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Caring for the patient with glaucoma

Damage to the optic nerve head and visual field due to glaucoma affects about 1 per

cent of the population over 40, rising to

10 per cent of those aged over 85 years.1 It is the most important cause of irreversible blindness in an estimated six million individuals worldwide.2 Even if the visual damage caused by glaucoma does not amount to blindness, many sufferers experience a markedly reduced quality of life. In the UK they may fail to satisfy the visual requirement of the DVLA driving test, and may find everyday tasks, especially mobility, much more challenging. There are also the inconvenience and the risks involved in both medical and surgical treatment of

the disease.

The most important risk factor for the development of glaucoma is abnormally raised intra-ocular pressure (IOP). The rise in IOP may be found with a normal eye drainage angle appearance (this is primary open-angle glaucoma or POAG), or with a drainage angle that is very narrow or ‘closed' by apposition of the iris and posterior cornea, hiding the angle structures from view (this is narrow-angle glaucoma or NAG).

In European and African-descended populations the most common type of glaucoma is POAG, with NAG approximately eleven times less common. NAG is, however, a more important cause in Chinese-descended populations.

There are rarer causes of open-angle glaucoma that are clinically important as they are associated with a worse prognosis. Pigment dispersion glaucoma affects young myopic patients and needs aggressive, often surgical treatment. The same is true of pseudo-exfoliative glaucoma, and glaucoma associated with topical and, rarely, systemic steroid treatment.

It is important to ensure that patients using long-term topical steroids to the face (especially steroid creams to the

periocular skin for eczema) and high-dose systemic steroids undergo periodic IOP checks. These can be conveniently performed by the patient's optician.

Primary open-angle glaucoma

POAG has a strong familial link, and up to 50 per cent of sufferers will have a first- or second-degree relative who is also affected.3 In recent years, researchers have identified specific genes that cause glaucoma in selected families. Most cases of congenital (usually infantile) glaucoma are caused by a small number of recessively inherited mutations, so usually arise within communities with a tradition of consanguineous marriage.

POAG typically has a slow, insidious onset with visual loss affecting the peripheral field first, with entirely normal Snellen acuity initially. Treatment of advanced disease is difficult, as so little visual field remains (tunnel-vision is experienced), that even with ideal control of IOP, very slow continuing visual loss results in a high risk of blindness.

Glaucoma is therefore ideally diagnosed at an early stage, before the patient notices any visual loss.

In the UK, the routine eye test performed by the high street optometrist includes tests to detect glaucoma, and is the main way that patients are referred to hospital for treatment. Unfortunately, many individuals do not regularly visit an optician and, even if they do, the tests are not infallible. Although there are approximately 250,000 diagnosed glaucoma sufferers in the UK, it is estimated that there are a similar number of undiagnosed cases.

The VISUAL FIELD AND OPTIC DISC

The optic nerve contains about one million axons and these axons exit the eye in a characteristic layered arrangement corresponding to the retinal location of their ganglion cell. Each ganglion cell serves a small area of the total visual field, so damage to neighbouring axons in the optic nerve head eventually results in confluent visual field loss. This has a characteristic arc-shaped pattern reflecting the arrangement of the axons at the point of damage.

Unlike neurological field defects, glaucoma field damage is often very asymmetrical across the horizontal midline. Glaucomatous visual field defects mainly occur within the central

25 degrees, and are best detected using automated static perimetry, for example the Humphrey Field Analyser (HFA).

The corresponding optic disc damage takes the form of an increase in the ratio between the optic disc diameter, and the optic disc cup (cup-disc or c-d ratio). This appearance is best appreciated using stereo indirect ophthalmoscopy, or imaging devices that generate a

3D topographic map of the optic nerve head, for example stereo-digital photography, or the Heidelberg Retinal Tomograph II (HRTII).

Other, more subjective signs of glaucoma disc damage include focal loss of optic disc rim, disc haemorrhages and atrophic changes in the adjacent retina. Skilled direct ophthalmoscopy also allows diagnosis, but would be impractical for detailed monitoring of the disease over time.

INTRA-OCULAR PRESSURE (IOP)

The level of the IOP is measured using either ‘contact' (Goldmann tonometry) or ‘non-contact techniques' (NCT).

The most rapid, and least invasive method is air-puff tonometry. A puff of compressed air is directed at the patient's cornea, and the evident brief flattening of the cornea is measured by the device. NCT is used by high street optometrists, however, the test is less accurate than contact tonometry, and artefactually elevated IOP is a common cause of inappropriate false-positive referrals to the ophthalmologist.

Contact tonometry using the Goldmann tonometer is more accurate, and is the method of choice in hospital eye departments, and also a growing number of optometrists.

