Pars Plana Anterior Vitrectomy

20   Pars Plana Anterior Vitrectomy

Lisa Brothers Arbisser

Every cataract surgeon encounters unplanned vitreous loss. Eyes respond differently, and surgeons are not automatons. Ideally, we would not lose vitreous the same way twice—a rarely achieved yet lofty goal. The incidence varies widely in the literature from 0.45%, reported years ago by Howard Gimbel, to reports that are logarithmically higher. A national registry measures it at 2.09%. A surgeon with a higher rate warrants remediation.1 It behooves us all to have a plan in mind, tools with which to execute the plan at the ready, and to be prepared for contingencies for every case. The earlier a complication is recognized and the stage of complication limited, the better the result. Rarely do we breach the posterior capsule without rupturing the anterior hyaloid. When limited to this stage, optimal outcomes uniformly result, assuming implant stability is achieved. Rupture of the anterior hyaloid membrane with prolapse of vitreous into the anterior segment increases the risk of late complications. Once vitreous is lost through incisions there is a greater likelihood of retinal tear or detachment; another set of actions is indicated. Depending on the timing, this problem may be associated with residual lens remnants.

Benjamin Franklin’s famous advice, as also cited in the previous chapter, bears repeating: “An ounce of prevention is worth a pound of cure.” When the surgery is optimally managed, outcomes can rival those of uncomplicated surgery. This chapter, based on both experience and laboratory exploration, discusses a set of cardinal principles that cannot be violated, and describes the tools and techniques for this procedure. Anterior segment surgeons are most comfortable with anterior incisions. Regardless of the incision site, there are universal principles for success. Although this chapter is intended to fully describe the pars plana approach to anterior vitrectomy, gaining experience in skills-transfer wet laboratories, apprenticing for a day with a vitreoretinal surgeon, or acquiring other hands-on experience is recommended prior to attempting a new technique in the setting of a complicated cataract case.

Guiding Principles

Avoid intra- and postoperative vitreous traction. Vitrectomy itself need not result in significant visual disability. Visual function is impaired by sequelae of retinal detachment, hemorrhage, and macular edema resulting from suboptimal vitreous management. Strictly embracing proper technique almost universally avoids impaired visual outcomes.

Maintain a Normotensive Globe

Employ tight incisions for anterior vitrectomy. Complications prolong surgery and hypotony invites hemorrhage, choroidal effusion, and subsequent edema. Alternating high and low intraocular pressure (IOP) can cause shear where choroidal vessels are tethered by anatomy; sudden hemorrhage may result. Phacoemulsification can fail without a controlled and stable environment; our vitreoretinal colleagues would not dream of using leaky incisions. The anterior segment surgeon must follow suit in handling complications.

Vitreous Always Follows a Gradient from High to Low Pressure

Once a complication is recognized, maintenance of the anterior chamber (AC) and avoiding collapse are essential. Think of vitreous as egg white in a bowl; tilt the bowl and it will come streaming out, running downhill. Vitreous follows an instrument withdrawn from an incision and may convert vitreous prolapse into vitreous loss. Infusion may easily displace vitreous. This is the logic for a biaxial approach to vitrectomy for any incision. Always separate the irrigation sleeve from the vitrector shaft and always discontinue infusion before exiting the eye. Incisions exceeding the diameter of the vitrector facilitate vitreous’s preferential egress through the leaky incision rather than into the vitrector port.

Never Fish Around the Complex and Wondrous Structure of Vitreous

Not typically visible in vivo, the vitreous body is composed of solid parts: cortex, septa, and cisternal walls. These surround and separate more liquid parts: canals, cisterns, and spaces. The equator-parallel and sagittal septa follow an incomplete spiral, which is mirror image in the left and right eye. The vitreous body resembles a snail shell in this way, as septa radiate between the 12 petal-like cisterns that surround the bursa premacularis and the cistern preoptica. In equator-parallel section, the vitreous body resembles a cut orange. Collagen fibrils and fibers interact with hyaluronic acid, as formed vitreous and the lamellae are interconnected by a loose mesh of fibers. The high water/low protein content is unique in the body. This complex structure we are manipulating acts like a toy slinky attached to wallpaper (the retina). It functions as both a filter and a barrier.2 Cataract surgeons must respect the vitreous. In the setting of unplanned loss, remove only the prolapsed vitreous that may adhere to anterior structures or incisions; disturb the unoffending structure as minimally as possible. Placing instruments other than a vitrector into the vitreous body to retrieve lens fragments courts disaster. Robert Machemer irrigated into the vitreous to create an animal model for retinal detachment research. Although taught as an option, levitation of a descended nucleus with either a spatula or ophthalmic viscosurgical device (OVD) cannula through the pars plana is risky both for vision and medicolegal outcomes (Fig. 20.1).

