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Glaucoma

Glaucoma

Types of Glaucoma Open Angle Narrow Angle Congenital The image “http://www.ihrfoundation.org/images/figure1_sm.gif” cannot be displayed, because it contains errors.

Statistics

About 0.5% to 1% of total population¹
66.8 million people worldwide¹
2 to 3 million Americans age 40 and over²
Second leading cause of vision loss³
Strongly age related
                                                                                                    1. Quigley HA. Br J Ophthalmol. 1996;80:389-393.

                                                                                                    2. Prevent Blindness America Web site. Accessed August 1, 200

                                                                                                    3. The World Health Report Web site. Accessed August 10, 2001

Visual Field Test
Used to diagnose glaucoma.

Glaucoma is one of the leading causes of blindness in the United States. It is a disease that typically affects older people, but it can occur at any age. Loss of vision is preventable if the disease is detected early and treatment begun. Visual Field Test Used to diagnose glaucoma. The eye has about 1 million tiny nerve fibers which run from the back of the eye to the brain. These nerve fibers allow us to see. Glaucoma is a disease which causes the destruction of these fibers. It was once thought that the loss of these fibers was due strictly to high pressure in the eye. It is now known that even patients with normal eye pressure can have glaucoma and loss of these nerve fibers. In many patients, the disease is not noticed in the early stages, because there is no pain and no noticeable change in vision. Early detection by an eye doctor is key to the prevention of vision damage from glaucoma. Routine eye examinations are recommended.

The problem with glaucoma is generally an increase of pressure inside the eye. Other types of glaucoma are possible without increase in the pressure. It is a complex subject. Generally, however, pressure is put on the optic nerve which causes it to have a depression in it, damaging the nerves coming into the eye as illustrated below.

Courtesy of MSD

Visual Field

Types of Glaucoma

Normal Eye Drainage

The reason that eye pressure is high in many glaucoma patients is that the drainage system in the eye is not working properly. The fluid in the eye, called the aqueous humor, does not flow out of the eye as quickly as it should. The drainage system lies in a part of the eye called the angle, located between the outer layer and the iris of the eye. The angle can be open or closed. Open Angle Glaucoma The Trabecular meshwork has reduced outflow, or the ciliary processes produce too much aqueous humor.

Normal Eye Drainage

Open Angle Glaucoma

There are several types of glaucoma. The most common form of glaucoma is called chronic open angle glaucoma. The drainage angle is open in these patients, but the eye fluid does not drain as quickly as it should. Closed angle glaucoma occurs when the drainage angle closes, and almost no eye fluid can escape. During closed-angle glaucoma, eye pressure can get very high and there is pain. Closed angle glaucoma constitutes a medical emergency and must be treated immediately.

Open Angle Glaucoma
The Trabecular meshwork has reduced outflow, or the cillary processes produce too much aqueous humor.

If the high pressure is allowed to continue for too long, blindness can result. Some persons are more likely to have glaucoma. These include the elderly or those who have nearsightedness, a family history of glaucoma, a past eye injury, diabetes or a past history of vascular shock. Furthermore, African-Americans are 6 times more likely to have the disease.

Glaucoma is treated with eye drops that lower the eye pressure. If the pressure does not fall to a low enough level with eye drops, then surgery may be necessary. Glaucoma surgery opens up the drainage system in the angle so that the eye fluid can flow more freely.

Narrow Angle Glaucoma

Another type of glaucoma is narrow angle glaucoma. In this type of glaucoma the iris is pushed forward by any one of several different mechanisms. This closes off the trabecular meshwork so that fluid that is manufactured in the ciliary body and ciliary processes cannot properly drain from the eye.

Narrow Angle Glaucoma

Congenital Glaucoma

As the name implies, congenital glaucoma is present at birth. Many times it is the result of and abnormality in the structures that drain the fluid from the eye, the trabecular meshwork. Children who have these eyes frequently have the appearance of wonderful big eyes. This is the result of the increased pressure causing the cornea to be larger, thus exposing the iris more. Frequently these children have chronic tearing. The cornea internal lining, the endothelium cracks and fluid pours into the stroma of the cornea, causing the cornea to become cloudy or white.

This condition is frequently helped by a "goniotomy" or making a cut inside the eye across the trabecular meshwork parallel to the iris. Pediatric ophthalmologists are trained to do this procedure, as well as many glaucoma specialists.

OHTS Study (Ocular Hypertension Treatment Study)

Recently the OHTS study has shown that some patients with elevated intraocular pressure that were thought to be benign, are actually at risk of developing glaucoma. Patients with thinner than normal corneas fall into this category.

Testing Devices for Glaucoma

There are a number of new testing devices that can pick up glaucoma more rapidly. The most sensitive of these is the Frequency Doubled Technology.

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Optic Nerve-Head and Nerve Fiber Analyzers

Other instruments that are being used to detect and follow glaucoma fall into the category of optic nerve-head and nerve fiber analyzers.

GDx (For Glaucoma Diagnosis)

The GDx works by measuring the birefringence of the retinal nerve fiber layer; hence it is technically a polarimeter. The so-called retardation that the polarimeter measures is directly related to the thickness of the NFL. No other available system computes thickness in this manner. Unfortunately the cornea also has birefringent properties that sometimes result in errors in the measurement of NFL birefringence. The new GDx Access calculates the contribution of each cornea and then subtracts its contribution. What remains is the birefringent properties of only the NFL.

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HRT II(Heidelberg Retinal Tomograph)

HRT is a confocal scanning laser ophthalmoscope used for the early detection and follow-up glaucoma. This latest HRT technology is a non-threatening, non-invasive, fast and easy examination- ensuring the highest quality and most accurate data.

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OCT (Ocular Coherence Tomography)

OCT, an emerging technology based on fiber optics, often uses a compact diode light source similar to those used in compact disc players. Thus, OCT technology can be robust, portable, low cost, and readily interfaced with optical fiber techniques to catheters, endoscopes, laparoscopes, and surgical probes. These attributes make it very attractive for medical and surgical diagnostics. Other advantages for using OCT in medical diagnostics are to

provide images of tissue structure at the micron scale in situ and in real time,
function as a type of optical biopsy, unlike the conventional histopathology that requires removal of tissue specimens for microscopic examination,
replace standard excisional biopsy when it is hazardous or impossible, possibly reducing sampling errors,
enable surgical guidance,
enable imaging of organ systems inside the body, and
provide image resolutions that are one to two orders above conventional ultrasound.

Courtesy of http://www.afrl.af.mil/techconn/index.htm

When the pressure inside the eye cannot be controlled by medications, surgery may be performed. There are a number of procedures that have been devised. The most common is called a filtering operation. In this operation a cut is made to open up the surface membrane over the white part of the eye, called the conjunctiva. Once the white part is exposed, an opening is made into the anterior chamber of the eye. The conjunctiva is then sewn shut. This causes a blister or bleb to form draining fluid from the inside of the eye to just under the surface of the conjunctiva. The blood vessels of the conjunctiva then carry the drained fluid back into the venous drainage of the body to be recycled.

Another type of surgery may be performed, laser trabeculoplasty. In this operation laser energy is bounced off a mirrored contact lens on the surface of the cornea into the trabecular meshwork. This creates little burns or openings on the trabecular meshwork that open it up and allow better drainage.

An operation for late stage glaucoma involves placing a tube tube inside the eye which then exits deep in the eye socket. The fluid is then picked up by the venous system. This is called a tube shunt. The fluid drains more than a traditional filter operation.