Assessing the angle: Getting back to basics

July 30, 2020

How gonioscopy can offer ophthalmologists an option in treating patients with glaucoma

This article was reviewed by Sunita Radhakrishnan, MD

Assessing the angle is important for glaucoma management. It can identify various mechanisms of IOP elevation, such as pigment dispersion and angle recession, and is critical for the diagnosis and staging of primary angle closure. It is also important in treating glaucoma.

“The angle must be visualized in order to perform laser trabeculoplasty, for example, at least when using the devices that are currently available,” said Sunita Radhakrishnan, MD, research director, Glaucoma Research and Education Group at the Glaucoma Center of San Francisco. “A new technology is in the pipeline that enables transscleral laser trabeculoplasty but for now, visualization of the angle is necessary to deliver laser energy to the trabecular meshwork. There are several minimally invasive glaucoma surgeries that also require angle visualization.”

In a clinic setting, physicians often use the Van Herick method, which can be performed rapidly, as a substitute for indirectly assessing the width of the angle. However, she questioned if this is the best route to take.

Radhakrishnan cited a study1 that evaluated the diagnostic capability of the Van Herick test when performed by observers with different levels of expertise who may all be involved in angle assessment. The study found that the sensitivity of the Van Herick test to detect gonioscopic angle closure ranged from 58% among technicians to 79% among residents.

“This is low sensitivity when screening patients for angle closure,” she noted.

A different study2 also addressed risk factors associated with misdiagnosis when using the Van Herick test to detect angle closure.

“The risk of misdiagnosis was found to be higher in patients with more severe angle closure, that is, those with primary angle-closure disease, instead of the primary angle-closure suspects, which is concerning,” Radhakrishnan said.

Indirect assessment using the Van Herick test cannot only miss angle closure but it also cannot detect angle pathology that could be present in patients with a deep anterior chamber and no obvious glaucoma risk factors. A classic example, Radhakrishnan said, is angle recession in a patient with an old and forgotten ocular injury.

These studies underscore the importance of directly viewing the angle structures with gonioscopy. This test provides a 360° assessment of the angle configuration.

“The observer can perform indentation in angle closure and identify peripheral anterior synechiae as well as plateau configuration. We can also detect various other causes of elevated pressure, such as pigment dispersion, neovascularization, and recession,” Radhakrishnan said.

Gonioscopy candidates

In her glaucoma practice, Radhakrishnan performs gonioscopy on every patient. However, there is an argument that gonioscopy should be performed even in patients who do not have known risk factors for glaucoma.

“This is a strategy to opportunistically screen for angle closure,” she pointed out, in light of the high visual morbidity of primary angle-closure glaucoma and the fact that population-based screening is not feasible for this disease.

It is also important to periodically repeat gonioscopy in patients with phakic IOLs because even those in whom the angle is open at baseline can ultimately develop angle closure as a result of lens thickening over time.

What to look for

When performing gonioscopy, the main landmark and usually the most easily recognized one is the scleral spur, which lies between the pigmented trabecular meshwork and pigmented ciliary body.

Radhakrishnan said when she performs gonioscopy, she first locates the scleral spur. If it is not visible in the first quadrant examined, then she looks in another quadrant in which the trabecular meshwork pigmentation may be more prominent. If the scleral spur still is not visible, Radhakrishnan suggests tilting the goniolens slightly; if the spur remains undetectable, then she performs indentation gonioscopy to differentiate appositional from synechial angle closure.

Very pale trabecular meshwork may increase the difficulty in visualizing the scleral spur. In such patients, she recommends starting at Schwalbe’s line (which can be identified with the corneal wedge technique) and working backward from there. Schwalbe’s line is often pigmented, especially in the inferior quadrant and can be mistaken for the trabecular meshwork, so care should be taken in interpreting the angle status to avoid misclassifying a closed angle as open.

When gonioscopy is performed in the operating room, different steps are necessary from those performed in the clinic.

