Glaucoma, the sneak thief of sight

The eyes are one of our most important sensory organs in this informative and visually demanding world. Unfortunately, there is a group of people who suffer from a specific entity of eye disease, in which their visual treasure is constantly being stolen by a ‘sneak thief of sight’, and which they cannot just get rid of, despite trying hard to protect themselves. They are miserable and depressive as they all know that they may not be able to see one day. They can hardly plan their future and some may lose their courage to face the problems and end up in tragedy. These are the patients suffering from advanced chronic glaucoma.

What is glaucoma?

Glaucoma refers to an irreversible, progressive optic neuropathy where intraocular pressure (IOP) and other factors contribute to the damage. There is a characteristic acquired atrophy of the optic nerve, and loss of retinal ganglion cells and their axons (Figures 1-3).

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Epidemiology of glaucoma in Hong Kong

Glaucoma is the second leading cause of blindness worldwide, accounting for 12.3% of the world's blindness [1]. In Hong Kong, glaucoma is the leading cause of blindness, accounting for 23%, or 92 out of 399 new registrations of permanent blindness in 2001/2002 [2]. Glaucoma can be further classified into acute and chronic types. The speculated age-specific prevalence rate of chronic glaucoma in the Chinese population is about 3% of the population aged 40 or above. According to the 2006 population statistics in Hong Kong, about 3.2 million of our population is aged 40 or above. In other words, there are about 100,000 chronic glaucoma patients in our society. On the other hand, the incidence rate of acute glaucoma in Hong Kong is about 500 per year, with reference to the Singaporean figures [3]. The most important risk factor for both acute and chronic glaucoma is age; the more advanced in age, the higher the chance of suffering from glaucoma.

Patients with acute primary glaucoma, if presenting early and treated properly, can usually expect a complete cure without much visual sequelae. Unlike acute glaucoma, patients with chronic glaucoma of any kind bear the threat of eventual blindness in their lives. The chance of complete cure is low and the goal of treatment is to keep their vision until the last day of their life. All the current treatment modalities in ophthalmology, namely medications, laser therapy and surgery are means to slow down the progression.

The clinical classification of glaucoma

Glaucoma can be classified according to the cause (primary vs. secondary), trabeculum status (open angle vs. angle-closure), onset mode (acute vs. chronic), IOP level (high tension vs. normal tension vs. low tension), and age (adult vs. juvenile vs. congenital). Each disease entity has its own clinical features and implications in management and prognosis. Among them, the most common types are chronic primary open-angle glaucoma (POAG), primary chronic angle-closure glaucoma (PCACG), primary acute angle-closure glaucoma (PAACG), and secondary glaucoma. In ophthalmology, the term glaucoma usually refers to the chronic types of the disease.

Primary open-angle glaucoma (POAG)

POAG is a chronic disease in which the eyes demonstrate characteristics of glaucomatous changes, including progressive optic atrophy and visual field defects. The anterior chamber angles of the eyes are graded as 'open' in gonioscopy and patients are usually adults. The disease is bilateral in the majority of cases, although asymmetry is common.

POAG is the most common type of glaucoma worldwide. The disease is usually asymptomatic, until, in the advanced stages patients begin to notice changes in visual functions. Studies in the USA found that about 50% of POAG patients were unaware of the disease until the late stages [4,5]. In other words, many patients missed the chance of early detection and control.

The major risk factors for POAG include higher IOP, older age, positive family history, lower central cornea thickness (CCT) and presence of POAG in the contralateral eye [6]. Other associated factors include diabetes mellitus, high myopia, vasospasm diseases like migraine, sleep apnoea, systemic hypertension and low diastolic perfusion pressures [6,7].

Even though many POAG patients' IOP is elevated compared with the 'normal' population, the level of IOP is not the most important parameter in the diagnosis of glaucoma. It is true that the higher the IOP, the higher the chance of glaucoma. However, people with elevated IOP may not develop glaucoma (ocular hypertension) and people with 'normal' IOP can suffer from normal-tension or even low-tension glaucoma. Therefore IOP alone is NOT diagnostic of glaucoma in most cases.

Secondary glaucoma

Secondary open-angle glaucoma refers to a sub-group of glaucoma with identifiable causes of IOP elevation. The persistent elevation of IOP causes the characteristic damage to the optic nerve head and results in glaucoma. Common sub-types of secondary glaucoma in the Chinese population include neovascular glaucoma, uveitic glaucoma, traumatic glaucoma, steroid-induced glaucoma, lens-induced glaucoma, developmental glaucoma, pigmentary glaucoma, etc. The mechanisms of the vast majority of secondary glaucoma (except ciliary body tumour with hypersecretion of aqueous humour) are due to changes in the resistance of the outflow pathway of aqueous humour, namely the trabeculum meshwork and collecting channels. This causes a shift (elevation) in the equilibrium IOP of the eye in order to re-achieve an equal rate of aqueous secretion and outflow. The elevation of IOP then damages the optic nerve head and results in glaucoma.

Primary chronic angle-closure glaucoma (PCACG)

This entity of glaucoma refers to conditions where gradual appositional and/or synechial closure of the anterior chamber angle lead to elevation of IOP and resultant glaucomatous damage. The prevalence of PCACG differs among ethnic populations and is high in the Chinese population [8]. Risk factors in Chinese peoples include advancing age (>60 years), female gender, family history of PCACG, hypermetropia, a small globe and a shallow peripheral anterior chamber [9].

