Tenon's Cyst Formation, Wound Healing, and Bleb Evaluation

Needling.

Using a needle and adjunctive compounds including antifibrotic medications and steroids to treat Tenon's cyst is well established.^37,38 This is performed in carefully selected patients by some surgeons at the slit lamp in the clinic, or alternatively under general or regional anesthetic. General anesthetic (GA) is most appropriate if the patient is a ‘squeezer’ or has difficulty with being still; all of the disadvantages of a GA are present, but excellent control of the eye and procedure is afforded, and the lack of iatrogenic subconjunctival fluid (as might occur following sub-Tenon's or peribulbar anesthetic) results in a great view of bleb extension in response to breaking the barrier to aqueous flow.

Pretreatment with weak adrenergic agonists, such as adrenaline 0.1% or alternatively a 50% solution of 2.5% phenylephrine in balanced salt solution or tetracaine, will constrict conjunctival blood vessels and decrease the risk of subconjunctival hemorrhage. Blood in the bleb area is not only distressing for the patient, but will also contain cytokines that are likely to promote bleb fibrosis and increase risk of surgical failure.

The general approach to needling a Tenon's cyst is to enter the conjunctiva some distance from the trabeculectomy site while avoiding blood vessels, advance the needle tip in the loose subconjunctival tissue towards the sclerostomy, and to then incise or perforate the posterior wall of the cyst with the needle tip (Fig. 78-1-4 and Video 78-1). There are multiple subtle variations according to individual preference: some surgeons prefer to make an individual puncture in the cyst wall with the idea that bleeding may be less likely, some like to make multiple punctures, and others prefer to make a large linear cut by moving the bevel of the needle in a sweeping motion. For needling of bleb encapsulation where the blockage to aqueous flow is peripheral to the scleral flap, it is not usually necessary to enter the anterior chamber with the needle tip, which improves the safety of needling in this situation. A conjunctival suture is generally not required, since the small entry site is well away from the sclerostomy and closes quickly.

Many surgeons feel that a smaller needle, such as 29-gauge or 29.5-gauge, gives sufficient flow while minimizing the risk of overdrainage and hypotony. Others may feel that a larger aperture resulting from a 23-gauge to 25-gauge needle is more likely to stay open, especially in the cases of a very thick encapsulation. Although literature on these decisions is sparse, there is evidence to suggest that if the IOP is below 12 mmHg the day after the needling, the needling is more likely to succeed.³⁸

It is usual to give adjunctive treatment with needling procedures. Most surgeons will give subconjunctival dexametasone and/or 5-FU; some use MMC, typically transconjunctivally. Another suggested alternative is use of 1 mg of bevacizumad (avastin) with needle revision.³⁹ Given the high pH of 5-FU, great care must be taken to avoid this entering the anterior chamber; 5-FU should be injected slowly with the needle tip well away from the sclerostomy. The anterior chamber should be observed during 5-FU injection and if 5-FU is seen to enter the anterior chamber in bulk, it should be immediately irrigated with balanced salt solution via a paracentesis.

The author prefers to use Healon GV (HGV) loaded into an insulin syringe, with an approximately 135° bend 6–10 mm from the bevel of the 29-gauge needle. This syringe and needle combination works well from a mechanical perspective, giving good access and angulation while allowing a surgeon with average hand size to perform the needling and depress the plunger with one hand. The HGV viscodissects adherent Tenon's and temporarily maintains the bleb space, prevents leakage of 5-FU from the needle entry to the ocular surface, avoiding epithelial toxicity, and the HGV–5-FU combination forms a depot of slow-release 5-FU.

Complications from needling procedures are relatively less than those of full incisional surgery to manage IOP, and include subconjunctival and anterior chamber hemorrhage, inadvertent buttonhole via needle tip perforation of the central bleb, accidental damage to the lens or corneal endothelium, hypotony, corneal epithelial toxicity from repeated 5-FU injections, and a low risk of infection. By far the most common complication is failure to control IOP, although needling procedures can potentially be repeated several times and the chances of success may be cumulative.

An alternative approach of using Nd: YAG laser instead of a needle to perforate the Tenon's encapsulation was described by Ofner and Smith in 1988,⁴⁰ but is not widely used.

Surgical Excision.

