Glaucoma is the second leading cause of vision loss globally, according to the World Health Organization. (1)
It slowly sneaks up on the eyes of its victims and robs them of their precious eyesight, without early warning symptoms. It is a progressive disease that causes preventable blindness and whose visual impairment effects can be arrested before causing significant damage.
Read on to find out what glaucoma is, its causes, diagnosis, treatment and what can be done to prevent it.
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What is glaucoma?
Glaucoma is a group of eye disorders that cause damage to the optic nerve, the nerve that connects the eye to the brain. The optic nerve, also known as the cranial nerve II or simply CN II, carries visual impulses formed by the retina to the brain, where visual information is interpreted. (2) Glaucoma affects both eyes, but one may be more severely affected than the other.
What causes glaucoma?
Glaucoma is linked to building up of higher than normal pressure (intraocular pressure) in the chambers of the eye. The range for normal pressure lies between 12 and 21mmHg, 14mmHg being the average. (3)
A pressure reading of more than 22mmHg is considered to be abnormally high. Not unless this pressure is controlled, the optic nerve and other parts of the eye become damaged, leading to loss of vision.
Types of glaucoma
There are several types of glaucoma, depending on the cause. The two main categories are open angle glaucoma and narrow-angle glaucoma. “Angle” in this case refers to the position between the iris and cornea, the location where aqueous humor drains out of the eye, into the circulatory system.
Aqueous humor is the fluid produced by the eye’s ciliary body into the anterior chamber (the region between the cornea and lens) through a trabecular meshwork and then into a drainage canal.
1.Primary open angle glaucoma
Primary open angle glaucoma (POAG) is the most common type. According to the American Academy of Ophthalmology, it affects more than 2 million people in the United States alone, and the unfortunate thing is that this figure is expected to increase to more than 3 million by 2020. (4) This accounts for more than 90% of the glaucoma cases. (5)
POAG occurs when the eye’s drainage canals are blocked. When drainage canals are blocked, pressure builds up within the eye. They blockage occurs slowly and is a lifelong condition. As mentioned earlier, this increased pressure ultimately destroys the optic nerve cells. Once a significant number of cells are damaged, blind spots begin to form in the field of vision.
In angle-closure glaucoma, the aqueous humor cannot drain properly because the drainage canal is either too narrow or completely blocked. In this type, the pressure can rise suddenly, and even be triggered by something simple like pupil dilatation. It causes sudden eye pain, headaches, blurred vision, and nausea. (6)
It has a closed or narrow-angle (45 degrees) between the iris and cornea, and the anterior chamber of the eye is usually smaller than average. The ability of the aqueous humor to pass through the iris and lens on its way to the anterior chamber then decreases. This then causes fluid pressure to build up behind the iris and consequently narrowing the angle further.
Angle-closure glaucoma is also known as acute glaucoma. It requires immediate medical attention or else it can deteriorate quite fast. (7)
3.Normal tension glaucoma
Although most cases of glaucoma occur when there is ocular hypertension, sometimes it happens when intraocular pressure is normal. That is the case in normal tension glaucoma (NGT)- the optic nerve is damaged although the pressure is not excessively elevated.
It develops slowly over time and is thought to be related to inadequate blood flow to the optic nerve, which leads to necrosis of the nerve cells.
Even though the intraocular pressure in normal tension glaucoma is usually normal or high-normal, there is a need to lower it, as the eyes remain susceptible to pressure-related damage. A lower than usual range is thus necessary to prevent vision loss. (8)
Normal tension glaucoma is also known as low-tension glaucoma. It is more common in people with a history of vascular disease.
Secondary glaucoma develops as a complication of other eye problems such as trauma, cataracts, diabetes, eye surgery or tumors.
Congenital glaucoma occurs in babies as a result of incomplete development of the eye’s drainage canals during prenatal development. In most congenital glaucoma cases, the condition can be successfully treated by microsurgery to correct the structural defects. (9)
Who is at risk of developing glaucoma?
Glaucoma tends to be inherited and is more likely to show up in later life. Other than genetic predisposition and advanced age (40 years and older), other risk factors include African, Latino or Asian ancestry, myopia, chronic illnesses such as diabetes and hypertension, previous eye injury, regular or long-term use of cortisone (especially eye drops), a thin cornea, and chronic eye inflammations. (10)
What are the signs and symptoms of glaucoma?
In its early stages, glaucoma has few or no symptoms. That means that by the time it manifests, it has progressed to a point where it’s difficult to stop or even to the point of irreversible vision loss. That is why eye specialists call glaucoma the “silent thief of sight.” (11) However, for angle-closure glaucoma, the clinical manifestations occur suddenly and require immediate specialist attention.
The signs and symptoms include:
- Hazy, fogy or blurred vision
- Gradual loss of peripheral vision
- Tunnel vision in the advanced sages
- Appearance or rainbow-colored circles (halos) around bright lights
- Severe throbbing eye pain
- Nausea and vomiting that is accompanied by eye pain
- Sudden vision loss
- Dilated pupils
- Eye redness
Because it takes time for most people to have a noticeable change in vision when there is glaucoma, it is important to undergo regular eye examinations by an eye specialist.
How is glaucoma detected?
