Definition of Intraocular Pressure
An Eye with Narrow Angle Closure Glaucoma
Intraocular pressure refers to the pressure inside the eye which is the result of two fluids that deliver nutrients and give the eye its shape and rigidity. These two different fluids are the aqueous and vitreous humor.
The aqueous humor (typically just called the aqueous) is found in the anterior chamber of the eye which is the space between the back of the cornea and the iris. It has the consistency of plasma and nourishes the cornea through the endothelium. It also maintains the shape of the anterior chamber and is constantly replenished and produced by the ciliary process.
The vitreous humor, or vitreous, is located in the posterior chamber of the eye behind the lens. It is a fairly thick gel-like transparent fluid that keeps the retina in place and gives the eye its shape. As we age, the vitreous starts to liquefy and collagen fibers begin to clump together forming what is most commonly known as vitreous floaters. Unlike the aqueous, the vitreous is not replenished constantly. It remains stagnant. This is why anything in the vitreous tends to stay in the vitreous such as floaters and hemorrhages.
Layers of the Cornea
The Flow of Fluid in the Eye
Aqueous is a gel-like, clear fluid that drains through the trabecular meshwork, a spongy tissue that allows the fluid to flow through a system of drainage tubes known as Schlemm’s canal.
The fluid then flows out of the eye. Together with the cornea and iris, these three structures make up what is known as the angle of the eye (see diagram).
Aqueous is constantly produced by the ciliary process which is located behind the iris on either side of the lens. Because production is constant, drainage needs to be consistent.
Intraocular pressure fluctuates considerably and continually, and it is typically highest in the morning and lowest just before bedtime.
The Structures of the Angle
Intraocular Pressure Testing - Tonometry
Tonometry is the procedure used to test the eye pressure and is an integral part of any eye exam. A tonometer is the device used to perform tonometry. The tonometry reading is taken by measuring how much pressure it takes to flatten the central portion of the cornea, and it is written in millimeters of mercury (mmhg).
Normal Eye Pressure Range
The normal range of pressure in the eye is anywhere from 10mmhg to 21mmhg. Some doctors will say 10mmhg to 20mmhg, but that is dependent on the individual doctor.
Anything below 10mmhg is considered to be too low, and the eye typically feels very soft. This can cause retinal detachments or lens dislocation among other things.
High pressures are typically anything over 20mmhg, but pressures over 26mmhg are dangerous. They can damage the optic nerve, causing loss of peripheral vision. This condition is known as glaucoma, and there are three main types:
- Open Angle Glaucoma
- Low - tension Glaucoma
- Narrow Angle Glaucoma
A Goldman Tonometer
Mires Seen During Tonometry
Open Angle Glaucoma
By far the most common type of glaucoma, primary open angle accounts for 90 percent of all glaucoma cases, and it can have a number of different causes including:
Open angle glaucoma can also be the result of other diseases such as diabetic retinopathy, or the use of certain medications, such as steroids. It can even develop after a surgical procedure such as cataract surgery or LASIK. When glaucoma develops as the result of a disease or treatment it is known as secondary glaucoma.
Low – Tension Glaucoma
This type, also called normal – tension glaucoma, is difficult to diagnose. The pressure remains within the normal range, but the optic nerve still sustains damage and loss of peripheral vision still occurs.
In order to make the diagnosis, the patient will need to complete a visual field test to check for peripheral (side) vision loss. The ophthalmologist may also recommend an OCT or HRT of the optic nerve to determine the extent of the damage.
Narrow Angle Glaucoma
This type of glaucoma is the result of an anatomical defect of the eye. The angle is the area where the cornea and iris meet (refer to diagram above). It is also the location of the trabecular meshwork, the drainage pipes we talked about earlier.
Narrow angle glaucoma is exactly what its name says it is. The angle is very narrow which makes it difficult for aqueous to drain properly. Most of the time this isn’t really a problem and patients may go through their entire lives without even knowing they have a problem. This is because there are varying degrees of angle narrowing.
