Gonioscopy in the diagnosis of glaucoma

Gonioscopy is a very important examination method for diagnosing and monitoring the treatment of patients with glaucoma. The main objective of gonioscopy is to visualize the configuration of the anterior chamber angle.

Under normal conditions, the structures of the anterior chamber angle are not visible through the cornea due to the optical effect of total internal reflection. The essence of this optical-physical phenomenon is that light reflected from the anterior chamber angle is refracted inside the cornea at the cornea-air boundary. A gonioscopic lens (or goniolens) eliminates this effect, since it allows the structures of the anterior chamber angle to be studied by changing the angle of the lens-air boundary.

Gonioscopy can be direct or indirect depending on the lens used, with a magnification of 15-20 times.

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Direct gonioscopy

An example of an instrument for direct gonioscopy is the Koeppe (Koerre) lens. To examine with this lens, a magnifying device (microscope) and an additional light source are required. The patient must be in a supine position.

Advantages:

• Direct gonioscopy is indicated for patients with nystagmus and altered cornea.
• Gonioscopy is used in children in a hospital setting under local anesthesia. Standard sedation therapy is possible if necessary. The Keppe lens allows examination of both the anterior chamber angle and the posterior pole of the eye.
• Direct gonioscopy provides a panoramic assessment of the angle, allowing comparison of different sectors, as well as two eyes if two lenses are installed simultaneously.
• Retroillumination is possible, which is very important for determining the nature of congenital or acquired pathology of the angle.

Flaws:

• Direct gonioscopy requires the patient to be in a supine position.
• The procedure is technically more complicated.
• An additional light source and magnifying equipment (microscope) are required, but the optical image quality is worse than with a slit lamp examination.

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Indirect gonioscopy

The angle is visualized with a lens combined with one or more mirrors, which allows its structures to be assessed opposite the installed mirror. For assessment of the nasal quadrant, the mirror is placed temporally, but the superior and inferior image orientations are maintained. The image is obtained with a slit lamp. Since the invention of the Goldmann method of indirect gonioscopy, which used a single-mirror gonio lens, many types of lenses have been developed. Lenses with two mirrors are used, which allow examination of all quadrants by rotating the lens 90°. Other lenses with four mirrors allow assessment of the entire anterior chamber angle without rotation. Goldmann lenses and similar lenses have a contact surface with a larger radius of curvature and diameter than the cornea, which requires the use of a viscous coupling agent. Zeiss lenses and similar lenses do not require a coupling agent, since their radius of curvature is the same as that of the cornea. These lenses have a smaller contact surface diameter, and the space between the cornea and the lens is filled with a tear film.

The correct choice of gonio lens type is essential for successful gonioscopy. Several points should be taken into account. Before using the goniolens, the depth of the anterior chamber can be estimated using the Van Herick-Schaffer method. If a wide-open angle is anticipated, any lens can be used as long as there is nothing to obstruct visualization of the anterior chamber angle.

On the other hand, if the anterior chamber angle is suspected to be narrow, a single- or double-mirror Goldmann lens or a Zeiss lens may be preferred. The mirrors in these lenses are positioned higher and more centrally, allowing visualization of structures that are otherwise not visible due to the anterior displacement of the iris-lens diaphragm.

Imagine an observer standing at point A, trying to see a house behind a hill. The hill in this example resembles the bulge of the iris. To solve this problem, the observer must move to a higher point - B, which will allow him to see the house, or move closer to the center (to the top of the mountain) - to point A' or to point B', which is even better, since it will open a full view of the house and its surrounding elements.

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Methodology of gonioscopy

An anesthetic is instilled into each eye, and a slit lamp examination is performed. Depending on the type of lens used, a viscous contact agent may be necessary. The goniolens is carefully placed on the eye, taking care to avoid distortion of the intraocular structures. To obtain a good image of the angle, the slit lamp beam should be perpendicular to the goniolens mirror.

It is necessary to adjust the slit lamp during the examination.

The patient is asked to look at the light source to assess the superior and inferior angles.

The light source is tilted forward and the goniolens is slightly displaced downwards, the patient is asked to look in the direction that is to be examined to assess the nasal and temporal angles.

These simple technical details are necessary for the evaluation of narrow angles and the identification of various angle structures, in particular the Schwalbe ring.

