Indentation Gonioscopy Revisited

Jeffrey M. Liebmann

Glaucoma diagnosis and management depends upon the collection and accu-rate interpretation of data. This process of information collection begins at the time of the initial patient encounter and involves the proper use of a wide variety of established and emerging techniques. Although we live in an age of increasing dependency on techno-logy, no technique is more important for the glaucoma patient than indentation gonioscopy.¹

Angle-closure glaucoma is a leading cause of preventable blindness worldwide, particularly in Asia.² Early detection and curative laser surgery can prevent irreversible damage in the majority of affected individuals. Conversely, failure to detect angle closure complicates the management of glaucoma and leads to unnecessary morbidity. Gonioscopy remains a cornerstone of glaucoma diagnosis and a thorough understanding of its clinical importance and technique is paramount to detecting and treating all forms of glaucoma. Evidence from clinical studies and imaging devices have enhanced our both understanding of the angle-closure process and the need to perform gonioscopy in darkened room conditions and with an indentation technique, as described below.

In the Ritch Angle-closure Classification System,³ angle closure can occur at the level of the iris, (secondary to relative pupillary and iris architecture (insertion, thickness, rigidity)), ciliary body (plateau iris, iridociliary cysts), lens (intumescence subluxation and dislocation) and posterior segment (malignant glaucoma syndromes, ciliochoroidal effusion, vitreoretinal disease). Each subsequent level of angle closure may have a relative component of the level preceding it. The appropriate treatment becomes more complex for each level of block, as each level may also require the treatments for lower levels of block.

Accurate diagnosis of the specific type of angle closure relies on two tenets. The first is indentation gonioscopy. Indentation gonioscopy permits dynamic evaluation of the angle and the assessment of appositional or synechial closure.¹ Using a 4-mirror gonioscopy lens with a diameter smaller than that of the cornea, pressure is applied on the central cornea, displacing aqueous towards the peripheral anterior chamber. If reversible iridocorneal apposition is present, aqueous is forced into the angle recess and results in discernable angle-widening and viewing of previously hidden angle landmarks. In addition, the behavior of the iris during indentation gonioscopy is specific to the level at which angle closure originates. Indentation gonioscopy can be difficult to perform when the diameter of the lens exceeds the diameter of the cornea.

In order to correctly discern a narrow, but open, angle from a closed one, it is of the utmost importance that gonioscopy be performed in a completely darkened room, as any light directed through the pupil will result in miosis, angle widening and may prevent detection of iridotrabecular apposition.⁴ For this reason, the smallest square of light should be used for a slit beam, ensuring that it does not project into the pupil. The quadrant to be assessed is first examined without pressure on the cornea and with the patient looking toward the mirror, allowing the examiner to see as deeply into the angle as possible. The widest quadrant is usually, but not always, the inferior angle (superior mirror), so it is logical to begin the examination at this location. Corneal indentation is subsequently performed and any changes in angle and iris configuration are noted. Pressure on the cornea during gonioscopy forces aqueous humor into the angle recess, thus making it wide enough to permit viewing over the iris convexity. The presence and extent of synechial closure, as well as the depth of the angle, can be determined by this method.

The pattern of indentation defines the type of angle closure. Apposition of the iris to the trabecular meshwork can be caused by any combination of anatomic, structural, or mechanical abnormalities in the anterior segment and can contribute to the risk of progressive trabecular damage, elevated intraocular pressure (IOP), peripheral anterior synechiae (PAS), glaucomatous optic neuropathy, and loss of vision. Understanding of the anatomic and pathophysiologic mechanisms involved in the etiology of angle-closure glaucoma assists in diagnosis and optimizing the treatment course.

In eyes with angle-closure caused by relative pupillary block, inden-tation gonioscopy easily moves the peripheral iris posteriorly, since it is only aqueous humor within the posterior chamber that has to be displaced. The angle opens easily and widely, and synechiae can be easily differentiated from iridotrabecular apposition. If the angle does not open easily and uniformly, other forms of angle-closure may be contributing to the disease process. Since every eye with angle closure has some component of relative pupillary block, laser iridotomy should be performed in all angle-closure eyes.

The anatomy of plateau iris configuration consists of an abnormal iridociliary relationship, with an obliterated ciliary sulcus and irido-ciliary contact. Consistent with knowledge visible during ultrasound biomicroscopy, the indentation pattern seen in eyes with plateau iris reveals complex iris motion. Indentation causes posterior movement of the iris onto the anterior lens capsule, concavity in the region of the posterior chamber where there is only aqueous to displace, and peripheral elevation of the iris over the anteriorly-positioned ciliary body, before the iris is again forced posteriorly into the angle recess. This ‘double-hump’ sign reveals the iris to be draped over the ciliary body.³ An ultrasound biomicroscope is not required for the diagnosis. The treatment for persistent appositional closure due to plateau iris in the presence of a patent iridotomy is argon laser peripheral iridoplasty.⁶ Indentation gonioscopy in the presence of an intumescent or anteriorly subluxed lens requires even more force than used to open the angle in plateau iris, since the entire lens needs to be displaced posteriorly.

In summary, indentation gonioscopy is indispensable for glaucoma management. The procedure should performed in a darkened room to avoid light-induced miosis, which may result in inadvertent widening of the angle. The indentation pattern will reveal the pathophysiology of the angle closure and allow appropriate treatment and care

References

1. Forbes M. Gonioscopy with corneal indentation. A method for distin-guishing between appositional closure and synechial closure. Arch Ophthalmol 1966; 76: 488-492.

2. Quigley HA, Broman AT. The number of people with glaucoma world-wide in 2010 and 2020. Br J Ophthalmol 2006; 90: 262-267.

3. Ritch R, Liebmann JM. Role of ultrasound biomicroscopy in the differentiation of block glaucomas. Curr Opin Ophthalmol 1998; 9: 39-45.

4. Barkana Y, Dorairaj SK, Gerber Y, Liebmann JM, Ritch R. Agreement between gonioscopy and ultrasound biomicroscopy in detecting irido-trabecular apposition. Arch Ophthalmol 2007; 125: 1331-1335.

5. Sakata LM, Lavanya R. Friedman DS. Aung HT, Gao H, Kumar RS, Foster PJ. Comparison of gonioscopy and anterior segment ocular coherence tomography in detecting angle closure in different quadrants of the anterior chamber angle. Ophthalmology 2008; 115: 769-774.

6. Ritch R, Tham CC, Lam DS. Argon laser peripheral iridoplasty (ALPI): an update. Surv Ophthalmol 2007; 52: 279-288.