What is glaucoma

  • Glaucoma is the leading cause of irreversible blindness worldwide.
  • 1 in 10 Australians aged over 80 years will develop glaucoma.
  • First degree relatives of glaucoma patients have an 8-fold increased risk of developing the disease.
  • At present, 50% of people with glaucoma in Australia remain undiagnosed.

Glaucoma is a group of diseases, where the optic nerve is damaged at the optic nerve head (where the optic nerve is connected to the eye). The cause of this damage can be multiple and varied, and the worldwide consensus among glaucoma specialists have moved from thinking of elevated intraocular pressure (pressure in the eye) as the only damaging factor in glaucoma, to thinking of it as only one factor in the damage causing glaucoma.

The optic nerve, made up of millions of nerve fibres – like an electrical cable, is critical for the transmission of information from the eye to the visual centers of the brain. Damage to the optic nerve results in the development of blind spots, but in glaucomatous optic neuropathy, because damage is confined at the optic nerve head, characteristic visual field defects results. Fortunately, in the majority of early glaucoma, the peripheral visual field is often affected before central vision is lost. As a consequence, glaucoma is often referred to as the “Sneak thief of sight” as individuals remain unaware of subtle progressive peripheral vision loss until too late when central vision becomes affected.

Worldwide, glaucoma is the leading cause of irreversible blindness. It affects 1 in 10 Australians aged over 80 years old. Up to 50% of people with glaucoma are unaware of their disease as it remains undiagnosed. Blindness from glaucoma is often preventable. Early detection and treatment by your ophthalmologist is the key to preventing optic nerve damage and blindness.

Types of glaucoma

Glaucoma is often categorized in terms of whether the angle is Open or Closed. This categorization is of importance to your treating ophthalmologist because the treatment offered is subtly different depending on the category of glaucoma.

figure1

Figure 1: The drainage angle. Image from the American Academy of Ophthalmology – Glaucoma patient education pamphlet

Where is the angle and what is its function?

The angle just describes the space inside the eye where the peripheral iris (coloured part of the eye) meets the peripheral cornea (clear window) on the internal surface at the front of the eye (Figure 1). The importance of the angle is that this is where the trabecular meshwork is found. The trabecular meshwork is a ring of tissue found at the entire 360 degree peripheral iris-corneal junction within the eye. The trabecular meshwork is the main outflow drainage pathway of aqueous fluid from the eye.

Aqueous fluid is constantly produced within the eye, and circulates to provide nutrients and remove wastes to sustain the delicate internal structures of the eye. Additionally, it maintains a constant pressure within the eye to keep the eyeball inflated. The reader should be aware that aqueous fluid is entirely different to tears, that are found and produced on the outside surface of the eye to lubricate, provide nutrients and remove waste and debris from the surface of the eye.

To maintain a stable pressure within the eye, the rate of aqueous inflow must equal the rate of aqueous outflow. When this becomes unbalanced, and the outflow of aqueous fluid does not keep up with its inflow, the pressure within the eye (intraocular pressure) rises. This elevated pressure in combination with the status of your optic nerve, genetic predisposition, age, and other risk factors determine the likelihood and rate of development of glaucoma.

Open angle glaucoma
Open angle glaucoma is the most common form of glaucoma in Australia, due to our large Caucasian population. In open angle glaucoma, the trabecular meshwork of the eye becomes less efficient at draining the aqueous fluid.

Open angle glaucoma can be associated with “high” or “normal” intraocular pressures – the arbitrary separation of High pressure glaucoma and Normal pressure (tension) glaucoma relates to dated definitions of glaucoma. Erroneously, glaucoma was once thought to be solely caused by high intraocular pressures. Our present understanding of glaucoma as an optic neuropathy (optic nerve disease) does NOT depend on a specific level of intraocular pressure. Recall, intraocular pressure is now considered one of the risk factors for glaucoma. Glaucoma can still affect individuals with “normal” intraocular pressures because the intraocular pressure is too high for that individual, given his/her specific susceptibility to glaucoma based on genetic predisposition, age, and optic nerve status.

Angle closure glaucoma
Angle closure glaucoma occurs when the drainage angle, hence the trabecular meshwork, becomes blocked by the peripheral iris (coloured part of the eye). Angle closure glaucoma is more common in persons of Asian descent, and those with hyperopia

(farsightedness). As a consequence of angle closure, no aqueous is able to exit the eye through the trabecular meshwork pathway and the intraocular pressure rises.

If angle closure occurs suddenly, the intraocular pressure may rise suddenly causing an ACUTE angle closure attack. Acute angle closure is an Ophthalmic Emergency requiring urgent treatment to prevent blindness that may occur over hours or days. The symtoms of acute angle closure is characterized by severe brow pain, blurred vision, halos, nausea, vomiting and a red painful eye.

