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Understanding glaucoma: an overview

Norma Ayres
RGN Dip Nursing OND
Glaucoma Nurse
Cavendish Hospital
Buxton
Derbyshire County Primary Care Trust

Glaucoma is an eye condition resulting in permanent damage to the head of the optic nerve and causing gradual reduced visual field. Norma Ayres discusses the importance of early detection and compliance with treatments to help prevent vision loss

Have you heard of "tunnel vision"? Curl your forefinger into the smallest circle possible. Look through the centre of the circle. That is how glaucoma may affect vision if it is not treated. Eventually there may be total loss of vision.

About glaucoma
Hippocrates first described glaucoma as blindness in advancing years with a glazed appearance of the pupil. He was probably describing a cataract. Richard Bannister differentiated this in 1622 describing glaucoma as a hard, blind painful eye.1 In fact, the most common type of glaucoma, open angle glaucoma, does not cause pain or any symptoms at all in the early stages. This is why it may go unnoticed until vision is severely affected. It is estimated that 10% of blindness in the UK is due to glaucoma.

Narrow (or closed) angle glaucoma is a different type and there is less incidence of this. It causes acute high intraocular pressure, a painful, red eye and is treated surgically as an emergency. This article will focus on open angle glaucoma.
Chronic open angle glaucoma is estimated to occur in approximately 2% of people over 40 years old and rises to about 10% in people over 75.2

It is only thanks to regular examinations by opticians that glaucoma is now picked up in the early stages. However, if people do not have regular eye check-ups it goes undetected until it is too late. Once sight is lost it cannot be restored. Fortunately, we tend to notice refractive changes in our vision around middle age so we visit the optician for glasses. The optician will also check the health of the structures inside the eye including the optic nerve head.

The optic nerve has 1.1 million nerve fibres, which are distributed in a common circular pattern in the retinal lining inside the eye. Light enters the eye through the pupil and stimulates the retinal nerve fibres. These impulses are transmitted along the nerve fibres to the brain, where they are interpreted into vision. If nerve fibres are damaged, the vision supplied by the nerve  in that part of the eye will be lost and cause a blind spot in the field of vision.

Everyone has a natural blind spot where the optic nerve enters the eye. This is called the optic nerve head. However, we are not aware of this blind spot. It is small and we are accustomed to it. Similarly, other blind spots caused by glaucoma are not noticed by people until they are quite extensive. People may then complain about a "shadow" in the periphery of their vision.

Central vision is usually retained until the last stages, and is commonly "normal" when assessed by eye test charts. If this were the only test performed then glaucoma would not
be detected.

People may be registered partially sighted if they have loss of peripheral field vision in both eyes which affects their daily activities. It is difficult for other people to comprehend a person's vision restriction and mobility difficulties when they are able to read and see directly ahead. Unfortunately, it is also difficult for someone with glaucoma field loss to accept that their vision is affected. Vision loss is usually slow and progressive over years; people adapt and do not always perceive themselves as having problems and this is particularly dangerous for drivers.

The Drivers and Vehicle Licensing Agency (DVLA) requires car drivers to inform them if they have an eye condition that affects the field of vision of both eyes.3 Results of binocular field tests are assessed by DVLA medical panels. There are stricter guidelines for heavy goods and passenger carrying vehicles who are not allowed to drive with any visual field defect. This is very distressing for all drivers affected and could  have consequences for a person's job and livelihood.  

Detection
Although glaucomatous vision loss cannot be restored, treatment can help prevent further damage and preserve the remaining vision.4

Treatment is by reducing intraocular pressure. Inside the eye there is a continuous production and drainage of fluid called aqueous humour. Aqueous is produced in the ciliary body in the posterior chamber situated behind the iris. It flows through the pupil into the anterior chamber and drains through channels called trabelculum situated in the angle where the cornea meets the iris (see Figure 1). The aqueous is absorbed into the venous drainage system.

[[Figure 1 glauc]]

If too much fluid is produced, or if the trabecular drainage system is blocked, the pressure inside the eye increases. The increase may only be a few millimetres of mercury, but if it is over a long period it may damage the nerve and may affect the circulation to the nerve head.

Other risk factors for glaucoma include: increasing age; being of Afro-Caribbean and African descent; having a close relative with glaucoma; systemic hypertension; steroid medication; and vascular disease. Glaucoma damage may be secondary to other eye conditions. Reducing the intraocular pressure is the only treatment.

Normal intraocular pressure may be between  10–22 mmHg, averaging 16 mmHg. However, a person may have an intraocular pressure reading up to 30 mmHg, and have no optic nerve damage. This is called ocular hypertension. Glaucoma is diagnosed if a person has ocular nerve damage with corresponding field loss. It is associated with raised intraocular pressure, but may be present with an intraocular pressure reading below 21 mmHg. This is called "normal tension" or "low tension" glaucoma. This may be confusing for patients. They may stop their drops if they think their pressure is "normal".5

A relatively recent diagnostic aid indicates previous pressure readings may be incorrect. Assessing intraocular pressure involves measuring how much pressure is required to flatten a 1 mm sphere of the cornea. If the cornea is thicker or thinner than the normal 555 microns, the estimated reading will not be correct. A thinner cornea will give a false low reading, whereas a thicker cornea will give a false high reading. An ultrasound instrument called a pachymeter measures the thickness of the cornea to aid diagnosis.

