Due to the availability of advanced imaging technologies, gonioscopy may not be used as much as it once was for the diagnosis and management of glaucoma. That said, its diagnostic value remains. Specifically, gonioscopy aids in the identification of angle pigmentation degree, the status of the ciliary body band, the quality and content of the trabecular meshwork width, peripheral anterior synechiae, neovascularization of the angle, minimally invasive glaucoma surgery (MIGS) candidacy and MIGS position, and scleral spur width.
Because assessing the filtration angle and identifying the angle structures using gonioscopy can be challenging, time-consuming, and uncomfortable for the patient, this article provides steps to facilitate its use. (See “Choosing a lens/types of procedures,” below.)
Choosing a lens/types of procedures
There are multiple gonioscopy lens options to choose from: single, two, three, four, or six mirror. Lens choice and whether to use a flange is based on practitioner preference.
A single-mirror lens has one 62° mirror. This allows for viewing of all the anterior chamber angles without rotation of the lens sans methylcellulose. Also, the single-mirror lens is designed for a laser trabeculoplasty procedure.
A two-mirror lens is designed to assess the anterior chamber angle and central retina. This lens has two opposing mirrors inclined at 62°, positioned 180° apart. No coupling solution is required to use this lens. The two-mirror lens may be used with a flange for trabeculoplasty or without a flange for gonioscopy.
A three-mirror gonioscopy lens is often the first lens the optometry student encounters. Its thumbnail mirror aids in angle assessment. The three mirror lens’s central and rectangular mirror provides a magnified view of any vitreal or retinal anomaly noted during ophthalmoscopy. The three-mirror lens requires coupling solution to allow suction of the lens onto the patient’s cornea. For successful use, the lens should be filled halfway; usually with
1% carboxymethylcellulose or 2.5% methylcellulose.
A four-mirror lens has a diameter similar to that of the human cornea, requiring no coupling solution. As a result, using this lens often makes applanation of the lens and the procedure easier for patients. Additionally, a four-mirror lens helps differentiate between the different types of angle closure during compression gonioscopy.
A six-mirror lens provides a 360° panoramic view of the anterior chamber angle without requiring coupling solution or a flange.
The gonioscopy procedures are indirect, direct, and compression:
Indirect. This involves the patient sitting upright at the slit lamp and the optometrist having an inverted image of the opposite angle due to light being reflected off the mirror and directed toward the patient. A single, two, three, four, or six-mirror gonioprism may be used to perform indirect gonioscopy.
Direct gonioscopy. This provides the viewer an erect view of the lenses and is usually performed when the patient is supine, such as during a surgery and/or an exam under anesthesia. A Koeppe or Barkan gonioprism are examples of lenses used for direct gonioscopy.
Compression gonioscopy. In this technique, gentle pressure is applied to the cornea to push the anterior chamber aqueous against the iris/lens diaphragm and open any appositional angle closure. A four-mirror lens is ideal here, as it aids in differentiating between the types of angle closure. This procedure is particularly important to understand and master, as it is helpful in considering whether a patient would benefit from a laser peripheral iridotomy (LPI). If there is no opening of the angle during compression gonioscopy, then it is unlikely that an LPI will open the patient’s angle, and incisional glaucoma surgery will be more effective for the patient.
Prepare the Patient
Gonioscopy is best performed in dim illumination, allowing for pupillary dilation, which is particularly beneficial in identifying narrow angles.
After ensuring appropriate lighting, the optometrist should educate the patient on the purpose of gonioscopy and what the patient can expect. This is because patients tend to express some degree of apprehension when having anything near their eye, and this apprehension can make identifying pathology difficult. (See the article, “Explaining glaucoma diagnostic devices.”)
