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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: firstname.lastname@example.org
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
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
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