With the growing popularity of newer angle procedures for the treatment of glaucoma, the use of gonioscopic lenses and other angle-imaging technologies in ophthalmic surgery has increased significantly. Efforts are underway to enhance the angle's visualization, improve angle surgery techniques, and achieve better surgical outcomes. This article reviews well-established and novel gonioscopic and anterior angle imaging technologies that may be useful in surgery.
HISTORICAL BACKGROUND
In 1900, Alexios Trantas described visualization of anterior
chamber angle structures in a living eye with keratoglobus.
In 1918, he coined the term gonioscopy for
examining the anterior chamber angle. Another pioneer
in the development of modern gonioscopy, Maximilian
Salzmann, became the first to use a corneal lens and indirect
ophthalmoscope to examine angle structures in 1915.
Koeppe improved upon Salzmann's goniolens shortly
thereafter by designing a steeper model. Curran is credited
with being the first to observe that angle closure
caused the IOP to rise. In the 1930s, Otto Barkan classified
glaucoma using direct gonioscopy and a Koeppe lens.
These significant contributions remain relevant today.1
PRINCIPLE OF GONIOSCOPY
The challenge of visualizing the anterior chamber angle
structures lies with the critical angle. When light passes
from a medium with a higher index of refraction to one
with a lower index of refraction, the angle of refraction is
larger than the angle of incidence, leading to internal
reflection of light. The critical angle for the cornea-air
interface is approximately 46°. Corneal lenses or mirrors
are needed to overcome the internal reflection of light. In
direct gonioscopy, the anterior curve of the contact lens is
such that the critical angle is not reached, and the light
rays are refracted at the contact lens-air interface. In indirect
gonioscopy, the light rays are reflected by a mirror in
the contact lens and leave the lens at nearly a right angle
to the contact lens-air interface (Figure 1).
INDIRECT GONIOLENSES
Zeiss and Posner Goniolenses
The Zeiss and Posner four-mirror lenses (Ocular
Instruments, Inc., Bellevue, WA) are the most commonly
used type of goniolenses in the clinical setting. The Zeiss
version of the lens is no longer manufactured and has
largely been replaced by the Posner goniolens.
The mirrors are tilted at 64° for evaluation of the angle, which eliminates the need to rotate the lens. The Zeiss and Posner goniolenses have an attached holding rod, which improves control at the slit lamp. This style of lens is convenient to use in the clinic, and it does not need a coupling agent such as methylcellulose. In addition, it enables clinicians to differentiate appositional from synechial angle closure. Learning to use this style of lens for gonioscopy is difficult, however, and it is possible to artificially open up the angle on examination.
Both of these four-mirror lenses can be used effectively in angle surgery (eg, goniosynechialysis). It is important to keep in mind, however, that they must be disinfected in glutaraldehyde or bleach and cannot be sterilized in an autoclave or heat disinfectant system.
Sussman and Khaw Goniolenses
With little-to-no rotation needed, both Sussman and
Khaw four-mirror goniolenses (Ocular Instruments, Inc.)
combine 360º of anterior chamber angle visualization.
These lenses have many similar characteristics to both
Zeiss and Posner goniolenses, but the former do not use
a rod for stabilization and are held directly on the eye
with the clinician's fingers.
Goldmann, Ritch, and Latina Goniolenses
The Goldmann single-mirror lens is the prototypical
gonioprism. The mirror in this lens has a height of 12 mm
and a tilt of 62° from the plano front surface. This lens
has superior clarity and functionality for angle laser procedures.
The Goldmann three-mirror lens provides two
mirrors for the examination of the peripheral retina and
one (tilted at 59°) for the anterior chamber angle. A viscous
agent for coupling is necessary to obtain good visualization.
Firm pressure on the eye with this goniolens
can artificially close the angle.
The Ritch trabeculoplasty lens (Ocular Instruments, Inc.) has four gonioscopic mirrors, with two inclined at 59° and 62° and a convex lens over one mirror in each set. The Latina SLT goniolens (Ocular Instruments, Inc.) features a 63° mirror and is popular for laser trabeculoplasty.