Protect Other Tissues from Collateral Damage

Although we must deal with vitreous, there is no justification for losing much-needed capsule support, chewing up the iris, or causing corneal edema by failing to protect the endothelium. The thinnest part of the retina and likeliest to tear is located anteriorly near the vitreous base. This attachment must be respected. The anterior segment surgeon must leave a clean anterior segment with a stable implant whenever appropriate, and a clean bag and a clear visual axis for rapid visual rehabilitation or to allow further timely management. Avoid vitreous incarceration in the pars plana sclerotomy just as we would an anterior incision to prevent postoperative traction. Visualization of the incision site with indentation ophthalmoscopy is mandatory within the early postoperative period.

Endophthalmitis Prophylaxis Is Critical

The incidence of infection is a multiple of that of a standard case.

Be Prepared

Code red, code blue, and amber alerts are accepted random routine preparation in the medical environment. Consider establishing and practicing Code V at the end of a random surgical day. Have a reusable clean vitrectomy kit for practice available. Make sure the surgeon and the staff know where the equipment is kept, how to set it up, and the parameters to check, and that instruments, medications, and devices are at the ready. The higher the volume of cataract surgery, the lower the complication rate and the less prepared for contingency a center may be without this Code V practice routine (Box 20.1).

First Signs of Complication

A rupture in the posterior capsule and, in particular, the anterior hyaloid changes the pressure relationship between the anterior and posterior chambers and the posterior segment. This change in the distribution of fluid will in turn affect the AC’s depth and, often, the pupil’s size; the pupil may suddenly bounce or snap. An increase or decrease in the AC’s depth during phacoemulsification or irrigation and aspiration (I/A) are both warning signs; unless there is a good explanation for the change, stabilize and explore.

Box 20.1 Vitrectomy Kit Items (Not Exhaustive)

  • Vitrectomy pack for phaco
  • Calipers
  • Microvitreoretinal (MVR) blades
  • Trocars
  • 23-gauge angled infusion cannula if not included in the vitrectomy pack
  • Triesence (Alcon, Fort Worth, TX)
  • Arbisser nuclear spears (Epsilon USA Instruments)
  • Scissors
  • Nonirrigating vectis
  • Rings and segments for zonular defects
  • Intraocular instruments (helpful)
  • Sutures: 8-0 polyglactin, 10-0 nylon, 9-0 and 10-0 polypropylene, CV-8 Gore-Tex (off-label)
  • Backup implants [three-piece AC intraocular lens (IOL)]
  • Extra OVD dispersive and cohesive
  • Acetylcholine
  • Preservative-free (PF) epinephrine
  • Trypan blue
  • 2% lidocaine for sub-Tenon’s anesthesia
  • 26-gauge straight and J-cannulas, syringes for dry cortex removal

A momentary spider of the posterior capsule is likely associated with a tear and must be inspected after stabilizing the chamber and protecting the hyaloid with OVD. An unusually clear appearance of the posterior capsule is usually a rent or hole.

Because vitreous follows a gradient from high to low pressure, it will always preferentially seek to flow into the phaco or I/A tip and obstruct its action. If lenticular material suddenly stops coming to the phaco tip, there is likely vitreous in the way. Vitreous cannot be refluxed out of an I/A tip but must be sharply cut to avoid traction.

The classic later signs of vitreous loss are an asymmetrically enlarged pupil and remote movement of the iris when touching the incision. Another ominous sign of vitreous loss is tilting of the nucleus’s equator or loss of mobility in a previously rotatable nucleus. Seeing clear space beyond the equator or having the equator come into view after removing the nucleus are sure signs of zonular loss with possible vitreous prolapse through the defect. A subtle sign of the presence of a forward strand of vitreous may be the inability to seal a properly constructed incision.

