What is Glaucoma?
Glaucoma is one of the leading causes of irreversible blindness in Singapore. It is a disease characterized by progressive damage to the optic nerve, which are made up of the nerve fibers that connect our eyes to our brain. Damage of the optic nerve leads to irreversible loss of our visual field, starting from peripheral field and progressing centrally to cause ‘tunnel-like’ vision. In severe cases, total vision loss and blindness may occur.
There are many different types of glaucoma with different causative mechanisms. Broadly, glaucoma can be divided into: Open-angle glaucoma and Angle-closure glaucoma. Simply explained, the angle of our eye is the drainage site of the fluid in our eyeball. Any compromise to the drainage system at our angles will lead to a build-up in eye pressure and consequently, glaucoma.
Glaucoma is frequently associated with high eye pressures, although it can occur even in people with seemingly normal eye pressures, due to a state of poor oxygen perfusion to their optic nerve. In most early cases, there are no symptoms of high eye pressure; which means most people are unaware that they even have an eye problem. That is why it is important to get your eyes checked annually to detect any signs of early glaucoma. This is especially so in people who have risk factors for glaucoma.
Risk factors for Glaucoma:
- Family members who have been diagnosed with glaucoma: We know that there is a genetic predisposition for glaucoma and the disease tends to run in families. Those with family members with glaucoma should get checked annually from the age of 40 years.
- Systemic Hypertension: It has been shown that people with high blood pressure may be more likely to have high eye pressure, hence may be at higher risk for glaucoma.
- Diseases with poor blood circulation such as diabetes mellitus, previous stroke, heart disease, migraine: When the blood circulation to our optic nerve is poor, the millions of tiny nerve fibres within it start to get starved of oxygen. Some of the nerve fibres will eventually die and blind spots in vision will appear. It is when all the nerve fibres die, that all vision will be lost.
- Obstructive Sleep Apnea: It is well-established that there is increased prevalence of glaucoma in obstructive sleep apnea. The mechanisms by which obstructive sleep apnea causes glaucoma are complex and are a combination of hypoxia, vascular and mechanical factors. It has also been shown that glaucoma progression is related to the severity of obstructive sleep apnea.
- High Myopia: It is well-known that people with high degree of short-sightedness are at higher risk of glaucoma. This could be due to the fact that myopic optic nerves are structurally weaker and hence more susceptible to nerve fibre loss than non-myopic optic nerves.
- Thin corneas: People who have thin corneas (less than 500 micrometres) will show artificially low eye pressure readings. This is dangerous because if actual eye pressure is higher than the measured eye pressure, such people are at risk of developing sight-threatening glaucoma without them or their doctor knowing. Without realizing it, a large percentage of our population have such thin corneas. These are the people who have had prior LASIK because LASIK is a procedure which uses laser to cut and shave down the thickness of cornea in order to correct refractive errors like myopia. Hence, people with prior LASIK would need continuous monitoring with a competent eye doctor who is meticulous about adjusting measured eye pressure readings accordingly.
- Prolonged use of steroid medication in any form: There is evidence that links steroid use to glaucoma. Prolonged use of steroid in the form of oral medication, nasal sprays, inhalers or eye drops can lead to an increase in the intraocular pressure. It is well-known that approximately 40% of our population are termed as ‘steroid-responders’, which means that they will develop a high eye pressure response to the use of steroids.
- Advanced cataracts: When the cataractous lens thickens, it tends to narrow the angles and compromise our eye’s drainage system. People who have very thick cataracts are more likely to suffer from angle-closure glaucoma.
Glaucoma could be related to Severe Snoring
Snoring is such a common phenomenon and yet did you know that people with severe snoring could really have obstructive sleep apnea? That means that in your state of deep sleep and muscle relaxation, your airway could get so narrowed that it becomes difficult to get enough air in – this commonly manifests as snoring. A lack of air leads to oxygen deprivation in many organs – hence obstructive sleep apnea is commonly related to early-onset hypertension, heart disease and even stroke.
