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Intraocular pressure (IOP)-lowering operations

In the field of surgical medicine, glaucoma surgery is something rather unique. Normally, a patient undergoing surgery seeks relief from pain, tumors, stones or other problems that are surgically correctable; he expects to be better off rather directly after the operation. A cataract patient, for instance, can be quite confident of gaining a better visual acuity following the lens removal.

In glaucoma, the situation is completely different: A glaucoma patient is usually not disturbed by his condition. Either his visual field defects are small or he has not noticed them at all. Nevertheless, he is expected to undergo an operation that will not result in better, but perhaps even in poorer vision. In particular, visual acuity might be mildly reduced for a short or perhaps even longer period. This situation requires a much more thorough patient education than other surgical procedures. It has to be made clear that the goal of the operation is not to improve the current visual function but rather to preserve vision and especially the visual field.

Education

The patient requires a thorough explanation of the benefits and the risks of the operation. Some aspects discussed here are the same topics covered in such an education. However, it should be stressed that this book is in no way a substitute for a personal discussion between the physician and patient. Each individual patient has unique concerns and each physician has his own methods and experiences. This book is meant to provide needed and tangible support for this crucial interchange, and to assist the patient in asking those questions that are of particular importance to him.

During this consultation, a decision has to be made regarding the appropriate time for the operation or whether it will be an out-patient or an in-patient procedure. Other points of discussion are the type of anesthesia that is recommended by the physician and acceptable to the patient. Compromises here are often possible. Other concerns include how the glaucoma should be treated until the day of surgery, and how frequently the patient should undergo post-operative examinations and who should perform them.

A Second Opinion

It is not uncommon for a patient to consult with another expert before consenting to an operation. Physicians completely understand this. Getting a second opinion is not considered a "vote of no confidence" to the treating ophthalmologist, but rather it may help both the patient and treating physician feel comfortable with the anticipated therapy.

The Goal of Surgery

Unfortunately, it is not possible to remove glaucomatous damage, therefore, the operation's goal is "only" the reduction and stabilization of intraocular pressure. If this goal is achieved, the likelihood for progression of visual field damage is considerably reduced. Following any procedure performed, frequent and constant examinations evaluating IOP, optic disc morphology and visual field remain crucial because progression is still possible, even when the IOP is normal. Other risk factors (Risk Factors for Glaucomatous Damage) must also be considered.

Indications of Surgery

As a general rule, operations are performed whenever the IOP is markedly increased even though medical treatment is administered, or when glaucomatous damage progresses in spite of conservative therapy. Another reason to consider surgery is intolerance to medical therapy. There are exceptions to these general rules: the patient's age, his general health, the condition of the other eye and the individual form of glaucoma have to be considered when making a decision about surgery. As has been seen (Glaucoma Associated with Pseudoexfoliation Syndrome), an eye suffering from pseudoexfoliation syndrome is more likely to be operated on at an earlier stage than an eye afflicted with Primary Chronic Open-Angle Glaucoma (POAG) . It is crucial that the patient accept the possible disadvantages associated with the surgery because of the accompanying long-term benefits.

Preparing for Surgery

The patient is requested to contact his primary care physician, who is asked to send the ophthalmologist a report on the patient's general health, including results from blood tests taken in the past and any special conditions that the patient has. The patient takes his glaucoma medication as well as any other drugs prescribed by his treating physician until the day of the operation. Whether or not these other drugs should be taken on this day is determined by the anesthesiologist. The administration of blood thinners (anti-coagulants) should be stopped, if possible, ten days before the operation. Sometimes this is not possible and requires a discussion between the surgeon and the treating physician to decide the best course. An operation while the patient is still under the influence of these drugs is basically possible, but requires certain precautions.

