THE BLEB AND BEYOND

Bleb-dependent filtration surgery remains the gold standard for reducing intraocular pressure (IOP) in eyes with open-angle glaucoma, but there are a range of alternative techniques for eyes in which the procedure is unsuitable, according to speakers at the European Glaucoma Congress in Nice.

Trabeculectomy does not appear to work equally well in all eyes and there is evidence suggesting that differences in the conjunctival tissue of the bleb may explain some of the variations in the technique’s efficacy, said Christophe Baudouin MD, PhD, Quinze-Vingts National Ophthalmology Hospital, Paris, France.

“We know that bleb formation is extremely important for the outcome of surgery and if we can understand how it works then we will also get a better idea of why it doesn’t work in some cases,” he added.

Dr Baudouin noted that his own research shows that the functionality of the bleb appears to correlate with the density of microcysts in the bleb’s conjunctival tissue, as detected by scanning laser microscopy. The microcysts’ density in turn appears to correlate with the concentration of goblet cells, which in the functioning blebs appear to contain aqueous humour instead of mucin, as is usually the case. (see Figure 1)

A more recent study they conducted using en face optical coherence tomography (OCT) appeared to confirm the correlation between the density of microcysts (see Figure 2) and the functionality of blebs, Dr Baudouin said. It also showed an inverse correlation between the amount of IOP reduction achieved preoperatively and the number of years a patient had received preservative-containing eye drops preoperatively.

Meanwhile, several new techniques have been developed over the years that use non-conjunctival routes to improve outflow of the aqueous from the anterior chamber.

“The question for the future will be whether it is better to improve glaucoma surgery with blebs, with its problems related to bleb formation and anti-metabolites, or whether blebless surgery will replace or partly replace bleb-dependent surgery,” said Dr Baudouin.

Endoscopic photocoagulation

An older form of minimally invasive glaucoma surgery that is undergoing renewed evaluation is endoscopic cyclophotocoagulation (ECP), a 15-year-old technique which aims to reduce IOP by both reducing aqueous production and increasing aqueous outflow, said Pavi Agrawal MD, Nottingham University Hospital, Nottingham, UK.

The ECP procedure involves the use of an 18-gauge endoscopic diode laser probe placed through a 1.5mm limbal incision to provide a visually targeted delivery of energy to the ciliary processes. The result is a coagulative shrinkage and necrosis of the pigmented epithelium. The technique also increases uveoscleral outflow through the inflammatory effects of the laser.

“The beauty of this technique is that it can be combined with phacoemulsification in elderly patients, hopefully avoiding the need for filtration surgery in some cases,” Dr Agrawal said.

ECP can achieve good reductions in IOP in many cases where medication and filtration surgery has failed. Success rates with the technique reported in the literature range from 75-90 per cent, varying on the definition of success. ECP can achieve up to a 40 per cent reduction in IOP, which is not far off the IOP reductions of 45-55 per cent achievable with filtration surgery.

The typical indications for cyclophotocoagulation include eyes in which previous medical and surgical therapy have failed and also patients who have a painful blind eye. However, there are several other types of cases in which the treatment has special advantages.

They include eyes with acute angle-closure crisis unresponsive to intravenous Diamox and iridotomy, and cases where the patient is pregnant, in whom anti-metabolites are therefore contraindicated.

Moreover, research suggests that the adverse effect on vision which some attribute to cyclophotocoagulation may actually be a result of the more advanced stage of glaucoma that is present in patients who undergo the treatment. The main complication of ECP is cataract progression, but when combined with phacoemulsification the main complications are an IOP spike and a fibrinous inflammatory reaction.

“Endoscopic photocoagulation is not a replacement for filtration surgery, but it is an important part of the glaucoma surgeon’s armamentarium. It has promising outcome data in terms of its effect on visual acuity, intraocular pressure and rate of complications,” Dr Agrawal added.

Defining success

While many years of research are needed to accurately assess the therapeutic value of any given surgical technique in glaucoma, one thing that could be achieved much more readily is a consensus as to what is meant by the “success” of the treatment, said Tarek Shaarawy MD, glaucoma sector head, Geneva University Hospitals, Switzerland.

“We need to ask their surgeons to adhere to a strict definition of success that we will agree on so that we can all speak the same language. We definitely need to include not only intraocular pressure reduction but also the amount of complications in our definition of success. We also definitely need to include visual function as outcome measures. There’s no use saying that the operation was a success when the patient went blind,” he added.

Christophe Baudouin:

baudoin@quinze-vingts.fr

Pavi Agrawal: pagrawal@doctors.org.uk

Tarek Shaarawy: Tarek.shaarawy@hcuge.ch