Dilated Pupil of Narrow Angle Glaucoma
Sometimes, however, the patient is unfortunate enough to suffer what is called a narrow angle closure. These “attacks” are extremely painful. When the angle is closed the aqueous can’t get from the posterior chamber where it is produced to the anterior chamber where it can drain. This causes the aqueous to build up in the eye, causing the pressure to rise rapidly. The pressure can reach 70 to 85mmhg very quickly, and this can cause a number of different symptoms including:
- Rainbows around lights
- Nausea and/or vomiting
- Photophobia (light sensitivity)
- Excruciating pain (which can be a sharp pain, or a feeling of extreme pressure that many patients say feels like their eye is going to explode)
- Extremely red sclera (the white part of the eye becomes very bloodshot)
- Blurred or hazy vision (caused by the swelling of the cornea)
- Pupil dilation (which is typically the cause of the narrow angle attack)
Medication Used in the Treatment of Narrow Angle Glaucoma
Narrow angle attacks typically occur when the pupil is dilated such as in a dark movie theater. The iris actually gets trapped in the angle and blocks the aqueous from reaching the anterior chamber and the trabecular meshwork. This is one of the main reasons technicians always check the angle prior to dilating a patient.
With artificial dilation, the pupil opens wider than it normally would in say a dark room. This increases the likelihood that the iris will get stuck in the angle. It is very important for the technician to check your angles before dilating you, especially if you are hyperopic (farsighted).
Treatment for Increased Intraocular Pressure
There are only two treatments for narrow angle glaucoma:
- Peripheral iridotomy
- Peripheral iridectomy
Both procedures involve creating a hole in the iris to allow the aqueous to flow from the posterior chamber to the anterior chamber.
A peripheral iridotomy can be performed two ways, either with a laser (the most common way), or surgically. A laser peripheral iridotomy, or LPI, is performed with a Nd:YAG (neodymium – doped yttrium, aluminum and garnet) or argon laser and is typically an outpatient procedure. It is also the most common method for treating narrow angle glaucoma.
The peripheral iridectomy involves removing a portion of the iris to allow the aqueous to flow properly. It is a surgical procedure that is usually performed in a surgery center or hospital. Although not as common as an LPI, a peripheral iridectomy is used when an LPI fails.
A Drop Used to Treat Open Angle Glaucoma
Open angle and low – tension glaucoma have several different treatments available including:
- A variety of medications, both eye drops and oral pills
- Laser surgery procedures (such as selective or argon laser trabeculoplasty)
- Conventional surgical procedures (such as trabeculectomy)
- Drainage implants
There are a number of different eye drops used to treat glaucoma. These include beta-blockers, carbonic anhydrase inhibitors, and prostaglandin analogs among others. There are also oral medications that can be used to treat glaucoma, which are typically diuretics the most common of which is Diamox.
Laser procedures such as argon or selective laser trabeculoplasty are used to stimulate the trabecular meshwork to allow more aqueous to exit the eye. Argon laser trabeculoplasty has been used for a long time to treat glaucoma; however, selective laser trabeculoplasty was only recently approved for treatment of glaucoma. Peripheral iridotomy can also be used to treat open angle glaucoma, especially if it is the result of pigment dispersion syndrome. However, this is fairly uncommon.
Surgical procedures, such as trabeculectomy, are typically used as a last resort because they are so invasive. Trabeculectomy involves removing a portion of the trabecular meshwork and leaving a “bleb” or permanent opening in the sclera through which the aqueous exits the eye. Trabeculectomy carries with it a high risk of infection and this risk remains as long as the bleb is open.
Long – Term Care for Glaucoma Patients
Patients with glaucoma require long – term care because their intraocular pressure must be monitored frequently. Most patients are seen every six months. Patients who have glaucoma that is uncontrolled are typically seen every three months or even every month in some cases.
Patients need to be monitored frequently because of the possibility of developing a tolerance to glaucoma medications, or for further laser treatment. Patients who have had a trabeculectomy must be watched to make sure the pressure doesn’t drop too low and to ensure the bleb does not become infected.
If you or someone you know has glaucoma, it is imperative that you keep your doctors appointments and take your medications as directed.
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© 2012 Melissa Flagg COA OSC