Pars Plana Vitrectomy

Modern pars plana vitrectomy was developed by Robert Machemer in 1970. ^{[1, 2]} Machemer created the vitreous infusion suction cutter (VISC), which was the first closed-system vitrectomy device with infusion and aspiration to control intraocular pressure during the surgical procedure. This was a monumental advance in ophthalmology because it allowed, for the first time, controlled access to the posterior segment of the eye.

Initially, vitrectomy was used primarily to clear the vitreous of opacities such as blood. However, technological advancement with better vitrectomy systems and advanced instrumentation allow for this procedure to be used in a much greater number of applications. Currently, vitrectomy surgery is fairly routine surgery for the vitreoretinal surgeon and can usually be performed safely as an outpatient procedure with excellent results.

Pars plana vitrectomy is appropriate whenever access to the posterior segment of the eye is necessary for treatment. Common indications include rhegmatogenous or tractional retinal detachment, ^[3] vitreous hemorrhage, retained lens fragments after cataract surgery, ^[4] endophthalmitis, epiretinal membrane, ^[5] macular hole, ^[6] vitreomacular traction, and intraocular foreign bodies. ^[7]

Pars plana vitrectomy is often performed under emergency conditions (eg, treatment of rhegmatogenous retinal detachment, management of endophthalmitis, or retrieval of an intraocular foreign body). In these conditions, the procedure may be contraindicated only if the eye has no light perception and if regaining any vision in the eye is impossible. Vitrectomy is contraindicated in the presence of suspected or active retinoblastoma or, in some cases, active choroidal melanoma because incision of the eye may be associated with systemic seeding.

In elective cases, such as epiretinal membrane removal or treatment of a macular hole, the use of a systemic blood thinner (eg, aspirin or warfarin) is a relative contraindication. To reduce the possibility of intraoperative and postoperative bleeding, patients should work with their primary care physicians and stop taking the blood-thinning agent.

Often, patients receiving warfarin are unable to discontinue its use; in such cases, a heparin or enoxaparin bridge can be employed preoperatively, and warfarin can be resumed postoperatively. A prothrombin time (PT) should be obtained on the day of surgery for any patient who has been taking warfarin, even if the drug was discontinued preoperatively, to ensure that the levels are low enough to permit surgery.

Local anesthesia with intravenous (IV) sedation is appropriate in most cases. A retrobulbar block consisting of an equal mixture of short-acting lidocaine 2% and longer-acting bupivacaine 0.75% can be used; hyaluronidase may be added to help with tissue dispersion.

Before the retrobulbar block is performed, IV propofol may be given by the anesthesiologist for short-term sedation during the block; 5-6 mL is usually sufficient. After the retrobulbar block is performed, pressure should be applied to the globe. Globe and orbit firmness should be monitored; excessive globe tightness is a sign of retrobulbar hemorrhage.

As an alternative to retrobulbar block, a subtenon block may be given before the start of the case. After topical anesthetic drops and povidone-iodine are instilled, the conjunctiva and the tenon capsule are incised in an oblique quadrant, and anesthetic is infused into the retrobulbar space through a blunt cannula. This method is less likely to give rise to retrobulbar hemorrhage.

In some cases, general anesthesia may be required. It should be considered for pediatric patients and overly anxious patients. General anesthesia should also be considered when the operating time is expected to be longer than usual or when a patient requests it.

Patients are brought to the operating room in an eye bed that has an appropriate head rest and the capability to have a wrist rest secured to it. Once the bed is positioned next to the operating microscope and locked, the bed is made completely flat, and the patient is positioned so that the head lies comfortably on the head rest.

The wrist rest is then appropriately secured so that its height is at the level of the patient’s zygoma and the apex of the patient’s head is about 1 cm from the rest. The patient’s arms should be appropriately secured so that they do not hang off the side of the bed. A bed sheet can be wrapped around the patient’s torso and secured with hemostats to prevent inadvertent movement during the procedure.

Pars Plana Vitrectomy

Before the procedure is begun but after the retrobulbar block has been placed, the eye to be operated on is thoroughly prepared with a 5% solution of povidone-iodine. Great care should be taken to wash the eyelid margins thoroughly and to bathe the conjunctiva adequately with the povidone-iodine solution. A hard metal shield is securely placed on the nonoperative eye for protection. The surgical field is then appropriately draped, and an eyelid speculum is placed in the operative eye.

Creation of sclerotomies

The older 20-gauge system or the newer 23-, 25-, or 27-gauge systems may be used for vitrectomy. Certain technical details are specific to the vitrectomy system used.