Opening the conjunctiva and surgically removing the dome-shaped encapsulation tissue should be considered a procedure of last resort in somebody who has vision-threatening visual field loss, an uncontrolled IOP, and has not responded to other management options. The usual technique is to elevate the conjunctiva and dissect it free from the Tenon's cyst surface (the author prefers a limbal incision and fornix-based conjunctival flap approach, although posterior approaches are also described and effective⁴¹), open and remove the top of the dome, and then close the conjunctiva in a watertight fashion. This approach may also remove resistance to aqueous flow and result in loss of IOP control, hence post-procedure hypotony and the complications that may result are common. Tenon's cyst may also recur following excision.

Reepithelialization.

As conjunctiva is resutured to cover the sclerostomy and subconjunctival tissues at the end of surgery, healing occurs by primary intention at the conjunctival–corneal interface in trabeculectomies with fornix-based conjunctival incisions. Although these defects are small, they can be of important clinical significance, as insufficient healing at this interface is a common cause of postoperative bleb leak.

Reepithelialization of the wound begins within hours of tissue injury and involves epithelial cell migration over the wound edge. First, the conjunctival epithelial cells differentiate into a more motile phenotype. Modifications include the loss of hemidesmosomal links between the epidermis and basement membrane,¹¹ alterations in integrin expression,¹² and the formation and assembly of intracellular α smooth-muscle actin filaments.¹³ After an initial lag of 1–2 days, epithelial cell proliferation will occur at the wound edge and provide more cells for migration.

Granulation Tissue Formation.

Granulation tissue formation is initiated by the release of growth factors synthesized by platelets, injured cells, and macrophages. The new matrix consists of loose connective tissue, fibroblasts, new blood vessels, and macrophages. Macrophages now secrete cytokines to stimulate fibroplasia and angiogenesis, whereas fibroblasts remodel the extracellular matrix to aid cell migration and proliferation. Angiogenesis leads to new blood vessel formation and provides necessary oxygen and nutrients for cell metabolism.

Angiogenesis.

Angiogenesis is initiated within days of wound formation in response to low oxygen tension and lactic acid build-up that follows from tissue trauma and surgical manipulation. Vascular endothelial cells proliferate to produce capillary buds, which network to form a capillary bed. Proangiogenic factors including vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) are secreted by inflammatory cells, particularly macrophages and platelets. Blocking antibodies to these two growth factors almost completely inhibits wound angiogenesis.¹⁴

Fibroplasia.

Fibroplasia is the deposition of extracellular matrix by large numbers of fibroblasts that have entered the wound site. Fibroplasia is initiated by growth factors, such as TGF-β and PDGF, that stimulate extracellular matrix production, fibroblast proliferation, and the differentiation of fibroblasts into myofibroblasts. Not all fibroblasts undergo differentiation, as in vitro studies have shown that successful fibroplasia depends on the presence of the fibroblast, rather than the myofibroblast.¹⁵ Myofibroblasts show altered responses to cytokines and are characterized by the presence of intracellular microfilament bundles containing α smooth-muscle actin (ASMA), the actin isoform expressed by smooth muscle cells.¹⁶ Myofibroblasts assist in wound closure by wound contraction and extracellular matrix production.

In addition to chemotaxis to soluble factors, fibroblasts are attracted to the wound by contact guidance and haplotaxis. Haplotaxis is the process of fibroblast migration up a surface-bound adhesion gradient, while contact guidance is migration along discontinuities in the matrix to which they are attached. This highlights the interdependence between fibroblasts and the extracellular matrix. While fibroblasts synthesize and remodel the extracellular matrix, the matrix in turn regulates fibroblast motility.

Fibroblast movement into and through the fibrin clot requires proteolytic cleavage of the extracellular matrix. As fibroblasts migrate over the fibronectin interface, traction forces are established with the underlying substrate inducing wound contraction. This process is facilitated by the release of matrix metalloproteinases (MMP).¹⁷ This family of enzymes degrade extracellular matrix, providing a ‘path’ for fibroblast movement. MMPs are secreted as inactive proenzymes that require activation to initiate extracellular matrix degradation. In addition, tissue inhibitors of matrix metalloproteinases (TIMPs) inhibit MMPs. The ratio of MMP to TIMP determines the balance between tissue degradation and extracellular matrix synthesis.

Fibroblasts bind to various components in the matrix, which include fibrin, fibronectin, and vitronectin through the use of cell-binding domains of the matrix proteins via integrins.¹⁸ Syndecans, transmembrane heparin sulfate proteoglycans, can also mediate cell–matrix interactions via the heparin-binding domain of fibronectin.¹⁹ This binding domain is structurally distinct from the integrin domain. These and other cell-binding domains collectively regulate the strength of cell binding and hence cell motility.²⁰

Collagen production is stimulated by the autocrine action of TGF-β and paracrine interleukin-4 (IL-4) from mast cells.²¹ Collagen is the major component of the wound matrix in animal models of glaucoma filtration surgery.^2,22

Wound Remodeling.