Out-Patient or In-Patient

If he feels well enough, the patient can get up immediately after the procedure. This leads to the question of whether the operation should be performed on an out-patient or in-patient basis. Out-patient procedures are safe and effective for a number of indications in ophthalmology, such as cataract surgery. In glaucoma surgery, the out-patient approach is possible, though the authors feel that it should not be recommended. Caring for the patient immediately after the procedure is as crucial for the long-term prognosis as the operation itself. Therefore, a hybrid protocol is recommended: The patient stays in the hospital but is free to go home on the first or second post-operative day if there are no complications. If problems do arise, the patient stays in the hospital for a few extra days; fortunately, this is a rare event.

Advantages of Surgery

If the operation is successful and leads to a decrease and stabilization of the IOP to an acceptably low level, the patient no longer needs medical therapy. Unfortunately, such an ideal situation does not always occur after an initial operation. Additional operations are occasionally necessary and always a possibility. Lowering and stabilizing the IOP significantly enhances the long-term prognosis for the visual field.

Disadvantages of Surgery

The patient has to invest a considerable amount of time in preoperative examinations, the operation itself and in those check-ups that are necessary after surgery. A transient, but sometimes even a permanent slight reduction in visual acuity is possible. In rare cases, a lens opacity (cataract) can develop more rapidly after glaucoma surgery than in a healthy eye. Cataracts, however, can be removed without creating any major problems for the patient.

By far the most common problem is a renewed increase in the intraocular pressure. A rise in the IOP, often slow to develop, can occur after months or even years. Since the introduction of mitomycin C (see below), this problem has become increasingly rare. The patient has to be aware, however, that sometimes IOP-reducing medications or another operation might become necessary. There is usually a relatively good prognosis after a second or even third operation. The IOP could also be too low after surgery; the corrective measures that have to be taken in this case are discussed in Complications of IOP-reducing Operations .

In many hospitals, an anesthesiologist is always present during the operation, even though the procedure is usually performed under local anesthesia [Gr. aisth?sis: sensitivity/Gr. anaisth?sis : insensitivity]. Before the operation, this specialist discusses anesthesia and general medical aspects with the patient.The authors usually operate under local anesthesia. General anesthesia is usually only required for children. Some patients express an initial preference for general anesthesia, but after discussing the advantages and disadvantages, they usually decide on local anesthesia. Even though both provide a pain-free operation, the local anesthesia is less cumbersome for the patient as a whole. Taking a sedative shortly before the procedure helps many patients.

After the operation is finished, patients often report that they had expected the procedure to be more painful and that they would no longer be afraid should a second operation become necessary. In uncomplicated cases, retrobulbar anesthesia is performed; if glaucomatous damage is advanced, subconjunctival anesthesia is preferred. During the procedure, the patient is able, and even requested, to relate if and when he suffers discomfort. The surgeon or the anesthesiologist can easily inject or instill additional anesthetic.

Depending on the individual situation, the patient may receive a preoperative sedative from the anesthesiologist. Immediately before administering the local anesthesia, the specialist will give a drug intravenously which will reduce anxiety and discomfort within seconds or a few minutes. This makes the injection of the local anesthetic into the eye less unpleasant.

In retrobulbar anesthesia, a drug is injected into the orbit, bypassing the eyeball

Fig. 7.13: In retrobulbar anesthesia, a drug is injected into the orbit, bypassing the eyeball

In subconjunctival anesthesia, the drug (local anesthetic) is injected beneath the conjunctiva

Fig. 7.14: In subconjunctival anesthesia, the drug (local anesthetic) is injected beneath the conjunctiva

Figure 7.13 shows the principle of retrobulbar anesthesia; and Fig. 7.14, the principle of subconjunctival anesthesia. In retobulbar anesthesia, the eye muscles are immobilized, the optic nerve is numbed (the patient no longer sees anything) and, most importantly of all, the fibers transmitting pain are switched off for a while: they no longer transmit their signals to the brain.

Subconjunctival anesthesia renders the area of operation completely painless. Some of the anesthetic diffuses backwards through the tissue and partially incapacitates the muscles. The patient is still able to see but will not be disturbed since the eye is rotated and fixated in a downward looking position. The patient is not blinded by the bright lights above the operating table.