20-gauge vitrectomy

With 20-gauge vitrectomy, the conjunctiva and tenon layer are incised to expose the sclera. This is done with Westcott scissors superonasally, superotemporally, and inferotemporally. Once bare sclera is exposed, light cauterization is applied over the planned sclerotomy sites to obtain hemostasis.

A caliper is then used to measure 4 mm from the limbus in phakic eyes and 3.5 mm in pseudophakic or aphakic eyes in the inferotemporal quadrant. This distance is marked on the sclera with the caliper, and 7-0 or 8-0 double-armed polyglactin suture is used to place 2 radial bites on either side of the mark. These bites should be about 1.5 mm long and 1.5 mm from each other. The suture is cut so as to leave tails approximately 2 cm long on each side (see the image below).

A sclerotomy is made with a microvitreoretinal (MVR) blade positioned between the 2 suture bites parallel to the limbus. The blade is aimed toward the middle of the globe and is inserted far enough that the tip is visible through the pupil before being pulled out.

The infusion line is then placed in the sclerotomy after it has been run to remove all air bubbles in the line. The infusion line is secured to the globe by engaging 1 flange of the infusion under the suture loop and tying the 2 free ends of the suture over the second flange. This knot should be tied as a temporary loop knot so that it can be permanently tied at the end of the case to close the sclerotomy after removal of the infusion line.

The light pipe is used to visualize the tip of the infusion line in the vitreous cavity. The tip should have a glistening metallic appearance; a dull-brown appearance may indicate that the choroid has not been penetrated. If the choroid has not been penetrated, a second sclerotomy is made superonasally with the MVR blade, and the tip of the infusion line is cleared by pushing the choroid away with the MVR blade. The infusion should never be turned on until the surgeon is confident that the tip is safely in the vitreous cavity.

Once the infusion line is in place, superior sclerotomies are made in the superotemporal and superonasal quadrants with the MVR blade. These should be made approximately 150^o apart at an appropriate distance from the limbus, depending on the phakic status of the eye.

23-gauge, 25-gauge, and 27-gauge vitrectomy

A 23-gauge, 25-gauge, or 27-gauge vitrectomy does not require incision of the conjunctiva and tenon layer to expose sclera. Peritomies are not created. Instead, trocars are placed through the conjunctiva and sclera to afford access to the vitreous. As in 20-gauge vitrectomy, this is done in the inferotemporal, superotemporal, and superonasal quadrants at an appropriate distance from the limbus, depending on phakic status. Before cannula insertion, conjunctiva is displaced with a cotton tip to keep the conjunctival puncture away from the scleral wound.

The cannula (on a trocar) is inserted into the sclera, usually at a 9-45^o angle (depending on gauge) and parallel to the limbus. After the trocar, but not the cannula, has entered the eye, the trocar is turned directly toward the center of the globe and is advanced until the hub of the cannula is flush with the sclera. The trocar is then removed, leaving the cannula in place. This maneuver allows a longer scleral wound and carries a lower risk of wound leakage.

The infusion line is attached to the inferotemporal stent, and the infusion is turned on once full penetration of the cannula into the vitreous cavity is confirmed.

Core vitrectomy

A light pipe and a vitrector are then passed through the superior sclerotomies. The first step is to perform a core vitrectomy to debulk the central vitreous (see the video below). This can be performed under direct visualization with the microscope. In patients with phakic eyes, the vitreous cutter should not be positioned too anteriorly, and the midline should not be crossed with any instrument, because this can cause lens touch and damage the crystalline lens.

Posterior vitrectomy

Once adequate core vitrectomy has been performed, the posterior segment is visualized (see the image below) with a wide-angle viewing system. Numerous systems are available, including both noncontact systems that are connected to the microscope and contact systems that are handheld or sutured in place with a lens ring.

When vitrectomy is performed, the infusion pressure must be monitored. If the vitreous cutter is cutting without engaging any vitreous, it can quickly outrun the infusion and cause the eye to collapse. The infusion pressure must be maintained at a level that is high enough to keep the eye formed but not so high that vitreous or retinal tissue is pushed out the eye (incarcerated) at the sclerotomy sites when instruments are passed in and out of the eye.

From this point, surgical management depends on the specific case. For a rhegmatogenous retinal detachment, inducing a posterior vitreous detachment (see the video below) is appropriate, if it is already not present. In a patient with diabetes, the posterior hyaloid should be incised and truncated 360º. If identifying the vitreous is difficult, triamcinolone acetonide may be injected into the posterior segment to highlight the gel. Detailed discussion of these and other techniques performed during vitrectomy is beyond the scope of this chapter.

