Purpose
To measure ophthalmodynamometric pressure (ODP) during vitrectomy in patients with proliferative diabetic retinopathy (PDR).
Design
Prospective, interventional, consecutive case series.
Methods
This study included 75 eyes of 75 patients undergoing vitrectomy for PDR. After core vitrectomy, the intraocular pressure was gradually raised using a vented-gas forced-infusion system (VGFI), and the optic nerve head was continuously monitored through a planoconvex contact lens. When the central retinal artery or its branches on the optic nerve head showed pulsations, the pressure was recorded as ODP. Diastolic blood pressure (DBP) and systolic blood pressure (SBP) were measured at the time of ODP measurement. Multiple regression analysis was performed to investigate the relationship between ODP and various explanatory variables: DBP, SBP, age, gender, body mass index, presence of hypertension, serum hemoglobin A1c, serum total cholesterol, fasting plasma glucose, presence of rubeosis iridis, and severity of PDR.
Results
ODP was 63.6 ± 11.5 mm Hg (range 15.5-84.4 mm Hg). The ODP significantly correlated with DBP (r = 0.570, P < .0001) and the mean arterial blood pressure (r = 0.522, P < .0001), but not with SBP (r = 0.121, P = .303). Multiple regression analysis revealed that ODP had a significant correlation with DBP ( P < .0001), presence of rubeosis iridis ( P < .0001), and severity of PDR ( P = .046).
Conclusions
We measured ODP using VGFI during vitrectomy in patients with PDR. The ODP was significantly associated with DBP. The ODP was lower in patients with rubeosis iridis and severe PDR.
Diabetic retinopathy is one of the most common causes of vision loss in industrialized countries. Visual impairment attributable to diabetic retinopathy is caused by macular edema and retinal neovascularization. Disturbance of retinal circulation exists even in the early stage of diabetic retinopathy. Changes in retinal blood flow are responsible for the development of nonperfusion area and retinal ischemia, eventually resulting in the development of proliferative diabetic retinopathy (PDR).
Ophthalmodynamometric pressure (ODP) means the minimum intraocular pressure (IOP) at which the first central retinal artery collapse occurs; this collapse is intermittent, that is, pulsating. Measurement of ODP is important in the assessment of blood perfusion into and out of the eye. In patients with retinal vascular occlusion diseases, such as central retinal artery occlusion, giant-cell arteritis-induced anterior ischemic optic neuropathy and unilateral ischemic ophthalmopathy, ODP is known to decrease. Thus, measurement of ODP is of clinical relevance in patients with retinal circulation problems.
Ophthalmodynamometry is an ODP measurement method in which IOP is elevated while observing the central retinal artery. Several methods have been used to raise and measure the IOP, such as external calibrated compression and direct cannulation. However, external calibrated compression cannot measure precise ODP because IOP is added to the external pressure applied to the eye. Repeated pressure application to the eye may change IOP. Moreover, the methods of pressure application and fundus observation are not sophisticated. In contrast, the direct cannulation method can measure ODP directly, but it is not easily applicable to living human eyes because of its invasiveness. We developed a new method to measure ODP using vented-gas forced infusion (VGFI; Accurus vitrectomy system, Alcon, Fort Worth, Texas, USA) during pars plana vitrectomy. This technique directly applies pressure into the vitreous cavity with the VGFI system. As our new method is comparable to the direct cannulation technique in principle, it seems that ODP can be measured with precision. The purpose of the present study is to measure ODP in patients with PDR using the VGFI system during vitrectomy, and to investigate factors related to ODP.
Methods
Patients
We analyzed 75 eyes of 75 patients with PDR who were undergoing pars plana vitrectomy at Tsukuba University Hospital from July 3, 2007 through December 16, 2008. The current study was a prospective, interventional, consecutive case series. Prior to inclusion in the study, all patients provided written informed consent after the nature of the study was explained to them. Exclusion criteria included preoperative IOP above 22 mm Hg or previously diagnosed glaucoma, age less than 18 years, and previous history of vitreous surgery. The indications for vitrectomy included recurrent or persistent nonclearing vitreous hemorrhage, traction or combined traction/rhegmatogenous retinal detachment, and adherent posterior hyaloid causing excessive macular traction.
