What is Open Angle Glaucoma?
Glaucoma refers to a group of eye diseases where an increase in eye pressure, or intraocular pressure (IOP), can harm the optic nerve and affect vision. But, it’s worth noting that not all cases of glaucoma involve increased IOP. There’s a subgroup of the disease, known as normal pressure glaucoma, where similar optic nerve and vision damage happens with normal IOP. Glaucoma diseases are classified into two types: open-angle and closed-angle glaucoma. These categories can either be primary or secondary causes and they can originate naturally or be caused by medical intervention.
In Open-angle glaucoma (OAG), the disease tends to be chronic, worsening over time, and can’t be reversed. It’s marked by specific eyes changes such as a open angle at the front of the eye, changes at the head of the optic nerve, thinning of the retinal nerve layer, and gradually losing side or peripheral vision (refer to Visual Field and Optical Coherence Tomography, OCT). As it worsens, it can even result in central vision loss and blindness. Another important element of OAG is the role of IOP, it’s big risk factor and something doctors target when treating patients.
Most of the time, the disease affects both eyes, but the severity can differ depending on the cause. Even though high IOP is a key risk factor for glaucoma, not all people with an IOP of more than 21 mm Hg will develop glaucoma or optic nerve damage. Interestingly, research has revealed that people with high IOP who don’t show any symptoms can maintain normal vision and healthy optic nerves.
What Causes Open Angle Glaucoma?
To understand the cause of Open-Angle Glaucoma (OAG), it’s important to know how a fluid called aqueous humor is produced and removed in our eyes. This fluid is continuously produced in the back of the eye by a part called the ciliary body and is drained into the front of the eye. The majority of this fluid escapes through a network of tiny canals in the eye called the trabecular meshwork, with a small portion draining through a different pathway.
The most common type of glaucoma, called Primary Open-Angle Glaucoma (POAG), is characterized by increased resistance to the drainage of this fluid through the trabecular meshwork. Despite this, the angle at which the cornea and iris meet remains normal. Because of this blocked drainage, the pressure within the eye gradually increases, resulting in damage to the optic nerve and loss of vision. Secondary OAG, caused by different factors, is less common than POAG.
Studies have shown several genes to be linked to OAG. The MYOC gene, for example, is connected with both juvenile and adult OAG. Its protein product, myocilin, has a role in the functioning of the trabecular meshwork where most of the eye’s fluid drains.
Another gene, the WDR36 gene, has been linked to adult-onset POAG and is found in various parts of the eye, including the lens, iris, ciliary muscles, retina, and optic nerve. This gene also has protein counterparts in the human heart, placenta, liver, muscle, kidney, and pancreas. Some research points towards this gene, suggesting that it might be responsible for about 6% of POAG cases. However, results are inconsistent across different populations.
The CAV1 and CAV2 genes also show a link with POAG. These genes, associated with cell processes that involve the plasma membrane, are expressed in several human cell types, including the scleral cells and retinal ganglion cells.
Another gene, the CDKN2B-AS1 gene, believed to impact the susceptibility of the optic nerve to glaucoma-related changes, has also been identified. The gene is associated with processes that regulate the cell cycle.
The OPTN gene, another essential one, has a role in many cell functions and is linked to adult-onset POAG. Finally, the LOXL1 gene, implicated in a condition called pseudoexfoliation syndrome, is crucial in certain processes of the extracellular matrix.
Studies have revealed over 40 new genes associated with different forms of glaucoma. Knowing about these genes and their roles helps us better understand the disease and develop potential new treatments.
Risk Factors and Frequency for Open Angle Glaucoma
Glaucoma is a worldwide health problem— it affects around 70 million people and is also the second leading cause of blindness across the globe. Of these individuals, a striking 74% suffer from a specific type called open-angle glaucoma (OAG). The United States has a higher percentage, with almost 80% of all glaucoma cases being OAG in type.
Recent studies show that approximately 68.56 million people globally have open-angle glaucoma, and more than half of these cases are found in Africa and Asia. It’s a serious condition— almost 10% of all people with glaucoma experience blindness in both eyes. As of 2020, about 5.9 million people were affected by bilateral blindness from OAG.
In the United States, a minimum of 2.7 million people aged 40 and over are diagnosed with the disease, and this number will likely increase. The disease tends to affect African American populations the most, but its prevalence is also rising among Latin American and Chinese individuals as they reach advanced ages.
