Introduction

Glaucoma is the leading cause of irreversible blindness worldwide, affecting an estimated 60 million people in 20101. More than 50% of sufferers in developed countries are unaware of their condition and this figure increases to over 90% in the developing world. The economic burden is substantial at almost $3 billion in the US in 20042. A recent estimate of the risk of progressing to blindness in patients with diagnosed glaucoma attending a specialist glaucoma clinic in the US is around 14% for unilateral blindness and 6% for bilateral blindness after 15 years3.

Glaucoma results in visual loss due to a progressive loss of retinal ganglion cells (RGC) with corresponding visual field loss and a characteristic ‘cupped’ appearance at the optic nerve head. Visual field loss usually begins peripherally, but sometimes affects the central visual field first. The only modifiable risk factor for glaucoma at present is the intraocular pressure (IOP) and the only proven method to delay or prevent glaucomatous optic neuropathy is IOP lowering. This may be achieved by medication, laser, or surgery.

Surgery for primary open angle glaucoma (POAG) has evolved in the past 50 years from full-thickness filtration surgery, in which a hole in the scleral wall at the corneo-scleral limbus drained aqueous from the anterior chamber to a subconjunctival ‘drainage bleb’. As this procedure carried a high risk of ocular hypotony, and bleb-related endophthalmitis4,5, it evolved to a partial-thickness scleral flap which provided an extra flow resistor (Fig. 34.1)6, prevented hypotony, and consequently lowered aqueous pressure within the subconjunctival drainage bleb lowering the risk of conjunctival breakdown and infection.

Fig. 34.1 Trabeculectomy (B) evolved from full-thickness filtration surgery (A) with the introduction of the scleral flap as an additional point of resistance. Note how the filtration procedure on the right has no scleral flap, rather a direct communication from the anterior chamber to the drainage bleb. Though both techniques create a conjunctival drainage bleb at the limbus (C). There were fewer bleb-related complications and less hypotony with the added resistance to aqueous flow provided by the scleral flap.

As a guarded filtration procedure, trabeculectomy was safer than the full-thickness procedure. However, as satisfactory long-term IOP control could not be achieved in a significant proportion of patients, pharmacological enhancement with anti-proliferative agents (e.g. 5-fluorouracil (5FU)7 and later, mitomycin C (MMC)) was introduced in the late 1980s. While these improved success rates some safety concerns, previously reduced by the evolution of trabeculectomy from full-thickness filtration surgery, reappeared8,9. The main concerns are hypotony10 and infection, especially as trabeculectomy-related infection has a predilection for more aggressive organisms than do other types of intraocular infections11. The alteration in conjunctival surface healing that resulted from the use of 5FU, and especially MMC, also resulted in prominent drainage blebs that could cause chronic discomfort or dysesthesia12. These and a higher incidence of cataract after trabeculectomy have promoted further evolution in glaucoma surgical techniques in four main directions.

General considerations

Medical versus surgical treatment

While medical and laser treatment are usually the first lines of therapy, there are some situations where early surgery is advisable. Surgery is the principal treatment for angle closure, developmental glaucomas, and a number of secondary glaucomas. Surgery is recommended for POAG in three circumstances (Box 34.1):

Box 34.1

Indications for surgery in primary open angle glaucoma

1 Inadequate IOP control

2 Progression of visual fields in spite of seemingly good IOP control

3 Intolerance of medication

4 Advanced glaucoma at presentation

5 Lack of availability and inability to afford cost of glaucoma medication

6 Patient preference.

1 Advanced visual field loss at presentation, especially in the younger patient. Age and stage of disease are important risk factors for severe visual loss.

2 Failure to achieve a level of IOP that will stabilize the visual field on two to three topical medications (Fig. 34.2).

3 Failure to tolerate or adhere to medical treatment or inability to use medical treatment (e.g. confused patients or patients physically unable to instill eye drops or able to do so only with difficulty) and where laser trabeculoplasty is either ineffective or insufficiently effective. Poor adherence and intolerance of medical therapy are both common problems. If non-adherence is suspected and especially if there is evidence of progression of glaucoma then laser or surgery should be considered.

Fig. 34.2 Greyscale portion of three sequential Humphrey 24-2 visual fields (SITA-standard algorithm) performed over a 2-year period showing progression of a paracentral visual field defect that was consistent on repeat testing, in a patient with treated IOP levels consistently in the normal range. Because of the central nature of this defect, the patient noted a positive scotoma. The defect stabilized following trabeculectomy surgery. In this patient’s case central vision was reduced due to the position of the defect, illustrating the high risk of visual loss with this type of defect and hence the argument for earlier surgery.

In the developing world, medication is often unaffordable, and surgery may be indicated as an opportunity to prevent progression with one intervention alone. However, the barrier to treatment of glaucoma in the developing world is often lack of awareness or understanding of the nature of the condition or that preventive treatment is important. Few therefore opt for treatment even when it is available or affordable.

The term maximal-tolerable medical therapy (MTMT) is often used to indicate the level of medical therapy above which there is likely to be no further benefit in terms of IOP-lowering, or above which the patient’s ability to tolerate the regimen is likely to diminish because of increasing side effects. The term is of limited usefulness: first it takes no account of an individual patient’s ability to sustain the maximal level of medication. This term usually denotes the maximal level of medication that a patient can tolerate at any one time, which is often higher than the level tolerated in the longer term. Second, the implication, perhaps unintended, is that it is in the patient’s best interest to take as many medications as possible before even considering surgery. This is inaccurate for the following reasons:

1 There is a law of diminishing returns with increasing levels of medication. Above 2–3 topical glaucoma medications, there is no evidence of increasing benefit by adding more. In addition, intolerance rises substantially with the use of 4–5 drugs.

2 Medical treatment on average achieves a 50% reduction in IOP. It can therefore be predicted whether or not a particular patient is more or less likely to achieve the desired target IOP level, based on the starting IOP. For example, a patient with an untreated IOP level of 37 mmHg on 2–3 medications is likely to achieve a treated pressure level of 18–19 mmHg. If the target level is deemed to be 12–15 mmHg then, while a proportion of patients may achieve this, less than 50% will do so sustainably. It is appropriate to try, but if the desired target level cannot be achieved reasonably quickly and consistently on 2–3 drugs then surgery should be contemplated, rather than persistence with medical therapy.