Vitrectomy Options

To best avoid vitreous traction, consider the best approach based on the particular condition of the eye. We need not always use an automated vitrector. If a small wisp of vitreous presents around zonules, it can be amputated with a scissor and reposited to the posterior segment with OVD.

A simple wisp that can be controlled is rare when vitreous prolapses through a broken posterior capsule. In the face of prolapse, automated vitrectomy is almost universally needed. In all cases the clear corneal paracentesis will be used for the irrigation cannula. When there is a small amount of prolapse without vitreous loss through incisions, vitrectomy can be nicely handled with the vitrector inserted through a clear corneal incision sized to fit the bare vitrector shaft. This is always the right choice when there is no view through the pupil or there is extreme or abnormal dimensional anatomy (Fig. 20.2).

In my experience, both in a laboratory setting and in the operating room, in the presence of copious vitreous prolapse, with vitreous loss through incisions or significant herniation around the bag equator, a pars plana sclerotomy approach to anterior vitrectomy is most efficient and preferable. The irrigating cannula is still placed through the clear corneal paracentesis. A direct entry sclerotomy under a fornix-based conjunctival flap created with an MVR blade and subsequently sutured whatever the vitrector gauge is worth learning to do safely. Alternatively, and theoretically the safest, a trocar system that enables a transconjunctival sutureless entry is best when the globe is closable or intact at the time of sclerotomy, as it requires pressure to insert. Trocars have the advantage of enabling repeated entry without trauma to sclera or proximity to the scleral wall and choroid. It is least likely to result in vitreous traction associated with incarceration at the incision site, as long as self-sealing, limbus parallel scleral tunnel is performed prior to entry perpendicular to the sclera. Any incision, anterior or posterior, must be closed when not in use. Sclerotomies are closed with a temporarily tied suture, preferably 8-0 polyglactin suture, a scleral plug, or, when available, the use of a valved trocar to maintain a closed environment and pressurized globe upon withdrawal of the vitrector shaft and during subsequent maneuvers (Fig. 20.3).

Rationale for a Pars Plana Approach to Anterior Vitrectomy

Because vitreous follows a pressure gradient from high to low, ideally the lowest pressure will always be in the posterior segment relative to the anterior segment once the hyaloid is ruptured and during the remainder of a procedure after vitreous presentation. The best way to accomplish this is with a pars plana exit. We also want to minimize traction by removing the vitreous close to its base. As instruments exit the eye, vitreous will tend to follow. If any vitreous follows the instrument to the incision, it will be right near the vitreous base, where we can eliminate incarceration at the pars plana, rather than up at the corneal incision.

This technique is the most efficient, because it calls the vitreous home or amputates the anteroposterior attachment to prolapsed vitreous, immediately relieving traction without increasing the size of the posterior capsule rent. It is least likely to encourage more vitreous prolapse and removes only the offending vitreous, sparing the general vitreous body structure. With an anterior incision, and a downward angled vitrector through a rent in the posterior capsule, the view can be compromised. Also there is a tendency to remove nonprolapsed vitreous, encouraging more to come forward, which almost always enlarges the posterior capsular rent. It is also very challenging to clear sheets of vitreous in intimate contact with posterior capsule or iris without damaging those structures from the anterior approach. The sheet tends to be broad, thin, and tightly adherent to these structures. Anteriorly placed irrigation and posteriorly located vitrector keeps a higher pressure in the AC. Multiple attempts in Liliana Werner’s University of Utah laboratory with eye bank and porcine eyes with Kenalog delineation confirm the biaxial pars plana approach with the vitrector posteriorly and irrigation anteriorly is vastly superior to an anterior approach under a bubble, as recommended initially by Steve Charles or vitrectomy by standard anterior limbal approach.3

The pars plana technique maintains low pressure posteriorly. Subsequent manipulation to remove cortex and implant a lens is least likely to result in re-presentation of the vitreous so long as the AC is maintained. It will not unzip the zonules when vitreous presents around the lens equator by calling more vitreous forward through the defect or risking a posteriorly inserted vitrector near the retinal wall through zonules. Finally, the pars plana approach facilitates amputation of the vitreous within incisions (Fig. 20.4). The method if classically taught, using a sweep from the side-port incision to drag entrapped vitreous from the incision actually creates more traction on the connection through the pupil rather than efficiently freeing the vitreous from incarceration in the wound. This practice is strongly discouraged.

