Coats of the eye

The conjunctiva is adherent to the sclera at the limbus, so the radial incisions to enter the subconjunctival plane have to be made just behind the limbus. The conjunctiva becomes friable with age and care must be taken to avoid tearing and buttonholing. Nontoothed forceps (such as notched forceps) are used to grasp the conjunctiva.

Tenon’s capsule is a layer of fascia that envelops the globe from the limbus to the optic nerve (Fig. 100.1). It is pierced by the extraocular muscles. A glove-like sleeve of fascia extends anteriorly (to the rectus insertions) and posteriorly (for several mm) along the muscles from the points at which they pierce Tenon’s capsule. Between the rectus muscles anteriorly these sleeves are joined by a layer of fascia: the intermuscular septum. The intermuscular septum and fascial sleeves of the recti are sometimes collectively referred to as the posterior Tenon’s capsule. The result of this complex arrangement is that several layers of tissue have to be incised to get access to the surface of the sclera (or sub-Tenon’s space). The anterior Tenon’s capsule may be dissected away from the sclera along with the conjunctiva (to which it is adherent by small fascial filaments). The intermuscular septum is then divided separately. Care must be taken when stripping fascia off the rectus muscles because ligaments from the recti to the wall of the orbit are functionally important in the actions of the muscle.⁷

Fig. 100.1 Tenon’s capsule. (a) Conjunctiva and (b) anterior Tenon’s have been dissected to reveal the (c) “posterior Tenon’s” glove-like extensions of tenons capsule along the recti and the associated intermuscular septum. Bare sclera (d) lies under this.

The thickness of the sclera varies. It is thickest around the optic nerve (1.2 mm) and thinnest under the recti behind their insertions so attempts to pass scleral sutures under the muscles are particularly hazardous. Where scleral mattress sutures are more typically passed, at the equator, it is approximately 1 mm thick. Passage of sutures is facilitated by the lamellar arrangement of collagen fibers, which allows spatulated (or “side cutting”) needles to follow a plane between lamellae.

Extraocular muscles

The recti are adherent to the sclera at the spiral of Tillaux. The location of this ring corresponds approximately to that of the ora serrata (Fig. 100.2).⁸ Circumferential scleral tires are therefore often placed as anteriorly as the rectus muscle insertions will allow. In this position they support the retina as far anteriorly as the ora serrata (“break ora occlusive buckling”).

Fig. 100.2 The ora serrata and the spiral of Tillaux (rectus insertions). The sclera has been rendered transparent to reveal the relationship between the ora serrata and the muscle insertions. Buckling anteriorly as far as the rectus insertions prevents anterior leakage.

The superior oblique muscle runs laterally from the trochlea to its insertion under the superior rectus. Passage of a superior rectus muscle hook from the temporal side of the muscle reduces the risk of inadvertently “hooking” the superior oblique as well as does keeping the sweep of the hook pre-equatorial. The superior oblique insertion is frequently encountered on the temporal side of the muscle where it can be an obstacle to scleral suturing. Division of a small (<) portion of the insertion to facilitate suturing seems to have little effect on ocular motility. Note that a vortex vein is usually present under the temporal edge of the superior oblique insertion.

The inferior oblique muscle passes under the lateral rectus muscle. The chance of inadvertently hooking it while passing a muscle hook under the lateral rectus is reduced by passing the hook from the superior side.

Choroidal vasculature

The long posterior arteries (as well as their corresponding nerves) run anteriorly from the equator at 3 and 9 o’clock and may be damaged by heavy photocoagulation or subretinal fluid drainage in these meridia (Fig. 100.3, available online).

Fig. 100.3 The choroidal vasculature. Surgical trauma should be minimized to all of these structures. A, anterior ciliary artery; B, long posterior ciliary artery; C, vortex ampulla; D, vortex vein; E, greater arterial circle of the iris.

