Glaucoma can be congenital, being present from birth, or acquired. Further subclassification depends on the mechanism by which aqueous outflow is impaired. The configuration of the drainage pathway, whether the filtration angle is open or closed, affects the prognosis and treatment. Glaucoma may also be primary or secondary, depending on the presence or absence of associated factors that have caused the pressure rise (Kanski 2007). For example in primary open angle glaucoma, the elevation in IOP is not linked with any associated ocular conditions.
The fluid or aqueous humour within the eye is made up mostly of water, with a solution of protein and other electrolytes. This helps to maintain the structure and clarity of the eye, provides nourishment to the tissues within and removes waste products. The aqueous humour is produced by the ciliary body and flows into the posterior chamber of the eye; it eventually works its way to the anterior chamber. This fluid is continually produced and replenished by the ciliary body and continuously drained by two routes.
The trabecular meshwork is one drainage pathway which accounts for 90% of the aqueous outflow (Forrester et al 1995, Kanski 2007). The uveoscleral route drains only 10% of the aqueous humour. Imagining the eye as a washbasin, the amount of fluid in the eye depends on the rate of aqueous secretion and the rate of aqueous outflow. As the eye wall is inelastic, the volume cannot increase. As a result, an increase in the volume of fluid in the eye will lead to an increase in pressure. If aqueous secretion and outflow are balanced, the IOP is maintained usually within a range of 11-21 millimeters of mercury. The level of IOP is inherited, so first degree relatives of patients with glaucoma are at greater risk. The absolute pressure does vary over the course of a day, tending to be higher in the mornings and lower in the afternoons and evenings. A single reading therefore may be misleading. Intraocular pressure is measured clinically using a 'tonometer' which measures the force required to flatten a known surface area of the cornea. This has a scale which indicates the IOP ( Forrester et al 1995, Kanski 2007, Allingham et al 2011).
The optic nerve consists of 1.2 million nerve cells funneling from the retina, through an opening in the posterior wall of the eye (sclera) to the brain (Forrester et al 1995). Each optic nerve transmits a signal about the light levels in one area of the retina. The optic nerve head can be visualised in clinic with a slit lamp and condensing lens. There are characteristic features that indicate that a disc has been damaged. The ophthalmologist is trained to recognise these features and any changes that appear over time, in order to implement changes in the treatment regime.
Various tests are used to assess the damage to the visual field that is the end result of damage to and loss of the nerve cells that run through the optic nerve head. Most commonly, the Humphrey perimetry visual field is used. This involves patients having their head placed in a large hemispheric bowl, with a target in the middle. The patients focus on this target and lights from various points on the bowl will flash. Patients press a thumb-switch to indicate that they have seen the flash, and in this way their field of vision is plotted, one point at a time.
In glaucoma, usually, the IOP is found to be elevated. This rise in pressure leads to damage to the delicate optic nerve head. Loss of a sufficient number of these optic nerves will result in a corresponding loss of visual field. If left untreated, all of the nerve fibers may perish and the patient will go blind (Kanski 2007, Allingham et al 2011).
Treatment depends on the cause of the glaucoma. The patient's age, IOP, degree of disc damage and visual field loss are important. The configuration of the drainage angle must be assessed. This is done using 'gonioscopy,' a technique that uses a contact lens with a prism or mirror (Figure 1). If the drainage angle is open then as a first line, topical therapy in the form of eye drops may be attempted to lower IOP. The drainage angle can be closed because of an anatomical predisposition (primary angle closure) or because of some other mechanism (secondary angle closure), such as inflammation or neovascularisation that pulls or pushes the angle shut (Trope 2005, Kanski 2007, Allingham et al 2011). The mechanism of primary angle closure is not fully understood, however treatment with a YAG laser, creating a pathway from the posterior to the anterior chamber has been found to treat primary angle closure glaucoma. This alternative pathway reduces the resistance to aqueous flow from posterior to anterior and therefore reduces the pressure on the back surface of the iris. This allows the iris to sink backwards and open the drainage angles.
[FIGURE 1 OMITTED]
When medical therapy fails in open angle glaucoma, or if medical therapy and laser YAG peripheral iridotomy fails in patients with angle closure glaucoma, surgery can be attempted. Trabeculectomy (see Figure 2) is an operation that creates an anatomical fistula between the aqueous and the subconjunctival space. The fluid drains into a bleb, which looks like an elevation in the conjunctiva and tenons layer adjacent to the cornea. The bleb is usually placed superiorly so that it is covered by the upper eyelid (Kanski 2007, Allingham et al 2011).