Glaucoma is not gender-selective; however, statistically, it affects women more. Women account for 55.4% of all open-angle glaucoma cases. Age plays a critical role as well. As people age, they are at increased risk of developing the disease— older populations are most commonly targeted by glaucoma.
In the future, OAG is expected to have the highest prevalence rate in Europe, followed by China and India. Due to its growing incidence in older populations, consideration of the longevity factor in epidemiological models is critical, especially for women and people in developed countries who tend to live longer.
- Key Risk Factors:
- Older age plays a significant role; the risk increases for African Americans over 40 years and Caucasians over 65 years.
- Race is another factor; people of African-American, Afro-Caribbean, and West African descent have a four times increased risk of developing OAG.
- Family history of the disease increases the risk; a study found that having first-degree relatives with glaucoma increases the risk by 9.2 times.
- Elevated intraocular pressure (IOP).
- Nearsightedness (Myopia).
- An increased cup-to-disc ratio in the eye.
- Disc hemorrhage.
- Thin central corneal thickness.
- Low ocular perfusion pressure.
- Both low and high blood pressure.
- Type 2 diabetes mellitus.
- High pattern standard deviation on visual fields is also a risk factor.
- Conditions like migraines or vasospasms.
- Low intracranial pressure.
- Use of oral contraceptive pills.
- Lifestyle factors, such as smoking, obesity, alcohol consumption, stress, anxiety, and sleep apnea also act as risk factors.
Signs and Symptoms of Open Angle Glaucoma
Open-angle glaucoma (OAG) often doesn’t show any symptoms at first. Detecting this disease early requires a detailed medical history and a comprehensive eye exam because early symptoms involve loss of peripheral, or “side,” vision. Most patients don’t notice this loss until a significant portion of their nerve fibers – nearly 40% – are affected, creating what’s often described as “tunnel vision.” These vision changes are usually the result of optic nerve damage and thinning of the retinal nerve fiber layer which occurs without any symptoms.
Older patients might hint at losing peripheral vision by talking about difficulties they are experiencing while driving or more frequently running into things at home. It’s imperative that if patients possess any of the mentioned risk factors, they must undergo an eye exam. The eye exam can reveal changes in the optic disc that typically occur before vision loss. The disease usually affects both eyes, and comparing the two optic discs can be helpful, but the damage can also be uneven.
If patients show any of the risk factors, symptoms or clinical findings listed below, doctors should refer them to an eye specialist for a diagnosis of glaucoma:
- Prior history of eye issues such as eye pain or redness, headaches, uveitis, diabetic retinopathy, cataracts, vascular occlusions, or colored halos around lights
- African-American descent
- Presence of refractive defect
- Long-term use of corticosteroids, either through eye drops or systemic use
- Prior eye surgery like photocoagulation, refractive procedures, cataract surgery, glaucoma surgery, and systemic surgery or medication use
- Past head or eye injury
- Using certain medications, including hypertensive drugs and topical or systemic steroids
- Medical history of diabetes, migraines, high blood pressure, vasospasm, cardiovascular disease, shortness of breath, or cardiac arrhythmia
- Family history (such as having a close relative with glaucoma, especially if that relative is a sibling) putting one at higher risk of developing glaucoma.
Older medical documents indicating high intraocular pressure (IOP), changes in the optic disk or visual field, can also be instrumental in diagnosing OAG.
Testing for Open Angle Glaucoma
Open-angle glaucoma (OAG) can be detected using a range of tools, with three main signs being critical:
* Changes in the optic disc or nerves in the back of your eye
* Alterations in your vision
* High fluid pressure within your eye
Here’s how different tests can help pinpoint these:
**Optic Nerve Test**
Your doctor can examine the optic nerve in your eye, which looks a bit like a donut. In healthy eyes, the bottom part is thickest, followed by the top, the side closest to your nose, and the side closest to your temples. In people with OAG, the top and bottom parts get thinner, messing up this order.
Other signs your doctor might look for include the ratio of the optic cup (the hole in the ‘donut’) being larger than 0.5, or the rim of the optic nerve thins out. They might also look for asymmetry of over 0.2 between the two ratios. Other anomalies, like hemorrhage or atrophy around the optic disc, can also be indicative of OAG.
**Visual Field Test**
This testing can be used to chart out what you’re able to see and recognize, helping doctors diagnose and track progression of OAG. This involves looking at two fields, trying to identify if there are any depressions or unusually low instances of points in areas commonly affected by glaucoma.