The anatomy of the vortex veins is somewhat variable but one tends to leave the globe either side of the vertical recti just behind the equator. They may be inadvertently hooked along with a vertical rectus muscle if the muscle hook is passed behind the equator. Injury to the vortex veins may result in interruption to the venous drainage from the choroid and choroidal detachment. When operating near the vertical recti the vortex veins should be identified to prevent injury.

The choriocapillaris itself is very vascular and prone to bleed when penetrated. Because the vortex veins tend to be located near the vertical recti subretinal fluid drainage is carried out closer to the horizontal than to the vertical recti whenever possible.

The anterior ciliary arteries are useful indicators of the meridia of the rectus muscle insertions (Fig. 100.4). As they supply the arterial circles of the iris surgical trauma (including diathermy) should be minimized.

Fig. 100.4 The anterior ciliary arteries. A useful indicator of the location of the rectus positions especially in reoperations. Note that the lateral rectus only has one.

Innervation

Sensory nerves from the globe and bulbar conjunctiva pass through the ciliary ganglion. Local anesthesia in this region, for example, sub-Tenon’s anesthesia, will therefore anesthetize the globe effectively. The innervation of the lids and palpebral conjunctiva is by the lacrimal, frontal and infraorbital nerves, which do not pass through the muscle cone. Blockage of the ciliary ganglion alone does not therefore reliably provide adequate analgesia for buckling surgery.

Finding the retinal break

Missed retinal breaks are an important cause of surgical failure so the preoperative examination should be very thorough.⁹ Even when a break has been found, it is essential to complete examination of the retina, as most retinal detachments have more than one break.¹⁰ Their location is carefully documented on a chart that can be referred to subsequently during surgery (Fig. 100.5). These drawings should show the location of retinal breaks in relation to easily visible retinal landmarks such as small hemorrhages, vascular bifurcations, and areas of pigmentation.

Fig. 100.5 A retinal drawing – note the emphasis on easily visible features that can act as guides to the location of breaks intraoperatively.

This carefully documented preoperative assessment has many advantages. If in doubt, an area of retina can be re-examined alternately with indirect ophthalmoscopy and slit lamp biomicroscopy to establish whether a break is truly present. The drawings made can be referred to if the retinal view becomes obscured during surgery. If the breaks are not easily seen the other retinal features on the drawings provide useful reference points to their location.

In children and uncooperative patients, it may be impossible to establish the position of the retinal breaks preoperatively. One is then forced to rely on the intraoperative examination under anesthesia.

Lincoff’s rules

Lincoff has shown how the location of retinal breaks determines the distribution of subretinal fluid (Fig. 100.6).¹¹ Review of the retinal drawings will therefore determine whether the break location is consistent with the subretinal fluid distribution. When the distribution of fluid does not seem to obey Lincoff’s rules reexamine the retina to ensure that no breaks have been missed.

Fig. 100.6 The location of retinal breaks can be deduced from careful analysis of the topography of the detachment as described by Harvey Lincoff. (A) An inferior detachment slightly higher on the temporal side pointing to a break on that side. (B) A subtotal retinal detachment – the break is usually close to the upper border of the fluid on the side it is highest. (C) The fluid crosses the midline superiorly implying a superior break near 12 o’clock. The fluid has tracked down further nasal implying the break is slightly to the nasal side. (D) The presence of bullae implies a superior break. A shallow sinus of fluid leads to a small superonasal break.

Scheduling surgery

In “macular on” retinal detachment, urgent surgery should generally be scheduled to prevent detachment of the macula. Cases with tractional tears tend to progress very rapidly and should be treated urgently. Asymptomatic retinal detachments and detachments without tractional breaks (e.g., those due to retinal dialyses, atrophic retinal breaks in young myopes and cases with “tide marks”) progress more slowly. Some delay, if it allows optimal conditions for surgery, may then be acceptable.