Extracellular matrix (ECM) remodeling defines the process where coincident ECM synthesis and breakdown refine the wound matrix. Plasminogen activators and matrix metalloproteinases are the chief mediators of ECM degradation. Remodeling initially consists of hyaluronan and fibronectin removal. Maturation of the remodeling phase is associated with deposition of proteoglycans and the replacement of collagen type III with collagen type I, which occurs synchronously with a reduction in fibroblast numbers. This reflects the transition of a matrix favoring cell proliferation and migration to that of high tensile strength and resistance to deformation.

Apoptosis.

A key event in the conversion of highly cellular granulation tissue to a less cellular scar is fibroblast death by apoptosis. Fibroblast apoptosis has been demonstrated in skin, kidney, and lung wounds.^23,25,26 The molecular pathways that induce fibroblast apoptosis are not fully understood. A reduction in mechanical tension of connective tissue has been proposed. Grinnell and colleagues have demonstrated that the release of mechanical tension is a trigger for fibroblast apoptosis in cell culture experiments. Fibroblasts in a mechanically loaded collagen matrix demonstrated little apoptosis whereas apoptosis was rapidly induced when the tension within the gel was released.²⁷ The antifibrosis agent mitomycin C has also been shown to induce Tenon's fibroblast apoptosis in cell culture, suggesting that the induction of fibroblast apoptosis may be a mechanism whereby this cytotoxic agent limits scar formation post trabeculectomy.^28,29

Precise regulation of fibroblast death is a critical factor in the terminal stages of the healing response.²⁹ Fibroblast apoptosis must not occur too prematurely to ensure that the defect is repaired, but also should not be delayed in order to limit functional impairment that would result from excess scar formation and wound contracture.

Apoptotic failure results in prolonged fibroblast survival and excess scar production.²⁴ Analysis of biopsies taken from keloid and hypercellular scars in human skin revealed persistent fibroblastic activity and inflammation for up to 10 years following the original surgery. This differed from normal scars, where biopsies taken 1 year after wound formation displayed minimal evidence of fibroblast activity.³⁰ In contrast, excessive or premature fibroblast apoptosis can result in insufficient wound healing. The impaired healing observed in full-thickness skin wounds of non-obese diabetic (NOD) mice was associated with higher levels of fibroblast apoptosis compared to identical wounds in control mice.³¹ Fibroblast apoptosis has not been demonstrated directly in subconjunctival wound healing in vivo. The dramatic reduction in fibroblast numbers observed in a rabbit model of glaucoma filtration surgery, 31 days after surgery, may be a consequence of apoptosis.²²

Vascularity.

Increased bleb vascularity, especially in the first weeks after surgery, has been associated with surgical failure.^33,34 Increased vascularity results from inflammatory mediators, dilation, and proliferation of local vascular networks. In the current literature, bleb vascularity usually refers to a general assessment of vascularity over the central bleb area. It is, however, becoming clear that specific regional areas of increased vascularity are also important: for example, hypervascularity of the tissues surrounding the central bleb area, 6 weeks after trabeculectomy with limbus-based conjunctival flap, was associated with a sixfold increased risk of bleb failure.³⁵

Corkscrew Vessels.

The presence of corkscrew vessels (Fig. 78-2-4) in the early weeks following glaucoma filtering surgery denotes an increased risk of surgical failure by scarring,³³ and this may be independent of the degree of vascularity.³⁶ Although the process is not well understood, it is likely to involve contraction of Tenon's tissue caused by migration of fibroblasts,^37,38 which produces a concertina effect on an existing, often dilated, vessel. Corkscrew vessels therefore probably signify recruitment of fibroblasts to the bleb, ongoing wound healing, and contraction of the bleb space, all of which may lead to bleb failure.

Dragged Vessels and Conjunctival Suture Line Contraction.

Dragging or straightening of conjunctival vessels (Fig. 78-2-5) also represents fibroblast-mediated tissue contraction.³⁷ The dragged vessels usually appear peripheral to the bleb space and the advancing line of fibrosis anterior to them typically decreases bleb size and function. In blebs formed from a limbus-based conjunctival flap, with a prominent and inflamed suture line, contraction of the suture line (Fig. 78-2-6) heralds shrinkage and possibly disappearance of the bleb. This process is slow, and unless efforts are made to recognize it by taking measurements or photographs, is typically not noted; if the bleb is thin-walled, IOP will remain low until very late in the process.

