Open Angle Glaucoma
What is open-angle glaucoma?
Open-angle glaucoma, or primary open-angle glaucoma (POAG), is a condition in which pressure inside the eye causes damage to the delicate tissue and nerves on the inside of the eye. Over time, this affects the ability to see and can cause blindness.
Glaucoma is a common disorder of the eye and is the second most common cause of blindness worldwide. Open-angle glaucoma is one of several types of glaucoma. Open-angle glaucoma most commonly affects adults and becomes more common with age. The exact cause is not well understood, but several factors may combine to cause this condition. Most people have no symptoms until significant damage to the eye has occurred. At this point, people with glaucoma notice that they cannot see clearly, especially at the sides of their vision.
Open-angle glaucoma can be treated with medications, laser therapy or surgery. If left untreated, it eventually results in blindness.
Effects of open-angle glaucoma
Open-angle glaucoma occurs when pressure inside the eye causes damage to the tissue and nerves inside the eye. This most commonly happens when pressure inside the eye increases, but can, in rare cases, occur with normal eye pressure. Pressure on the delicate tissues and nerves at the back of the eye causes damage and vision problems.
Symptoms of open-angle glaucoma
Open-angle glaucoma does not produce any early symptoms, therefore many people are unaware that they are affected. Because the vision loss that results from glaucoma tends to affect peripheral vision first, the unaffected eye may be able to compensate for the vision loss in the affected eye. This gives the impression that vision loss is not occuring.
Over time, however, both eyes become affected. When approximately 40 percent of the optic nerve fibres are damaged, the vision loss becomes noticeable. Vision in the central field is usually the last to be affected.
For this reason, screening for glaucoma is extremely important. People who are younger than 50 years of age should be screened by an optician once every five years, and people who are 50 years of age or older should be screened every two to three years.
Risk factors for open-angle glaucoma
The causes of open-angle glaucoma are not well understood, but it's likely that the increased pressure is caused by a combination of factors.
People over the age of 40 are more likely to develop glaucoma, and it becomes more common with age. People who are short-sighted, who have diabetes, who have African heritage and people who have a family member who has glaucoma are more likely to develop this condition.
In the eye:
- Histories of eye pain or eye redness
- Halos, glowing or blurred border around objects
- Previous ocular diseases, such as cataracts
- Uveitis, inflammation of one of the layers of the eye
- Previous eye surgery
- Trauma to the head or eyes
- Severe myopia
Elsewhere in the body:
- Vascular occlusion, blocked blood vessels in the body
- Diabetes, type 1 or type 2
- Headache or migraine
- Vasospasm, the sudden tightening of a blood vessel
- Cardiovascular disease
- Obesity, a BMI of 30 or higher
- Medication for hypertension
- Use of corticosteroids
- Alcohol use
- Stress and anxiety
- Sleep apnea
- Previous surgery
Unlike narrow/closed-angle glaucoma, open-angle glaucoma does not cause a rapid rise in intraocular pressure (IOP). In narrow/closed-angle glaucoma, the trabecular meshwork at the rear of the eyeball closes, cutting off the drainage of the aqueous humor of the eye, and increasing IOP.
Diagnosis of open-angle glaucoma
When glaucoma is present, the loss of vision and increased pressure inside the eye is often noticed during a routine checkup by an optometrist. Diagnosis is usually confirmed by an ophthalmologist (eye doctor). The doctor will check for signs of nerve damage, vision loss and elevated pressure inside the eye. Other tests may be needed to exclude any possible underlying causes for the condition, including blood tests and a CT or MRI scan of the head.
- A slit lamp examination, in which a slit lamp, which is a combined light and magnifier, is used to examine the optic disc, the point at which the optic nerve leaves the eye. The object of this is to determine whether the disc has taken on a ‘cupped’ appearance.
- Tonometry, which measures the pressure inside the eye. .
- Field of vision testing, to determine the extent of vision loss.
- Opthalmoscopy, which allows visualisation of the retina and optic disc.
- Pachymetry, a test of corneal thickness. A thickening cornea may indicate glaucoma.
- Gonioscopy, an examination of the trabecular meshwork, to assess the drainage of the aqueous humor. In open-angle glaucoma, this test typically does not find an obstruction. If an obstruction is present, the disorder is more likely to be closed-angle glaucoma.
In addition, if it is suspected that open-angle glaucoma is the result of an underlying disorder, a complete blood count and a test of the erythrocyte sedimentation rate may be ordered. In cases where the person affected may have syphilis, a microhemagglutination treponema pallidum test may be ordered. If an underlying autoimmune disorder is suspected, a serum protein electrophoresis test may be ordered.
Treatment of open-angle glaucoma
Treatment aims to decrease eye pressure with medication, laser therapy or surgery, in order to slow down or prevent worsening of the condition. Lowering intraocular pressure by 20 to 40 percent is the only treatment for open angle glaucoma.
Eye drops are a common form of first-line treatment.
The eye drops may contain beta-blockers, which reduce the aqueous output of the eye. People who are already using systemic beta-blockers should ideally use eye drops containing beta-blockers from a different class. Other drugs that can be administered as eye drops include various prostaglandin analogs, which increase the outflow of aqueous humor. Cholinesterase inhibitors, cholinesterase agonists and carbonic anhydrase inhibitors may also be used. Oral osmotic diuretics, such as glycerin and mannitol, may also be prescribed. These work by drawing fluid from the eye into the bloodstream. All of these can be used as first-line therapy, depending on what other medications the affected person is already using and their tolerance of side-effects.
- Laser trabeculoplasty may be used as the first surgical option for patients who do not respond well to eye drops, or if eye drops have successfully reduced intraocular pressure without arresting optic nerve degeneration. In these procedures, a laser is used to improve drainage through the trabecular meshwork.
- Surgical trabeculectomy is an option for patients whose open-angle glaucoma has not responded to eye drops or surgical trabeculectomy. In this procedure, a small channel is created between the front of the eye to the conjunctiva, allowing drainage to bypass the trabecular network.
- Guarded filtration procedure and partial-thickness procedures are surgical procedures aimed at reducing intraocular pressure by improving drainage from the eye.
Prevention of open-angle glaucoma
Screening people (testing patients without symptoms) who may be at risk of glaucoma can help to identify the condition early and prevent the complications of open-angle glaucoma.
People younger than 50 years of age, who do not have a family history of open-angle glaucoma, should be screened once every five years. People who are older than 50, or younger but with a family history of glaucoma or other risk factors, should be screened once every two to three years. People with open-angle glaucoma will need to see an optometrist or opthalmologist at least once a year and possibly every several months, based on the progression of the disorder.
International Glaucoma Association. “Primary open angle glaucoma (or chronic glaucoma)”. Accessed 8 May 2018. ↩ ↩