Reducing eye and head movements seems to reduce the rate at which subretinal fluid accumulates. Bed rest, eye patching and rectus sutures have all been used to reduce the amount of subretinal fluid. This may prevent extension of subretinal fluid to the macula and make it easier to identify retinal breaks. It may also avoid the need for subretinal fluid drainage.¹² Such measures are rarely practical now, as most retinal detachment care takes place in an ambulatory or semi-ambulatory setting.¹³

Anesthesia

General anesthesia provides ideal operating conditions for younger patients, uncooperative patients and for reoperations (when local anesthesia may be less effective).

Peribulbar administration of a 50 : 50 mix of lidocaine and bupivacaine, particularly when used with adjunctive Hyalase, provides excellent anesthesia and akinesia. Its action is not limited to the nerves which travel in the muscle cone or sub-Tenon’s space.

In theory, sub-Tenon’s anesthesia alone should be insufficient (see above). In practice, sub-Tenon’s anesthesia gives anesthesia equivalent to other techniques,¹⁴ presumably due to overspill of anesthetic agents to the peribulbar space. A particular advantage of sub-Tenon’s anesthesia is the ease with which it can be “topped up” intraoperatively.¹⁵ Sub-Tenon’s anesthesia may also be a useful adjunct to general anesthesia¹⁶ both to block the vagus (preventing bradycardia or asystole from rectus muscle traction) and for analgesia in the immediate postoperative period. The presence of buckles and scarring may make this technique more difficult and less effective in reoperations.

Positioning the head for surgery

Surgical access is best when the orbital rim is horizontal. This is achieved by extending the neck slightly and tilting the nose away from the operated eye (Fig. 100.7, available online). Once in the correct position the head may be fixed with a loop of clinical adhesive tape.

Fig. 100.7 Improving surgical access by flattening the orbital rim: (A) extension of the neck; (B) slightly tilting the head.

Preparation and draping

The skin and lashes are cleaned with a disinfecting solution such as povidone iodine. A diluted aqueous iodine solution can be instilled in the conjunctival sac (avoiding the use of undiluted or alcohol-based preparations for this). The skin should be thoroughly dried before application of a sterile self-adhesive drape. Providing these steps are taken it should be possible to tuck the eyelashes under the lid when the lid speculum is introduced and it is therefore unnecessary to trim the lashes preoperatively.

If the palpebral aperture is deemed too narrow a lateral canthotomy may be performed.

Conjunctival peritomy

Neat incisions with later anatomical reposition of the conjunctival edge prevents a number of unpleasant sequelae. Conjunctival misalignment with heaped scars may cause tear film dysfunction. Buttonholing the conjunctiva may lead to Tenon’s prolapse and delay healing. Recession of the peritomy edge leaves bare sclera and makes subsequent reoperation difficult.

A circumferential limbal peritomy with radial relieving incisions is made.¹⁷ The conjunctiva 3–4 mm behind the limbus is grasped with forceps and gently lifted creating a radial pleat of conjunctiva (Fig. 100.8). A blunt-tipped spring scissors is used to make a vertical radial cut. A second cut is often needed to extend the incision through Tenon’s capsule down to the sclera. Great care is taken not to tear the conjunctiva. The spring scissors can easily be used in either hand and the orientation varied as the incision progresses. A gentle spreading action of the scissors under the conjunctiva breaks the weak trabecular adhesions to the episcleral tissue.

Fig. 100.8 Limbal peritomy. (A) The conjunctiva is grasped with a nontoothed forceps and elevated to create a pleat. Radial division of this with spring scissors initiates the peritomy. (B) Spreading action with blunt scissors to break the trabecular adhesions to the sclera. (C) Extension of incision around the limbus. (D) 180° peritomy for segmental buckle.

A slight modification of this technique with the circumferential incision 2 mm behind the limbus leaves a frill that may be useful during closure (Fig. 100.9).

Fig. 100.9 A 180° peritomy with a 2-mm limbal frill of conjunctiva to facilitate closure.

The extent of the peritomy depends on the size of the buckle planned. A 360° peritomy is not required if a local explant is planned. Note that it is not necessary to perform a 360° peritomy purely to sling all four muscles as rectus bridle sutures can be passed transconjunctivally.