Elevation.

With the exception of flat or nonexistent blebs, lower blebs tend be associated with better long-term outcomes. A low bleb with large surface area without focal thinning or demarcation by fibrotic tissue tends to give excellent IOP control; hypovascular thin-walled ‘cystic’ blebs are undesirable as discussed below. In Tenon's cyst, or bleb encapsulation, with thickened bleb walls, bleb height is typically increased,³⁴ occasionally to dramatic levels. The amount of elevation may or may not reflect IOP, but in a significant proportion of encapsulated blebs the IOP is raised because aqueous cannot easily diffuse through the thickened bleb wall, and this may be associated with poor bleb outcomes.³³ The contour by which the maximum elevation is reached also has an influence on the success of surgery. A steep contour at the anterior edge of the bleb may produce areas of bleb-edge tear film bubbles, best seen immediately after a blink (Fig. 78-2-7) and these areas of nonwetting can predispose to dysesthesia and dellen formation.

Bleb Area and Extent.

There is a complex interrelationship between bleb wall thickness, transmural permeability to aqueous, the area of the bleb, and IOP. In general, large bleb areas are desirable and associated with good IOP control and a lower risk of long-term complications. There are, of course, exceptions. A small-area, thin-walled bleb with high transmural flow, which at the extreme might become a ‘sweating’ or leaking bleb, will usually produce a low IOP. A very large encapsulated bleb can have a huge surface area, but high resistance to transmural flow and thus high IOP. It is helpful to distinguish between separate bleb areas; traditional descriptions of blebs do not allow for the possibility of mixed-morphology blebs,^39,40 such as a central thin-walled demarcated area surrounded by a large area of diffuse conjunctival elevation (Fig. 78-2-8). Whereas the localized or focal parts of the bleb are easy to discern, the full extent of diffuse conjunctival elevation may be more difficult. Using ultrasound biomicroscopy to measure the total area of the bleb, Jinza et al.⁴¹ reported an inverse relationship between bleb area and IOP. Blebs that extend circumferentially to the inferior bulbar conjunctiva (Fig. 78-2-9) are relatively common in recent times when antimetabolite use has become more widespread. These rarely result in any significant complications but frequently produce a mild foreign body sensation, and patients may notice that their eyes appear ‘wet.’ These eyes are also prone to spontaneous subconjunctival bleeds.⁴² Blebs may also extend onto the cornea. Such blebs may be asymptomatic but can also lead to dysesthesia and foreign body sensation (Fig. 78-2-10). Corneal blebs tend to be thin-walled and well demarcated posteriorly, typically with a scalloped anterior edge.

Wall Thickness.

Wall thickness is inferred from the degree of translucency or transparency. Wall thickness determines the amount of transconjunctival flow. Blebs that appear to be less translucent tend to have lower transmural outflow facility and higher IOPs, but this is not an absolute relationship³⁶ since the arrangement of connective tissues should also alter permeability, as proposed by Maumenee in the 1960s.⁴³ Few blebs have uniform wall thickness, so resistance to transmural flow will vary across the bleb. Aqueous flow will take the route of least resistance and most will pass through the most permeable part of the bleb area. It is not known whether thicker parts of the bleb with less transmural flow and therefore less exposure to potential pro- or antifibrotic factors in the aqueous are more likely to fibrose, and similarly whether thinner areas might be exposed to more wall tension and different cytokine compositions, which may therefore tend to further thin or reinforce the wall in this area. Whatever turns out to be the case, it is well known that blebs tend to remodel significantly over time.

Avascular Areas (‘Cystic Blebs’).

Although much of the literature in recent times has concentrated on the role of antimetabolites in producing thin-walled, leaking blebs, surgical technique also plays a significant role.⁴⁴ Thin and avascular, or ‘cystic,’ bleb areas were common in full-thickness procedures (Fig. 78-2-11) prior to trabeculectomy, long before antimetabolite use entered glaucoma surgery.⁴⁵ The importance of recognition of avascular and thin areas is that they predispose to many of the long-term complications of initially successful trabeculectomy including bleb leaks (Fig. 78-2-12), hypotony, dysesthesia, and bleb-related endophthalmitis (Fig. 78-2-13). Examination of these blebs over time, for example by serial photographs, shows that they are far from static and are continually remodeling themselves. The processes that drive remodeling are not well understood but mechanical stress,⁴⁶ MMP activity,³⁸ and fibroblast apoptosis probably all contribute.^28,31

Subconjunctival Blood.