[FIGURE 2 OMITTED]
Surgery is performed as a last resort. Patient selection is a crucial factor in any surgical procedure. The main reasons for performing a trabeculectomy are if medical therapy is not successful, if the patient is not compliant or has side-effects from medical therapy, and if better control is required especially in young patients or those with only one eye.
The risks and benefits may be discussed fully so that informed consent can be obtained. There is no absolute indication to perform a trabeculectomy. It is important to explain that this operation is being performed not to improve vision, but to prevent further deterioration. Immediately postoperatively, the vision will be worse due to the inflammation and there is a risk of loss of vision in the eye altogether due to choroidal detachment, haemorrhage or from infection (Trope 2005, Allingham et al 2011). Even if the pressure is left high, loss of vision will occur over several years. If the patient's life expectancy is short, it may not be worth undergoing an operation to maintain vision for longer, given the risk of immediate loss of vision.
Other surgeries may also be considered. For example, if there is no visual potential for the eye, and IOP is elevated causing pain, diode laser destruction of the ciliary body can be attempted to reduce production of aqueous fluid. This can be done under local anaesthetic in a short period of time and requires less patient co-operation. It can be repeated until the desired effect is achieved (Martin & Broadway 2001, Allingham et al 2011).
Trabeculectomy surgery requires several steps, each of which should be executed carefully and successfully for the next to work correctly. The pupil should be miosed (constricted) with pilocarpine preoperatively.
* A flap is created in the conjunctiva and Tenon capsule (Figure 2a). This flap can be along the border of the cornea (limbal-based) or further posteriorly (fornix-based). Care must be taken not to damage the conjunctiva by using toothed forceps--Moorfield's forceps are the instrument of choice for handling conjunctiva.
* Episcleral tissue is cleared and wet-field cautery is used to close blood vessels and seal areas of bleeding over the proposed site of the sclera flap (Figure 2b).
* Before the next step, anti-metabolites (e.g. 5-fluorouracil or mitomycin C) may be used depending on the patient's individual needs. Incisions are made up to two-thirds thickness of the sclera to create a 'trapdoor' lamellar sclera flap (Figure 2c).
* This flap is dissected forwards until clear cornea is reached (Figure 2d). A paracentesis, or small incision is then made through the trapdoor flap, into the anterior chamber of the eye and a gush of aqueous fluid is seen.
* A peripheral iridectomy is taken, removing a portion of the iris to help the circulating aqueous fluid pass more easily through the new pathway and out of the eye. The technique for the surgical iridotomy is first to exert gentle pressure on the sclera to prolapse the iris (Figure 2e). This iris is grasped with the Colibri forceps and pulled until the pupil margin is brought halfway up to the limbus. The De Wecker scissors are used to excise the tented-up iris with one cut.
* The superficial sclera flap is sutured to the sclera at its corners so that it is positioned lightly adjacent to the underlying bed. At this stage, releasable sutures can be used. The advantage of these is that the flap may be sutured more tightly to reduce the rate of postoperative scleral flap leakage (Figure 2f). A small amount of balanced salt solution is inserted through a paracentesis to re-inflate the anterior chamber, test the patency of the fistula and check for any holes or leaks in the flap.
* Conjunctiva and Tenon's capsule flap is sutured (Figure 2g) and irrigation through the paracentesis is repeated to inflate the bleb (Figure 2h). A steroid and antibiotic is injected under the conjunctiva to reduce postoperative inflammation and infection (Trope 2005, Kanski 2007, Allingham et al 2011).
Often combined trabeculectomy and phacoemulsification is performed. This is only considered when there is cataract is present and glaucoma is well controlled. If not, surgery is performed as two separate procedures.
Postoperative recovery and follow up
Postoperatively, there is a recovery period of 2-3 months. The postoperative follow-up is particularly important because the success of the surgery will depend on the rate and extent of the conjunctival healing process (Kwon et al 2006).
Appointments are arranged for the day after the surgery and for several weeks thereafter, on a weekly basis initially but sometimes more frequently if necessary. Postoperative medications generally include antibiotic and steroid drops to reduce the rates of infection and control the severity of the inflammation. A cycloplegic drop is also used to maintain pupil dilatation and manage the pain. The frequency and adjustment of these drops will be directed by the surgeon (Trope 2005, Kanski 2007).