Risk factors for Primary glaucoma

Age

Advancing age is a major risk factor for the development of glaucoma. Data from the Melbourne Visual Impairment Study (MVIP) showed that in its population, the prevalence of glaucoma rose exponentially with age from 0.1% at ages 40 to 49 years to 9.7% in persons aged 80 to 89 years. Data from the Blue Mountains Eye Study (BMES) found a prevalence of glaucoma in persons aged 49 years and over as 3.0% (95% confidence interval, 2.5-3.6%).

The number of Australians aged 50 and over in the year 2000 with glaucoma was estimated to range between 144 000 persons (BMES) to 167 000 persons (MVIP). With the aging population, it has been predicted that the number of Australians aged 50 and over with glaucoma in the year 2030 could rise to as many as 307 000 persons (BMES) to 337 000 persons (MVIP).

Family History of glaucoma

A family history of glaucoma puts an individual at greater risk of developing the disease. Individuals with a close relative with primary open angle glaucoma have a 3 – 6 times prevalence of glaucoma than standard population.

Evidence strongly supports that all individuals with a first-degree relative diagnosed with glaucoma are considered at high risk of developing glaucoma themselves. It is recommended that such individuals undergo a full ocular examination and ongoing monitoring for glaucoma. Additionally, all patients diagnosed with glaucoma should alert first-degree relatives of the benefits of ocular examination and screening for glaucoma.

Intraocular pressure

The medical literature is clear that high intraocular pressure is a significant risk factor for glaucoma. Different individual’s optic nerves vary in their susceptibility to elevated intraocular pressure. This susceptibility depends on other risk factors such as the health of the optic nerve, corneal thickness, age, family history etc.

Strong evidence supports the decreased risk of glaucoma development and progression with intraocular pressure reduction. The target level of intraocular pressure will need to be individualized depending on each person’s risk profile and other concurrent health conditions. Your treatment may include any combination of medications, laser surgery or incisional surgery.

Ethnic origin

People of African descent have been identified to have an age-adjusted prevalence of primary open angle glaucoma 4.3 times greater than Caucasians. In contrast, for primary angle closure glaucoma, individuals of Asian or Inuit descent are reported to have 3 – 10 times greater rates compared to other ethnic groups.

Refractive error

Myopia (nearsightedness) is a risk factor for development of primary open angle glaucoma, with pooled data from population studies estimating almost a doubling in relative risk of glaucoma in myopes compared to non-myopes.

Hyperopia (farsightedness) is a risk factor for development of primary angle closure glaucoma. This is may be consequent of cataract development in an anatomically predisposed eye.

Central Corneal Thickness

Thickness of the cornea (window at the front of the eye – see Figure 1) affects calibration of the instruments used to measure intraocular pressure. This is because all instruments effectively “touch” the cornea – even those that use a puff of air. The mechanical properties of the cornea therefore affect our intraocular pressure readings.

Data from the Ocular Hypertension Treatment Study suggest that persons with thinner corneas are at increased risk of developing glaucoma. Assessment of your central corneal thickness is a useful component in your assessment of risk for glaucoma.

Diabetes

There is conflicting data on the association between type 2 diabetes mellitus and primary open angle glaucoma, but evidence from many population-based studies including the Blue Mountains Eye Study (Australia) and Beaver Dam Eye Study (USA) have shown a greater risk of glaucoma in persons with type 2 diabetes.

In any event, all persons with type 2 diabetes should undergo regular ophthalmic review to monitor their retina for development of other diabetic eye diseases.

Systemic hypertension (high blood pressure)

There remains a paucity of evidence linking high blood pressure and primary open angle glaucoma. The relationship is complex and may involve optic nerve perfusion pressure dependent on relationships between systemic blood pressure (pressure within blood vessels of the body), intracranial pressure (pressure inside the skull around the brain and optic nerves) and intraocular pressure.

Low systemic blood pressure, especially overnight when asleep, may pose a risk factor for progressive optic nerve damage in normal tension glaucoma. Your ophthalmologist may sometimes organize a 24-hour blood pressure monitor to check for dips in systemic pressure overnight.

Long-term steroid users

Corticosteroids are the main source of drug-induced glaucoma. Corticosteroids administered by any route can result in increases in intraocular pressure that can lead to glaucoma. Steroid-like substances can also be found in some natural and traditional medicines. You should inform your ophthalmologist if you are taking any of these. Evidence indicates that long-term steroid users are at increased risk of glaucoma and should undergo surveillance.

Migraine and peripheral vasospasm

Vasospasm has been proposed as a potential mechanism for optic nerve damage in glaucoma, particularly in normal tension glaucoma. Migraine has been identified in several international studies such as the Blue Mountains Eye Study and the Ocular Hypertension Treatment Study as a risk factor for glaucoma progression. Evidence indicates that individual with vasospastic conditions such as migraine and Raynaud’s syndrome are at increased risk of glaucoma.

Previous Eye trauma

All individuals with a history of significant blunt eye trauma or penetrating eye injury are at increased risk of developing glaucoma.

