What is Glaucoma?
This may be surprising to most people, but the definition of glaucoma has now moved away from the inclusion of intra-ocular pressure. Glaucoma is now considered a multi-factorial optic neuropathy, with accelerated loss of optic nerve fibres. This causes both:
- A structural change in the optic nerve (assessed with SLE and optic nerve imaging.
- A functional change in the visual field (assessed with automated perimetry.
The intra-ocular pressure (IOP) is a balance between:
- Aqueous production by the ciliary body.
- Aqueous egression from the anterior chamber, either through the Trabecular Meshwork or through the alternate uveo-slceral pathway (about 10%).
Whilst glaucoma may or may not be associated with raised IOP, it is the only modifiable risk factor that we can clinically treat in practice. The mean pressure in the population is 16mmHg, with standard deviations meaning the range most quote is 11-21mmHg. Irrespective of the presenting IOP, in the presence of glaucomatous optic neuropathy, most glaucoma specialists will attempt to treat by reducing the IOP.
Too many doctors fall into the trap of diagnosing ‘glaucoma’ without adequately phenotyping patients. The critical determinant for making good management plans is making correct diagnoses usually at presentation: for instance, in Oxford University Hospital we run screening clinics run at the Fellow/Consultant level. The key for correct diagnoses is targeted history taking and interpretation of clinical signs, particularly gonioscopy. Trainee doctors should get into the habit of trying to classify the glaucoma they are evaluating:
- Primary versus secondary
- Open versus closed angle
- Acute versus chronic
- Congenital/juvenile versus adultAqueous
In the short term, the limitation to aqueous flow is the scleral flap. Hence if the sutures are too tight, the resistance is too high: hence the post-operative pressure will be too high. Ways to combat this:
- Fixed sutures should not be overly tight.
- Put in releasable sutures: if the pressure is high, in clinic these can be taken out to reduce resistance at the slit lamp.
- If still the pressure is high, laser the fixed sutures.
Ultimately, the aqueous will collect under the conjunctiva in a so called ‘bleb’. The resistance to flow at the sub-conjunctival location will be fibrotic reactions. These can be circumvented by:
- Using mitomycin C at the time of surgery
- Needling the bleb and administrating 5-FU at intervals, if clinically indicated
- Be wary of a thin conjunctiva at the time of surgery, as this can lead to bleb ‘leaks’, with subsequent risk of bleb related endophthalmitis or bleb related infection (BRE or BRI)
Glaucoma Drainage devices (GDD)
- The basic design of all GDD is the same with a silicone tube connecting the anterior chamber (or vitreous cavity in the case of pars plana tubes) to the plate, anchored in the sub-conjunctival space.
- At Oxford, I use the Baerveldt tube which is a non-valved silicone device. It has with a large pliable silicone plate, available in 250, 350 mm2. To avoid instant hypotony upon implantation, I occlude the lumen with 3.0 supramid/prolene suture. At around six weeks, a capsule forms around the plate, limiting the filtration rate and thus causing the intra-ocular tension to rise. It is at this point that the patient is taken back to theatre to remove the suture.
- The BVT will necessitate a general anaesthetic, predominantly as we sling the lateral and superior recti muscle, which stimulates the oculo-cardiac reflex.
- Those patients unable to tolerate a general anaesthetic could have an Ahmed valved tube or BVT-250.
- Ensure patients are motivated and physically able to administer drops and attend frequent follow up in the acute post-operative period. Otherwise, trans-scleral diode laser would be a suitable alternative.
- Turn off the tap: i.e. reduce the production of aqueous
- The laser probe is inserted inside the eye via a tiny incision; usually at the same time as cataract surgery.
- The advantage of ECP is that it allows for direct visualisation of the eye structure needing treatment, thus improving accuracy and reducing the amount of laser energy required.
- Oxford University Hospital Trust is one of only 20 centres in the UK with access to the laser and I offer this procedure as part of the extensive Glaucoma Service.
- The ECP will take up to six weeks to exert its maximal effect; hence anti-hypotensive drops should continue in the interim.
- I use ECP in the following scenarios:
- If filtration surgery is likely to fail, i.e. exquisitely thin conjunctiva which may not easily close.
- Refractory glaucoma
- Cataract with early glaucomatous optic neuropathy, on single/dual therapy
Lens extraction in primary angle closure (PAC)
Optimise the natural outflow by restoring the natural anatomy
- Gonioscopy is mandatory in phenotyping all new patients referred to clinic. I ask all trainees use the Spaeth criteria to give as much information of the irido-corneal angle as possible. Not only is this diagnostically critical, in the age of minimally invasive glaucoma surgery options, the angle morphology drastically changes the management plan.
- In the setting of PAC or PACG, clear lens extraction may be useful either if:
- YAG iridotomy doesn’t adequately alter the angle morphology (gonioscopy following laser PI is mandatory, ideally by the physician performing the assessment prior to laser).
- As first line treatment, as evidence now suggests that this may result in improved pressure control (mean 1.2mmHg lower than the subset who had PI first) and reduce the need for further treatment (both medical and surgical).
- Very important to consent patients that this is NOT a refractive procedure, the indication is strictly medical. Hence carefully consent that distance unaided vision may be worse after the surgery and spectacles may be required for both near and to refine the distance vision.
- In the case of trabecular failure, lens extraction alone will not be enough and patients may go on have filtration surgery.
Allergan Xen® gel stent
- Create new outflow pathway, with a sub-conjunctival bleb
- A 6mm long, gelatin coated tube
- It is an ab-interno procedure, meaning that the procedure is approached from inside the eye (traversing across the anterior chamber). A preloaded, single use injector is positioned 2mm beyond the limbus in the sub-conjunctival space. Tip:
- It is vital to ensure mobility of the stent in this space after implantation: it cannot be kinked or engulfed in Tenon’s.
- Having performed a good number of implantations at OUH, I would go as far as say that this is almost a hybrid procedure. I did Glaukos iStent® and ALCON CYPASS® as during my fellowship and would classify these as MIGS: they spare the subconjunctival space. Conversely the Xen is not as invasive as filtration surgery, but does create a bleb and the challenges of managing this in the post-operative period.
- Patient selection is critical. At the JR, I have developed the following criteria for indications:
- Easy access, with prominent cheeks and deep set eyes logistically challenging
- Open angle, with at least Spaeth C30R. I have found doing gonioscopy and consenting patients in clinic personally key.
- Minimal risk of scarring, best reserved for Caucasian patients.
- Patients not on anti-coagulants.
- Avoid in patients unable to commit to post-operative steroid drops.
- Avoid in advanced GON or secondary glaucoma.
- An ideal candidate would be early POAG on mono/dual therapy who requires cataract surgery.