The optic nerve is assessed by viewing through the pupil. Drawings and photographs record the optic nerve head to compare optic nerve disease progression with visual field loss. A three-dimensional image is required to accurately assess nerve fibre loss.

Instruments obtaining laser imaging of the retinal nerve fibre layer and the optic nerve head now aid more reliable diagnosis. The images obtained detect loss of retinal nerve fibres and also measure the thickness of the nerve fibre layer. Glaucoma nerve fibre loss will correspond with a visual field defect.

Computerised assessment of visual fields is also now more reliable. Previous testing relied on the operator's judgment and technique, and the patient's cooperation. The field of vision was assessed by the operator manually moving a target into a person's field of vision until it was seen. Blind spots and a field loss were plotted manually. This kinetic testing may still be used for people unable to perform computerised tests that assess test reliability.

Treatment
Both surgical and medical treatments for glaucoma are effective and have few side-effects. Eye drops are the preferred choice and surgery is normally only considered if these are not effective. A variety of eye drops are available that act by reducing aqueous humour production or increasing aqueous outflow via the trabecular meshwork. Some drops also stimulate circulation to the nerve.

Categories of drops include beta-blockers, alpha agonists, sympathomimetics, parasympathetics, carbonic anhydrase inhibitors and, more recently, prostaglandin analogues. These drugs may cause ocular and systemic side-effects. Allergies, irritation and transient blurred vision are not uncommon. Systemic absorption of beta-blockers, for example, may cause respiratory depression. A person's health and other medications are considered before eye drops are prescribed.

One can understand patients' noncompliance with treatment if drops are causing problems when glaucoma is asymptomatic initially. Patient and carer education and follow-up evaluation are vital. Considering patient circumstances and discussing lifestyles when deciding on treatments also helps
with compliance.6

Drops are most effective if they are absorbed through the cornea into the eye. Some of the drop is absorbed through the conjunctiva vessels and some may drain via the lachrymal duct into the throat and be absorbed systemically. If drops are absorbed into the circulation they are more likely to cause side-effects. Closing the eye and occluding the punctum (blocking the corner of the eye) after instilling the drop for one minute helps to prevent this. If multiple drops are used, an interval of at least 10 minutes between drops should be given to avoid washout of the first drop.

The drops will aid constant control of intraocular pressure if they are instilled at regular intervals throughout the day as prescribed; for example, every 12 or eight hours. Fluctuation of intraocular pressure may be more damaging to the optic nerve.7

Several major studies have investigated effects of treatment, including the Early Manifest Glaucoma trial, which concluded that treatment reduced disease progression by 27.4%.4

Conclusion
Glaucoma is an impairment causing disability and handicap. Regular eye tests in people over the age of 40 aid detection of glaucoma.

Public and patient education may raise awareness of the effects of glaucoma. The International Glaucoma Association (IGA) (www.iga.org.uk) provides patient information and resources for patient support groups. This may improve compliance with regular glaucoma assessments and prescribed treatments to help reduce risk of progression of visual loss. Glaucoma is difficult to diagnose in the early stages and early detection and treatment can reduce progressive damage to the optic nerve and preserve sight.

References
1. Glaucoma. Available from: http://www.mrcophth.com/Historyofophthalmology/glaucoma.html
2. National Institute for Health and Clinical Excellence. Glaucoma: draft scope. London: NICE, 2007. Available from: http://www.nice.org.uk/guidance/index.jsp?action=download&o=34368
3. Driving and Vehicle Licensing Agency. At a glance guide to the current medical standards of fitness to drive. Swansea: DVLA; 2008. Available from: http://www.dvla.gov.uk/media/pdf/medical/aagv1.pdf
4. Heijl A, Leske MC, Bengtsson B et al. Reduction of intraocular pressure and glaucoma progression: results from the Early Manifest Glaucoma Trial. Arch Ophthalmol 2002;120(10):1268–79.
5. Royal College of Ophthalmologists. Guidelines for the Management of Open Angle Glaucoma and Ocular Hypertension. London: RCO; 2004. Available from: http://www.rcophth.ac.uk/docs/publications/published-guidelines/glaucoma...
6. Tsai JC. Medication adherence in glaucoma: approaches for optimizing patient compliance. Curr Opin Ophthalmol 2006;17(2):190–5.
7. Satilmis M, Orgül S, Doubler B, Flammer J. Rate of progression of glaucoma correlates with retrobulbar circulation and intraocular pressure. Am J Ophthalmol 2003;135,5: 664–9.