Blood contains multiple inflammatory mediators, and provokes inflammatory and healing responses all over the body. Blood is a nearly ubiquitous finding in the subconjunctival space immediately following glaucoma filtering surgery but minor blood presence is thought to have minimal impact on outcome. One might expect the impact of minor peri- and intrableb subconjunctival hemorrhage to be trivial, since cytokines and other mediators are not continuously released and there is dilution from aqueous flow. This is supported by a study by Sacu et al.³⁶ Larger volumes of subconjunctival blood, enough to form a clot and persist, may provoke more local tissue response, provide a depot of inflammatory mediators, and also provide a scaffold for fibroblasts to bridge the sub-Tenon's space. Recurrent inferior subconjunctival hemorrhages in patients with blebs that extend to the inferior bulbar conjunctiva are potentially concerning but usually benign and self-limiting.⁴²

Microcysts.

This is the sign most commonly associated with good bleb function (Fig. 78-2-14), and almost certainly reflects transconjunctival aqueous flow.^47–49 Satisfactory IOP control and the presence of microcysts are consistently associated in the literature.^33,36,50 In some very low, homogeneous, and diffuse blebs, the only signs of bleb function may be the presence of microcysts and the detection of a tiny optically empty space using a high-power, very narrow slit beam. Microcysts are usually not distributed evenly throughout the entire bleb area, which may reflect local variations in transmural flow. Although the relationship between bleb function and microcysts is strong, not all well-functioning blebs have visible microcysts, and microcysts can still be present when the bleb is underperforming and the IOP is very high. Microcysts are evidence of transconjunctival filtration and hence flow, but are not evidence of adequate bleb function.

Bleb Leak.

Bleb leaks are best detected by a variant of the Seidel test. The basis for this is that concentrated fluorescein, for example 2% solution or a moistened strip, does not fluoresce under cobalt-blue light, but dilute solutions will. After instilling topical anesthetic drops, the authors prefer to use a fluorescein strip, slightly dampened with more topical anesthetic and, holding the upper lid up in downgaze to expose the bleb, carefully apply the flat of the strip to suspect areas of the surface. In this way multiple leaks will not be missed, and mislocation of the leak because of high flow and intermittent ‘flooding’ can be avoided. Alternatively, a drop of 2% fluorescein can be applied, with the bleb surface well exposed, and the area observed for fluorescence as aqueous dilutes the high concentration of fluorescein in the tear film. If the IOP is very low, lack of flow may mask a leak; gentle digital pressure can produce some flow through the leak and make it visible, although low IOP is not just a result of a bleb leak, as discussed below.

Early Bleb Leak.

The viability of the bleb depends, at least in part, on its inflation by aqueous flowing from the anterior chamber, and so a watertight conjunctival closure at the end of surgery is imperative. Leaks from the wound in the first days after surgery provide a low-resistance alternative to aqueous egress via the sub-Tenon's space. If the leak allows bulk flow of aqueous, the rest of the bleb can collapse, allowing apposition of the inflamed inner bleb walls, which are likely to adhere. If such adherence is over most or all of the bleb area, bleb failure is probably inevitable. Most of the time, the leak is small enough that the bleb space is maintained, and these can be managed conservatively with good results, although early leak may be a risk factor for subsequent bleb failure (see also Chapter 75).⁵¹ Hypotony can occur in the presence of bleb leak, in particular where the scleral flap does not provide adequate resistance to flow.⁵² Early resistance to flow in the first weeks after surgery is largely due to resistance at the level of the scleral flap and related to scleral suture tension. However, if there is bulk flow from the anterior chamber because of poor scleral flap construction or closure, a weak conjunctival closure will often leak also; thus, hypotony is often seen in association with early postoperative bleb leaks. These are often confused. The decision to revisit the scleral flap should be made on IOP levels, and the decision to resuture the conjunctiva should be made on bleb elevation and extent.

Late Bleb Leak.

Bleb leaks after the first few weeks may rarely occur from unusual causes, such as conjunctival erosion over a retained suture end, but are almost universally from thin and avascular areas, usually referred to as cystic blebs (see also Chapter 81). The leak may be relatively slow, with the Siedel test appearance of a ‘sweating’ bleb, or fast enough for the patient to complain of intermittent or continuous watering. Once the fistula from the anterior chamber to the bleb is well established there is usually only a little resistance to flow into the bleb, so leakage from blebs late after surgery may result in hypotony. Bleb-related endophthalmitis, which may be rapid and devastating, is associated with late bleb leaks and thin bleb walls.