The relationship between the height of the IOP and the risk of developing glaucoma is complex. There is considerable overlap between the normal population IOP distribution, and the IOP distribution of patients with evident glaucoma damage.4 Therefore, the traditional diagnostic abnormal IOP ‘cutoff' of >21mmHg is not highly sensitive or specific. A third of glaucoma patients never show IOP of >21mmHg, and are labelled ‘normal-tension glaucoma' (NTG).

Nonetheless, very high levels of IOP over >30mmHg do constitute a high risk of developing glaucoma, and most ophthalmologists would treat this level regardless of the appearance of the optic disc or visual field.

The decision to treat lower IOP levels depends on whether glaucoma damage is evident at the optic disc or visual field. Most ophthalmologists would attempt to lower the IOP in patients with any signs of ongoing damage, regardless of the initial IOP level. The ultimate therapeutic aim would be to lower the IOP by at least

30 per cent of the presenting level – evidence from randomised controlled trials in both POAG and NTG would support this approach.5

Patients with mildly elevated IOP (between 21mm and 30mmHg) but no signs of disc or field damage are described as ‘ocular hypertensives' (OHT), and are at higher risk of eventually developing glaucoma than the rest of the population. The absolute risk depends on the presence of other risk factors, for example precise appearance of the optic disc, corneal thickness, age and family history.6 Some of these patients are treated to lower the IOP, and even those low-risk patients who are left untreated are often monitored, perhaps annually, to detect the earliest signs of any glaucoma damage.

Clearly, on-going treatment and monitoring of both established glaucoma patients and ocular hypertensives, (treated or not) constitutes a substantial follow-up commitment for hospital ophthalmology departments. At least a third of the activity in ophthalmology departments is glaucoma-related, and only 10 per cent of this clinical load are new patients.7

The well-documented demographic changes in Western countries will substantially increase this workload in the coming years.

treating Narrow-angle glaucoma (NAG)

Many NAG patients have asymptomatic, chronic IOP elevation, and are detected routinely by the high street optician using NCT, optic disc examination, perimetry and slit-lamp microscopy detection of the typical shallow anterior chamber-narrow drainage angle. Most patients with NAG are ‘long-sighted', or hypermetropic (examination of the patients distance glasses will reveal magnifying lenses), and some of these patients are relatively young at diagnosis.

NAG is very rare in myopic, or

‘short-sighted' patients, and almost never occurs in patients who have had cataract extractions.

treating Acute angle-closure glaucoma

An important subgroup of NAG is acute angle-closure glaucoma (AACG). These patients do present, sometimes to their GP directly, with symptoms of acute, or sub-acute, excessive IOP elevation. The symptoms vary and patients with lower peak IOPs may present with an ‘incomplete' clinical picture, however, symptoms and signs include:

• Eye pain and headache, and redness of the eye (caused by high IOP, typically >40mmHg)

• Blurred vision, often described as ‘misty'. Patients may notice haloes around light sources (caused by corneal oedema, secondary to the excessive IOP). Patients may also report several minor,

self-limiting attacks of blurred vision and eye pain in the past months

• Nausea, vomiting and sometimes, abdominal pain (mediated via the vagus nerve) and

• Fixed, mid-dilated pupil, which appears oval in shape, and is difficult to examine because of the corneal oedema (caused by iris ischaemia secondary to the excessive IOP).

AACG is a medical emergency as there is a risk of blindness if it is not treated promptly. Patients are treated immediately with IV acetazolamide, which reduces aqueous production, and so lowers the IOP rapidly, producing dramatic relief of symptoms. In AACG, the underlying mechanism is obstruction of aqueous flow to the drainage angle by peripheral iris. This is corrected by use of pilocarpine drops, which constrict the pupil, and flatten the iris profile.

Beta-blocker drops such as timolol are also used to reduce aqueous production.

The definitive treatment both to the affected eye, and the fellow eye (which is also at risk) is neodymium YAG laser iridotomy. This laser puncture of the peripheral iris allows aqueous to flow from the posterior chamber to the anterior chamber, and reduces the tendency in NAG for the iris to be forced forward, so obstructing the drainage angle (see

figure 5). Patients who do not respond fully to an iridotomy may need chronic medical therapy (see options below) to control persistent IOP elevation, or possibly surgical treatment.

Systemic treatments, especially those with anti-cholinergic actions causing pupil dilation can very rarely induce AACG in susceptible patients. Implicated drugs include many anti-depressants, and even nebulisers, for example ipratropium. The same potential, albeit very small, risk exists for those patients undergoing pupil dilation for retinal examination, for example diabetics. In practice, it is probably impossible to identify all potentially susceptible patients in advance.

