WHO has estimated that 4.5 million people are blind due to glaucoma
Glaucoma is the second leading cause of blindness and the leading cause of irreversible blindness worldwide.
The term ‘glaucoma’ relates to a group of conditions that can be classified in a variety of ways, but all of which are characterised by optic nerve damage (often referred to as cupping) and visual field loss (often with an arcuate pattern of mid-peripheral loss), secondary to retinal ganglion cell damage and death. The most common types of glaucoma are Primary Open Angle Glaucoma (POAG) and Primary Angle Closure Glaucoma (PACG).
It has been estimated that by 2020 there will be approximately 80 million people with glaucoma, an increase of about 20 million since 2010. Furthermore, it is thought that at present over 8 million people are bilaterally blind due to glaucoma, a figure that is set to rise to over 11 million by 2020 with the increasing prevalence, unless improved screening and effective treatment strategies are successful.
Risk factors for the development of glaucoma include intraocular (eye) pressure (although it is important to recognise that glaucoma frequently develops with so-called ‘normal’ intraocular pressure [IOP]), increasing age, positive family history for glaucoma and ethnicity. POAG is most common in white Caucasians and black individuals of African origin. PACG is most common in South-East Asians and worldwide about 33% of individuals with primary glaucoma have PACG. PACG is associated with a greater risk of blindness in comparison with POAG. Although elevated IOP is the major risk factor for glaucoma the condition is not considered to be a direct consequence of the pressure, but to relate to IOP-associated risk factors such as stress susceptibility of the optic nerve supporting structures and optic nerve blood flow, that are affected by IOP. Certain individuals can sustain a degree of IOP elevation without the development of glaucoma and are referred to as having Ocular Hypertension (OH), although as a group such individuals remain at increased risk of developing glaucoma with time. Small eye size (low axial length, often associated with a hypermetropic refraction) and other anatomical or pathophysiological ocular features that increase the risk of pupil-block (increased resistance to flow of aqueous from the posterior to anterior chamber), are the major risk factors for PACG. The secondary types of glaucoma, associated with other ocular or systemic conditions, are much less common than primary glaucoma, but can be more devastating to visual function. Congenital and paediatric types of glaucoma are uncommon, but have significant life-long implications for those affected.
Most forms of glaucoma are bilateral, but asymptomatic in their early stages, although sub-acute angle closure ‘attacks’ may alert individuals to their condition of susceptibility to, or manifest, PACG. However, most cases of PACG (>75%) are chronic in nature and the highly symptomatic acute angle closure glaucoma remains relatively rare. Most people with glaucoma have a slowly progressive, but irreversible course of deterioration. Late presentation with significant irreversible loss of vision remains a major problem in certain populations. Mass population screening for glaucoma may not be economic, whereas screening high-risk subsets might prove to be beneficial. Screening for glaucoma requires optic nerve examination, visual field analysis, measurement of IOP and, for classification purposes, a full ophthalmic examination including an assessment of the irido-corneal drainage angle (eg gonioscopy). At present, glaucoma case-finding is generally opportunistic and processes to achieve this vary greatly from country to country. In the future it is hoped that improved optic nerve imaging and/or visual function analysis will make economic screening for glaucoma (with high specificity and sensitivity) a realistic proposition. Increasing public awareness about glaucoma and the consequences of late diagnosis or inadequate treatment is essential. About 50% of all glaucoma cases are undiagnosed in developed countries, this figure being as high as 90% in the developing parts of the world.
Knowledge about glaucoma has increased due to several large epidemiological studies and large multicentre clinical trials (see Additional Resources) that have added to our knowledge about the natural history of open angle glaucoma and the effect of treatment. Current basic science and clinical research continues in an attempt to unravel the many unknowns relating to aetiology and management of glaucoma on a worldwide basis.
The mainstay of treatment for open angle glaucoma involves reducing IOP by 20-40%, which can be achieved using medical, laser or surgical strategies. At present the majority of treated patients are prescribed topical medications (prostaglandin analogues, beta-blockers, carbonic anhydrase inhibitors or alpha-agonists; alone or in combination). Availability and persistent adherence to topical medications remains a significant problem and side effects both local and systemic can limit their use. Laser treatment of the trabecular meshwork (eg selective laser trabeculoplasty) is virtually free of adverse effects, but long-term effectiveness has yet to be determined and lasers are not available throughout all countries. There are a variety of highly successful surgical procedures to lower IOP, but the risks of iatrogenic visual loss, although rare, are higher in comparison with non-surgical treatments, so that surgery has not become a popular initial strategy in the management of glaucoma. Laser partial ablation of the ciliary body (eg cyclodiode) is frequently effective in lowering IOP, but is an irreversible method and the perception of many is that this risks hypotony (over-treatment) and associated loss of vision. At present management choices throughout the world depend on availability of the various therapeutic modalities, this correlating highly with the socioeconomic status of the particular country.
The mainstay of treatment for (or prevention of) PACG is the provision of a peripheral laser iridotomy or surgical iridectomy (ie a hole in the iris that connects the posterior and anterior chambers, so bypassing any potential, or established, pupil block to aqueous flow). In early PACG (acute or chronic) the provision of a laser iridotomy can be curative. Prophylactic laser iridotomy procedures are carried out in individuals considered to be at risk of PACG, but the predictive power of tests to identify those at particular risk are poor at present, resulting in many patients having what is effectively unnecessary intervention. Once PACG is manifest and there is significant permanent irido-corneal closure (with peripheral anterior synechiae), treatment strategies are essentially the same as for POAG. Lens extraction (cataract surgery) is an effective method by which pupil-block can be prevented and current research is assessing the role of cataract surgery in the management of PACG.
The treatments for secondary glaucoma have to be tailored to the individual and address the management of the associated comorbidity in addition to the glaucoma. Advances have been made in the management of childhood glaucoma, a situation where early surgery is frequently considered essential.
In the future it is hoped that the development of sustained-release, gene and/or stem cell therapies will improve the outcomes of anti-glaucomatous therapy. Eventually, it is hoped that effective novel neuro-protective or ideally neuro-regenerative therapies will become available.
(Contributed by Professor David C Broadway BSc(Hons) MD FRCOphth DO(RCS) Consultant Ophthalmic Surgeon, Norfolk & Norwich University Hospital)
In February 2016, the International Council of Ophthalmology (ICO) published the first edition of its Guidelines for Glaucoma Eye Care. Download the ICO Guidelines for Glaucoma Eye Care from the ICO website at: http://www.icoph.org/glaucoma
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