with a normal distribution. The level of statistical significance was set at P < 0.05. P < 0.0083 was considered significant after Bonferroni correction for multiple endpoints (analysis software: SAS 9.1; for Windows; SAS Institute Inc, Cary, NC).