Impediments to Small-gauge Vitrectomy: Facts and Fiction
Almost all of the earlier problems have been overcome
Marc J. Spirn, MD
Although microincisional vitrectomy surgery (MIVS) was first conceived in 1995 and has been commercially available since 2002, there are still many vitreoretinal surgeons who are reluctant to adopt smaller-instrumentation techniques. As we approach the 10-year anniversary of microincisional vitrectomy, it is worth considering the remaining obstacles to successful conversion from 20-gauge surgery to either 25-gauge or 23-gauge techniques.
ORIGINAL DRAWBACKS TO MIVS
When 25-g vitrectomy was first introduced, there were many initial drawbacks. Some of these weaknesses included flexible instruments, poor illumination and rudimentary special instruments. Because of these limitations, many conditions could not be treated with MIVS, and cases were limited to simple indications, such as macular hole, macular pucker or vitreous floaters.
Over time, these initial drawbacks were improved upon. Instruments became stiffer with second-generation 25-g instruments — and stiffer yet with the introduction of 23-g tools. Illuminating sources improved with the advent of xenon light sources. Instrumentation improved quickly with the introduction of multiple forceps types, curved illuminated lasers, and even vertical and horizontal scissors, among other innovation. By early 2007, because of these improvements, most surgeries could be performed using MIVS techniques, including more difficult rhegmatogenous and tractional retinal detachments.
With there being many more surgical indications that can be approached with MIVS, the question is why anyone would want to use 23-g or 25-g, instead of conventional 20-g instrumentation. Twenty-gauge vitrectomy has been around for over 30 years. As such, its instrumentation, techniques and indications are time-tested and mature.
For those surgeons thinking about switching to 25-g or 23-g techniques, there are several advantages. For the surgeon, it results in quicker, more efficient surgeries. For the patient, the benefits are faster healing (because of smaller wounds), improved postoperative comfort (because of the lack of sutures), and potentially decreased induced astigmatism. In its most evolved, current form, it may even be safer than 20-g techniques (more on this below).
Although most retinal conditions can be treated with MIVS, there have been several concerns about small-instrumentation surgery from the beginning. Some of the fear around MIVS concerns the possible increased risk of endophthalmitis, a higher incidence of hypotony, and increased intraocular flow rates. All of these drawbacks have been mitigated recently, with newer techniques and instruments.
PERSONAL MIVS EXPERIENCE
In order to best elucidate the pros and cons of MIVS, it may be helpful to follow my own personal evolution with small-instrumentation surgery. I began with MIVS in 2006, during my fellowship at the Wills Eye Institute. At that point, complex cases were being performed with 20-g techniques but most others were being performed using 25-g instruments on Alcon's Accurus vitrectomy machine.
Intraoperative impediments during 25-g cases at the time revolved mostly around poor illumination and difficult laser techniques. We used halogen lighting, and the laser was sufficiently flexible such that the treatment of superior retinal pathology was difficult. Twenty-gauge instruments were chosen mostly for cases involving complex proliferative vitreoretinopathy, tractional detachments, oil cases, or cases in which lens removal was necessary.
In early 2007, after the commercial introduction of 23-g vitrectomy, I moved almost exclusively to 23-g surgery. Around this time, we also started using xenon illumination. With a faster cut rate of 2,500 cuts per minute (cpm) and with stiffer instrumentation, peripheral dissection was easier and safer than with 25-g. I found that endolaser was also significantly easier and more closely approximated 20-g techniques.
In addition, I found that on momentary mode with a low cut rate (of about 350 cpm) and high vacuum, I could remove the vast majority of retained lens material if desired, though the fragmatome was still useful for larger, denser lenses. From 2007 to the present, there have been very few cases in which I have not elected to use 23-g techniques.
Nevertheless, there are still cases in which 20-g instrumentation may be preferable. These cases include oil removal, which I find easier and quicker by opening one 20-g port, and cases in which a fragmatome or MPC scissors is necessary.
Despite the fact that nearly all surgical indications can be performed using 23-g techniques, over the years, there have been some reservations about using 23-g vitrectomy. One major concern is whether endophthalmitis is more common with MIVS. Early studies suggested that the rate of endophthalmitis may be higher with 25-g than with conventional 20-g techniques. Possible reasons for this conclusion include that wounds are often not sutured, vitreous gel may be left near the sclerotomy, resulting in a possible vitreous wick, and the more common incidence of postoperative hypotony.
Techniques have evolved with time to combat these factors. Initially, sclerotomies were made with a straight stab incision using a chevron-shaped blade. Nowadays, a beveled incision, using a smaller linear trocar, is preferable. Smaller wound size and beveled incisions likely reduce bacterial entry into the eye.
Whenever there is any concern about the integrity of the wound, a partial-thickness suture through the conjunctiva and sclera should be considered. Additionally, taking care to remove any gel near the sclerotomy site can prevent vitreous from prolapsing through the sclerotomy when the cannula is removed. This vitreous may act as a wick, drawing bacteria into the eye postoperatively.
Additionally, adding air or gas at the end of the case and positioning the patient with his or her head up may help to close the internal os of the sclerotomy, while also helping to minimize the influx of bacteria postoperatively. Using some of these techniques, the rates of endophthalmitis have been recently shown to be similar to 20-g surgery.
Another concern with MIVS is that of postoperative hypotony. Again, because the wounds are not sutured, there is the possibility of wound leakage, leading to hypotony. The introduction of beveled wounds has helped to minimize this complication. In addition, it appears that leaving the eye partially or entirely filled with air or gas can also limit the likelihood of hypotony. Hypotony is problematic because it can result in choroidal detachments.