I use this clear, concise explanation of why gonioscopy is being performed: “I’m going to perform something called gonioscopy. The reason we are doing it today is because your angles, your eyes’ drainage system, appear narrow, and gonioscopy will allow me to get an enhanced view of this system and ensure that I can not only safely dilate your eyes, but you won’t develop what is called angle closure in the future. First, I will instill an anesthetic in both eyes, so that this procedure is more comfortable for you; then I will place the gonioscopy lens on the front of your eyes. You will feel me touching your eyelids, but you should not feel anything else. If you do, please let me know immediately. I will then have you hold on to the handlebars here on the slit lamp and make sure you are comfortable before I begin. Any questions before I get started?”
Ready Yourself and Your View
The OD’s elbow should rest on the slit lamp table for support. For an added brace, I also rest my pinky and ring fingers on the patient’s orbital bone.
Next, I start at 10x magnification and increase to 16x if I need to see more detail of the angle between the iris and the cornea. A scenario of when I increase the magnification: A patient in whom I am highly suspicious of neovascularization of the angle and want to be able to identify those small, critical, new vessels. In patients who have a narrow angle or suspected angle closure following Van Herick angle assessment, I use low slit lamp illumination to prevent pupillary constriction from artificially opening the angle.1
Some structures, such as Schwalbe’s line, may be challenging to visualize due to a decreased amount of pigment visible in the angle or the iris bowing forward, such as in the case of an iris bombé. Don’t sweat it! There are a couple techniques that can help overcome these tricky angles:
1. Instruct the patient to look slightly in the direction of the angle you want to view. This allows for a secondary view in narrow-angle cases.
2. Use a corneal wedge. This enables a clear view of the superior and inferior angle. To do this, form a bright optic section beam 10° to 20° off axis from the slit lamp. This will form two beams: a bright beam that follows the iris through the angle and onto the internal corneal surface and a fuzzier, broader beam that follows the external corneal surface as it meets the sclera. Where these two beams meet is Schwalbe’s line, which, as a reminder, is the anterior border of the trabecular meshwork.
Know Where/What to Look For
I start with the inferior angle to get my bearings. This is because its increased pigmentation due to gravity allows me to identify the structures easier. Within the inferior angle, I begin with the most posterior structure, the iris. The reason: The iris is the most easily identifiable structure within the angle, facilitating orientation during the exam.
Iris: An iris that has a flat, concave (inwards) insertion is indicative of pigment dispersion syndrome. An iris that has a convex (outward) insertion is more indicative of iris bombé. A plateau iris configuration displays the typical “double hump” sign due to the aqueous humor being pushed behind the iris.2 Iridodialysis, or tears in the iris, can occur as a result of blunt trauma.
Ciliary body: If this structure is broadened, this suggests angle recession.
Schlemm’s canal: Blood in Schlemm’s canal can occur if the episcleral venous pressure (EVP) is higher than the patient’s IOP. Normal EVP is 8 mmHg to 10 mmHg. Normal IOP is 10 mmHg to 20 mmHg.3 The cause of this high EVP can be the pressure exerted from the gonio lens itself or from hypotony. To distinguish the difference, the OD should apply less pressure. If the blood recedes along with any accompanying Descemet’s folds, blood in Schlemm’s canal is the root cause. If blood in Schlemm’s canal is still present, ocular signs and symptoms from the exam, such as recent ocular surgery, blurred vision, and low IOP, can help indicate that this is due to hypotony.
Increased EVP may also be due to other conditions, such as a dural or carotid cavernous sinus fistula, superior vena cava obstruction, Sturge-Weber syndrome, or thyroid ophthalmopathy.
To help determine the specific cause, a diagnostic workup, including orbital ultrasound, orbital CT and/or MRI, orbital venography, and fluorescein angiography are recommended.4
Schlemm’s canal should particularly be examined after MIGS, as blood can reflux into it, causing a hyphema. A hyphema can cause occlusion of the stent with blood, or iris tissue, which can be visualized via gonioscopy.
Scleral spur: This structure is the site of attachment of the longitudinal muscles of the ciliary body band. It appears as a thin, pale line that may yellow with age. A shorter scleral spur has less trabecular meshwork attachments compared to healthy eyes, leading to increased outflow resistance and, ultimately, elevated IOP.
