Proficiency in surgical gonioscopy is essential to obtain a clear working view and to optimize surgical outcomes. Like all glaucoma surgeries, trabecular bypass interventions have a substantial failure rate. A glaucoma specialist is typically in the best position to perform these procedures and manage any adverse outcomes expediently. There are scenarios where a non-fellowship-trained ophthalmologist can provide the best care by offering MIGS surgery, but I recommend planning this with a local (or at least regional) glaucoma specialist beforehand, to ensure there is a backup when needed.
The following surgical gonioscopy pearls are focused on direct gonioscopy, which is typically used to visualize the nasal angle from the temporal sitting position. Indirect gonioscopy can be helpful during goniosynechialysis procedures but is not specifically discussed here. Moreover, the surgical gonioscopy pearls will be contrasted with clinical gonioscopy to highlight and emphasize key differences. A surgeon who is new to surgical gonioscopy should naturally be comparing the experience to what they know from clinical gonioscopy at the slit lamp. The skills are complementary, but distinct differences should be noted.
Know the anatomy
Translating knowledge from clinic to the operating room is key. The comfort from frequent, routine clinical gonioscopy exams is what allows the surgeon to manipulate angle tissue confidently and accurately in the operating room. In clinic, I use the Sussman lens to examine the angle, with compression when there is irido-trabecular contact. It may seem redundant, but I recommend routinely re-examining the angle of a patient you are considering for angle-based surgery. You may even want to assess one final time in surgery before making wounds, to ensure the landmarks are distinct. On the other hand, a softer anterior chamber after wound construction can elucidate a hypopigmented meshwork by encouraging blood reflux into Schlemm’s canal.
With respect to clinical gonioscopy and anatomical landmarks, when at the slit lamp with a 4-mirrored indirect gonioscopy lens (e.g., Sussman), the physician has access to 360 degrees of angle visualization. Contrastingly, surgical direct gonioscopy is limited to approximately 100 degrees of angle across from the surgeon—most often the nasal angle—and thus requires a more facile recognition of key anatomical landmarks.
Optimize corneal clarity
There are several ways to keep a pristine cornea for optimal gonioscopic views. Avoid preoperative viscous lidocaine gel, and use tetracaine drops instead. Be careful to keep the eye closed during povidone administration to the lids and lashes, and make sure not to abrade the corneal surface during draping and lid speculum placement. Cornea-protecting viscoelastics (e.g., HD Eyefill) should be applied generously to maintain clarity and to optimize the contact surface for the lens. For filling the anterior chamber, most surgeons have a preferred viscoelastics or soft-shell combinations, but there is not a wrong choice here if corneal clarity is maintained.
Consider the amount of pressure inside the anterior chamber; too little viscoelastic and the cornea will be soft and prone to wrinkles, too much and collapse of Schlemm’s may make goniotomy or canulation difficult. The gonioscopy lens must be held on the cornea with enough pressure to maintain a clear gel meniscus, but excessive exterior pressure, especially in larger (e.g., myopic) eyes, will cause wrinkles and a blurred view.
As described above, one can surmise that during surgical gonioscopy, the image clarity fluctuates due to variations in lens pressure on the cornea, changes in the volume of viscoelastic in the anterior chamber, movements of instruments through corneal wounds, and the arrival of new obstructions into the view (e.g., blood reflux). During clinical gonioscopy, corneal clarity and image stability can be more readily achieved without the additional confounding variables present during surgical angle examination.
Construct and utilize corneal wounds
Variations in corneal wound construction can make a case easier or more challenging. Short wounds may compromise anterior chamber maintenance, but instruments may deform longer wounds, compromising your view. A wound placed too anteriorly can deform the cornea or interfere with the gonioscopy lens, but more posterior wounds are more likely to produce blood from limbal vessels, which can track onto the cornea. In cases of suboptimal wound construction, copious application of corneal viscoelastic will displace blood and clear your view. Be conscious of the “oarlock” relationship of instruments in corneal wounds. In cataract surgery, poor technique with an instrument pushing against a wound edge can result in excessive eye movements, poor fluid dynamics, and inferior technique. However, in glaucoma angle surgery, an instrument impinging on the wound can obliterate the view making surgery near-impossible.
Without a need to assess corneal wounds, a clear clinical gonioscopy slit lamp image is easier to obtain and to adjust rapidly to change level of focus, illumination, and zoom. At the slit lamp, it takes a few seconds to put down the gonioscopy lens and resume a normal anterior segment exam. In the operating room, transitioning between standard and gonioscopic visualization takes additional time and involves rotating both the scope and the patient’s head.
Align head and scope for success
For ophthalmology residents and glaucoma fellows, this can be a counterintuitive concept: it is possible to clearly distinguish the angle anatomy, yet still be in a poor position for surgery. The patient’s head is tilted away, and the microscope must be rotated toward the surgeon beyond the amount needed just to visualize the angle. When the view is truly perpendicular (akin to en face imaging) to the angle structures, maneuvers like goniotomy and canulation become much more facile. Finally, increasing surgical microscope zoom and light intensity will further aid the best possible visualization.
Comparing these ergonomic elements, in clinical gonioscopy, the physician’s body position is nearly identical to that for holding a 78D or 90D lens for fundoscopy at the slit lamp. Clinical gonioscopy does not require the specific hand and arm positioning necessary for successful surgical gonioscopy.
Practice makes permanent
Before your first glaucoma angle surgery, practice surgical gonioscopy near the end of about a dozen cataract surgeries in patients with a history of glaucoma or glaucoma suspects. Do this after the intraocular lens has been placed in the capsular bag but before evacuation of viscoelastic. Progress can be controlled and stepwise: several cases of just bringing the nasal angle into sharp focus, followed by several cases during which the view is extended further superonasally and inferonasally; finally, several cases in which a viscoelastic canula is introduced into the anterior chamber and the cannula is brought near - but not touching - the angle and swept superiorly and inferiorly to mimic surgical maneuvers.
Ultimately, the goal with surgical gonioscopy is to develop increasing skill in surgical maneuvers such as moving instruments toward, into, and along the meshwork, and using viscoelastics to clear the view when necessary. I believe these surgical gonioscopy pearls will benefit your development in surgical gonioscopy and optimize surgical outcomes. Moreover, by contrasting surgical gonioscopy with clinical gonioscopy, one can gain insight into the unique aspects specific to surgical viewing of the anterior chamber angle.
I must credit several of these pearls to my local colleague, Dr. Jamie Taylor, who helped introduce me to angle surgery. Lastly, I recommend the necessary ongoing self-directed learning—greatly aided by access to surgical videos—on the topic of angle surgery to foster proficiency.