It was once assumed that the use of general anesthesia might be the best way to protect the optic nerve during the operation; however, subconjunctival anesthesia is now considered the safest method.

Patients often worry that they won't be able to keep the eye still during the operation; this is not a problem. The chance of an unintentional eye movement is small due to the effect of the anesthetic and the fact that the eye will be fixed by a suture. Even if the patient does move a bit during the procedure, this hardly constitutes a threat to the operation. When the patient feels the urge to move - such as before sneezing - he should tell the surgeon who will simply remove his instruments from the eye for a moment.

The goal of all IOP-lowering operations is to enhance the aqueous humor outflow. There are numerous techniques that cannot be discussed in detail here. What is important is to know that these are not techniques that are either intrinsically good or bad; but rather the technique is chosen and adapted to the needs of the individual eye, while at the same time, corresponds to the personal experience, skill and training of the surgeon. The procedure, so to speak, is a surgeon's trademark signature. The authors' method of choice for an IOP-lowering procedure is a modified trabeculectomy, a procedure discussed more elaborately below. Figure 7.15 shows the principle of trabeculectomy. The aqueous humor is directed through an artificial opening beneath the conjunctiva; this leads to the formation of a "bleb," depicted in Fig. 7.16.

In trabeculectomy, a new outflow (under the conjunctiva) is created for the aqueous humor

Fig. 7.15: In trabeculectomy, a new outflow (under the conjunctiva) is created for the aqueous humor

The bleb is usually covered by the upper lid

Fig. 7.16: The bleb is usually covered by the upper lid. On the picture at the right, the conjunctiva, which has been soaked with aqueous humor (bleb), is indicated by the circle

For this procedure, the skin surrounding the eye is first sterilized and draped with sterile cloths; next, a lid retractor is put in place. This keeps the eye open throughout the entire procedure. The eye is then fixed to the retractor by two sutures (so-called Bridle sutures), which are placed at the limbus, the transition zone between the cornea and sclera. By rotating the eye downward, the patient is not disturbed by the bright light of the operating lamp and the surgeon has a perfect view of the surgical field.

Both the conjunctiva and the underlying Tenon's capsule are lifted with a forceps and incised with scissors. (Tenon's capsule is a slightly movable and extensible connective tissue layer between the conjunctiva and sclera.) Both layers are gently pulled back, which exposes the sclera. Should a second operation become necessary, this is the area where the surgeon will encounter adhesions that will have to be dissected with scissors. Next a small plastic sponge is soaked with mitomycin C and placed between Tenon's capsule and the sclera. The duration of this application depends upon a host of factors, as discussed later in this chapter. After the sponge's removal, the involved area is thoroughly irrigated, ensuring that the mitomycin C has been completely removed.

The conjunctiva is then fixed with the Bridle suture, thus exposing the sclera. In case of bleeding, the small vessels are coagulated by diathermy, an instrument at whose tip electricity is transformed into heat. The hemorrhages are not at all dangerous; they just make the surgeon's job a bit more difficult.

Next a corneoscleral tunnel is prepared using a special diamond scalpel; the sclera and cornea are split into two different layers. The inner wall of this tunnel is opened with a punch instrument.

A tunnel is thus created which provides direct access to the anterior chamber and whose outside remains covered by a scleral layer. Pushing a small forceps gently though this tunnel, the iris is held in place and a small part is cut off with scissors. This is called an iridectomy (see Acute Angle-Closure Glaucoma ).

Then the outward opening, the scleral flap, is closed with sutures. This suture is necessary to keep the IOP from going down to zero immediately after the operation (before the wound has started healing), sometimes even keeping it rather high. This is meant to prevent the numerous complications that come with a sharply reduced intraocular pressure, also called ocular hypotony. The sutures generally used are made of resorbable [Lat. sorbere: to swallow] material (meaning they are broken down and gradually digested by the body). After 10 - 15 days, these sutures gradually resorb, thereby spontaneously further opening the tunnel. Two or three sutures using material that is not resorbable are also made. These keep the tunnel relatively closed after the other sutures have dissolved, thus preventing the IOP from dropping to extreme lows. If, on the other hand, the IOP remains elevated after weeks or months, the non-resorbing sutures are cut with the tip of a needle or with a laser, thereby evoking a reduction in the intraocular pressure.