Most of us are now aware that obstructive sleep apnea results in a state of hypoperfusion (or insufficient oxygen) to our tissues while asleep. However, many of us are NOT aware that such hypoperfusion states in our eye tissues could lead to various eye diseases. There has been increasing evidence that obstructive sleep apnea is associated with glaucoma, ischemic optic neuropathy, central serous chorioretinopathy and other retinal diseases. Since glaucoma is one of the most serious sight-threatening diseases common to our ageing population, let us look at the risk factors of glaucoma, how it is related to obstructive sleep apnea and why we should get checked early.
Although complex, we now know that these various factors are responsible for the development of glaucoma in obstructive sleep apnea:
- Hypoxia: Oxygen is important for the proper functioning of all cells. Hypoxia describes the lack of oxygen and subsequent oxidative stress which takes place in cells. Prolonged episodes of hypoxia, which is what happens in obstructive sleep apnea, directly damages the optic nerve fibres and results in glaucoma.
- Vascular factors: Obstructive sleep apnea is well-known to be associated with various vascular changes including hypertension, atherosclerosis of carotid artery and increase in intracranial pressure. This is also related to definite vascular changes and dysregulation of the optic nerve. Such vascular changes disturb the blood flow to the optic nerve and is exacerbated by states of nocturnal hypotension in obstructive sleep apnea.
- Mechanical factors: Apart from the supine position that is adopted at night to sleep, obesity is also another risk factor for high eye pressures due to excessive intraorbital fatty tissue and increased episcleral venous pressure.
What are the Symptoms of Glaucoma?
Very often, there are little to no symptoms of glaucoma. This is because elevated eye pressure (more than 21mmHg) is insidious and undetectable by patients. Even vision loss which takes place from the peripheral visual field goes unnoticed until too late. It is only when eye pressure is extremely high above 35 or visual field is severely narrowed, that patients are alerted. At that stage, glaucoma may be too advanced and visual loss may be irreversible.
How is Glaucoma Detected?
Patients who are at risk of glaucoma should have the following assessed by their eye specialist:
- Eye pressure: An eye pressure above 21 mmHg is considered high and at risk of glaucoma. Patients with a thin cornea should have an even lower threshold than 21 mmHg.
- Cornea thickness: A person’s actual eye pressure must be adjusted according to his/her cornea thickness.
- Angle assessment: How open or narrow a person’s angles will contribute to his/her glaucoma risk.
- Optic nerve appearance: By looking at the optic nerve, a well-trained eye specialist will be able to assess the risk of glaucoma. A healthy optic nerve has optimal color and thickness; while a glaucomatous optic nerve has pallor and thinning of the nerve rims.
- Retinal Nerve Fiber Layer Thickness: With advanced eye imaging technology, we are able to measure the exact thickness of the retinal nerve fiber layer thickness and compare this to the normal population data to a determine whether there is glaucoma risk. This measurement is also useful for subsequent follow-up visits because the machine is able to track the thickness progression over years.
- Visual Field assessment: Patients with glaucoma will have areas of their visual field that are affected and appear diminished to them. By performing a visual field test, we are able to map out the area of visual field defects that are present in glaucoma and determine whether the glaucoma disease is progressively deteriorating.
How is Glaucoma Treated?
Treatment of glaucoma is targeted at treating the underlying cause and reducing the eye pressure. It is thought that with a decrease in eye pressure, blood perfusion to the optic nerve can be improved.
The first line of therapy is usage of eye pressure-lowering eye drops. There are a multitude of glaucoma eye drops, each with its own specific benefits and side effects. Glaucoma eye drops should be individualized and customized according to the individual’s characteristics and needs.
When glaucoma cannot be adequately controlled by eye drops, it is then prudent to consider glaucoma laser therapy or glaucoma surgery for better glaucoma control. Laser and surgery are not without risks, hence the decision to proceed should be made after detailed discussion with your eye specialist.
Since there is no cure for glaucoma and visual loss is irreversible, the importance of early glaucoma detection cannot be further emphasized. In fact, it is recommended that patients with obstructive sleep apnea should get screened for glaucoma and other eye diseases by an eye specialist.
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