Methods of gonioscopy

Definition

Gonioscopy is biomicroscopic examination of the anterior chamber angle of the eye, where aqueous humor gains access to Schlemm’s canal. It enables the glaucomas to be classified into two main groups, angle-closure glaucoma and open-angle glaucoma.Gonioscopy is helpful diagnostically, prognostically, and therapeutically in glaucoma.1–4

Methods of gonioscopy

Equipment

Because of the curvature of the cornea and the difference in the index of refraction between the eye and the air, light rays coming from the far peripheral iris, the angle recess, and the trabecular meshwork undergo total internal reflection (Fig. 6-1), which prevents the clinician from examining these structures without the use of a contact lens to eliminate the air–cornea interface. Three types of gonioscopic contact lenses are available: (1) those whose surface is slightly larger than the cornea and that require a gonioscopic coupling gel (e.g., Goldmann lens); (2) those whose surface is smaller than the cornea and that use the patient’s tear film as a coupling agent (e.g., Zeiss or Sussman four-mirror lens); and

(3) those whose surface is quite large, that use saline or similar fluid as a coupling agent, and that necessitate that the patient lie supine (e.g., Koeppe lens) (Fig. 6 2). Devices from the first two groupings are most popular because they can be used under standard examination circumstances, with the patient sitting at the slit lamp. Advantages and disadvantages of the direct and indirect methods are listed in Table 6-1.

Goldmann and Zeiss lenses (indirect method)

The Goldmann and Zeiss types of lenses are termed indirect gonioscopic lenses because they have mirrors by which the angle is examined with reflected light (Figs 6-3 and 6-4). The patient can be examined with the light and magnification of the slit lamp and corneal microscope. The magnification obtained depends on the power of the microscope and should be 163 to 203.

Koeppe lens (direct method)

The Koeppe lens allows the observer to look directly at the angle (Fig. 6-5).The curvature of this lens adds 1.53 to the magnification of the angle image. Lighting is usually obtained by a Barkan hand illuminator or fiber optic light source, and magnification is obtained by a supported, counterbalanced microscope having 1.63 objective lenses and 103 ocular lenses. With the 1.53 magnification of the Koeppe lens, a 243 magnification of the trabecular area is obtained.

A hand-held microscope may be used for less exacting gonioscopy, but without support the depth of focus is too critical at a magnification above 163, and 6 to 103 ocular lenses should be used.

Technique

Indirect gonioscopic lenses

The patient sits upright at the slit lamp, with the head firmly against the headrest. When the Goldmann type of lens is used, a drop of 1% methylcellulose is placed in the corneal curve of the lens.With the patient looking up, one edge of the lens is positioned in the lower fornix. The upper lid is elevated, the patient is instructed to look straight ahead, and the lens is rotated against the eye (Fig. 6 6). This is a familiar maneuver to clinicians because this model

Fig. 6-1  Rays of light originating at the anterior chamber angle. These rays undergo total internal reflection by the cornea.

(A)

(B)

(C)

(D)

Fig. 6-2  Gonioscopic contact lenses. (A) One-mirror Goldmann; (B) threemirror Goldmann; (C) Koeppe; (D) hand-held Zeiss.

Fig. 6-4  Rays of light emerging through a Goldmann lens.

of gonioscopic lens is similar to that used for retinal evaluation and treatment. Devices such as the Zeiss lens are available with a handle and a special clamp to secure the lens, so that the physician merely advances the lens to the point of contact with the cornea of the eye to be examined, while the opposite eye follows the fixation light (Fig. 6-7). Other models include the 4-mirrored lens in a circular casing without a handle, for direct apposition to the cornea; special attention should be taken to not inadvertently indent the cornea.

The mirrored arrangement of both of these types of lenses causes the observed image of the angle to be reversed but not crossed.3 In other words, that which is seen in the mirror is 180º away, but its detail is not altered with respect to right and left (Fig. 6-8).The other important adjustment the novice gonioscopist need master is to apply the absolute minimum amount of pressure of the contact lens on the cornea, especially while maneuvering the slit-lamp beam and the lens to maximize visualization. Routine use of gonioscopy in many normal eyes will eventually eliminate such artifacts as inadvertent corneal folds, air bubbles in the coupling agent, and disorientation with respect to where the patient should gaze during the examination to most effectively reveal different aspects of the angle.4

(A)

(B)

Fig. 6-6  (A) The Goldmann lens is brought into contact with the inferior sclera. (B) The Goldmann lens tipped up into position.

(From Alward WLM: Color atlas of gonioscopy, San Francisco, Foundation of American Academy of Ophthalmology, 2000.)

Fig. 6-7  Zeiss four-mirror lens held in a diamond configuration. This position is more natural for some examiners, but the corners of the lens against the patient’s eyelids can feel uncomfortable.

(From Alward WLM: Color atlas of gonioscopy, San Francisco, Foundation of American Academy of Ophthalmology, 2000.)

Another crucial gonioscopic skill for the clinician to master is to minimize the artifact of light-induced miosis during the examination, which, especially in narrow-angle eyes, can alter the assessment of the angle. Always perform slit-lamp gonioscopy in a darkened room; and practice using the smallest possible slit-lamp beam, in both height and width, to view the angle structures without throwing light into the pupil.

Eye

Gonioscopic image

(B)

Fig. 6-8  (A) Schematic of indirect gonioscopy mirror, with images viewed 180º reversed but oriented the same right-to-left (e.g., 11 o’clock, 12 o’clock, 1 o’clock positions). (B) Example of indirect gonioscopy, showing location of surgical iridectomy, iris nevus, and neovascular vessels as they appear on the iris and as an image in the gonioscopic mirror.

Indentation (compression) gonioscopy5

By deliberately varying the amount of pressure applied to the cornea with a tear-coupled indirect (e.g., Zeiss) contact lens, the physician can observe the effects on angle width. Increased pressure indents the central cornea and displaces fluid into the angle, opening it wider (Figs 6-9 and 6-10). To the experienced examiner, this technique is valuable in evaluating the status of the angle and the presence of synechiae. The ability to visualize angle structures by indentation may be reduced in the presence of elevated intraocular pressure.

Fig. 6-9  Indentation gonioscopy. Pressure on the cornea displaces the iris to widen a narrow or closed anterior chamber angle. This maneuver exposes additional anatomic landmarks and is useful in determining the presence or absence of peripheral anterior synechiae. Synechiae,

if present, can sometimes be separated. (A) Without pressure. (B) With pressure.

(A)

(B)

Fig. 6-10  (A) An eye with appositional angle closure. No trabecular meshwork is visible. (B) With indentation gonioscopy, parts of the trabecular meshwork are visualized (small arrow) but there is a broad peripheral anterior synechia (large arrow), which precludes visualization of the remainder of the trabecular meshwork.

(From Alward WLM: Color atlas of gonioscopy, San Francisco, Foundation of American Academy of Ophthalmology, 2000.)