Microincisional Vitrectomy Surgery for the Repair of Retinal Detachment
SEENU M. HARIPRASAD, MD
Transconjunctival sutureless microincision vitrectomy surgery (MIVS) was introduced by Fujii and colleagues in 2002, permitting sutureless vitrectomy for the first time.1 This major advance, which used 25-gauge instrumentation, provided several advantages over conventional 20-gauge vitrectomy, including shorter surgical time, less surgically induced inflammation, and reduced risk of postoperative corneal astigmatism, factors that ultimately led to improved patient comfort and faster visual recovery.2-4 The reduced operative time with MIVS can be attributed to the elimination of the substantial time spent opening and closing the conjunctiva and sclera, which more than compensates for the additional time spent on vitreous dissection through a 25-gauge probe.
Nonetheless, the original MIVS procedure did pose some unique difficulties. The major issue arose from the excessive flexibility of the first 25-gauge instruments, making it difficult for surgeons to adequately shave the vitreous base and to rotate the eye. The smaller diameter of its light probes also created a problem with inadequate illumination. Postoperative leakage of gas or silicone oil from the sclerotomies into the subconjunctival space was another concern with this new procedure. Furthermore, an incomplete repertoire of 25-gauge instruments severely limited the types of procedures performed by MIVS. For instance, the lack of a 25-gauge curved laser probe made peripheral photocoagulation difficult. In addition, since MPC scissors and illuminated instruments were not available in 25-gauge, MIVS was not used in cases of severe PVR or tractional retinal detachment. Other procedures, such as removal of silicone oil or dense vitreous hemorrhage, were not performed with MIVS due to the low flow rate caused by the smaller inner lumen diameter.
In 2005, Eckardt introduced 23-gauge MIVS, which addressed the major concerns of 25-gauge vitrectomy.5 The stiffer shaft of the 23-gauge instrumentation feels much like that of a 20-gauge instrument, improving both vitreous gel removal and vitreous base shaving relative to 25-gauge instruments; in addition, the larger diameter of the 23-gauge provides improved illumination and higher flow rate, increasing the ease with which silicone oil and other dense materials can be removed. Furthermore, it retains all of the advantages of 25-gauge vitrectomy over the 20-gauge procedure.
In addition to the improvements made in MIVS with the introduction of 23-gauge surgery, other enhancements made recently have increased the attractiveness of both 23-gauge and 25-gauge small-incision vitrectomy. These include the introduction of a xenon light source, which further improved illumination; minimization of gas leakage via better wound construction (due to the use of beveled incisions and recently released improved trocar blade designs); and the expansion of the array of MIVS instrumentation, including the Chow cannula for silicone oil injection and curved endolaser probes for peripheral photocoagulation. These improvements have led to a wider acceptance of MIVS in the past several years, with almost 90% of retinal specialists responding to the 2006 ASRS Practices and Trends survey reporting at least one experience with smallincision vitrectomy and almost half reporting frequent use.6
Small-incision vitrectomies, however, are not without their challenges. An increased risk of transient hypotony has been reported with both 25-gauge7-9 and 23-gauge vitrectomies.10,11 However, some recent reports12,13 have not supported these findings, and others9,14 have demonstrated that the now-standard oblique incision reduces the incidence of hypotony relative to the original perpendicular incision. Endophthalmitis is also a concern with MIVS. The association between 25-gauge vitrectomy and an elevated incidence of endophthalmitis is controversial, however, with two large (N≥4400) retrospective studies reporting an increased risk15,16 and three similar studies (N≥3500) reporting no difference in endophthalmitis rates between 20-gauge and 25-gauge vitrectomies.17-19 This variance may be due to the type of incision used with the 25-gauge surgeries. Both of the studies showing no difference in endophthalmitis rates reported using primarily oblique incisions, whereas at least one of the two studies demonstrating increased endophthalmitis rates with 25-gauge vitrectomy used primarily perpendicular incisions. Only one large study (N= 4021) examining endophthalmitis rates with 23-gauge vitrectomy has been published, and it found no increase in endophthalmitis.20 Thus, careful, prospective research is needed before conclusions can be drawn regarding a potential connection between endophthalmitis and small-incision vitrectomy using modern wound construction.
|Seenu M. Hariprasad, MD, is an associate professor, director of clinical research, and chief, Vitreoretinal Service, University of Chicago, Department of Surgery, Section of Ophthalmology and Visual Science, Chicago, Illinois. He is a consultant to GlaxoSmithKline, Allergan, Alcon, Genentech, OD-OS, Pfizer, Ocular Therapeutix, Alimera Sciences and Bayer Pharmaceuticals. Speaker's Bureau Participation: Alcon, Genentech and Allergan. Dr. Hariprasad can be reached at firstname.lastname@example.org.|
23-GAUGE vs 25-GAUGE
The introduction of 23-gauge instruments addressed the major drawbacks of the 25-gauge system — excessive flexibility, poor illumination, and low flow rate — making it the clear choice for small-incision surgery. Since that time, however, advancements in MIVS have erased most of these advantages. For instance, manufacturers have developed 25-gauge instruments that are substantially more rigid than their original counterparts (Figure 1), minimizing the problems caused by instrument flexure. Furthermore, improvements in flow rates, illumination, wound architecture and instrumentation diversity have benefited both 23-gauge and 25-gauge systems alike. Thus, the race for superiority is a close one, with fewer characteristics distinguishing the two systems than in the past. The larger wound of the 23-gauge system theoretically increases the risk of leakage, hypotony and infection, whereas 25-gauge instruments are still slightly more flexible than their 23-gauge counterparts.