Radhakrishnan referred residents, fellows, and novice surgeons to the American Academy of Ophthalmology website for tips on gonioscopy in the operating room (https://www.aao.org/clinical-video/intraoperative-gonioscopy).

“The main consideration is that the microscope illumination changes from coaxial for cataract surgery to being directed along the iris plane and onto the angle,” she explained. “To achieve this, the microscope is tilted about 30° toward you and the patient’s head is turned about 30° to 40°away from you. This increases the working distance by several inches, which takes a little time to become accustomed to.”

Another difference from the examination performed in the clinic is that a surgical (direct) goniolens is used in contrast to the indirect lens in clinic. A coupling agent is used on the cornea as well as adequate viscoelastic in the anterior chamber. She prefers Healon GV (Johnson & Johnson Vision) and noted that the product name is now Healon GV Pro to indicate a change in the formulation; the new formulation was recalled by the FDA and Healon GV has not been available for several months. Some recommendations that have been suggested for replacing Healon GV are Healon 5 and ProVisc (Alcon Laboratories), Radhakrishnan recounted.

Gonioscopy alternatives

Despite being the best way to directly evaluate the angle, gonioscopy does have limitations. “It is a subjective procedure, and it needs a slit lamp as well as a highly trained observer. In addition, there are potential sources of error regarding detection of angle closure because of the use of illumination and the contact with the eye,” Radhakrishnan said and noted that angle imaging is an alternative that overcomes some of the limitations of gonioscopy.

Cross-sectional imaging of the angle can be performed with optical coherence tomography (OCT) and ultrasound biomicroscopy (UBM). The former is a noncontact technology that facilitates assessment in the dark and the entire anterior chamber is visualized in 1 scan. The disadvantage is that visualization of structures behind the iris is not possible.

OCT provides several options to image the angle. The dedicated anterior-segment OCT (Visante OCT, Carl Zeiss Meditec) is best for visualizing the angle recess, but this machine is not widely available. Angle imaging can also be done with a retinal OCT device, but the scleral penetration is not as good as with the Visante. Radhakrishnan uses a Visante OCT in her practice and also has access to Cirrus OCT (Carl Zeiss Meditec) with an anterior-segment external lens kit that can be attached to the machine to obtain images of the angle.

UBM provides excellent visualization of the angle structures both in front of and behind the iris. However, traditional UBM with an immersion bath is time consuming, and a highly trained ultrasound technician is needed.

With both technologies, as with gonioscopy, the scleral spur is the key landmark to locate.

Radhakrishnan typically uses imaging as an adjunct to gonioscopy to identify primary versus secondary angle-closure mechanisms. However, imaging can also be a substitute for gonioscopy for patients who cannot tolerate a contact procedure or if corneal pathology does not permit visualization of the angle by gonioscopy.

She also relies on imaging to educate patients about their disease and to explain why laser or surgery might be recommended in certain situations.

“OCT is my first-line imaging device because it is easy to use and comfortable for patients,” Radhakrishnan said.

UBM is reserved for viewing structures posterior to the iris and for evaluating plateau configuration.

“I would like to encourage clinicians to perform gonioscopy more often,” she concluded. “Know the landmarks and use imaging technologies as adjuncts to gonioscopy. Remember that gonioscopy performed in the operating room is different from that in clinic.”

References
  1. Johnson TV, Ramulu PY, Quigley HA, Singman EL. Low sensitivity of the Van Herick method for detecting gonioscopic angle closure independent of observer expertise. Am J Ophthalmol. 2018;195:63-71. doi: 10.1016/j.ajo.2018.07.026
  2. Atalie C. Thompson, Daniel M. Vu, Lisa A. Cowan, Sanjay Asrani. Risk factors associated with missed diagnoses of narrow angles by the Van Herick technique. Ophthalmology Glaucoma. 2018;1(2);108-114. doi.org/10.1016/j.ogla.2018.08.002
Sunita Radhakrishnan, MD
E: frontdesk@glaucomasf.com
Radhakrishnan has no financial interests to disclose.
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