It’s also important to ensure that these results are consistent over multiple tests. If you’re at risk of developing glaucoma or already have high fluid pressure in your eyes, regular testing is recommended.
**Tonometry Eye Pressure Test**
Tonometry measures the pressure within your eyes. However, these readings can differ slightly between different measurements or examiners due to variables like corneal thickness. Higher pressure in the morning can also impact the reading. It’s also good practice to compare readings from previous tests and to make sure that the measurement is consistent. Additionally, anyone who records consistently high eye pressure readings (over 21mm Hg) is often considered glaucoma prone.
Despite being a risk factor, high eye pressure does not necessarily confirm an OAG diagnosis.
**Gonioscopy**
This examination allows an ophthalmologist to see the angle between the cornea and iris to ensure that it is wide enough for fluid to flow between the chambers of the eyes. If the angle is considered open (around 20° to 45°), then a diagnosis of open-angle glaucoma can be made.
**Optical Coherence Tomography**
Another diagnostic tool is optical coherence tomography (OCT), which creates detailed images of your eye including the retina and optic nerve. OCT is widely used to manage glaucoma cases by showing if the optic nerve layer is thinning out.
**Corneal Photokeratoscopy**
This test examines the structure of the front of your eye and can be useful in managing OAG. Preliminary results indicate that increased fluid pressure can cause changes to the structure of your eyes, which can be detected by this test.
Remember, OAG is most effectively diagnosed using a combination of the above diagnostic criteria, based on a thorough clinical evaluation and examination.
Treatment Options for Open Angle Glaucoma
The aim of treating Open-Angle Glaucoma (OAG) is to prevent both worsening changes in the optic nerve (the nerve connecting the eye to the brain) and deterioration of the field of vision. This is usually achieved by lowering and maintaining a target level of eye pressure (IOP), which aims to prevent further damage to the vision and optic nerve, thus helping preserve a person’s quality of life.
There are different views on when to start treating OAG. Some doctors begin treatment when the IOP reaches above 21 mm Hg, while others wait until there’s evidence of optic nerve damage or if the patient is at high risk of glaucoma getting worse. Treatment is usually initiated if there are signs of damage due to OAG, such as optic disc hemorrhage, nerve fiber layer defects, asymmetric cupping, vertical vocalization, or notching of the cup, or if symptoms of raised IOP such as halos around lights, blurred vision, or eye pain are present.
Doctors set a target IOP based on various factors like the severity of damage, baseline IOP, the patient’s age, race, family history, corneal thickness, and so on. Follow-up appointments, as well as the frequency of required check-ups, are scheduled depending on these factors and the rate of IOP reduction. If the patient’s condition worsens while on medication, the doctor needs to confirm that the patient is using his or her medication correctly. Other health issues that can influence glaucoma, such as diabetes and smoking, should also be managed.
Various topical eye medications can be prescribed, including Prostaglandin analogs (like Latanoprost and Bimatoprost), Adrenergic agents (like Brimonidine), Beta-blockers (like Timolol), Carbonic anhydrase inhibitors (like Dorzolamide), and Cholinergic or parasympathomimetic agents (like Pilocarpine). Some medications may cause side effects and are chosen considering various factors including patient preference, and their health conditions. In some cases, systemic agents (like Acetazolamide and Mannitol) are used if eye drops aren’t effective.
For some patients, laser therapy might be considered. This can be used as the primary therapy for OAG, to reduce the number of glaucoma drops, or when medications don’t work effectively. It can also be used as a less invasive alternative to surgery. The main types of laser therapy include argon laser trabeculoplasty, selective laser trabeculoplasty, and micropulse diode laser trabeculoplasty.
Surgical treatment for glaucoma is advised when IOP remains high after compliance with medication or if the glaucoma is advanced. Different surgical options available include trabeculectomy, glaucoma drainage devices, and non-penetrating glaucoma surgery. Minimally invasive or micro-invasive surgeries are also starting to be used for less severe cases of glaucoma.
Ultimately, the treatment chosen depends on the individual patient’s situation and the severity of the glaucoma. By careful monitoring and individual treatment plans, progression of glaucoma can often be slowed and vision loss prevented.
What else can Open Angle Glaucoma be?
