The Education Committee carefully selects presentations from the WGC-2019 for your benefit. This month Parul Ichhpujani introduces the sessions: Blood flow measurements in glaucoma: Ready for Prime Time? and Tackling challenging glaucoma: A case-based approach.
Blood flow measurements in glaucoma: Ready for Prime Time?
Presentation 1: Invasive angiography: New and Old By Dr Osamah Saeedi
Presentation 2: Application of OCT-A in glaucoma – Facts and Artifacts By Dr Christopher Leung
Presentation 3: Systemic Assessment of Vascular Regulation By Dr Konstantin Gugleta
Dr Osamah Saeedi spoke about the need for building upon the already known angiographic findings about retinal vasculature (“standing on the shoulders of giants”).
What Conventional Angiography contributed: In patients with glaucoma, Fundus fluorescein angiography (FFA) has shown hypoperfusion and prolonged arteriovenous passage time. Indocyanine angiography showed decreased peripapillary choroidal filling.
What New Non-Invasive Angiography can contribute: The new kid on the block, Erythrocyte Mediated Angiography (which uses ICG loaded erythrocyte ghosts) has shown that at some point of time the erythrocytes pause in retinal vessels; they are not constantly in motion. This technique was initially used to study In vivo vasomotion (in nonhuman primate) and ongoing autoregulatory phenomenon in ocular blood vessels.
Dr Saeedi shared his research on studying vasomotion in humans using HRT II (with Erythrocyte mediated ICG angiography) and development of automated velocimetry for retinal microvasculature. He concluded that although, this technique is useful for validation of noninvasive angiography but still in process of development and hence not ready for prime time.
Dr Christopher Leung discussed the role of OCT-A in diagnosing as well as assessing progression of glaucoma. OCT-A has shown 4 individual layers of capillary network (which are interconnected). In glaucoma, loss of capillary bed has been identified by OCT-A. Literature is conflicted about the value of RNFL thickness as measured by OCT and vessel density parameters of the corresponding areas measured on OCT-A. But since most studies had variability in OCT resolution and measurement protocols, therefore no concrete conclusion can be drawn from them. There is also difference in the time to scan the retina; OCT for RNFL thickness takes just 2-3 seconds while OCT-A measurements may require 11-16 seconds. Diurnal variation in flow measurements has also been seen using OCT-A.
On standardizing the region of interest and the retinal layers for measurement, inner macular vessel density and inner macular thickness measurements, it has been found that the inner macular thickness parameters outperform macular vessel density. He concluded that the OCT-A of macula may have a limited role in diagnosis of glaucoma.
Not much literature exists regarding role of OCT-A in detecting glaucoma progression. Dr Leung suggested that, what needs to be remembered for cross sectional studies using OCT-A in glaucoma patients is that a large amount of data may be irrelevant for serial analysis because of motion artifacts. Therefore, at this time point, the role of OCT-A in studying progression is uncertain.
Prof Konstantin Gugleta pointed out that the autoregulation capacity depended both on Blood pressure (BP;much stronger component) and intraocular pressure (IOP). This capacity can be ascertained by Retinal dynamic vessel analyser. He also talked about the concept of neurovascular coupling and flow mediated dilation.
Retinal Veins have been considered to be passive conductors; but recent studies have shown that they have contractile elements and thus can be modulated both by vasocontraction and vasodilation agents. Although there are no new methods to measure Retinal Venous pressure (RVP), it can be measured by Ophthalmodynamometry. RVP is of significance in Normal Tension Glaucoma and venous occlusion.
He also talked about the Translaminar pressure. Unfortunately, none of the currently available methods to measure it can be used in clinical practice. He suggested that retinal venous pulsations can be a surrogate for translaminar pressure. To ascertain, what is the ‘Pacemaker’ for retinal venous pulsation, he talked about a study that used ECG gated recording of IOP and retinal vein pulsations and found similar pattern in retinal vein diameter and IOP.
He said that despite ongoing research, still there is no concrete parameter(s) of vascular dysregulation, which would be easily measurable, therapeutically influenceable as directly as possible, and predictive in terms of future glaucomatous damage.
Rapidly developing technologies such as OCT-A may be more predictive of future glaucoma damage by virtue of combining functional and morphological information. But we are far away from prime time.
Tackling challenging glaucoma: A case-based approach?
Presentation 1: Big eyes by Dr Norman Aquino
Presentation 2: Post corneal surgery by Dr Thasarat Vajaranant
Presentation 3: Post vitrectomy by Dr Augusto Paranhos Jr.
Presentation 4: Small eyes by Dr Jimmy Lai
Presentation 5: Expulsive choroidal hemorrhage by Dr Arvind Neelkantan
Dr Norman Aquino spoke about the association between myopia and glaucoma. The odds for development of glaucoma increase with increase in severity of myopia. He discussed about clinical dilemma one faces especially in young myopic patients with normal IOP, average CCT and peripapillary atrophy, whether to consider a refractive procedure or not, keeping in mind likelihood of developing glaucoma later in life. Propensity for myopes to mount a steroid response further compounds the problem.