Nomenclature: Vitrectomy Mode (I/Cut/A) Versus Cortex Mode (I/A/Cut)

The nomenclature is confusing. The lack of standardization among manufacturers compounds the confusion. This chapter, therefore, refers to the order in which each function is engaged by the foot pedal. The best machine parameters for performing vitrectomy are the same regardless of what incision location is used for the vitrector needle. Employ the settings that most effectively reduce vitreoretinal traction and prevent followability. For the majority of phacoemulsification machines, bear in mind that foot position (FP) 1 is irrigation only, FP 2 engages both irrigation and cutting by activating the guillotine, and aspiration ensues only as FP 3 is entered, resulting in irrigating, cutting, and sucking simultaneously, ensuring that no vacuum is applied to the vitreous without chopping it off in tiny bites, minimizing traction in any part of the FP sequence.*

All machines have an alternate setting that is not the default, wherein FP 1 initiates irrigation as before, but FP 2 allows vacuum, and cutting mode only activates on FP 3. This setting, in this chapter’s nomenclature I/A/Cut versus the default I/Cut/A, goes by different names on different platforms and is useful when followability is desired during removal of the residual cortex, once prolapsed vitreous is dispatched. This useful setting allows the surgeon to remain in FP 2 while removing lens material where followability is desirable but can allow near-instant activation of the cutter in the event vitreous presents.

Always employ the default setting when there is any likelihood that vitreous will be encountered.

Machine Settings

Cutting Rate and Flow

Steve Charles has coined the term port-based flow limiting. This describes the goal of achieving the highest cut rate possible, the lowest effective flow rate, and the lowest vacuum that generates the removal of vitreous. As the vitreous is engaged, the faster the guillotine opens and closes, the more traction is reduced because a lower volume of vitreous enters with less followability. The highest cut rate possible on some older phaco machines is 400 cuts per minute. Newer models achieve up to 16,000 cuts per minute. Faster cutting leads to less traction and a smoother removal of vitreous. Always use the available machine’s fastest cut rate for vitrectomy. The higher rates are one of the reasons that three-port total planned vitrectomy has become safer over time. When close to the retinal surface the higher rates are critical. For anterior vitrectomy where we remain within the pupillary aperture, this speed is less critical and still acceptable with any phaco machine for the cataract surgeon’s purpose. The technique, however, will vary slightly depending on the cut rate. At lower rates the episodic pull of the vitreous is almost visible, as is the opening and closing of the guillotine. With these lower cut rates it is very important not to drag vitreous around and critical to keep the vitrector handpiece steady in one place until the advantage of that location has concluded removing accessible prolapsed vitreous. The needle can then be moved to a new location in FP 2 to be certain no vitreous is dragged along by the flow, and then FP 3 is reengaged to remove prolapsed vitreous in this new location. Waving the needle around and rapidly moving its position is discouraged. With the higher cut rates the action of the guillotine is such a blur that only the sound provides the feedback of activity. The effect is more like erasing than aspirating vitreous and the activity at the vitrector tip can be more efficient and dynamic without causing traction. This simultaneously promotes safer and quicker surgery. The aspiration flow rate (for peristaltic pump platforms) is generally set at 15 to 20 cc/min depending on the vitrector gauge. The logic is simply to make things happen but not too fast.