Anterior chamber angle elements

The structures of the anterior chamber angle can be divided into two groups.

• The fixed part consists of the Schwalbe ring, trabecular meshwork and scleral spur.
• The movable part, including the anterior superior surface of the ciliary body and the place of attachment of the iris with its last fold.

The examiner should conduct a general examination to assess important aspects.

• Iris plane - the iris can be flat (wide mil) or very convex (narrow mil).
• The last fold of the iris and its distance from the Schwalbe ring are two elements for assessing the amplitude of the angle. The upper part of the angle is usually narrower than all its other parts.
• The root of the iris is the point where the iris attaches to the ciliary body. It is the thinnest part and is most easily displaced when the pressure in the posterior chamber rises. In myopia, the iris is larger and thinner, with a large number of crypts, and is usually attached posteriorly to the ciliary body. On the other hand, in hyperopia, the iris is thicker, attached anteriorly to the ciliary body, which creates a narrower angle configuration.
• Nodules, cysts, nevi and foreign bodies of the iris.

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Classification of angles

Gonioscopy determines the amplitude of the angle, as well as the type of glaucoma, open-angle or closed-angle, each of which has its own epidemiology, pathophysiology, treatment and prevention. The Schaffer classification evaluates the amplitude of the angle between the last fold of the iris and the trabecular meshwork-Schwalbe's ring.

• Grade IV - 45°.
• Grade III - 30°.
• Grade II - 20°, angle closure possible.
• Grade I - 10°, likely angle closure.
• Gap - angle less than 10°, more likely to close the angle.
• Closed - the iris fits tightly to the cornea.

The Spaeth classification also takes into account details concerning the periphery of the iris, as well as the effect of indentation on the configuration of the angle.

Uveitis. With uveitis, areas of uneven pigment deposition can be seen, giving the appearance of a "dirty" angle.

Closed-angle glaucoma. In closed-angle glaucoma, patchy areas of pigment deposition can be seen on any element of the anterior chamber angle, their presence indicates that the iris is attached to this area, but there is no permanent attachment. Pigment spots and a narrow angle may be evidence of a previous episode of acute closed-angle glaucoma.

The angle is usually devoid of vascularity. Occasionally, small branches of the arterial circle of the ciliary body may be seen. These branches are usually covered by the uveal meshwork, forming a circular tortuous structure or may converge radially toward the iris sphincter. In neovascular glaucoma, abnormal vessels cross the ciliary body and branch in the trabecular meshwork. Contraction of the fibroblast myofibrils accompanying the abnormal vessels causes the formation of peripheral anterior synechiae and closure of the angle.

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Use of gonioscopy in trauma

Contusion. When a blow is applied to the cornea, a wave of fluid is formed abruptly. This wave moves into the angle, since the iris-lens diaphragm serves as a valve, preventing fluid from moving backwards. This movement of fluid can damage the structures of the angle, the severity of the damage depending on the force of the blow. Detachment of the iris from the scleral spur at the site of attachment - iridodialysis.

Angle recession. Angle recession occurs when the ciliary body ruptures, leaving its outer wall covered by the longitudinal portion of the ciliary muscle.

Cyclodialysis. Cyclodialysis is a complete separation of the ciliary body from the sclera, resulting in communication with the suprachoroidal space. Cyclodialysis is often accompanied by hyphema.

Iridodialysis. Iridodialysis occurs when the iris is torn off at the point where it attaches to the scleral spur.

Causes of errors in gonioscopy

When performing gonioscopy, the researcher must remember that some actions distort the results of the study. The gonioscopic lens increases the amplitude of the angle (deepens it), too much pressure on the sclera causes the fluid to move to the angle.

Compression gonioscopy is invaluable in the evaluation of closed-angle glaucoma, particularly in differentiating iris overlap from true synechiae. Zeiss goniolenses are recommended for this type of gonioscopy. Compression gonioscopy mechanically applies pressure to the aqueous humor, causing corneal indentation, allowing the examiner to dynamically change the relative position of the iris. This procedure helps differentiate a narrow angle from a closed angle, as well as determine the risk of angle closure. Excessive pressure causes folds in the Descemet membrane, making it difficult to examine the angle.

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