Following these steps, Tenon's capsule and the conjunctiva are separately closed with a resorbing suture in a continuous fashion. The Bridle sutures are then removed. The patient is given local steroids to prevent post-operative inflammation, antibiotics to minimize the risk of infection and atropine, an effective preventive measure against ciliary block, which works by relaxing the ciliary muscle (see Ciliary Block Glaucoma ). If a bandage is placed on the eye, this is only done on the first day.

Alternative Approaches

As mentioned, there are many different types of trabecular surgery, and these differ to varying degrees from the method just described. All these approaches have their merits but describing them here is not within the scope of this book. Many surgeons prefer, for instance, the classic trabeculectomy where a scleral flap is created. Under this scleral flap, an opening into the trabecular meshwork is cut with a scalpel or scissors. The authors do not perform this procedure because of the higher incidence of extremely low IOP experienced during the initial post-operative days, and the increased corneal astigmatism encountered long-term. Another approach is to remove the inner layer including the trabecular meshwork with a trepan, a procedure called goniotrepanation [Gr. trypanon: drill].

There are other ophthalmic surgeons who prefer the deep sclerectomy. During this procedure, there are no penetrations into the anterior chamber [Lat. penetrare : to penetrate] and part of Schlemm's canal and the inner layer, the trabecular meshwork, remain in place. Some surgeons implant a piece of collagen between the scleral flap and the inner scleral wall. The advantage is seen in the fact that a deep sclerectomy (with or without a collagen implant) rarely leads to very low IOP levels, and that post-operative visual function is quickly restored. This makes the operation more acceptable to the patients. The considerable disadvantage is a relatively small pressure reduction. Therefore, the increased post-operative IOP might be too high for many patients.When performing viscocanalostomy, Schlemm's canal is "washed out" with a substance of high viscosity. This mechanical opening of the canal likewise lowers the IOP in some patient, but not in all.

Since the goal in most patients is to achieve a relatively low IOP level that will remain low for years to come, the authors rarely use these latter methods.

New techniques are continuously being developed. A high percentage of these variations disappear within a few years, and only a few find a lasting place in glaucoma surgery.

One must distinguish between complications that occur:

a) during the operation,

b) in the first few post-operative days,

c) some time after the procedure.

 

a) Intraoperative complications:

In extremely rare cases, a pronounced arterial hemorrhage within the choroid occurs which is a potential threat to the entire eye. Fortunately, the authors have not seen this type of dramatic complication within the past 10 - 15 years. Other complications are less dangerous, although they do render the operation more difficult and time-consuming, and being of a technical nature, are not discussed here.

b) Complications during the first few post-operative days:

There is sometimes bleeding into the anterior chamber. While this temporarily reduces the visual function for some time, it is basically harmless. There might be an increase in the IOP or even a drop to 0 mm Hg. The IOP rise is no problem if diagnosed and treated in time with medication. The initial IOP gives no indication about how the pressure will develop in the years to come. A drop to zero itself is no catastrophe but requires intense supervision by the ophthalmologist. A shallowing of the anterior chamber must be identified in time since any contact between the lens and the posterior (inward) layer of the cornea has to be prevented.

A much-dreaded complication is "malignant" glaucoma, caused by the Ciliary Block Glaucoma . Since the ophthalmologist can anticipate which eyes are more likely to develop ciliary block, precautions can be taken. In most cases, a ciliary block can be prevented by administering a few drops of atropine post-operatively. During the past ten years, this group has seen malignant glaucoma develop only after one single trabeculectomy, and this problem was immediately solved by quick intervention.

c) Complications within the first three post-operative months:

In the first few weeks and months, the IOP can still fluctuate or remain too high or too low. If, after resorption of the resorbable sutures, the IOP is still dangerously high, the physician will prescribe IOP-lowering drugs and refer the patient to the surgeon who, in most cases, will cut the non-resorbable sutures in order to open the tunnel. Other groups that use the scleral flap technique seek to achieve immediate filtration; they try to avoid any IOP-lowering drugs in the immediate post-operative period. If the IOP is too high, they resort to massage of the eyeball with the desired goal of opening the trabeculectomy.

