With the improvements in phacoemulsification, high vacuum levels are available to complete surgeries quickly. However, high vacuums carry the risk of anterior chamber instability, unless accompanied by increased infusion pressure produced by elevation of the irrigation bottle height or forced infusion. Increased infusion pressure may lead to increased IOP and the potential reduction of the vascular perfusion pressure. To the best of our knowledge, this is the first report of in vivo DOPP fluctuations during simulated steps of phacoemulsification.
The measured static IOP in the present study was very close to the theoretical value based on the bottle height, perhaps due to the silicone sleeve that prevented leakage of fluid from the main incision and the minimal outflow through the side port incision at the start of the procedure. Our results were similar to those in a previous study by Khng et al.
6 They measured IOP in cadaveric eyes while using commercially available a phacoemulsification system (Sovereign; Advanced Medical Optics [AMO], Santa Ana, CA) and documented good agreement between the static and theoretical IOPs.
Phacoemulsification surgery can induce substantial fluctuations in IOP.
6 7 9 11 12 Grinbaum et al.
7 measured IOP continuously during in vivo phacoemulsification operations with a column height at 65 cm above eye level. They did not change the settings during the different parts of the operation and found that IOP varied from 38 to 2–3 mm Hg during phacoemulsification. Khng et al.
6 conducted IOP measurement across eight stages of phacoemulsification in cadaveric eyes. They documented that IOP exceeded 60 mm Hg during 48% to 85% of the standard coaxial or bimanual microincision phacoemulsification procedure time. In the present report, IOP fluctuated from 13 mm Hg at baseline to 96 mm Hg during the static conditions of setting A, which simulated the cortical cleanup and viscoelastic removal stages. The reported variations may be due to different aspiration flow and vacuum settings during the course of each operation. It is worth noting that in our study, the IOP was greater than 60 mm Hg, which is the retinal artery perfusion pressure,
6 during the three simulated steps of phacoemulsification. The simulated cortical cleanup and viscoelastic removal stages generated high transient pressures. Even the pressures during simulated anterior capsular polishing were close to the retinal perfusion pressure. These results may correlate with the clinical experience of pain and intermittent visual phenomena reported by some patients when the I/A tip or phaco tip is inserted into the anterior chamber.
13 A postocclusion surge during the occlusion break from the phacoemulsification tip is a common phenomenon in phacoemulsification procedures. Theoretically, IOP decreases to the lowest level when the postocclusion surge occurs. Khng et al.
6 simulated the postocclusion surge by placing a kink in the aspiration tubing of the phacoemulsification handpiece to completely occlude it during coaxial phacoemulsification in cadaveric eyes, causing the lowest IOP. Although we could not simulate an occlusion in vivo, we hypothesize that IOP would drop to about the baseline level if the decrease IOP was similar to that in Khng’s study, which ranged from 40.7 to 59.6 mm Hg.
Rapid fluctuations in IOP above a 30-mm Hg range could lead to compromised posterior segment blood flow.
14 Geijer and Bill
15 found that high elevations in IOP reduced the perfusion pressures and caused marked reductions in optic nerve blood flow in monkeys. In healthy volunteers, optic nerve head blood flow decreased more than 80% due to rapid and large decreases in the mean ocular perfusion pressure of 100% or more.
16 In the present report, the IOP at all static and dynamic points was more than 39 mm Hg above baseline IOP. During a modern cataract procedure, nuclear disassembly, cortical cleanup, capsular polishing, and viscoelastic removal may take a few minutes, during which the IOP might be significantly elevated. Although increases of IOP are typically transient, the cumulative effects of elevated IOP could exceed perfusion pressure and reduce ocular blood flow, resulting in damage to the optic nerve head. Riva et al.
16 suggested that the recovery of optic nerve head blood flow to baseline was affected by the duration of IOP elevation. Although it is the common clinical experience that most eyes do well after phacoemulsification surgery,
17 previous studies have suggested that cataract extraction increased the incidence of NAION.
18 19 20 The onset of NAION may result from the transient elevations of IOP that occur immediately after cataract surgery.
4 5 There is also another report of a cataract extraction and closed vitrectomy with complications of retinal and choroidal ischemic infarctions.
21 The authors indicated that transiently increased IOP could be one important factor contributing to the changes in ocular blood perfusion.
Previous studies have speculated that autoregulatory responses in ocular arteries occur within certain limits.
12 16 22 23 Garhofer et al.
23 reported that a short-term increase of IOP up to 43 mm Hg does not alter retina or optic nerve head regulation. However, large fluctuations in IOP overwhelm the ocular autoregulatory capacity and cause the reduction of ocular perfusion.
12 16 There may be a difference between the effects of chronically elevated IOP and those caused by intermittent, acute IOP spikes. The association between long-term, low DOPP (< 55 mm Hg) and an increased prevalence
24 25 and incidence
14 of primary open-angle glaucoma (POAG) has been demonstrated from large-scale clinical data analysis. The authors suggested that low diastolic blood pressure and abnormal ocular blood flow autoregulation play important roles in the genesis and progression of POAG.
14 24 25 However, the relationship between short-term reduced ocular blood flow, as occurs during cataract surgery, and the damage of visual function remains unclear. Trible and Anderson
26 suggested that the degree of acute visual field depression during acute periods of elevated IOP depends on ocular perfusion pressure. Bui et al.
27 demonstrated that a gradient of retinal functional deficits can be caused by acutely elevated IOP. In the current research, during simulation of cortical cleanup and viscoelastic removal, DOPP was less than zero in all cases, and during simulation of nuclear disassembly, DOPP was usually less than zero. Even when aspiration was performed, DOPP was still less than zero in more than half of the cases. Retinal ischemia occurs when retinal perfusion pressure is reduced to less than approximately 20 mm Hg.
15 Logically, vascular perfusion of the retina and choroid was blocked when DOPP was less than zero. Thus, ocular perfusion is likely blocked intermittently during phacoemulsification procedures, especially in patients with diastolic blood pressure lower than 70 mm Hg.
There are many cataract patients with compromised optic nerves, such as occur in glaucoma and atrophic optic nerve. They are more prone to have optic nerve damage during phacoemulsification.
18 20 Nguyen et al.
18 evaluated seven patients with nonarteritic anterior and posterior ischemic optic neuropathy after cataract extraction and found that all patients had vascular risk factors. A history of NAION in the fellow eye has been suggested as an additional risk factor for the incidence of NAION after cataract extraction.
20 Thus, it seems that column height should be set lower for these patients; however, the suitable height and vacuum during phacoemulsification remains unclear until further studies are undertaken.
A main limitation of the present study was that the measurement of IOP was not performed continuously. For the safety of the cataract eyes, the pressure transducer was not present in the anterior chamber continuously during the procedure to prevent trauma to the corneal endothelium, iris, or lens capsule. However, the results of static IOP measurements in the present study were very close to the theoretical values. The irrigation and aspiration phase produced mean pressure readings that were similar to those during nuclear disassembly in both bimanual and coaxial eyes.
6 The discontinuous measurement of IOP in the present study may limit us from determining the full range of IOP and DOPP magnitudes. However, this limitation does not affect our conclusions because we focused on the fluctuations of IOP and DOPP at selected time points during simulated steps of phacoemulsification.
In conclusion, IOP and DOPP fluctuated widely in this study during three simulated steps of phacoemulsification. The transient changes have the potential to adversely affect blood flow to the optic nerve, retina, and choroid.
The authors thank Britt Bromberg, PhD, Xenofile Editing, for providing editing services for the manuscript, and the operating theater nurses for coordinating the study.