Most UK patients with glaucoma will have POAG, in whom these drugs will be harmless. Any patients with myopia or who have undergone cataract surgery in both eyes will almost certainly be safe. The potential ‘at risk' group are a tiny group of individuals who are awaiting iridotomy treatment for underlying chronic NAG, or (probably more likely) those patients with, as yet, undiagnosed chronic NAG.

I would recommend caution in very long-sighted patients (strong magnifying spectacles for distance vision), those with an existing diagnosis of NAG, or a previous attack of AACG. I always warn all patients of the possible symptoms of angle-closure glaucoma, and the need for immediate attention. Advice can always be sought from the patient's ophthalmologist if there is any remaining doubt in individual glaucoma patients.

DRUG TREATMENT options

About two-thirds of POAG patients will be adequately managed with medical

(eye-drop) treatment, and a third will eventually require at least one surgical treatment, though even these patients will be treated medically initially. Medical treatment to lower the IOP is directed at reducing the production of aqueous humour and increasing the drainage.

Many of the drugs have effects on both production and drainage, and the distinction is rarely important clinically. Medical options for treating glaucoma have greatly improved in both efficacy, and patient acceptability in recent years. These include:

• The prostaglandins (bimatoprost, travaprost and latanoprost) are all relatively recently developed prostaglandin-derivatives, and are all highly effective IOP-lowering treatments. The introduction of these drugs is probably responsible for a fall in the number of glaucoma operations in the UK in recent years. They are all more expensive than the alternative glaucoma treatments, but are notably more effective.

All need only once-daily administration and have minimal adverse effects, which include increased eyelash growth, darkening of the patients iris colour (usually symmetrical) and often slight redness of the eyes. In practice these adverse effects are rarely problematic. There are no important interactions with systemic medications, and the only systemic adverse effect seems to be slight worsening of wheeze in some asthmatic patients, though severe exacerbation is excessively rare.

• Beta-blockers are effective in reducing aqueous production, but are less well tolerated than the prostaglandins because of the uncommon sometimes serious systemic adverse effects. These effects are common to systemic beta-blocker therapy and include worsening of asthma (rarely life-threatening), bradycardia and heart block, worsening of peripheral vascular disease and Raynaud's, erectile dysfunction and depression. There is some evidence that the lower-dose

beta-blockers are safer, with equally effective IOP lowering properties.

• Alternatives to the prostaglandins and beta-blockers are generally less effective or more prone to cause adverse effects. These include brimonidine, which is highly effective but is associated with troublesome allergic reactions causing red eye.

If patients are affected, the advice is to stop the drops, and arrange an ophthalmological review to allow an alternative eye to be tried. A few weeks without the offending drop is highly unlikely to result in noticeable further visual loss in all but the most advanced glaucoma cases.

Topical carbonic anhydrase inhibitors, are highly effective in some patients, but tend to produce inadequate IOP lowering in most patients, if used alone. Both drops are, however, very safe and well tolerated.

• Combination treatments are now available. A popular preparation is timoptol 0.5 per cent combined with dorzolamide. Others include latanoprost and timoptol 0.5 per cent, and brimonidine and timoptol 0.5 per cent. These combinations have the advantages and disadvantages of the constituent drugs, but probably do improve compliance.

It is most important to appreciate that all of these combination treatments contain a relatively high dose of

beta-blocker. Patients may complain of potentially serious beta-blocker adverse effects, but the physician may not always appreciate the presence of beta-blocker as none of these drops have names which end in ‘ol'.

It is vital with combination drops, and naturally also those containing

beta-blocker alone, for example timoptol, to always stop the drops at once if potential beta-blocker associated adverse effects are suspected. The risk to vision of a week or two without the beta-blocker is slight, but the consequences of failing to stop the drops can be serious, though fortunately this is rare.

reducing the risk of visual loss

The care of glaucoma patients is directed at a reduction in the risk of problematic visual loss during the patient's lifetime, combined with avoidance of reduced quality of life caused by the various treatments. The key principles are:

• Timely detection of glaucoma or high glaucoma risk, usually detected by screening undertaken by the patient's optician in the setting of the routine

eye test.

Occasionally, however, patients are referred directly from the GP, usually because of the onset of symptomatic AACG or the presence of very advanced symptomatic visual loss from POAG (or NTG/NAG) in patients who never visit

the optician.

Ironically, these are often patients with lifelong previously excellent spectacle-free distance vision who purchase ready-made reading glasses from the supermarket, and patients from a lower

socio-economic background who are generally less attentive to health issues.