If there is a tendency to scar, a low concentration of mitomycin C is injected into the bleb. Some other ophthalmologists prefer to inject 5-FU under the conjunctiva far from the bleb. If, on the other hand, the IOP remains too low after two or three months and the patient suffers from this condition, either the patient's blood or an agent of high viscosity is injected into the bleb to initiate the process of scarring. Injections into the bleb are possible at any stage.

Infections of the bleb are always a dreaded consequence, but fortunately, are quite rare. If the eye is red or when ocular pain is present, the patient should immediately consult his doctor to rule out an infection.

Mitomycin C was originally developed as an antibiotic. Today it is primarily used by oncologists (doctor s specializing in tumor treatment) because of its cytostatic effect in cancer therapy. A cytostatic is a drug that is capable of inhibiting cell division (i.e. the multiplication of tumor cells).

In ophthalmology, diluted mitomycin solution is applied to the operation field. In the near-term it reduces inflammation. Even more important, in the long-term, it prevents or at least greatly reduces the chance that the bleb will scar and therefore the IOP will rise.

Even though the use of mitomycin C is extremely effective, it is not without its own dangers. When this drug was first used in ophthalmology, severe complications were reported. However, it was soon shown that these complications occurred especially when the medication concentration employed was too high or it was used for a prolonged period of time. Another danger arises when the drug is not adequately rinsed out after its application. Nevertheless, with the correct dosage and duration, it has proven quite useful. The authors have been using mitomycin C for the past 10 years in very low, but constant, concentrations; the application time is adjusted to the particular situation which, in itself, depends upon the patient's age, the duration and time of the antecedent pressure-reducing local therapy, etc. Especially long application times are required, for example, with neovascularization glaucoma (see Secondary Angle-Closure Glaucoma ); medium-long application is undertaken with, say, inflammatory secondary glaucoma; while very short administration times are used in initial operations on older patients.

Before the age of mitomycin, a long-term increase in the intraocular pressure was observed in about 50% or even more of the operated patients. Indeed, after re-operation, or in special cases, such as young patients or patients of African decent, the long-term prognosis was even poorer. Since the introduction of mitomycin, the pressure prognosis after an operation has significantly improved.

Because post-operative monitoring is crucial to the outcome of the operation, it is advantageous if the surgeon himself can perform the first and even second post-operative check-ups.At the authors' clinic, the operation is usually performed in the morning; the first check-up takes place on the same afternoon, and then, as needed, once again in the same evening. The intraocular pressure is measured; if it is increased, this is not dangerous in light of the operation just performed. However, to prevent damage to the optic nerve from occurring, the pressure is normalized using medications. On the other hand, if the pressure is too low, frequent eye examinations are necessary. If the anterior chamber has flattened and the iris and lens are tending toward the cornea, special procedures must be undertaken because the lens should never come into contact with the posterior corneal surface.

After the operation, local antibiotics are prescribed. Additionally, in the first two to three days, atropine is also administered, and during the first weeks, drugs to reduce inflammation are also given, the doses depending upon the degree of eye inflammation. One must consider that locally administered steroids can reversibly increase the IOP, even after an operation (see Additional Causes ). Therefore, we often use non-steroidal anti-inflammatory agents.

If the eye pressure is still too high several weeks after the operation or if it is normal but only with concomitant use of pressure-reducing medications, then the non-resorbable sutures on the corneoscleral tunnel are cut. If the pressure still remains high or again increases after weeks or months, then the conjunctiva is injected with a highly diluted mitomycin C solution. The authors have had good results with this procedure. Occasionally adhesions can form that require additional types of drugs or another operation.