Slinging rectus muscles

Between two and four rectus muscles are slung depending on the planned size of the buckle.

A closed pair of blunt scissors is pushed though the intermuscular septum between two recti. The opening created is enlarged by spreading the blades (Fig. 100.10). In this way, the sub-Tenon’s space is opened and bare posterior sclera is exposed.

Fig. 100.10 Opening the intermuscular septum by spreading with blunt scissors. This opens sub-Tenon’s space exposing the posterior sclera.

The muscle is engaged with a sweeping posterior and circumferential movement around the globe employing a specialized muscle hook (Fig. 100.11, available online). Very posterior “sweeps” risk damage to the vortex veins. Once a rectus has been successfully hooked the globe moves with the hook. If the muscle is inadvertently split a second hook can be passed from the opposite side of the muscle. The remaining anterior fibers of the intermuscular septum are now cut off or swept off. This dissection should be limited to the fascia required to visualize the sclera.

Fig. 100.11 Passage of muscle hooks to avoid the obliques – the lateral rectus from above (A) and the superior rectus from the temporal side (B).

A large braided (e.g., 2/0 silk) bridle suture is passed under the muscle. There are various ways of achieving this involving reverse passage of the suture under the muscle (to avoid engaging the tip in the sclera). Alternatively, a modified muscle hook with a threading eyelet at its tip can be used.¹⁸ These bridle sutures can be clipped to the surgical drape to position and stabilize the globe thereafter (e.g., while suturing). When free movement of the globe is desirable (e.g., when searching for breaks) the clips are released.

The sclera is now inspected for dark ectatic areas (Fig. 100.12). Suturing and even cryotherapy or indentation at these sites can be perilous so their early identification is essential.

Fig. 100.12 Scleromalacia. The sclera is examined for any areas of thin sclera at the start of surgery. Surgical manipulation is avoided in these areas because of the risk of scleral perforation.

Reoperations

Special precautions must be taken during revision surgery.

Adhesions are invariably present between the conjunctiva and sclera. A cutting action with the scissors is more effective than a spreading action in breaking these. Care must be taken to avoid delaminating into the sclera or buttonholing conjunctiva.

A fibrous capsule forms around old explants. Incising this capsule exposes the surface of the explant. The plane thereby opened up is useful for slinging rectus muscles. The muscle hooks are placed around the muscles over the explants before these are removed. The rest of the peri-explant capsule is then trimmed flush with the sclera. The sclera is often very thin in the bed of longstanding buckles, especially encircling elements. No attempt is made therefore to dissect capsule off the bed of the buckle. Particular care must be taken when dissecting the capsule from the very thin sclera under the rectus muscles. The scissors are gently lifted away from the globe while closing the blades.

Examination under anesthesia and break localization

A careful indented examination under anesthesia of the whole peripheral retina is now carried out to confirm the location of the retinal breaks. The preoperative drawings provide a useful reference if they are difficult to locate.

The location of each break is marked on the sclera. This essential step is carried out while the cornea is clear and allows planning of the rest of the operation. The sclera is indented under indirect ophthalmoscopic indentation using a fine (but not sharp) tipped instrument such as a Gass scleral indenter.¹⁹ Once the indent is seen to correspond to the position of a retinal break sustained (for several seconds) indentation is applied. The resulting transient scleral thinning produces a focal area of scleral translucency and the underlying choroid shows through. This point is then marked with very gentle diathermy or a surgical marker pen. If a marker pen is used the sclera is dried both before and after the application to prevent the dye spreading. Several marks are made for larger breaks.

Errors in break localization may lead to buckle malposition. Localization errors tend to be radial (i.e., determining how far back a break is) rather than circumferential (determining its clock hour). For example, if a retinal break is highly elevated (as in a bullous detachment) parallax errors may make the break seem more posterior than it truly is (Fig. 100.13).

Fig. 100.13 Break localization error. When breaks are highly elevated, they appear more posterior than they really are due to parallax. A, apparent location; B, actual location.