An innovative scheme in The Forest of Dean, Gloucestershire, involves enhanced payment to opticians to undertake a more detailed glaucoma screening examination, and then direct referral of those glaucoma patients detected to the consultant ophthalmologist. We have demonstrated an increased detection rate, reduced false-positives and a more streamlined route for the patient.8

• Confirmation of glaucoma by a senior ophthalmologist, and initiation of treatment if necessary. Prostaglandin drops are increasingly the first-line treatment choice. This step is essential to avoid missed cases of rare secondary glaucomas, and chronic NAG (needing laser iridotomies or surgery). It is also important to discharge those patients who have normal eyes (false-positive referrals) safely, and to establish a reasonable follow-up plan to monitor those patients on treatment, and those who require regular monitoring only.

• A well-planned protocol to undertake repeat IOP checks, with visual field testing and optic disc imaging to ensure that patients are well-controlled on the chosen medication, and are not deteriorating at a rate likely to result in problematic visual loss during their lifetime. Glaucoma specialists are therefore more concerned with the progress of the young patient with already advanced POAG, than the elderly patient with OHT. The former patient is clearly at greater risk of eventual blindness as he has less vision to lose, and more time in which to lose it. As a guide the former patient might be reviewed every four to six months, whereas the latter would usually be checked annually. These guidelines will clearly be flexible, depending on the height of the treated IOP, and the presence of any evidence of disease progression in the visual fields or optic disc photographs.

• A protocol that ensures rapid review by a senior glaucoma specialist of patients who are at high risk of blindness, but who are not responding to medical treatment. These patients may benefit from surgical treatment, for example a trabeculectomy, possibly combined with cataract surgery if a cataract is also present. Trabeculectomy is a microsurgical operation that involves the creation of a surgical fistula to allow aqueous to drain into a reservoir beneath the conjunctiva (a ‘bleb'), and therefore circumvent the eye's natural trabecular drainage system.

Modern developments including the use of anti-scarring preparations

(5-flourouracil and mitomycin C) to reduce the risk of conjunctival fibrosis and ultimate trabeculectomy failure, as well as releasable scleral flap sutures have increased the success rate, and safety, of the surgery.9 Most trabeculectomies are now performed as day-case, local anaesthetic, procedures in UK units.

• In response to the advances in glaucoma diagnostic and monitoring technology in recent years, and the structural changes in the NHS, the personnel involved in the care of glaucoma patients is slowly changing. There is increasing involvement of professions allied to medicine (PAMs), especially in the glaucoma monitoring process.

practice-based commissioning

The predominant professional group involved are optometrists by virtue their existing skills and practice equipment levels. There are several successful models of ‘shared-care' between both

hospital-based, and community–based optometrists working in concert with the hospital-based ophthalmologist.

There may be a role for the GP, especially with practice-based commissioning (PBC), who already has some experience of ophthalmology, though involvement would require some update in skills, and perhaps more pertinently, a substantial capital investment to provide the essential equipment. This would include a slit-lamp microscope, tonometers, biomicroscopic lenses, automated perimeter and optic disc imager. The total cost would exceed £50,000. The cost-effectiveness of these ongoing changes in healthcare provision is currently uncertain. Novel shared-care arrangements with interested groups of PAMs, and perhaps even GP practices undertaking PBC, may not always cost the NHS less, even if the quality of care and patient satisfaction, remain high.

References

1 Tielsch JM, Katz J et al. A population-based evaluation of glaucoma screening. Am J Epidemiol 1991;134;1102–10

2 Quigley HA. Number of people with glaucoma worldwide. Br J Ophthalmol 1996;80:339–93

3 McNaught AI, Allen J et al. Accuracy and implications of a reported family history of glaucoma. Arch Ophthalmol 2000;118(7): 900–4

4 Sommer A, Tielsch JM et al. Relationship between intra-ocular pressure and primary open-angle glaucoma among white and black Americans. Arch Ophthalmol 1991;109:1090–5

5 The AGIS investigators. The advanced glaucoma intervention study (AGIS): 7. The relationship between control of IOP and visual field deterioration. Am J Ophthalmol 2000;130:429–40

6 Kass MA, Heuer DK et al. The ocular hypertension treatment study. Arch Ophthalmol 2002;120:701–13

7 Personal observation, Glaucoma activity audit, Gloucestershire, 2003

8 Personal communication, Lisa Collins, Countywide Head of Optometry, Gloucestershire Direct Referral scheme, 2006

9 Khaw PT, Wells AP et al. Surgery for glaucoma in the 21st Century. Br J Ophthalmol 2002;86:710–1

USEFUL WEBSITES

International Glaucoma Association

glaucoma-association.com

Fight For Sight

fightforsight.org.uk

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