If the pressure remains too low for a longer period, then the bleb is injected with either the patient's own blood or a highly viscous gel. As long as the pressure remains low, vision is significantly impaired, however, after pressure has returned to normal, in almost all cases the sight soon follows.

Activities after the operation

There used to be strict guidelines that the patient had to follow after a glaucoma operation. However, with today's advanced techniques, the risks have become much smaller. The patient is usually told that he can go back to leading a normal life a few days after the operation. Depending on the visual function, a patient might not yet be able to drive a car (this should be discussed with the ophthalmologist). Showering and hair washing are permitted after about the 2nd or 3rd post-operative day. For those patients who work, when they can return to work is naturally a primary concern. This can only be decided by assessing the individual's particular case and depends not only on how well the operation went, but also on the patient's profession. Because both eyes are never operated on simultaneously, many activities (e.g. office work) can soon be taken up again. Naturally, there is the prerequisite that the non-operated eye has a satisfactory visual function. Caution is recommended with activities that require stereoscopic vision.

Trabeculotomy and goniotomy

Trabeculotomy and goniotomy, performed for congenital glaucoma, are described in the section: Congenital Glaucoma.

Iridectomy

Using laser iridotomy as an example, it was noted that there are cases (even though they are very rare) when the iris can no longer be opened with a laser beam. In these cases, surgery is required. Even here, many variations are possible. The authors prefer entry via the cornea. The iris is held with forceps, and a small piece is cut off with scissors. Then the cornea is sutured.

Cyclodialysis

Cyclodialysis is an operation that used to be performed quite frequently. Using a scoop, a connection is fashioned between the anterior chamber and the choroid. This enables the aqueous humor to flow directly into the subscleral space. The reason that this technique is only rarely used today is that the effect can only be poorly controlled: the pressure is frequently very low right after the operation, but then it suddenly spikes to high levels.

Plastic implants

Because there is a natural tendency for scar tissue to form, the intraocular pressure often rises again after operations where fistulae [Lat. fistula: tube-shaped connection] are formed. For decades, attempts have been made to implant "pressure valves" in the eye. There are now numerous implants available for this purpose, of which Fig. 7.29 is an example. We very rarely apply these techniques as experience has shown that a re-operation with mitomycin C is, in most cases, a safer alternative than inserting a plastic implant. However, this is another area where many views are possible, and in some countries, these plastic valves are implanted quite frequently [Lat. implantatio: implantation].

The plastic implant directs the outflow of aqueous humor from the anterior chamber over the sclera

Fig. 7.29: The plastic implant directs the outflow of aqueous humor from the anterior chamber over the sclera.

Combined cataract / glaucoma operation

If both glaucoma and a cataract are present at the same time, then a cataract operation can be combined with an IOP-reducing operation. The advantage is that only one surgical procedure per eye is required for both diseases. However, the disadvantage is that the long-term prognosis for the pressure reducing effect is significantly poorer than with a pure trabeculectomy. If there is a large degree of glaucomatous damage present or if the damage occurs with only a moderate pressure rise, then the goal of the operation becomes achieving as low an IOP as possible. Each individual case must be uniquely assessed as to whether a combined operation or two individual operations would be better for that particular patient. If a patient has a cataract with an IOP that is only slightly increased and no glaucomatous damage, then usually only a cataract operation is performed. If the glaucoma is the primary problem and the lens is only slightly clouded, then usually only a trabeculectomy is performed. If both must eventually be performed, sometimes a combined operation will be considered. If the glaucoma problem is by far the more serious of the two, either because the pressure is very high or the damage is advanced, then a glaucoma operation is performed first and a cataract procedure is then done at a later time.The reason that the sequence of operations is glaucoma first, cataract second, is that with glaucoma, the damage is continuously progressing, and once present, cannot be alleviated. A glaucoma operation should therefore not be postponed for too long. However, a cataract operation can successfully be performed at any time, even after the glaucoma operation. For this reason, the primary focus is to avoid progression of glaucomatous damage.

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