Vitreous base shaving

Once a posterior vitreous detachment is induced and the posterior hyaloid is removed, shaving the vitreous base is often necessary. The vitreous base must be shaved because its adherence to the retina prevents it from being completely removed. Vitreous base shaving is done in coordination with scleral depression performed by a skilled assistant. This step is always necessary in a rhegmatogenous retinal detachment procedure.

Other vitreoretinal maneuvers

After an adequate amount of vitreous is removed, numerous intraocular procedures may be indicated, depending on the reason for the vitrectomy. These include perfluorocarbon injection, membrane peeling, endolaser treatment, retinotomy or retinectomy, air-fluid exchange, gas-air exchange (usually involving sulfur hexafluoride [SF₆] or octafluoropropane [C₃ F₈]), and silicone oil injection.

Completion and closure

At the end of the procedure, the sclerotomies are closed so that they are watertight. For 20-gauge vitrectomy, this involves suturing each of the sclerotomies with 7-0 or 8-0 polyglactin suture in a mattress fashion. The infusion line is withdrawn, and the sclerotomy is closed with the preplaced polyglactin suture. Conjunctiva is carefully reapposed with either polyglactin or chromic gut suture to make sure that the sclerotomies are adequately covered.

The 23-gauge, 25-gauge, and 27-gauge stents are designed to create self-sealing sclerotomies. As a cannula is withdrawn, a cotton tip should be used to apply pressure to the sclerotomy site. If wound leakage is suspected, a single polyglactin suture can be passed through both conjunctiva and sclera to ensure watertight closure of the sclerotomy.

Finally, physiologic intraocular pressure (IOP) is confirmed by means of palpation or tonometry. Antibiotics and steroids can then be administered through subconjunctival injection or topical application of ointment to the eye. The eye is taped and shielded.

The following technical points should be kept in mind in the performance of pars plana vitrectomy:

• In phakic eyes, do not cross the midline with an intraocular instrument, or the crystalline lens may be damaged and a cataract may ensue

• Lower the infusion pressure when removing an instrument from the eye to prevent vitreous or retinal incarceration

• When performing 20-gauge vitrectomy, clear vitreous near the sclerotomy entrance at an early point to prevent sclerotomy breaks or vitreous or retinal incarceration in the sclerotomy

• When injecting perfluorocarbon, do not inject directly over the fovea; also, do not inject too quickly, or the central retinal artery may become occluded

• Upon completion, examine the peripheral retina with scleral depression to ensure the absence of iatrogenic breaks or retinal detachment

• During air-fluid exchange, fogging of the posterior surface of an intraocular lens can often occur; to regain visualization, apply viscoelastic to the back of the intraocular lens

The major complications of vitrectomy are bleeding (0.14-0.17%), ^{[8, 9]} infection (0.039-0.07%), ^{[10, 11]} and retinal detachment (5.5-10%). ^{[12, 13]} If possible, blood thinners should be discontinued preoperatively. Intraoperatively, adequate IOP must be maintained; drops in pressure that cause globe collapse are a major risk factor for the development of a choroidal hemorrhage.

Eyes should be meticulously washed with a dilute povidone-iodine solution before the operation. Immediately after the operation, subconjunctival or topical antibiotics should be administered before the eye is patched. Topical antibiotic drops should be prescribed for at least 1 week after the procedure. Endophthalmitis is less common in postsurgical vitrectomized eyes because the vitreous has been removed.

Retinal detachment can occur during vitrectomy if an iatrogenic retinal break is made during the procedure (as when inadvertent retinal touch causes a break or sclerotomy tear). These detachments should be repaired in a manner similar to primary retinal detachment repair.

Equipment

Pars plana vitrectomy requires highly specialized equipment that is found only in an operating room (OR) that is specially equipped for vitreoretinal surgery. Generally, the following are needed:

• An eye bed on which a wrist rest for the surgeon can be secured

• An operating microscope

• A mechanical vitrector

• A wide-angle viewing system

• Calipers

• Westcott scissors, forceps, and needle holders

• An argon indirect laser or endolaser device

• An endoillumination system

• A bipolar cautery

• Intraocular instruments (eg, forceps, scissors, and flute needle)

• Scleral depressor

• Sulfur hexafluoride (SF₆) and octafluoropropane (C₃ F₈) gases

• Silicone oil