MICROINCISIONAL VITRECTOMY SURGERY FOR THE REPAIR OF RETINAL DETACHMENT
Small-incision vitrectomy is well suited for repairing retinal detachments, with its improved patient comfort and faster visual recovery relative to traditional vitrectomy. Because MIVS preserves ocular structures and minimizes scarring of the conjunctiva, sclera, and tenon's layer, it is an excellent choice for patients who have had multiple previous ocular surgeries or may require surgery in the future (i.e., a glaucoma patient with an existing, or a likelihood of a future, trabeculectomy). MIVS, however, may not always be optimal in certain circumstances; nonetheless, small-gauge instrumentation need not be abandoned, even in complicated cases. For instance, in the rare case where MIVS is unsuccessful at repairing a retinal detachment, one of the ports can be enlarged to 20-gauge and the procedure can be completed. This type of hybrid arrangement is also ideal for silicone oil removal. Because only one port needs to be larger - the one used to remove the silicone oil - the infusion port and the illumination port can remain 25 or 23-gauge. This retains most of the advantages attributed to the small-gauge procedure, while eliminating the disadvantage of slow and cumbersome silicone oil removal. The need to place a scleral buckle does not preclude the use of MIVS either. In these situations, the surgeon can place the trocar-cannula system through the sclera after the conjunctiva and tenon's layer are still down.
SURGICAL EQUIPMENT CONSIDERATIONS
The three leading vitreous surgery systems — Alcon's Accurus Surgical System, Alcon's Constellation Vision System, and Bausch & Lomb's Millennium Transconjunctival Sutureless Vitrectomy System — are all options for the MIVS procedures described here. My preference is for the Constellation, which evolved from the Accurus. The Accurus was the first vitreous surgery system to provide a vitrectomy probe with 2500 cpm, as well as the ability to simultaneously change cut rate and vacuum with a foot pedal. However, the Constellation has introduced unique new technologies. For instance, its dual pneumatic drive vitrectomy probe is capable of 5000 cpm, compared to 2500 cpm for the Millennium and the Accurus (1500 cpm for the 25-gauge Accurus probe). This ultrahigh speed cut rate effectively reduces vitreous traction, which decreases the chance of creating retinal tears and permits the surgeon to work remarkably close to a detached retina with less movement of the mobile tissue.
Another feature unique to the Constellation is IOP compensation via direct control of infusion pressure; this produces less fluctuation of IOP throughout the procedure, improving patient safety. The Constellation is also the only system to provide direct control of the duty cycle (percent of time that the port is open during a complete cutting cycle); this feature allows the surgeon to control flow rate while keeping cut rate constant, which in my opinion provides a measure of safety when cutting near the retina. Finally, the Constellation makes use of second-generation 25-gauge instruments. These instruments are 50% to 90% more rigid than first-generation instruments depending on the distance into the eye, alleviating a major drawback of original 25-gauge systems, and also have much higher flow rates.
Other features of the Constellation enhance its convenience — these include an integrated laser that offers surgeon control of power and ready/standby settings through a multi-function footswitch, an automated fluid/air exchange feature (rather than a stopcock), its proportional intraocular diathermy, making it easier to perform posterior drainage retinotomies and to mark the edges of a retinal break, and lastly, continuous reflux of BSS from the vitrector port rather than a pulse reflux, allowing one to more effectively force blood off of the retinal surface as well as spread steroid over the retinal surface when highlighting membranes.
MIVS is becoming increasingly established as a safe and effective alternative to 20-gauge surgery, producing greater patient comfort and quicker visual recovery. It is preferred over 20-gauge in cases where conjunctival scarring is a concern, such as in patients with past or upcoming ocular surgeries. Recent advances in the technology of vitreous systems as well as the expanding array of small-gauge surgical instruments have greatly increased the acceptance of MIVS by vitreoretinal surgeons. Whether you choose 23-gauge or 25-gauge instrumentation, MIVS is progressively obviating the need for conventional 20-gauge vitrectomy, particularly with the use of hybrid port arrangements for those special cases requiring a full armementarium of instrumentation such as a fragmatome, MPS scissors, or silicone oil extraction cannula. The potential disadvantages of MIVS — increased rates of hypotony and endophthalmitis — have had limited impact on its use, due to the questionable clinical significance of transient hypotony and the unconfirmed nature of the reported increase in endophthalmitis. MIVS has allowed me to achieve excellent outcomes in the repair of retinal detachments while producing less trauma to the eye. RP
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