Primary Open-Angle Glaucoma (POAG) is a chronic progressive optic neuropathy with characteristic morphological changes at the optic nerve head and retinal nerve fiber layer, in the absence of other ocular or congenital anomalies.
It is the second leading cause of blindness in the world after cataract and the leading cause of permanent blindness. Glaucoma treatment consists of lowering intraocular pressure (IOP), which can be performed by topical eye drops, laser, or surgery. Even with treatment, there is 15.5% blindness per 7.5±5.5 years.
Considering the irreversible nature of the glaucomatous damage, it is fundamental to treat it early and effectively. Surgery is more effective in decreasing IOP (48% mean decrease) than medication (18-35%) and laser (25.6%).
Surgery is usually considered when medical therapy is deemed inappropriate, not tolerated, insufficient, and either glaucoma progresses (documented by visual fields, OCT, or other) or has a high risk of progression.
Trabeculectomy is a traditional glaucoma surgery modality. Trabeculectomy consists of opening a guarded hole in the eye within the trabeculum, to allow drainage of the aqueous humor and consequently to decrease the IOP. Trabeculectomy is invasive and has a high complication rate.
There are many non-penetrating alternatives to trabeculectomy that aim to have the same effectiveness in decreasing IOP but fewer complications.
The most widely known non-penetrating surgery is deep sclerectomy.
Filtering surgery evolved from the classic trabeculectomy, in which penetration into the anterior chamber is a necessary step, toward nonpenetrating deep sclerectomy.
The first procedure presents several serious complications, such as durable hypotony, hyphema, flat anterior chamber, choroidal detachment, endophthalmitis, and surgery-induced cataract.
To avoid such drawbacks, a novel nonpenetrating technique was designed to improve the predictability of the intraocular pressure (IOP)-lowering action, while reducing the incidence of the immediate postoperative complications encountered with the penetrating method.
This surgery works by building up new outflow pathways for the drainage of the aqueous humor while maintaining the integrity of the anterior chamber. Deep sclerectomy acts at the bulk of main resistance to aqueous humor egress, located at the juxtacanalicular meshwork and at the inner wall of Schlemm's canal.
It consists of dissection of these two structures while keeping a thin filtering membrane through which aqueous humor is drained. The membrane prevents overfiltration and ensures a reproducible postoperative IOP.
This surgery is indicated for most glaucomas, with the exception of angle closure and neovascular cases. The procedure consists in opening the conjunctiva and Tenon's capsule and creating a 5 × 5-mm limbus-based superficial scleral flap.
A deeper scleral flap measuring about 4 × 4 mm is dissected and the roof of Schlemm's canal is removed. A space maintainer is inserted and the flap and conjunctiva are closed.
Results after 10 years are good, with an IOP of 12.2 ± 4.7 mmHg and an overall success rate of 77.6% with few complications.
In recent decades, safety concerns have fuelled progress in glaucoma surgery techniques. Standard trabeculectomy (TE) has well-known complications such as hypotony; choroidal detachment; flat anterior chamber; hyphema; acute or late endophthalmitis; and, in some cases, surgery-induced cataract.
Many new methods, including non-penetrating surgical procedures, have been developed in the search for an alternative approach.
Deep sclerectomy (DS) has become one of the most widely used non-penetrating surgeries in primary open-angle glaucoma (POAG), and there is good evidence as to its efficacy and safety.
In DS, removing the inner wall of Schlemm’s canal and the juxta-canalicular trabecular meshwork enhances aqueous outflow. The trabeculo-Descemet’s membrane (TDM) remains intact to control and prevent excessive aqueous outflow, which offers an advantage in comparison with TE.
The low complication rates of DS offer an opportunity to perform the surgery at an earlier stage of glaucoma, and it can be considered as first-line therapy in cases when eye drops are not enough to control the IOP or if laser treatment is unavailable.
DS can also be considered if the compliance of the patient is uncertain. Furthermore, DS is preferred in uveitic glaucoma with open angle as it causes less inflammation than penetrating procedures.