Diagnosis of glaucoma

Glaucoma can only be diagnosed on the basis of multiple sources of information, quite like a jigsaw puzzle, that includes a comprehensive history, assessment of risk factors, and a full ocular examination including the structural and functional characteristics of the drainage angle and optic nerve head. Intraocular pressure measurements are only one small component in this process, and should not be relied upon for a diagnosis. Sometimes, before a diagnosis of glaucoma can be made, you may require repeated evaluation of optic nerve structure and function to look for evidence of change or progression over time.

Why then do we treat intraocular pressure in glaucoma?

Most eye doctors would now agree that glaucoma is actually a group of conditions, characterized by a particular form of optic nerve damage that is often but not always associated with elevated intraocular pressure. The reason that glaucoma management still largely depends on reducing intraocular pressure is that intraocular pressure is the most reversible of the multitude of risk factors that leads to glaucoma, and that reducing this risk factor has the strongest medical evidence for showing treatment benefit and preventing the progression of glaucoma. Treating ophthalmologists therefore individualize treatment to optimize an intraocular pressure that is considered too high for the continued health of the eye.

As previously mentioned, the term “normal tension glaucoma” is a relic of the medical profession’s older terminology and definitions of glaucoma. Normal tension glaucoma can be considered a subset of primary open angle glaucoma, and its management is largely the same as the high tension glaucoma – principally of reducing intraocular pressure. One issue with normal tension glaucoma is that because the starting ocular pressure is already low, it is harder to effect a further significant reduction in intraocular pressure. There is ongoing research into alternative strategies such as neuro-protection or optimizing optic nerve blood flow as potential strategies to manage these difficult to treat cases.

How do we lower intraocular pressure?

Current glaucoma management is based on the triad of medications, laser surgery, incisional surgery, or any combination of the three.

Medication

Medications help by either reducing the production and inflow of aqueous fluid into the eye, or by increasing the outflow pathways to allow aqueous fluid to drain more effectively from the eye. Some medications have the ability to do both. The critical factor with medications is that it requires cooperation from patients. Remember
– drops can only work if you use them.

Laser

Laser surgery may be performed in some cases of glaucoma. Different lasers are used to treat open and closed angle glaucoma. Laser can be applied to the iris or the trabecular meshwork to allow aqueous fluid to flow more effectively within the eye, and drain better by the normal drainage channels within the eye. Laser surgery, unlike incisional surgery, will not create a physical opening between the inside of the eye and the outside of the eye.

Incisional surgery (trabeculectomy or glaucoma drainage device)

Incisional surgery may sometimes be required if the disease cannot be controlled using medications or laser, or the patient is intolerant of the above strategies. The requirement for incisional surgery becomes more urgent the more aggressive or advanced your glaucoma becomes. Incisional surgery creates an alternate pathway for aqueous fluid to exit the eye. This fluid is drained by physically creating a

pathway from within the eye to the space just under the whites of the eye under the upper eyelid (subconjunctival and subtenon space).
Aqueous fluid will then be absorbed by the fine blood vessels that are naturally found on the whites of the eye.

Summary

Glaucoma is the leading cause of irreversible blindness in the world. If detected early, blindness is usually preventable. Today, up to 50% of the population with glaucoma still do not know that they have the disease. We now understand the disease process more clearly, and research has continued to identify risk factors. If you are aware of any of these risk factors, talk to your general practitioner or eye care practitioner to seek a full assessment.

References

  • NHMRC Guidelines for the Screening, Prognosis, Diagnosis and Prevention of Glaucoma, 2010
  • Glaucoma Australia (www.glaucoma.org.au)
    • Glaucoma information booklet – Glaucoma the thief of sight
  • American Academy of Ophthalmology (www.aao.org)
    • Preferred Practice Pattern – Primary Open Angle Glaucoma, October 2010
    • Preferred Practice Pattern – Primary Angle Closure, October 2010
    • Glaucoma patient education handbook – Glaucoma: An In- depth look, 2008
    • Glaucoma patient education pamphlet – Glaucoma: A closer look, 2009
  • Tarek M. Shaarawy, Mark B. Sherwood, Roger A. Hitchings, Jonathan G. Crowston eds. Glaucoma. Volume 1 Medical Diagnosis and Therapy. Philadelphia: Saunders Elsevier; 2009.
  • Wensor MD, McCarty CA, Stanislavsky YL, Livingston PM, Taylor HR. The prevalence of glaucoma in the Melbourne Visual Impairment Project. Ophthalmology. 1998 Apr;105(4):733-9.
  • Rochtchina E, Mitchell P. Projected number of Australians with glaucoma in 2000 and 2030. Clin Experiment Ophthalmol. 2000 Jun;28(3):146-8.
  • Mitchell P, Smith W, Attebo K, Healey PR. Prevalence of open-
    angle glaucoma in Australia. The Blue Mountains Eye Study. Ophthalmology. 1996 Oct;103(10):1661-9.