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Review Glaucoma Gonioscopy—A Primer Syed Shoeb Ahmad Queen Elizabeth Hospital, Kota Kinabalu, Malaysia A ssessment of the anterior chamber angle (ACA) is an indispensible investigation for evaluation of glaucoma. The most commonly performed method for the determination of the ACA is gonioscopy. This technique, while being simple, is often hampered by the subjective nature of the procedure, especially in inexperienced hands. This review is intended to improve the knowledge, attitude, and practice among the practitioners regarding the procedure of gonioscopy. Keywords Gonioscopy, anterior chamber, glaucoma angle-closure, glaucoma open-angle Disclosure: Syed Shoeb Ahmad has nothing to disclose in relation to this article. This study involves a review of the literature and did not involve any studies with human or animal subjects performed by any of the authors. No funding was received for the publication of this article. Acknowledgements: Syed Shoeb Ahmad wishes to thank the Secretariat of the Greek Glaucoma Society, Dr Anton Hommer and the Austrian Glaucoma Society, as well as Dr Khoo Say Peng from the Dr Kong Specialist Eye Centre, Kota Kinabalu, Malaysia for the images and Ms Emmeline Stanislaus for editing the manuscript. Authorship: All named authors meet the International Committee of Medical Journal Editors (ICMJE) criteria for authorship of this manuscript, take responsibility for the integrity of the work as a whole, and have given final approval to the version to be published. Open Access: This article is published under the Creative Commons Attribution Noncommercial License, which permits any noncommercial use, distribution, adaptation, and reproduction provided the original author(s) and source are given appropriate credit. Received: December 2, 2017 Accepted: February 7, 2017 Citation: US Ophthalmic Review, 2017;10(1):42–5 Corresponding Author: Syed Shoeb Ahmad, Ophthalmology Department, Queen Elizabeth Hospital, Jalan Kebajikan, 88587, Kota Kinabalu, Malaysia. E: syedshoebahmad@yahoo.com Gonioscopy is a requisite investigation for all patients with glaucoma. It is a procedure for evaluation of the anterior chamber angle (ACA), utilizing special instruments known as gonio-lenses or -prisms. Alexios Trantas (1867–1961) was the first to use the term “gonioscopy” in 1907 (Figure 1). The term was derived from the Greek word “gonia” meaning angle and “skopein” to observe. Trantas used a direct ophthalmoscope and digital pressure at the limbus to observe the ACA in a patient with keratoglobus. Later, Maxmilian Salzmann (1862–1954) used indirect gonioscopy with a contact lens for examination of the angle (Figure 1). 1 Therefore, both Trantas and Salzmann are called the “Fathers of gonioscopy”. 1,2 Gonioscopy helps to categorize the type of glaucoma, that is, open- or closed-angle. This directs the ophthalmologist towards the subsequent management of glaucoma. The procedure may also identify any secondary causes of outflow obstruction such as pigment, pseudoexfoliative material, new vessels, angle-recession or foreign bodies in the ACA. This concise review of gonioscopy is intended for residents and junior doctors who are often intimidated by the complex nature of this procedure. The article describes the optical principles forming the basis of gonioscopy, the types of gonioscopy, the procedure for performing indirect gonioscopy, the difference between direct and indirect gonioscopy, identification of angle structures, grades of angle width, dynamic gonioscopy, flowchart for gonioscopy, and the common errors during performing the procedure. Principle of gonioscopy When light passes from a medium with a greater index of refraction to a medium with lower index of refraction, the angle of refraction (r) becomes greater than the angle of incidence (i). i reaches a “critical angle” when r is equal to 90º. If i becomes more than the critical angle, light is reflected back into the first medium (Figure 2). The critical angle for the cornea-air interface is approximately 40o. Light rays coming from the angle of the anterior chamber exceed the critical angle and are therefore reflected back into the AC. This prevents visualization of the ACA on slitlamp biomicroscopy. This deficiency can be overcome by optically replacing the cornea with another interface. The refractive index of the contact lens is the same as that of the corneal epithelium so that there is minimal refraction at the interface of these two surfaces (contact lens-cornea). This removes the optical effect of cornea. The light rays from the ACA reach the contact lens. Subsequently, they are made to pass through the new contact lens-air interface. In direct gonioscopy (gonio-lens) the anterior curve of the contact lens is such that the critical angle is not reached. The light rays are refracted at the contact lens-air interface to the observer’s eye. In indirect gonioscopy, the light rays are reflected by a mirror in the contact lens (gonio- prism). The light rays leave the lens at nearly a right angle to the contact lens-air interface. 2 42 TOUCH ME D ICA L ME D IA