During follow-up visits, adjustments can be made to control the IOP. High post-operative pressure can be corrected by manipulating the trabeculectomy flap and holding sutures. There are several ways of manipulating the flap. Massage can be applied, which loosens the sutures and allows more aqueous outflow. The sutures can be cut by laser (Argon laser suturelysis) or, if releasable, sutures can be 'pulled' which has the same effect. If excessive scarring is thought to be the culprit, antimetabolites may be used. 5-fluorouracil (5-FU) can be injected under Tenon's layer in first few post-operative weeks to reduce conjunctival scarring down on to the sclera.
This may therefore help to keep the communication between the anterior chamber of the eye and the subconjunctival space (Trope 2005, Kanski 2007, Allingham et al 2011).
Occasionally, the surgeon may elect to 'needle' the bleb post-operatively if there is excessive conjunctival scarring process. A small gauge needle is used to break up the scar tissue to allow more filtration of the aqueous fluid. This is usually performed in theatre under local anaesthesia, although some surgeons may also perform this as a minor procedure under topical anaesthetic in the outpatient department(Broadway et al 2004, Feldman & Tabet 2008).
If the IOP is too low, the surgeon may reduce the amount of steroid drops to allow additional healing process. There is a wide range of postoperative scenarios for which there are a number of possible treatments. Thus, it is very important for the patient to have a proper postoperative follow-up under the guidance of the treating surgeon. Complications of trabeculectomy There are several complications after glaucoma surgery. These can occur in the early postoperative stage or after a longer period of time, and may potentially result in temporary or even permanent reduction in vision. Occasionally, a further surgical procedure may be required to correct the complication.
The most common short-term complication of trabeculectomy surgery is the failure to adequately lower the IOP. This may occur due to the excessive scarring of the conjunctival tissue with decreased filtration of the aqueous fluid out of the eye. The pressure can even increase postoperatively. This might be due to aqueous misdirection where, instead of draining forward, the aqueous drains posteriorly. This pushes forward the lens and the iris and blocks the drainage angle, causing an increase in the IOP (Kanski 2007).
On the other hand, the IOP may be too low, termed as 'hypotony', due to the excessive filtering of the aqueous fluid or a leaking wound. This may cause blurred vision and can be associated with shallowing of the anterior chamber, cataract formation, and greater risk of intraocular fluid accumulation (called choroidal effusion) or intraocular bleeding (suprachoroidal hemorrhage). The suprachoroidal hemorrhage is a particularly feared complication after trabeculectomy, because it is frequently associated with pain, elevated IOP, and permanent decrease in vision. It often requires additional surgery to drain the blood (Trope 2005, Allingham et al 2011).
There can also be long-term complications of having a trabeculectomy bleb around the eye. If there is a leak from the bleb, the pressure may become too low. In addition, the bleb leak can increase the risk of infection. An infection in a post-trabeculectomy eye can be serious because the surgical opening in the sclera may allow direct access of the offending micro-organism to the inside of the eye. Such intraocular infection can seriously compromise the vision and even the integrity of the eye itself. Therefore, any symptoms of infection in a post-trabeculectomy eye such as pain, decreased vision, redness, and purulent discharge, should be reported and examined promptly in order to exclude a bleb infection (called blebitis or bleb-associated endophthalmitis) which may occur even years after the surgery (Trope 2005, Kanski 2007, Allingham et al 2011).
Cataract formation and progression of pre-existing cataract can take place after filtration procedures, being the main cause of early visual loss after filtration surgery.
Even a successful trabeculectomy operation may not give good IOP control forever; the surgery is considered successful if it controls the IOP for a period of 7-8 years. If the first trabeculectomy fails, it can be repeated a second (and rarely a third) time to control glaucoma. Subsequent trabeculectomies usually have a higher chance of failure than the primary trabeculectomy surgery. If trabeculectomy fails to control glaucoma adequately, the surgeon may consider other options such as glaucoma drainage devices (Trope 2005, Kwon et al 2006, Allingham et al 2011).
Glaucoma drainage implants
Glaucoma drainage implants or aqueous shunts create an alternate aqueous pathway from the anterior chamber by channeling aqueous fluid out of the eye through a tube to a subconjunctival bleb (Figure 3a).
[FIGURE 3 OMITTED]
These devices are useful when glaucoma is uncontrolled with medications, laser, or trabeculectomy. They are also used when there is not enough healthy tissue to proceed with trabeculectomy or in cases where trabeculectomy would be likely to fail (Kanski 2007, Shetty 2008).
