Ординатура / Офтальмология / Английские материалы / Retinal and Vitreoretinal Diseases and Surgery_Boyd, Cortez, Sabates_2010

Anterior vitrectomy is a crucial tool in the skill set of the anterior segment surgeon. Although a planned anterior vitrectomy may be performed in such settings as traumatic cataract removal or secondary IOL placement, this procedure is most often an unplanned —and unwelcome—addition to a cataract surgery. Even the most experienced surgeon will occasionally be inadvertently faced with vitreous prolapsing into the anterior segment. Thus, a surgeon’s comfort with basic anterior vitrectomy principles and techniques can defuse intraoperative stress and improve patient outcomes when complications involving the vitreous occur.1

During cataract surgery, it is imperative to quickly recognize complications such as

posterior capsular tear with associated vitreous loss. One must attempt to maintain a controlled working environment while avoiding subsequent problems, including hypotony and vitreoretinal traction.

The main goals of anterior vitrectomy are to free the anterior segment of vitreous and to release any vitreous traction, which may cause breaks or tears in the retina (Figure 1). Important matters to consider when performing anterior vitrectomy include the accessibility of vitreous and retained lenticular tissue, minimization of secondary complications, and optimization of visual and surgical outcomes.

Vitreous Anatomy

A basic knowledge of vitreous anatomy is helpful for understanding the intraoperative behavior of the vitreous. While the vitreous is 98 to 99 percent water, it also contains a network of fine collagen bundles suspended in coils of mucopolysaccharide hyaluronic acid. These collagen and hyaluronate components impart a gel-like consistency and a degree of elasticity to the vitreous. While a small amount of traction can be absorbed by the vitreous, a larger amount may be transferred through its collagen bundles to the posterior and peripheral retina, resulting in retinal tears and macular edema.2 The vitreous adheres most firmly to the retina at the optic nerve and at the vitreous base. It extends approximately 2 millimeters anterior and 4 mm posterior to

the ora serrata. Looser attachments also occur along the retinal vessels, in the perimacular region and in the periphery of the posterior lens capsule (Figure 2).

The Role of Vitrectomy in

Anterior Segment Surgery

Vitrectomy is a word the anterior segment surgeons really do not want to say nor hear. It conjures certain complications as capsulerupture,fallinglensfragments,anterior chamber IOLs, surgically induced astigmatism, cystoid macular edema and retinal detachment. However, surgeons can be taught to do better. There are definite indications for vitrectomy, and proper performance can rescue surgical results.Advances in techniques and technology

mean that vitrectomy no longer has to be a dirty word, or a daunting procedure.3

Lens extraction performed through two paracentesis-type incisions offers unique advantages that enhance surgical control and safety (Figure 3). The crucial difference is not the size of the incisions; it is the separation of inflow and outflow. This same separation of inflow and outflow facilitates the perfor-

mance of anterior vitrectomy, for the reasons mentioned above.

Bimanual phaco also provides significant advantages in the management of complications. If the posterior capsule is compromised during surgery, the first goal of the surgeon is to maintain stability of the anterior chamber to prevent both posterior migration of lens material and anterior prolapse of the vitreous.4

By maintaining infusion in the anterior chamber, it becomes safer to use the phaco needle, aspiration tip or vitrector to remove residual lens tissue. Irrigation should never be brought down into the capsule or vitreous space, where it may dislodge lens tissue, enlarge the capsular tear or engage the vitreous.

Advantages

Pars plana anterior vitrectomy has several advantagescomparedwithcurrentmethodsfor dealing with vitreous loss. Anterior vitrectomy employs continuous infusion and aspiration through the same instrument and incision. This circumstance may cause further vitreous prolapse toward the incision.4 The cutter is

usually used at a slow rate with a large aspiration port, which can hydrate the vitreous and exert greater traction on the peripheral vitreous and retina.5

Completevitreousremovalfromtheanterior chamber may be difficult due to fluid flowing out of the wound and limited accessibility.

With a pars plana lensectomy, the pars plana incision can be placed in a position that offers optimal access to the remaining lenticular material. By allowing the surgeon to pull down prolapsed vitreous from the anterior chamber, the posterior approach can reduce the amount of vitreous removed overall (Figure 4). Removing less vitreous from the eye may lower the likelihood of postoperative hypotony.

In addition, a pars plana approach facilitates the removal of retained lenticular material near the posterior capsule, iris, and ciliary body and thus lessens the chance of secondary inflammation and cystoid macular edema.6

Because the procedure is performed in a closedchamber,reducedintraocularturbulence minimizes IOP fluctuations and lessens the risk of suprachoroidal hemorrhage. Furthermore, pars plana vitrectomy reduces corneal edema

that occurs secondary to trauma from anterior vitrectomy performed through a phaco incision. According to Chalam and Shah5 96.5% of eyes studied achieved a BCVA ≥ 20/40. Furthermore, 72.4% achieved a BCVA ≥ 20/20 postoperatively at 3 months. Vitreous loss with posterior capsular rupture occurred in all eyes, and cystoid macular edema was noted in two (6.8%) eyes. These results compare favorably with those of cataract surgery in which no vitreous loss is noted or anterior vitrectomy is used for complicated cataract surgery.7

When nuclear remnants fall through a rent in the posterior capsule (but remain within the surgeon ́s view), or vitreous prolapses to the wound, then the phaco surgeon should consider pars plana vitrectomy. The advantage of this approach over vitrectomy through a corneal incision resides in the fact that the vitreous gel is pulled in a posterior direction by the vacuum, thus limiting vitreous loss, preventing incarceration of vitreous strands in the incision and facilitating a thorough clean-up of the anterior segment.

