Gonioscopy: A Text and Atlas Tanuj Dada
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GONIOSCOPE TEXT

Introduction and Principle1

Gonioscopy is a clinical biomicroscopic technique to examine the angle of the anterior chamber of the eye with the use of a special contact lens known as the gonioscope. It was Trantas in 1907, who first visualized the angle in a case with keratoglobus by indenting the limbus and coined the term gonioscopy. The goniolens was introduced by Salzmann in 1914 and the gonioprism by Goldmann in 1938. Gradle and Sugar were the first ones to try and quantitate the angle configuration in 1940, a goal not yet reached. Forbes in 1966 gave description of pressure gonioscopy with the Zeiss 4 mirror lens.
The anterior chamber angle is formed between the peripheral part of the cornea and the periphery of the iris and is not visible on routine clinical examination. It is not possible to view the iridocorneal angle of the normal eye directly, because light from the angle strikes the cornea at an angle of incidence > 46°, which is the critical angle (cornea-air interface) for total internal reflection. Thus light rays coming from the anterior chamber angle exceed this critical angle and are reflected back (Figure 1.1) into the anterior chamber, thereby preventing visualization of the angle (there is a rare exception to this rule in eyes with a very steep cornea and a deep anterior chamber like keratoglobus and keratoconus where the angle structures may be directly visualized). A gonioscope facilitates examination by obviating the air cornea interface (Figure 1.2), thereby allowing light from the angle to exit the eye. Since the index of refraction of the contact lens approaches that of the cornea, there is very little refraction at the interface between these two media, which eliminates the optical effect of the front corneal surface.
Gonioscopy is an invaluable tool in diagnosing and planning management for glaucoma cases. The purpose of gonioscopy is to determine the topography of the anterior chamber angle. One of the most common indications for performing gonioscopic examination is to identify angles at risk of closure and distinguish between primary angle closure and open angle glaucoma (Table 1.1). In routine clinical practice gonioscopy identifies eyes at risk of closure, an important prerequisite before dilating a predisposed patient.
It also helps us to identify developmental anomalies in the anterior chamber angle. Angle abnormalities leading to a secondary glaucoma are also easily identified with gonioscopy.3
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Figure 1.1: Light ray from the angle undergoes total internal reflection
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Figure 1.2: Use of the gonioscope eliminates the corneal optical power allowing the light rays from the angle to pass through the new lens-air interface
One can visualize new vessels in neovascular glaucoma, pigment and debris in pseudoexfoliation and pigment dispersion glaucoma, angle recession and angle foreign bodies in cases of trauma, copper deposition on Descemet's membrane, vitreous strands, etc. It is important to remember that the anatomy of the angle can be altered by surgery, time and drugs and therefore gonioscopy may need to be repeated on a regular basis during the long-term follow-up of a glaucoma patient. The gonioscope also aids in documenting any obstruction of the fistula created during glaucoma filtering surgery and studying the patency of laser iridotomy.4
Table 1.1   Indications for gonioscopy
Diagnostic
• To study topography of the anterior chamber angle
• To assess degree of opening of anterior chamber angle recess
• To assess risk of closure on dilatation of pupil
• Visualization of congenital anomalies
• Classification of glaucoma (Primary / Secondary)
• To note the presence and extent of angle neovascularization
• Assessment of abnormal angle pigmentation
• Visualization of pseudoexfoliative material in the angle
• To look for post-traumatic angle recession, cyclodialysis
• Rule out foreign body in the angle after open globe injury
• Neoplastic invasion into angle structures (ciliary body tumor)
• Diagnosis of epithelial down-growth
• To look for vitreous strands incarcerated in the surgical wound
• To view copper deposition on Descemet's membrane
• To study patency of trabeculectomy fistula
• To view a peripheral laser iridotomy
• To visualize the internal ostium of a glaucoma drainage device
• To see orientation of haptics of an anterior chamber IOL
Therapeutic
• Laser trabeculoplasty
• Excimer laser trabeculotomy
• Goniotomy/gonioplasty
• Laser goniophotocoagulation
• Reopening of a blocked trabeculectomy opening
• Nd-YAG laser after deep sclerectomy
• Laser of suture tied around tube of a glaucoma drainage device
• Indentation gonioscopy to break an acute attack of PACG
In addition to its various diagnostic uses, gonioscopy is also used for therapeutic purposes when performing procedures like goniotomy, laser trabeculoplasty, goniophotocoagulation, etc.
In general gonioscopy is contraindicated whenever there is any suspicion of an open globe injury or immediately after a concussion injury as pressure on the globe can aggravate the injury caused to ocular tissues. Examination is also to be avoided in the early postoperative period after any intraocular surgery and in cases with traumatic hyphema until the blood has cleared to permit an adequate view of the angle structures. One should avoid gonioscopy if the eye has a corneal abrasion or an external ocular infection.5
Gonioscopic examination is an important tool in the examination of patients with ocular disorders and must be incorporated into the routine ophthalmic evaluation as a standard protocol. An incorrect diagnosis is often made if one omits to examine the anterior chamber angle, resulting in improper therapy. The gonioscopic evaluation provides a clear insight into the pathogenesis of the glaucomatous pathology and facilitates appropriate medical, laser and surgical treatment. Mastering gonioscopy is also a necessary requirement for the performance of laser procedures on the angle structures. Competent performance of gonioscopy is an art and science acquired only through experience as it requires considerable eye-hand coordination and a knowledge of the normal and abnormal gonioanatomy. and the ability to avoid artifactual observations.
All ophthalmologists must acquire training in the art of gonioscopy and perform it regularly in their out-patient service.