The following preoperative information was obtained for each patient: age, gender, body mass index, presence of hypertension (HT), serum hemoglobin A1c (HbA1c), fasting plasma glucose, and presence of rubeosis iridis. By means of preoperative retinal photographs, fluorescein angiography, and intraoperative retinal findings, the severity of PDR was graded by Early Treatment Diabetic Retinopathy Study (ETDRS) final retinopathy severity scale. Data on the patients’ characteristics are presented in Table 1 .
| Number of eyes | 75 |
|---|---|
| Male/female | 53/22 |
| Age (years) a | 57.4 ± 11.8 |
| Body mass index a | 23.6 ± 3.8 |
| Hypertension (+)/(-) | 47/28 |
| Serum HbA1c (%) a | 7.2 ± 1.8 |
| Fasting plasma glucose (mg/dL) a | 152 ± 67 |
| Rubeosis iridis (+)/(-) | 15/60 |
| Severity of PDR b | |
| Level 61 | 0 |
| Level 65 | 5 |
| Level 71 | 11 |
| Level 75 | 14 |
| Level 81 | 16 |
| Level 85 | 18 |
a Values are presented as the mean ± standard deviation.
b Severity of PDR refers to ETDRS final retinopathy severity scale.
Surgical Procedures
All surgeries were performed under sub-Tenon local anesthesia by 2 experienced surgeons. The crystalline lens was removed with phacoemulsification and intraocular lens implantation when required, followed by 20-gauge 3-port pars plana vitrectomy. We used a bottle of balanced saline solution injected in 0.5 mg of epinephrine during surgery as an infusion fluid. Using contact lenses, posterior hyaloid separation and removal of the posterior vitreous membrane were performed, and then bimanual delamination, en bloc dissection, and segmentation techniques were used to remove proliferative tissues. Membrane dissection and segmentation were performed when necessary to eliminate all tangential tractions. Peripheral vitrectomy and panretinal endophotocoagulation were routinely performed. Air-fluid exchange was conducted when an iatrogenic retinal tear and/or rhegmatogenous retinal detachment were identified intraoperatively.
Measurement of Ophthalmodynamometric Pressure
We measured ODP during vitrectomy using the VGFI system. This system controls the perfusion pressure by delivering the pressurized air (0-120 mm Hg) into the bottle of balanced saline solution, instead of changing the height of the irrigation bottle. After core vitrectomy, we confirmed that balanced saline solution was not leaking from each sclerotomy. Then, intraocular pressure was gradually raised using VGFI, and the optic nerve head was continuously monitored through a planoconvex contact lens. When the central retinal artery or its first branches on the optic nerve head showed pulsations, the pressure was recorded as ODP. The measurements were repeated 3 times, and their mean values were used for data analyses.
Systemic blood pressure was measured at the same time as ODP measurements. Systolic blood pressure (SBP) and diastolic blood pressure (DBP) were recorded with an indirect blood pressure measurement (oscillometric methods). Mean arterial blood pressure (MBP) was defined as DBP plus one-third of the difference between SBP and DBP.
Statistical Analysis
The mean and standard deviation were calculated for each variable. The relationships between ODP and systemic blood pressures were examined with the Pearson correlation coefficient. Mann-Whitney U test was performed to compare ODP between patients with and without rubeosis iridis. Multiple regression analysis was performed to investigate the relationship between ODP and various explanatory variables. Variables tested were DBP, SBP, age, body mass index, presence of HT, serum HbA1c, presence of rubeosis iridis, and the severity of PDR. All tests of association were considered statistically significant if P < .05. The analyses were carried out using Stat View (version 5.0, SAS Inc, Cary, North Carolina, USA).
Results
ODP measured during vitrectomy was 63.6 ± 11.5 mm Hg (range 15.5-84.4 mm Hg). DBP, SBP, and MBP measured at the same time were 75.8 ± 9.7 mm Hg (range 50.0-98.0 mm Hg), 149.6 ± 18.4 mm Hg (range 102.0-187.0 mm Hg), and 125.6 ± 12.0 mm Hg (range 94.0-149.7 mm Hg), respectively ( Figure 1 ). ODP was significantly correlated with DBP (r = 0.570, P < .0001) and MBP (r = 0.522, P < .0001), but there was no significant correlation between ODP and SBP (r = 0.121, P = .303) ( Figure 2 ). ODP was significantly lower in patients with rubeosis iridis than those without rubeosis iridis ( P < .005, Figure 3 ). Multiple regression analysis showed that DBP ( P < .0001), presence of rubeosis iridis ( P < .0001), and the severity of PDR ( P = .046) had a significant positive correlation with ODP, whereas other explanatory variables showed no relationship with ODP ( Table 2 ).
