Gonioscopy

Gonioscopy is performed during an examination of the eye to evaluate the internal drainage system of the eye, also called the anterior chamber angle. The corner is where the cornea and iris meet. This is where the fluid inside the eye (aqueous humor) flows out of the eye into the venous system. Under normal circumstances, the angle cannot be seen on examination. A special prism contact lens, which is placed on the surface of the eye, allows visualization of the angle and drainage system.

A gonioscopy is an eye examination that is used to look at the front of the eye (anterior chamber) between the cornea and the iris. A gonioscopy is a painless examination to see if the area where fluid drains from the manhole (called the drainage angle) is open or closed. This is often done during regular eye exams, depending on the patient’s age and whether they are at high risk of developing glaucoma. Gonioscopy is done if the doctor thinks the patient should be tested for glaucoma.

Why is it necessary?

The pressure inside the eye is maintained by the constant production and drainage of fluid. If the drainage system is not working properly, pressure inside the eye, also known as intraocular pressure, can increase. High intraocular pressure can cause damage to the optic nerve, which sends images from the eye to the brain. This type of damage is called glaucoma, the second leading cause of blindness worldwide.

By examining the angle, doctors can determine if it is open or closed, and if there are abnormal blood vessels, adhesions (synechia), or damage from a previous injury to the eye. A closed angle is an anomaly that may predispose the patient to a sudden or rapid increase in intraocular pressure. This increase in pressure can cause a very serious acute form of glaucoma that can be treated and even prevented with laser treatment (iridotomy) if the angle abnormality is recognized by gonioscopy.

In addition, gonioscopy allows the ophthalmologist to note finer features of the eye’s drainage system in order to guide the patient’s diagnosis and treatment plan.

How it’s done?

During gonioscopy, the head is placed in a slit lamp (a special microscope is used to examine the eyes). After numbing the eyes with drops, a special contact lens is placed directly on the eye and a beam of light is used to illuminate the corner. Although the eyelids may feel the presence of the lens, there is usually no pain associated with this examination. Examination of both eyes usually takes several minutes.

Anterior chamber angle

The anterior chamber is an angular space bounded anteriorly by the posterior (inner) surface of the cornea and posteriorly by the anterior surface of the iris and part of the ciliary body.

The anterior chamber has a depth of 3 mm and contains 0,25 ml of aqueous humor. The depth of the anterior chamber is less in the hypermetropic eye than in the myopic. It is also smaller in children and the elderly.

Information about the anterior chamber of the eye:

• volume: 220 μl;
• chamber volume decreases by 0,11 µl/year of life;
• depth: 3,15 mm (2,6-4,4 mm);
• chamber depth decreases by 0,01 mm/year of life;
• the depth of the chamber is less in the hypermetropic eye than in the myopic;
• the chamber is deepened by 0,06 mm for each myopic diopter;
• camera depth is slightly reduced during placement, partly by increasing lens curvature and partly by direct lens translocation;
• wide anterior chamber angle refers to an eye in which the angle between the iris and the surface of the trabecular meshwork is between 20 and 45 degrees.
• angles less than 20 degrees are called narrow angles.

Indications for the procedure

Gonioscopy is performed for:

• examining the front of the eye to check for glaucoma
• checking whether the drainage corner of the eye is closed or almost closed;
• determining the type of glaucoma in a patient;
• detection of scratches or other damage to the drainage angle;
• glaucoma treatment;
• checking for birth defects that can cause glaucoma.

During gonioscopy, the laser beam can be directed through a special lens at a drainage angle. Laser treatment can reduce pressure in the eye and help control glaucoma.

Glaucoma is a group of similar diseases that cause damage to the optic nerve, which transmits information from the eye to the brain.

In the early stages, glaucoma usually has no symptoms, which makes it so difficult to treat. In most cases, glaucoma is associated with higher than normal pressure inside the eye, a condition called ocular hypertension. But it can also happen when intraocular pressure is normal. If the disease is untreated or uncontrolled, glaucoma first causes peripheral visual impairment and can eventually lead to blindness.

According to the American Academy of Ophthalmology, the most common type of glaucoma, called primary open-angle glaucoma, affects an estimated 2,2 million people in the United States, and that number is expected to rise to 3,3 million by 2020.

And because most cases of glaucoma have few or no early symptoms, about half of people with glaucoma don’t know they have the disease.

In most types of glaucoma, increased intraocular pressure is due to damage to the optic nerve at the back of the eye.