For vitreous removal, the cut rate should be set high, at 500 to 600 cpm, with low to moderate aspiration. A high cut speed for vitreous removal causes vitreous to flow continuously into the cutter, resulting in less pulsatile stress being placed on the retina.8 The vitrector is then placed through the capsular tear just below the capsule with the aspiration port facing up toward the cornea. The cutter should be maintained in a fairly central position and not moved peripherally beyond the plane of the iris root to avoid undue stress on the vitreous base. The vitreous is removed to a level just posterior to the capsule (Figure 4).

To perform a pars plana vitrectomy with 23G / 25-gauge instrumentation, irrigation is maintained in the anterior chamber with reduced pressure (lowered bottle height) as a stab incision is made with an MVR blade or other suitable instrument 4 mm posterior to the limbus, usually in the superotemporal or inferotemporal quadrants. The conjunctiva is pushed to one side so that intact conjunctiva will cover the sutureless incision afterwards (Figure 5). A small-gauge, high speed cutter is introduced and the lens fragments removed.9 Irrigation is titrated to maintain a stable anterior chamber.

According to David Chang, M.D., in many cases it is recommended to perform bimanual anterior vitrectomy with a separate limbal side-port infusion and high cutting rate through the use of pars plana sclerotomy for the vitrectomy cutter.

Following vitrectomy, irrigation is maintained while cortical remnants are aspirated and the capsule cleaned as necessary. Finally, a cohesive viscoelastic is introduced through a corneal incision while continuous irrigation is still maintained, preventing further vitreous prolapse.

The haptics of the intraocular lens can then be inserted into the ciliary sulcus and the optic captured behind the capsulorrhexis to avoid any rupture of the posterior capsule (Figure 6).10

Following removal of viscoelastic with irrigation and aspiration, the surgeon should ensure a vitreous-free anterior chamber.

Converting a difficult situation into a familiar situation remains the goal of most approaches to challenging cataract cases. Pars plana vitrectomy and vitreous staining provide the anterior segment surgeon with valuable tools in a variety of cases. Adopting these relatively simple techniques will improve outcomes and allow greater flexibility in the operating room.

Vitreoretinal pathologies are serious conditions primarily affecting traumatized patients with an estimated overall prevalence of 0.14 percent in the Caucasian population.1,2 Pars plana lensectomy has been shown to be effective in achieving visual improvement during surgery.3 Since the first report of surgical repair through vitrectomy by Kelly and Wendel,4 several modifications of the original technique have been described. The modifications involve many unresolved controversies associated with vitreous surgery. These include the appropriate vitreous substitute used for intraocular tamponade,5-9 the optimal duration of intraocular tamponade and postoperative positioning,10-12 the benefits of adjuvant therapy,13-20 the role of internal limiting membrane (ILM) peeling, the optimal staining material,21-30 and the extent of vitrectomy needed to achieve a successful surgery.31

Indications for Lensectomy

Pars plans lensectomy with an ultrasonic fragmatome system is one of the latest advances both in retinal surgery and cataract surgery. In cases of congenital cataracts and cataracts in young adults, the lensectomy can be the procedure of choice. Complications like after cataract, vitreous loss, bullous keratopathy, epithelial down growth, anterior synechiae, secondary glaucoma and wound dehiscence which are met with during conventional surgery, are either absent or seen in significantly small numbers, after pars plana lensectomy. Since this procedure utilizes a small scleral incision and a closed system to maintain intraocular pressure during the surgery, preoperative reduction of intraocular­ pressure is not necessary.32

Early ambulation of the patient is possible since no corneo-scleral section has been made. The most important advantage of this procedure is that vitreous loss does not occur. Lensectomy can also be done in cases where vitrectomy or scleral buckling procedures are indicated in order to obtain a clear media, if the lens is cataractous (Figure 1).

Lensectomy and Anterior

Vitrectomy in Pediatric

Cataracts

Usually, the basic surgical techniques for pediatric cataract surgery have been lensectomy and anterior vitrectomy (LAV).33-38 These techniques provide a clear visual axis but needs rehabilitation of aphakia by the use of spectacles or contact lenses. The approaches

for LAV are either limbal or via pars plana (Figure 2). Most surgeons prefer limbal approach to minimize the risk of damaging the peripheral retina and to prevent vitreous from becoming incarcerated in the wound. This approach is particularly used for the management of pediatric cataracts associated with uveitis.

With the general acceptance and the advances in design of intraocular lenses as a mode of aphakic correction especially in children (older than 2 years of age), LAV is losing ground to modern cataract surgery techniques because it does not allow the option of placement of a posterior chamber IOL. Some authors have described a technique of pars plana lensectomy where a peripheral rim of anterior capsule is left intact for a sulcus fixation of the IOL (Figure 3).