DIRECT GONIOLENSES
Koeppe/Barkan Goniolens
The Koeppe lens (Ocular Instruments, Inc.) is the prototypical
diagnostic goniolens used in children. It is available in different
diameters and radii of posterior curvature. A handheld
biomicroscope or tilted operating microscope is required for
adequate visualization of the angle. The Barkan goniolens
(Ocular Instruments, Inc.) has served as the prototypical surgical
goniolens. Neither of these lenses is moved by a rod.
Swan-Jacob Goniolens
The Swan-Jacob goniolens (Ocular Instruments, Inc.) has
been modified for goniosurgery and is now one of the more
popular models for angle surgery. Clinicians use an attached
rod to manipulate the lens for improved visualization. The
lens provides excellent visualization for goniotomy, ab interno
trabeculotomy using the Trabectome (NeoMedix
Corporation, Tustin, CA), trabecular micro-bypass iStent
surgery (not FDA approved; Glaukos Corp., Laguna Hills,
CA), and placement of the CyPass suprachoroidal device
(not FDA approved; Transcend Medical, Inc., Menlo Park,
CA). Use of the lens requires tilting the patient's head and
operating microscope as well as a viscoelastic coupling
agent (Figure 2). The Khaw, Hill, and Ritch surgical gonioprisms
(all from Ocular Instruments, Inc.) are competing
products that feature modifications of this lens.
RECENT ADVANCES
The Volk G-6 Goniolens
The new Volk G-6 goniolens (Volk Optical, Inc.,
Mentor, OH) uses six rather than four mirrors potentially
to provide true panoramic 360° visualization (see “Volk's
New Gonio Lens Is First With Six Mirrors”).
Ahmed 1.5X Surgical Goniolens
The Ahmed 1.5X Surgical Goniolens (Ocular
Instruments, Inc.) is an indirect goniolens with
enhanced magnification, field of view, and brightness. It
is designed of low-index, low-dispersive optical glass
with a curved anterior surface to give a 1.5X magnification
of the angle structures, which is offset to reduce
aberrations. This lens is specially designed for surgical
procedures and is steam sterilizable (Figure 3).
Endoscopic Angle Visualization
Endoscopic visualization of the angle using an endoscopic
probe (Endo Optiks, Little Silver, NJ) can allow
physicians to examine the angle with an endoscopic
camera and light source attached to a viewing monitor.
An additional incision is required to place a straight or
curved probe into the anterior chamber (Figure 4).
EyeCam Fiber Optic Visualization of the Angle
Clinicians can view the angle on a monitor when using
an external, fiber optic, high-resolution camera system
(Clarity Medical Systems, Inc., Pleasanton, CA) and a coupling
medium placed on the cornea. The EyeCam or
RetCam has been used in clinical settings for retinopathy
of prematurity and angle visualization, and these units
can be adapted to an OR setting. They also enable physicians
to document angle structures with video or photographs
(Figures 5 and 6).
CONCLUSION
The trend toward more minimally invasive glaucoma
surgery has encouraged the development of improved
techniques for visualizing the anterior chamber angle.
Newer goniolenses and imaging technologies enhance
ophthalmologists' ability to perform angle surgery.
Future technological advances will be driven by the
need to improve visualization of the angle, reduce
manipulation of the patient and operating microscope
during surgery, provide better control of the eye, and
facilitate easier surgical maneuvers inside the anterior
chamber. Ultimately, these innovations will likely allow
ophthalmologists to improve long-term surgical outcomes
for their glaucoma patients.
Iqbal Ike K. Ahmed, MD, is an assistant professor at the University of Toronto. He is a consultant to Clarity Medical Systems, Inc.; Endo Optiks; and Glaukos Corp. Dr. Ahmed may be reached at (905) 820-3937; ike.ahmed@utoronto.ca.
Steven D. Vold, MD, is chief executive officer of and a glaucoma and cataract surgery consultant at Boozman-Hof Eye Clinic, PA, in Rogers, Arkansas. He acknowledged no financial interest in the products or companies mentioned herein. Dr. Vold may be reached at (479) 246-1700; svold@cox.net.