Serous or hemorrhagic choroidal detachments can occur either intraoperatively or postoperatively. When they occur postoperatively, it is most often a result of transient or prolonged hypotony. Making sure wounds are watertight is paramount to preventing this complication. However, if patients rub their eyes or if a wound spontaneously leaks, then hypotony may occur.
A potential intraoperative cause of choroidal detachment occurs when the infusion line retracts into the suprachoroidal space. Unlike with 20-g surgery, in which the infusion line is sewn in place, with MIVS, the line is prone to small movements, particularly during scleral depression or with excessive eye manipulation. Instead of infusing into the vitreous cavity, the line may instead infuse into the suprachoroidal space. This process results in a progressive, quickly enlarging serous choroidal detachment.
Despite this complication being very scary and potentially devastating, there is a quick and easy fix. The surgeon should disengage the inferotemporal infusion line from the cannula, leaving the cannula in the suprachoroidal space. The infusion line should then be inserted into one of the superior cannulas and the pressure elevated. This resulting rise in intraocular pressure will pump the suprachoroidal fluid out through the inferotemporal cannula, which will gape the suprachoroidal space.
This intraoperative complication is somewhat more likely because of the popularity of beveled incisions. Because the beveled incisions result in a longer intrascleral course of the cannula, there is less room for error. If the cannula in the vitreous cavity moves even a little, it is more likely to end up in the intrascleral space.
High intraocular flow is another of the concerns with current 23-g surgery. Intraocular flow of fluid is influenced by several major factors: the size of the infusion line, the aspiration rate, and the outflow resistance. During vitrectomy, when the cannulas are occupied by the light pipe, infusion line and vitrector, the aspiration capacity determines the inflow. Thus, 25-g has less flow potential than 23-g, which has less than 20-g vitrectomy.
However, when most of the gel has been cleared and the instruments are removed from the eye, in order to keep a constant pressure, the inflow equals the outflow. Assuming the sclerotomies are well constructed, the outflow in a 20-g eye should be minimal when the instruments are removed because the sclerotomies are slit-like. Conversely, with 25-g and 23-g vitrectomy, the cannula system gapes the wound, facilitating outflow through that sclerotomy when the instrument is removed, and there is no vitreous to plug the internal os.
This increased outflow ability results in much greater inflow and, thus, in much greater intraocular flow. In some situations, this increased flow can be helpful. High flow can be used to liberate blood or retained lens fragments positioned on the posterior pole. Nevertheless, increased flow also has its drawbacks. When there is increased inflow, it is possible that the infused balanced salt solution or air will hit and damage the retina and cause subsequent visual field defects.
High flow can also be problematic when using perfluorocarbon liquid (PFCL). If one of the ports is left open, for example, during self-depressed illuminated endolaser, then the high inflow can create bubbles at the top of PFCL, making visualization difficult. More importantly, the high inflow can push PFCL under the retina. Although this is most likely to occur at the edge of a retinectomy, the subretinal perfluorocarbon may then migrate posteriorly during a perfluorocarbon-air exchange.
NEWEST ADVANCES IN MIVS
There have been several major recent advances that have made the drawbacks stated above much less likely. These improvements include newer instrumentation and improved cannulas, trocars, and vitrectomy machines. Of these improvements, the new vitrectomy machines have received the most press. Over the last two years, Alcon has introduced the Constellation and Bausch + Lomb has introduced the Stellaris.
The benefits of these newer machines include improved IOP monitoring and control, variable duty cycle control and increased cut rates. With cut rates of 5,000 cpm, peripheral dissection can now be performed more safely than ever. For those surgeons who prefer 25-g over 23-g because of its smaller size and decreased relative flow, Alcon has introduced a 25-g plus system that is stiffer, more closely approximating 23-g surgery.
Newer instruments include illuminated picks and 23-g MPC scissors, which can be very helpful for delaminating and segmenting tractional retinal detachments. As a result of these instruments and many that preceded them, such as 23-g scissors, picks, and forceps, the most difficult maneuvers can now be accomplished with 23-g techniques.
The most recent change that I have instituted in my own surgical practice is the use of valved cannulas. Valved cannulas have several major advantages over the open type. Because valved cannulas prevent outflow, they result in a closed system. This closed system limits intraoperative hypotony and restricts the outflow of fluid. By limiting the outflow of fluid, inflow through the infusion line is also limited. This reduces the likelihood of going through more than one bottle of BSS, it prevents perfluorocarbon from being agitated and bubbling up, and most importantly, it prevents perfluorocarbon from being pushed under the retina.
Today, looking back, it is evident that there have been many advances in the way MIVS is performed compared to its early introduction. Using beveled wounds, valved cannulas, improved microincisional trocars, and better instrumentation — such as illuminated picks, curved illuminated lasers, and 23-g MPC scissors, among others — even the most complicated surgical cases can be safely performed using microincisional techniques.
Additionally, with improved surgical methods, the rates of endophthalmitis and hypotony are now likely comparable to those of 20-g surgery. The benefits of MIVS for the surgeon include increased efficiency and decreased operative times. For patients, MIVS techniques result in more comfortable postoperative courses. As these techniques continue to move forward, there will be fewer and fewer impediments to successful conversion from 20-g vitrectomy to MIVS. RP
|Marc J. Spirn, MD is an attending surgeon at Wills Eye Institute and instructor of ophthalmology at Thomas Jefferson University, both in Philadelphia. He reports no financial interest in any products mentioned here. Dr. Spirn can be reached via e-mail at email@example.com.|