Parallax errors may be avoided by draining subretinal fluid and then reforming the globe with air (the DACE operation) (Fig. 100.14).^20,21 The view of the retina through the gas bubble can be challenging for those not experienced with this technique.

Fig. 100.14 Bullous retinal detachment: (A) managed with the DACE (drain air cryotherapy explant) procedure. (B) The operation starts with subretinal fluid drainage and air injection. This aids subsequent break localization.

Retinopexy

The indent from the explant closes retinal breaks but retinopexy is required to produce an enduring bond between the retina and the retinal pigment epithelium that will persist even if the indent disappears.²²

Retinopexy was initially achieved using diathermy in association with lamellar scleral dissection and scleral implants.⁴ Cryotherapy has supplanted diathermy because it can be performed without scleral dissection and the treatment can be monitored ophthalmoscopically. More recently photocoagulation has also been used.

Cryotherapy

The technique of cryotherapy is described in detail in Chapter 106, Prevention of retinal detachment. The aim is to produce freezing of healthy retina surrounding all the retinal breaks. The treatment is monitored using the indirect ophthalmoscope. When the indent from the tip of the cryoprobe is seen under a retinal break the cryoprobe is activated. After a few seconds one observes whitening of the retina. Smaller breaks can be treated with a single application. The break is seen as a darker area within the freeze and this is useful in confirming that the whole break has been treated. Larger breaks may need several applications. These are applied by working methodically around the edges of the break to ensure contiguous burns with minimal overlap (Fig. 100.15). Refreezing and freezing of the bare central RPE in large breaks are avoided to reduce the risk of RPE dispersion.²³

Fig. 100.15 Confluent cryotherapy burns with minimal overlap.

One can envisage the immediate tissue reaction as an ice ball expanding outwards progressively in every direction from the tip of the probe for as long as the cryoprobe is active. Some factors (the insulating effect of an intervening rectus muscle, the heat sink effect of the choroidal circulation) impede the development of a visible reaction on the retina. Others (reduced choroidal blood flow in high myopia, the insulating effect of intraocular gas) hasten the development of retinal freezing. The effect of subretinal fluid is particularly important.

In the presence of shallow subretinal fluid, the indentation of the tip of the cryoprobe approximates the pigment epithelium to the retina and both freeze almost simultaneously. If the breaks are more elevated the pigment epithelium cannot be apposed to the retina. Freezing of the pigment epithelium can then be clearly seen to precede freezing of the retina, sometimes by several seconds (Fig. 100.16). In this situation, what is the optimal end point of treatment: freezing of the pigment epithelium or the retina? This is a particularly important question given the concern about potential adverse effects of excessive cryotherapy on other tissues.^24–26 In an experimental model the adhesions produced by freezing of the retinal pigment epithelium alone lacked the microvillous interdigitations normally present between pigment epithelium and retina. The resulting chorioretinal adhesion was weaker than when the freeze was allowed to extend to the retina.²⁷ In practice, the chorioretinal adhesions that develop from freezing of the pigment epithelium alone seem to be sufficient. A further advantage of freezing the retina however, is that the “lighting up” of the retinal breaks is a useful confirmation that all the edges of the break have been treated.

Fig. 100.16 Cryotherapy in the presence of subretinal fluid. Freezing of the retinal pigment epithelium is seen to occur first (A). As the ice ball gets bigger freezing of the retina with lighting up of the breaks (B).

In the presence of bullous subretinal fluid, it may be impossible to approximate the retina to the retinal pigment epithelium. In this situation, subretinal fluid drainage is probably safer than very heavy cryotherapy which may cause severe postoperative vitritis.

The tip of the cryoprobe must be allowed to thaw completely before attempted withdrawal, otherwise choroidal hemorrhage and even scleral avulsion may occur.