When trying to diagnose certain eye or neurological conditions, doctors must consider a range of possibilities. These can include:
- Conditions related to the optic nerve (the connection from the eye to the brain), such as:
- A deep, healthy-looking pit in the optic nerve with no vision problems, which might be simply due to the optic disc being larger than average
- Buildup of tiny, rock-like deposits called drusen on the optic nerve
- Structural abnormalities in the optic disc (the spot where the optic nerve enters the eye)
- The optic disc is misplaced or angled unusually
- The blood supply to the optic nerve is reduced, causing damage (ischemic optic neuropathy)
- Diseases affecting the retina (the light-sensitive layer of tissue at the back of the eye), such as:
- Blockage of small veins or arteries in the retina
- A group of genetic disorders causing loss of vision (retinitis pigmentosa)
- Treatment involving widespread laser to the retina (panretinal photocoagulation)
- Conditions related to the brain and its functioning, such as:
- A tumor in the pituitary gland (a small organ at the base of the brain) which may cause vision loss in specific parts of the visual field
- A decrease in brain fluid pressure due to brain fluid drainage tubes or other neurological issues
- Foster Kennedy Syndrome, a rare condition involving swelling in one optic nerve and atrophy in the other
- Brain blood flow issues or injuries
- Multiple sclerosis, a disease of the central nervous system that affects the brain and spinal cord
It’s very important for doctors to consider all these possibilities and to carry out appropriate tests to reach an accurate diagnosis.
What to expect with Open Angle Glaucoma
Advanced Primary Open-Angle Glaucoma (POAG) can lead to severe damage to the optic nerve, even to the extent where an individual might not perceive light, although most people with this condition will not lose their vision completely in their lifetime. Several factors can accelerate the progression of this disease, these include:
* Older age
* High intraocular pressure (pressure in the eye)
* Increased cup-to-disc ratio or small area of the optic nerve
* Atrophy near the optic nerve
* Bleeding in the optic disc
* Thin corneal thickness at the center
* Reduced flexibility of the cornea
* Low blood pressure in the eye
* Inconsistent use of prescribed treatment
* The presence of abnormal, flaky material on the lens of the eye (Pseudoexfoliation)
Research has shown about a 35% chance of developing severe glaucoma in at least one eye over ten years if the condition remains untreated.
Possible Complications When Diagnosed with Open Angle Glaucoma
Glaucoma can lead to the following complications:
- Blindness: Typically this occurs without pain
- Painful blind eye or absolute glaucoma: This can happen if open-angle glaucoma leads to a blockage of the central retinal vein. This blockage can cause a condition known as neovascular glaucoma, resulting in a painful blind eye
Preventing Open Angle Glaucoma
Glaucoma is a condition that can cause blindness, but it’s preventable with the right care and management. One of the best ways to manage and slow down the progression of this condition is through effective treatment, which can maintain the patient’s vision and prevent permanent damage to the optic nerve.
However, successfully following through with the recommended medical therapy can be difficult for patients. These treatment plans usually involve daily steps to keep the pressure in the eye (IOP) under control. This ongoing pressure management can be a significant challenge for patients, as it’s something they’ll have to pay attention to for their entire lives.
Moreover, many people struggle with the daily routine of medication use. Not using medication consistently doesn’t adequately control eye pressure. Some patients may try to use their eye drops daily but might not apply the drops correctly, meaning the medication won’t be properly absorbed – this is particularly a problem for older individuals at risk. If patients don’t consistently use eye drops or oral medication over time, their chances of progressing towards blindness may increase.
Research has found that the more frequently a patient has to take a medication, the less likely they are to stick to their treatment plan. When asked why they struggled to stick to their medication regimen, many patients pointed to reasons like forgetfulness and having other priorities. A lack of information and emotional factors also played a role for some.
Interestingly, patients were more likely to stick to their medication plans when they were better educated about their condition and the reasons behind their treatment. That being said, a 2018 study found that simply mailing information and educational materials to patients didn’t significantly improve adherence to treatment in older glaucoma patients.
The importance of patient education in managing glaucoma is clear, but the best way to educate patients is still up in the air. Research has found no link between a patient’s adherence to their medication plan and their experience of side effects, which suggests that side effects probably won’t discourage patients. One of the difficult things about glaucoma is that patients often don’t notice any symptoms until it’s too late. This means an integrated approach is needed. Health care workers like nurses, therapists, social care workers, and primary care clinicians will often be the first line of defense, as not all patients with glaucoma visit their eye doctor on a regular basis.