There are several models of glaucoma drainage devices. All such shunts consist of a tube attached to an end plate that acts as a surface for bleb formation. Some may also contain pressure-sensitive valves for regulation of aqueous flow (Minckler et al 2008, Shetty 2008).
Molteno, Baerveldt, Ex-PRESS R50, the Schocket , Krupin and Ahmed implants (Figure 3b) are examples of such devices, with the latter two containing a valve structure (Minckler et al 2008, Shetty 2008, Gedde et al 2009).
As in trabeculectomy, a patient who receives a glaucoma drainage implant will require antibiotics and steroids to prevent infection, inflammation and scarring.
Other potential complications of such procedures include:
* too low pressure due to over filtration and wound leak
* raised pressure caused by obstruction or kinking of the tube
* poor tube positioning
* erosion of the tube
* swelling of the cornea and retina
* double vision
* hastening of the cataract formation (see Shetty 2008, Sarkisian 2009).
The outcome of the surgery will depend on the type of glaucoma and is variable. Additional glaucoma medications or surgery may be needed if the IOP remains higher than desired.
Glaucoma is a very serious, sight threatening eye disease that can lead to significant disability and reduction in the patient's quality of life. A wide spectrum of treatment options are available including topical therapy, laser and surgical treatment, but an individual management plan must be chosen, based on patient's unique situation and requirements. In most cases where glaucomatous damage progresses despite medical treatment, intraocular pressure lowering surgery becomes necessary. Postoperative care of the patients is just as important as the operation itself in order to achieve an optimal control of the intraocular pressure, to reduce the progression of the disease and its potential sight threatening complications.
Provenance and Peer review: Commissioned by the editor; Peer reviewed; Accepted for publication October 2010.
Allingham RR, Damji KF, Freedman S et al 2011 Shields' Textbook of Glaucoma Philadelphia, Lippincott, Williams and Wilkins
Broadway DC, Bloom PA, Bunce C, Thiagarajan M, Khaw PT 2004 Needle revision of failing and failed trabeculectomy blebs with adjunctive 5-fluorouracil: survival analysis Ophthalmology 111(4) 665-673
Feldman RM, Tabet RR 2008 Needle revision of filtering blebs Journal of Glaucoma 17(7) 594-600
Forrester A, Dick D, McMenamin P Lee W 1995 The Eye: basic sciences in practice London, WB Saunders
Gedde SJ, Schiffman JC, Feuer WJ et al 2009 Tube Versus Trabeculectomy Study Group. Three-year follow-up of the tube versus trabeculectomy study. American Journal of Ophthalmology 148(5) 670-684
Kanski JJ 2007 Clinical Ophthalmology: a systematic approach Oxford, Butterworth Heinemann
Kwon YH, Fingert JH, Greenlee EC 2006 A patient guide to glaucoma MedRounds Publications Available from: www.medrounds.org/glaucoma-guide/2006/02/ table-of-contents-patients-guide-to.html Accessed November 2010
Martin KR, Broadway DC 2001 Cyclodiode laser therapy for painful, blind glaucomatous eyes British Journal of Ophthalmology 85(4) 474-476
Minckler DS, Francis BA, Hodapp EA et al 2008 Aqueous shunts in glaucoma: a report by the American Academy of Ophthalmology Ophthalmology 115(6) 1089-1098
Sarkisian SR 2009 Tube shunt complications and their prevention Current Opinions in Ophthalmology 20(2) 126-130
Shetty R 2008 Glaucoma, drainage devices Available from: http://emedicine.medscape.com/article/1208066-overview Accessed November 2010
Trope GE 2005 Glaucoma Surgery London, Informa Healthcare
MBBS, BSc, DHMSA
Specialist Registrar in Ophthalmology, Royal Eye Infirmary, Plymouth
Opthalmology specialist trainee year one, The Royal Eye Infirmary, Plymouth
Adam P Booth
BSc (Hons), MB ChB, FRCOphth, PhD
Consultant Ophthalmic Surgeon, Glaucoma Specialist, Royal Eye Infirmary, Plymouth
No competing interests declared
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Correspondence address: Neil Modi, Specialist Registrar in Ophthalmology, Royal Eye Infirmary, Plymouth, PL4 6PL. Email: firstname.lastname@example.org
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|Title Annotation:||CLINICAL FEATURE|
|Author:||Modi, Neil; Vahdani, Kaveh; Booth, Adam P.|
|Publication:||Journal of Perioperative Practice|
|Article Type:||Disease/Disorder overview|
|Date:||Jan 1, 2011|
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