Cryotherapy to the disc or macula (Fig. 100.17) occurs when the indentation from the shaft of the probe is mistaken for its tip (“shaft indentation”) (Fig. 100.18). This cognitive problem can be avoided by encouraging trainees to intentionally indent posteriorly (without actually activating the cryoprobe!) (Fig. 100.19). They then become familiar with the distinctive appearance of shaft indentation.

Fig. 100.17 Inadvertent cryotherapy to the posterior pole.

Fig. 100.18 Shaft indentation – the cause of posterior cryotherapy.

Fig. 100.19 The appearance of shaft indentation (right) compared with tip indentation (left).

Beginners find cryotherapy on anterior breaks challenging because the cryoprobe has a tendency to slip over the surface of the eye. This can be overcome with counter-traction from the bridle sutures on the opposite recti (Fig. 100.20). Alternatively, the cryoprobe is intentionally placed anterior to the break and the globe rotated using the tip of the probe. The pressure of the probe is then slightly released while the indent from the probe is viewed using indirect ophthalmoscopy. As the globe returns slowly to the primary position the tip indentation is seen to move. When the indentation of the tip is under the break a very small increase in the pressure applied stabilizes the globe and the cryoprobe is activated.

Fig. 100.20 Additional bridle sutures to stabilize the globe during treatment of large anterior breaks, e.g. a dialysis.

Choice of scleral explant

Knowledge of the location of all the retinal breaks and their elevation allows planning of the rest of the operation including the choice of scleral explant.

The aim of placing an explant is the creation of an indentation in the wall of the eye which provides adequate support to all the elevated retinal breaks. Areas of detachment where there are no breaks do not require support. Breaks in areas of attached retina can be treated with retinopexy alone.

Historically, a number of materials have been used in the manufacture of explants.³³ None of these seem to be as well tolerated as silicone rubber which is biologically inert and can be left in situ indefinitely.^6,34

Two types of cylindrical silicone explant are currently in regular use, solid silicone tires and silicone sponges consisting of air-filled cells (Fig. 100.21). Early silicone sponges had communicating air cells and an increased risk of infection³⁵ but this problem has been much reduced by the use of sponges with closed air cells.

Fig. 100.21 Various types of explant may be used. Solid silicone tires of various sizes and profiles (A–C) are trimmed to the desired size (D–F). Silicone sponges also come in various sizes (G,H) and may have a circular or oval cross-section (I). Watzke sleeves (J) are used with bands (K) and silicone tires to create encirclements (L).

Solid silicone is, for practical purposes, non-compressible. Silicone sponges, because of their cellular composition are easily deformable and compressible. When a sponge is initially sutured the sponge is compressed and the intraocular pressure rises. As the intraocular pressure subsequently falls to physiological levels the sponge expands facilitating closure of elevated retinal breaks. (Details of the mechanism of action of scleral buckles are described in Chapter 99, The effects and action of scleral buckles in the treatment of retinal detachment.)

The height of the indentation produced by an explant is a factor of the bite separation and the diameter of the explant. Silicone tires are generally thinner than sponges and therefore produce lower indents. When used to treat elevated breaks an adjunctive measure such as subretinal fluid drainage or gas injection is more likely to be necessary if a circumferential tire is used.

The profile of the resulting indent also differs. Tires tend to produce broad indentation of uniform, albeit relatively low height. Sponges, particularly those with a circular cross-sectional profile, are much higher centrally. Accurate localization in the transverse meridian is therefore critical when using a sponge with a circular cross-sectional profile.

The design of solid silicone tires reflects their original use as circumferential scleral implants. There is a groove on the upper surface with a rectangular profile to accommodate an overlying band. It was subsequently found that they can be used as explants.³⁶ Some surgeons dispense with the band and use them as purely local explants.

Silicone tires are usually oriented circumferentially. Sponges may be oriented either circumferentially or radially. The geometry of the indent favors radial orientation of sponges when treating retinal tears.² While the choice between tires and sponges is to some extent a matter of surgeon preference their different physical properties favor their use in different situations depending on the number, size and location of retinal breaks. This can be illustrated by some examples:

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