Another indication with which DS is the more appropriate choice of procedure is high myopia, which has a higher risk of choroidal detachment. In the presence of risk factors for inflammation such as chronic blepharitis, or for patients with dementia, DS could be considered to reduce the risk of endophthalmitis.
Cataract surgery in glaucoma patients is a good indication to reduce the IOP through combined surgery (phaco with DS). On the other hand, neovascular glaucoma is considered as a contraindication to DS, since the TDM filtration can be decreased or stopped by the fibrovascular membrane over the irido-corneal angle.
Similarly, cases of iridocorneal endothelial syndrome are contraindicated for DS. A narrow angle is considered as a relative contraindication because of possible anterior synechia formation or iris incarceration following surgery.
Eyes with damaged trabeculae (e.g., post-traumatic angle recession, post-laser trabeculoplasty) are also relatively contraindicated for DS, as surgical success depends on the integrity of angle structures.
The different biometric changes following TE and DS, which may be important in cases such as combined surgery, are discussed as follows. Biometric changes after TE and DS Astigmatism Claridge et al. studied and controlled 29 patients admitted for TE.
Subjective and automated refraction, manual keratometry and corneal topography were assessed pre- and postoperatively, and the results confirm that glaucoma surgery has an influence on astigmatism.
After TE there was an increase in vertical keratometry producing with-the-rule (WTR) astigmatism. Moreover, the topographic changes lasted for at least 12 months after surgery.
These changes could have a significant effect on visual function in some patients. Rosen et al. used corneal topography preoperatively and at 12 weeks postoperatively to show a WTR shift of 1.5–2.5 D of cylinder, which was underestimated by keratometry.
This was thought to relate to tight sutures, excessive cautery or a large drainage bleb. This change in astigmatism would interfere with precise measurement of the IOL in combined surgery.
On the other hand, in a report by Corcostegui et al. reviewing 38 eyes of 35 patients, there was no clinically significant refractive change following phaco-DS surgery.
This could be down to the fl at bleb after DS. Moreover, the sutures in DS should not be tight like those in TE. A recent review by Chan et al. suggests that the astigmatism change stabilises at 3 months after TE.
Anterior chamber depth (ACD) is another variable after glaucoma surgery. Husain et al. analysed 122 patients over 5 years after TE and found that the mean decrease in ACD from baseline was 0.11 mm at all postoperative visits.
However, Bouhéraoua et al. evaluated parameters including ACD in 20 eyes of 20 patients who underwent DS for POAG. Measurements were taken 1 day pre-op, then on Days 1, 7 and 30 post-op, and surgery was not found to aff ect ACD.
This could be a result of the controlled fi ltration through TDM in DS. Axial length Husain et al. reported that, 5 years after TE, axial length (AL) was shorter by about 0.16 mm compared with the value before surgery. This has an eff ect of about 0.4 D, which is sometimes signifi cant visually.
On the other hand, García et al., who studied 22 patients who had undergone DS, reported that the decrease in AL 1 year after DS was 0.08 mm. The difference is thought to be related to the amount of fi ltration after glaucoma surgery.
Phaco surgery can result in damage to the endothelium in the cornea. Glaucoma surgery is another wellknown cause of endothelial cell loss (ECL), but it occurs less with DS than with TE.
Arnavielle et al. studied 62 eyes of 62 patients and reported ECL values of 7% after TE and 2.6% after DS 3 months after surgery, then, 12 months after surgery, 9.6% with TE and 4.5% with DS.
Some surgeons prefer to include paracentesis in DS surgery to reduce the aqueous humour in the anterior chamber, which reduces the chances of perforating the TDM.
This procedure may be the cause of more ECL occuring in DS. Reduced endothelial cell count may lead to corneal decompensation in the long term and this should be considered in combined surgery: it is wise to measure the endothelial cell count before surgery.
Regardless of this, DS offers an advantage to the patient with reduced endothelial cell count. Overall, DS has many advantages that may make glaucoma surgery or combined surgery in many patients safer and more successful.
This is particularly relevant in developing countries, where minimally-invasive glaucoma surgery (MIGS) might be difficult to afford or unavailable; also, MIGS might not reduce the IOP enough to reach the target pressure.