Linear Versus Fixed Vacuum Setting

Vitreoretinal surgeons who frequently work in the posterior segment prefer to use linear vacuum for vitrectomy, and therefore this is usually the default for phaco machines as well. Familiarity leads to facility. Surgeons may adjust the vacuum on the fly based on how vitreous is behaving. They may wish to be more or less aggressive with vacuum, and the nuance of where they are in FP 3 is intuitively controllable. Anterior-segment surgeons who rarely perform vitrectomy are not usually as adept at these maneuvers and often get nowhere due to a light foot that may not even venture into FP 3. Staying in FP 2 accomplishes nothing because there will be continuous cutting without suction. For those with a heavy foot, the nuance of applying more or less vacuum during vitreous removal is lost, and they may use more vacuum than necessary and may cause more traction. Anterior-segment surgeons can consider a panel/fixed setting for vacuum instead of a linear vacuum setting for vitrectomy. Find and maintain the lowest level of vacuum that moves the vitreous; there is no reason to use higher or lower vacuum once vitreous removal is evident. The panel setting allows us to either go pedal-to-the-metal in FP 3 with suction, or come up into FP 2 without suction. The vacuum default settings on today’s phaco machines are usually set at 150 mm Hg, ideal perhaps for primary three-port total vitrectomy. In unplanned vitrectomy, however, the anterior segment surgeon is almost always removing vitreous in a sea of dispersive viscoelastic, causing an effective level to be closer to 250 mm Hg for 20-gauge and 350 mm Hg for 23-gauge vitrectomy on average.

Adjusting Irrigation Inflow

The irrigation bottle must be kept moderately high to maintain a normotensive eye. If there is forced infusion, then a normotensive setting in the range of 25 mm Hg should be chosen. Most phaco machine’s default settings place the bottle low. The appropriate bottle height depends on the size of cannula we use as well as the vacuum level. Most anterior-segment surgeons opt for a 23-gauge cannula, which is delivered with most newer vitrector kits. The port of the irrigation cannula should be held sideways, neither irrigating down into the vitreous nor directly up toward the endothelium. To control the IOP, have the scrub nurse stand with one hand on the bottle’s button, ready to raise it as needed, and one hand on the vacuum button. Start the vacuum for 20-gauge vitrectomy at around 200 mm Hg, and ask the scrub nurse to progress by 10-mm increments to 250 mm Hg or stop once the vitreous begins to move. As soon as movement toward the vitrector port is seen, with a finger on the globe from the nondominant hand holding the irrigation cannula, the surgeon can instruct the scrub nurse to raise the bottle until homeostatic normotension is achieved. Ideally, just prior to removing the vitrector at the end point, when not aspirating any longer, the bottle would once again be lowered to reduce the IOP when there is no material being removed and just before irrigation is turned off and the vitrector exited from the incision to reduce pressure variation.

Visualization: Particulate Staining

Although slit or tangential illumination with a light pipe can help to visualize the invisible vitreous, nothing compares to triamcinolone acetonide as a tool to particulate stain the vitreous. Essentially we “throw a sheet over the ghost” (Fig. 20.5), as originally devised by Gholam Peyman,4 and then popularized in the anterior segment by Scott Burk5,6 using preserved Kenalog off-label. Kenalog must be washed of the preservative. However the commercially available nonpreserved Triesence (triamcinolone acetonide injectable suspension; Alcon Laboratories, Inc.) preparation is Food and Drug Administration (FDA)-approved and billable. When instilled intracamerally, the suspended particles are individually trapped in the vitreous matrix but will rinse out of aqueous. It will not adhere to OVD but can be blocked by it. Triesence is best diluted 10 to 1 with balanced salt solution (BSS) to prevent a whiteout effect within the AC, obscuring intraocular structures. The dilution also provides enough volume for repeated use during the procedure. The suspension can settle out of its diluent in the syringe, as there is no chemical to keep it suspended. A small pearl: have the scrub nurse introduce an air bubble into the syringe to facilitate shaking up and resuspending the particles. Of course, expel the air prior to handing it to the surgeon. In the best of all worlds, Triesence would be prepared preoperatively and kept on the back table, ready to be used immediately upon suspicion of a complication to be followed by OVD. However, as OVD is always at hand, it is preferentially used immediately to stabilize the environment when a complication is suspected. The Triesence then is usually used after initial vitrectomy, once the OVD is also removed, to provide a critical visual end point for vitrectomy. Triesence instillation into the AC of a minim or two should always be one of the last maneuvers in the complicated case to rule out any unsuspected vitreous representation. As an added benefit, the drug has the therapeutic effect of reducing postoperative inflammation.

May 13, 2018 | Posted by in OPHTHALMOLOGY | Comments Off on Pars Plana Anterior Vitrectomy
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