The way to diagnose glaucoma is by doing a comprehensive eye examination, thorough history taking and blood tests if needed. During the eye examination, the ophthalmologist assesses the following:
- Inner eye pressure
- Drainage angle
- Shape and color of the optic nerve
- The complete field of vision
- The thickness of the cornea
The tests used include:
Tonometry is a quick and fast test to check intraocular pressure by measuring the tone or firmness on the surface of the eye. (12)
The specialist uses anesthetic eye drops to numb the eye, then applies some light pressure on the surface of the eye, to slightly indent the cornea. The resistance of the indentation is then measured using a tonometer. The firmer the tone of the eye, the higher the pressure reading will be.
Ophthalmoscopy helps the doctor examine the back part of the eye, known as the fundus. It consists of the retina, optic disc, and blood vessels.
The specialist uses a unique instrument known as an ophthalmoscope. Before the procedure, the pupils are dilated using eye drops, to make them larger and easier to look through. The doctor then looks through the ophthalmoscope lenses to examine the shape and color of the optic nerve. (13) If the intraocular pressure is elevated and the optic nerve does not appear normal, the doctor orders for additional tests.
The perimetry test is used to measure the visual field function and to detect dysfunction in central and peripheral vision. To do the test, the specialist asks an individual to sit inside a perimeter (bowl-shaped instrument). (14)
Then one gazes at the center and is asked to press a button each time one sees a flash. A computer then records the spot of each flash and whether one pushed the button at that specific spot. At the end of the test, a print out is made to show the areas of vision (visual fields). From the results, the doctor can then determine whether there are any patterns of vision loss.
With gonioscopy, the doctor can determine whether the angle where iris meets the cornea is wide, narrow, open or closed. (15)
During the test, the doctor first numbs the eyes then uses a gonioscope to look at the angle. The doctor is able to tell whether there is an abnormality of the angle in all the four zones, whether there is evidence of past angle closure or torn places from previous injuries.
Using a pachymeter, the medical practitioner can measure the thickness of the cornea. Corneal thickness is important since it can cover the real reading of intraocular pressure.
A thin cornea may show low intraocular pressure when in reality, the actual reading is high. A thicker cornea, on the other hand, shows a higher intraocular pressure reading than there is. (16)
During a pachymetry test, the doctor gently places a pachymeter in front of the eye which does the reading. Normal corneal thickness is about 540 to 560 microns. (17)
How is glaucoma treated?
The primary goal of treating glaucoma is to prevent loss of vision by lowering intraocular pressure. Although the damage caused by glaucoma cannot be reversed, the good news is that if caught in its early stages, its progress can be slowed or stopped.
Depending on the cause and stage of glaucoma, treatment options include eye drops, laser treatment or surgery. (18)
In the majority of the cases, the initial treatment is usually eye drops. They help lower eye pressure by improving how aqueous humor drains or by decreasing the amount of fluid produced by the eyes. Prescription eye drops include prostaglandins, beta blockers, carbonic anhydrase inhibitors and miotic agents. (19)
The eye drops can cause stinging sensations, eye redness, change in eyelash growth, darkening of the iris, rashes, dry mouth or even an irregular heart rate. Despite these side effects, it is crucial to stick to the recommended dosage and frequency, for a favorable outcome.
If eye drops alone do not adequately lower eye pressure, oral medications may also be prescribed. The current Food and Drug Administration (FDA) approved glaucoma medications are the carbonic anhydrase inhibitors (CAIs).
They work by reducing fluid production in the eyes. CAIs may cause frequent urination, tingling sensation in the fingers or toes, and have a metallic taste. (20)
When medications do not achieve the desired results, surgery becomes the next best option. (21) Surgery may eliminate the need for using eye drops, but it is not always the case. Some people may need to continue using them, to prevent recurrent pressure buildup.
To increase the outflow of eye fluid, the ophthalmologist performs laser trabeculoplasty to create microscopic holes at the region where the iris meets the cornea.
The procedure is painless, takes 10 to 15 minutes and can be done on an outpatient basis. It is performed by passing laser beams (high energy light beams) through the eye’s drainage canal. The eye’s drainage system is then changed in microscopic ways, to allow aqueous fluid to pass more freely.
Laser trabeculoplasty causes minimal heat damage to adjacent tissues, meaning that it can be repeated safely. (22)
When neither medication nor laser surgery lowers intraocular pressure, the doctor may suggest the traditional conventional trabeculectomy procedure. (23)
During this procedure, the surgeon creates an artificial passage in the sclera, by partially removing part of the eye’s drainage system. The passage then creates a controlled leak of eye fluid.
Can glaucoma be prevented?
Early detection by thorough and routine eye examinations is the key to preventing vision damage caused by glaucoma. (24)
The eyes should be tested:
- Every 2 to 4 years before the age of 40
- Every 1 to 3 years from 40 to 54 years
- Every 1 to 2 years from 55 to 64 years
- Every 6 to 12 months after the age of 65
For people with high-risk factors such as elevated intraocular pressure, diabetes and family history, testing should be done at least annually or once every two years after the age of 35.
Other self-care tips include:
- Know your family eye health history and take measures for regular eye care as glaucoma tends to run in families
- Exercise regularly (30 t0 45 minutes) 3 to 5 times in a week. Physical activity not only reduces eye pressure but also improves overall health
- Sleep with the head elevated at about 20 degrees, to reduce intraocular pressure when asleep.
- Only use eye drops that are prescribed by a healthcare provider.
- Wear eye protection for instance when participating in high-risk sports or using power tools. Severe eye injuries are a predisposing factor to glaucoma.