Trabecular meshwork: Heavy and homogenous pigmentation throughout the angles signal pigment dispersion syndrome. Heavy and patchy pigmentation throughout the angles signals pseudoexfoliation syndrome.
Fine new blood vessels found over the trabecular meshwork that cross the scleral spur are signs of neovascular glaucoma.
Large hyphemas secondary to ocular surgeries or trauma can fill the trabecular meshwork with blood, causing elevated IOP.
Finally, in ghost cell glaucoma, an unusual secondary open-angle glaucoma following a vitreous hemorrhage, the red blood cells diffuse through the vitreous cavity and settle into the trabecular meshwork, causing elevated IOP.5 These cells appear khaki-colored on exam and usually settle into the inferior angle due to gravity.5
Schwalbe’s line: In some patients, such as in those who have pseudoexfoliation or pigment dispersion syndrome, pigment granules may settle in this area. This is termed Sampaolesi line. Posterior embryotoxon is an anteriorly displaced Schwalbe’s line that may be found in 8% to 32% of the normal population.6
Posterior embryotoxon may be a benign finding or associated with the ocular condition, Axenfeld-Rieger syndrome.7,8 Axenfeld-Reiger syndrome is often accompanied by other ocular manifestations, such as ectropion uveae, iris corectopia, or iris atrophy. These patients may have either microcornea or megalocornea and often have systemic manifestations, such as craniofacial dysmorphism and dental abnormalities.
Document Finding
Multiple grading systems are available to record your angle assessment. They are Scheie, Spaeth, and Shaffer. (See “Angle assessment tables,” below.) There is no best system. For other optometrists, describing in the patient’s chart what they saw may be easier, i.e. “ciliary body band 360°.”
Pro tip. Remember when performing indirect gonioscopy, your mirror is 180° away from the angle you are viewing.
An Essential Tool
As illustrated, gonioscopy remains an important diagnostic tool. While performing it can be challenging, as is the case with the other skills optometrists have to learn, it just takes time and practice to master. OM
References
1. Kim JM, Park KH, Han SY, et al. Changes in intraocular pressure after pharmacologic pupil
dilation. BMC Ophthalmol. 2012;12:53. Published 2012 Sep 27. doi:10.1186/1471-2415-12-53.
2. Kiuchi Y, Kanamoto T, Nakamura T. Double hump sign in indentation gonioscopy is correlated with presence of plateau iris configuration regardless of patent iridotomy. J Glaucoma. 2009;18(2):161-164. doi:10.1097/IJG.0b013e31817d23b5.
3. Jonas JB, Aung T, Bourne RR, Bron AM, Ritch R, Panda-Jonas S. Glaucoma. Lancet. 2017;390(10108):2183-2193. doi:10.1016/S0140-6736(17)31469-1.
4. Jørgensen JS, Guthoff R. Zur Genese der einseitig dilatierten episkleralen Gefässe und Erhöhung des intraokularen Drucks [Pathogenesis of unilateral dilated episcleral vessels and increase in intraocular pressure]. Klin Monbl Augenheilkd. 1987;190(5):428-430. doi:10.1055/s-2008-1050426.
5. Campbell DG. Ghost cell glaucoma following trauma. Ophthalmology. 1981;88(11):1151-1158. doi:10.1016/s0161-6420(81)34892-1.
6. Rennie CA, Chowdhury S, Khan J, et al. The prevalence and associated features of posterior embryotoxon in the general ophthalmic clinic. Eye (Lond). 2005;19(4):396-399. doi:10.1038/sj.eye.6701508.
7. Shields MB. Axenfeld-Rieger syndrome: a theory of mechanism and distinctions from the iridocorneal endothelial syndrome. Trans Am Ophthalmol Soc. 1983;81:736-784.
8. Reis LM, Maheshwari M, Capasso J, et al. Axenfeld-Rieger syndrome: more than meets the eye. J Med Genet. 2023;60(4):368-379. doi:10.1136/jmg-2022-108646.