Additionally, structure function correlation doesn’t stand true in most cases. He suggested that models based on Deep Learning and Artificial Intelligence may give better insight to such relationship. He concluded that the key to diagnose and follow up glaucoma in big, myopic eyes is to establish a ‘reliable’ baseline.
Dr Thasarat Vajaranant talked of increased incidence of glaucoma with complexity of corneal surgeries. She suggested to watch out for steroid response after any corneal surgery, which may add insult to inflammation and tilt the scales towards glaucoma.
Ideal tonometers for this set of patients are Pnemotonometer or Tonopen; to be used in non-diseased area, as close to the center as possible. Since these compromised eyes can have a rapid loss of fixation, so she emphasized that one must exercise extra caution and have a low threshold of treatment. Goldmann perimetry and ASOCT can fill in gaps in dilemma.
When planning glaucoma surgery in eyes with keratoplasty or keratoprosthesis (KPro), consider implanting tube of the drainage implant in sulcus or in pars plana.
Dr Vajaranant showed a patient with monocular vision and uncontrolled intraocular pressure who had undergone corneal surgery with drainage implants in three quadrants. The dilemma was what to consider next. She chose to implant a third drainage implant (AGV FP7; smaller size) in superonasal quadrant with a sulcus placement. In cases where tube (in AC) is touching the cornea, you can put the tube in the sulcus after extending the tube using an angiocatheter.
For KPRo patients, consider tube of drainage implants in pars plana as anterior chamber is already crowded with a functional AC depth of less than 1 mm. She prefers a Combined Vitrectomy with Pars plana implantation of a Baerveldt glaucoma implant with KPRo, in a single sitting. In such a scenario the implant plate is put first followed by the temporary KPro fitting by the cornea surgeon followed by 360 degrees vitrectomy by the retina surgeon, after which the cornea surgeon replaces the temporary KPro with the permanent KPro. The tube is then inserted in the pars plana (ligated), and patch graft is put and closed.
Dr Augusto Paranhos Jr. talked of glaucoma and secondary ocular hypertension post vitrectomy.
He mentioned that in his experience I-CARE tonometer worked best for IOP measurement after vitrectomy. The postoperative rise of intraocular pressure (IOP) is most likely in eyes with proliferative vitreoretinopathy changes, combined vitrectomy with lensectomy and with Silicon oil tamponade.
The key to a good retinal surgery with a stable postoperative IOP profile is good shaving of vitreous base. Dr Paranhos suggested that when glaucoma occurs after vitrectomy, retina surgeon should revisit and shave the vitreous base very well.
In cases with silicon oil induced glaucoma, AC maintainer should be used at the time of Silicon Oil removal to prevent migration of oil microglobules. For surgical management of glaucoma, he said did not prefer trabeculectomy as silicon oil globules migrates under the conjunctiva, causing bleb failure. Tube in pars plana or micropulse laser are his preferred modalities for managing refractory cases. In advanced, refractory cases with poor visual potential he suggested using traditional cyclophotocoagulation.
Dr Jimmy Lai spoke about small eyes viz.,eyes with narrow palpebral fissure, smaller orbital volume, nanophthalmos and microphthalmia.
From the point of view of glaucoma, he mentioned that the only trouble with eyes with narrow palpebral fissure is compromised surgical field. Nanophthalmic eyes on the other hand are predisposed to intraoperative suprachoroidal hemorrhage, while microphthalmic eyes have associated ocular defects that may compromise disease course and assessment. Internal disproportion inside a small globe can erupt through pressure difference between its structures.
Small eyes pose difficulty in tonometry and indentation gonioscopy. Small Optic disc size pose diagnostic challenge. Glaucomatous small eyes have rapid visual field progression; hence aggressive IOP reduction is warranted.
If Glaucoma drainage implant (GDI) is planned for these eyes, placement of the GDI plate may pose a challenge and the leading edge of the GDI can be too close to limbus. He suggested using an adult size Ahmed Glaucoma Valve (AGV) and trimming the implant just behind the valve thus reducing its size by a third. Dr Lai said that the modified implant works better than the pediatric AGV as even trimmed implant has more surface area than the former.
Dr Arvind Neelkantan talked about a dreaded complication, expulsive suprachoroidal hemorrhage (SCH). He cited that the Glaucoma surgery related SCH is more likely to occur postoperatively (1.6 – 6.1%) than intraoperatively (0.15% in incisional glaucoma surgery).
SCH is more likely to occur in both small and big eyes, elderly, aphakes, cases associated with vitreous loss and/or prolonged intraoperative hypotony. A surgeon should get alert if on table patient complains of intolerable pain and there occurs loss of red reflex and AC shallowing. Stopping surgery stat would help to build up the intraocular pressure and that would tamponade the vessels. Prompt wound closure with excision of prolapsed vitreous and non-viable iris tissue should be done. The role of Posterior sclerotomy in inferotemporal quadrant is controversial. Follow up on B Scan ultrasonography and seek help from your vitreoretinal colleague to drain SCH only after a week or more.