Diagnosis of glaucoma

As with all medical conditions, diagnosis of glaucoma can only be made with careful history taking, physical examinations and appropriate investigations. The aim is to identify the risk factors of glaucoma, information about medical health for choice of treatment options, documentation of physical signs, and the anatomical and functional status of the eyes (Table 1).

In diagnosing glaucoma, we take into account of all the clinical parameters. Elevation of IOP (above 21 mmHg) is observed in the majority of glaucoma patients. However, IOP alone is not diagnostic of glaucoma in most cases. In fact, glaucoma can be considered a mismatch of IOP and the susceptibility of the optic nerve head. When the IOP is higher than the optic nerve head can withstand, there will be progressive optic nerve head damage, resulting in glauco- matous changes. Therefore by diagnosing glaucoma based on IOP alone, we can miss patients with normal-tension or low-tension glaucoma, while over-treating those with ocular hypertension alone. In general, people, after a comprehensive examination and investigation, can be categorized into glaucoma patients, glaucoma suspects or non-glaucomatous.

Management of glaucoma

The goal of treating glaucoma is to stabilize the optic nerve and retinal nerve fibre status and hence maintain functional vision throughout the patient's life. Although modification of risk factors may be beneficial, the only well-proven, effective measure is to lower the IOP to a 'safe' level [10,11]. This objective is achieved by medical treatment, laser therapy and surgery as appropriate.

Medications

Medications in glaucoma include systemic and topical ones. Acetazolamide is used systemically to achieve a rapid and effective reduction in IOP. It is used in urgent or semi-urgent conditions for a short period of time, and its use is limited by the side effects. Hyperosmotic agents like mannitol, isosorbide and glycerol are useful in acute glaucoma for rapid IOP control.

Topical medications are the first-line treatment of choice in the majority of cases. They work by increasing aqueous outflow and/or decreasing aqueous inflow (Table 2). The first-line medications include beta-blockers and prostaglandin analogues. They are effective in reducing IOP without many local and systemic side effects. The second-line medications include topical carbonic anhydrase inhibitors (CAIs), sympathomimetics and miotics. They are less effective and usually work together with the first-line medications. In general, the choice of medication(s) depends on the target IOP, the patient's tolerance, general health and monetary status.

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Laser treatments

Several types of laser treatment are available for glaucoma (Table 3). In the Chinese population, the most frequent application of laser treatment is in primary and secondary angle-closure glaucoma, and depends on the mechanism of angle closure. Laser peripheral iridotomy (LI) can relieve and prevent pupil block types of acute angle-closure status. Laser peripheral iridoplasty (LPI) is used to modify the morphology of the peripheral iris to re-open an appositional closure of the trabeculum angle. In open-angle glaucoma, laser trabeculoplasty (LT) to modify the filtration function of the trabeculum is an option. However, its effectiveness in Chinese patients is about that of a second-line medication [12]. Another type of laser treatment is trans-scleral cyclophotocoaglation (TSCPC). It reduces aqueous production by destruction of ciliary epithelium in those advanced glaucoma patients without other better treatment options.

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Laser treatments are relatively safe and effective, especial-ly in Chinese patients with acute and chronic angle-closure glaucoma. LI and LPI have been proven superior as first-line treatments in primary acute angle-closure glaucoma for rapid reduction of IOP and prevention of recurrence, com-pared with the empirical medical approaches [13]. In LT and TSCPC, the effects may not be long lasting. Regular moni-toring is required and the procedures repeated as indicated.

Surgical intervention

Glaucoma surgeries are reasonably safe and effective means for IOP control. The most common surgeries are filtrating surgery and glaucoma drainage devices. Filtrating surgeries can be penetrating, like trabeculectomy, and non-penetrating, like viscocanalostomy/deep sclerectomy. They are commonly indicated in all types of glaucoma that cannot be controlled in a non-invasive manner. They lower IOP by providing an alternative drainage pathway for the aqueous humour and hence achieve a new equilibrium IOP. Another alternative is the use of drainage devices. As with all other surgical interventions, each glaucoma procedure has its own rates: success, quality success, and short-term and long-term complications. Factors contributing to the success of surgery include age, ethnicity, type of glaucoma, use of topical medications, previous ocular surgery and trauma, ocular conditions, use of anti-metabolites, etc.

Patient education and psychosocial support

Chronic glaucoma, like other chronic illnesses, is not curable in most cases. Patients with glaucoma face the possibility of blindness in the future. Besides the physical debility, mental stress is also significant in affecting their overall health. Anxiety and depression are not uncommon in glaucoma patients [14]. Moreover, as the disease is asymptomatic until late, it is important that patients understand the need for treatment, in order to enhance compliance. Those patients judging the effects of treatment subjectively are less likely to comply with prescriptions and clinical visits. They are bothered by the side effects of treatments without having better vision. Hence doctors should spend more time explaining the condition to their patients rather than just prescribing medications. A good understanding of the disease for patients is an important step in starting life-long treatment. Furthermore, a multidisciplinary approach involving general practitioners, ophthalmologists, psychiatrists, social workers and patients' self help societies are beneficial to patients in need.

Conclusion

Glaucoma is a chronic illness requiring life-long management. The goal of treatment is to enable patients to have useful vision in their lifetime and a reasonable quality of life. Besides treating the physical condition by medical means, patients' social and psychological health are equally, if not more, important. A multidisciplinary approach for patients with special needs is desirable.