Glaucoma is the second leading cause of blindness in the US (behind macular degeneration) and the second leading cause of blindness worldwide (behind cataracts).

How to prepare?

If the patient wears contact lenses, they should be removed prior to this examination and not put back on for 1 hour after the test or until the medication used to numb the eyes wears off. If the eyes can be dilated during the examination, arrangements should be made for the patient to have someone drive them home after the test.

What can the patient expect during the procedure?

A gonioscopy is usually done by a doctor who treats eye conditions (ophthalmologist). The drops are used to desensitize the eyes so that the patient does not feel the lens touching the eye during this painless examination. Gonioscopy is usually done in a doctor’s office. During a gonioscopy, the patient may be asked to lie down or sit in a chair. A microscope (slit lamp) is used to view the eye. The patient should place the chin and forehead on the support bar and look straight ahead. A special lens is positioned slightly at the front of the eye and a narrow beam of bright light is focused into the eye. At this time, the doctor looks through the angle of the anterior chamber with a slit lamp. The survey takes less than 5 minutes. Gonioscopy usually does not cause discomfort. Drops that are instilled into the eye before the procedure may cause a slight burning sensation. Also, an unpleasant sensation may be due to the fact that it takes some time not to blink.

Risks of the procedure

If the pupils have been dilated, vision may be blurred for several hours after the examination. Do not rub your eyes for 20 minutes after the test, or until the medicine has been completely removed from your eyes. There is also a very small risk of eye infection or an allergic reaction to eye drops used to numb the eyes.

Gonioscopy results

Gonioscopy is an eye exam that is used to look at the front of the eye (anterior chamber) between the cornea and the iris. During gonioscopy, the drainage angle of the eye is checked. The doctor measures the angle of the drain, its width, and checks whether it is open or closed. The following research results are possible:

1. Normal – Drainage angle looks normal, wide open and not blocked.
2. Pathology – the angle of the drainage looks narrow, is a gap or is closed. This means that the corner is partially or completely blocked, or there is a risk that the corner will be closed in the future.

A partially or completely blocked anterior chamber angle may mean that the patient has closed glaucoma. There are many reasons why the anterior chamber angle may be blocked. These include scar tissue, abnormal blood vessels, injury or infection, and extra color pigment in the iris.

What affects the test?

Reasons why a patient cannot take a test, or why results may not be appropriate, include: the patient cannot sit or lie down during the test; the patient is allergic to the medication used to numb the eyes during the study.

In these cases, other tests may be done to check for glaucoma or other eye problems. These tests include: slit lamp examination of the eye, tonometry (which measures the pressure inside the eyeball), ophthalmoscopy (which tests the optic nerve), and perimetry (which tests peripheral vision).

Research algorithm

The test, which is done for all glaucoma patients, is to look at the corner where the aqueous humor leaves the eye. Since the angle is inside the eye, essentially around the angle on the inner surface, it can only be seen with a large contact lens called a gonioscope. There are several types of gonioscopes, but they are all placed on the eye after they have been anesthetized and have mirrors to allow the doctor to see a few things.

First, the doctor determines if the angle is open, closed, or somewhere in between? The angle passes around the eyes in a circle, so 4 zones (top, bottom, side of the nose, temporal side) are examined by turning the gonioscope.

Second, doctors look for places where the iris is permanently attached to the mesh. These are signs of past, significant angle closure (called peripheral anterior synechiae).

Third, look for other signs of abnormality, such as new blood vessels or ruptures from past trauma, that alert the doctor to the possibility of secondary glaucoma.

Unfortunately, ophthalmologists don’t always do gonioscopy when appropriate, as many studies have found when examining charts of glaucoma patients from across the United States. Nearly half of these cards had no documentation that the doctor had done the gonioscopy or knew whether the angle was open or closed.

Alternative Methods

Gonioscopy is the main method of research, which tells whether the closure of the angle is possible. No doctor can classify every angle for sure. But it is in the few intermediate relationships that one must use judgment and decide with the patient on the best course. The decision in case of angle closure is whether to make a laser hole in the iris immediately or wait to see how the eye performs without laser iridotomy.

For several years, glaucoma centers such as the Wilmer Institute have added new research methods to evaluate the angle. The most recent and promising of these is called anterior segment optical coherence tomography or ASOCT. This is a painless imaging test that does not require anesthesia and uses dim light that reflects off the eye to take a cross-sectional image of the angle along with the cornea and iris. Studies using this technique have shown that the iris acts differently in those with a closed angle – it is less “spongy” – that’s one way of describing it.