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While the so-called ‘gold standard’ for raised IOP has been surgical trabeculotomy, this technique is now generally acknowledged to provide relatively poor clinical outcomes for stabilising IOP. One development within conventional trabeculotomy surgery, however, has been the emergence of the Trabecutome surgical technique where a miniaturised diathermy tool is used to ablate tissue within specific angular zones of the trabecular meshwork (typically between 60 and 120 degrees). This technique, initially developed by Dr Baerveldt,1 effectively removes layers of tissue which restrict fluid outflow and during the surgical technique ablated tissue fragments are removed from the eye by the aspiration channel of the device.
Recent studies such as that of Ting et al2 confirm the relative effectiveness of the Trabectome technique for exfoliation versus primary open-angle glaucoma and in combination with cataract surgery and intraocular lens (IOL) implantation. In this study, for the combined procedure, the success of outcome in controlling IOP one year after the procedure was around 90 per cent. Higher success rates after lens extraction are probably associated with the increased ‘openness’ of the trabecular meshwork. A limitation to the uptake of the procedure is possibly the cost of each disposable hand piece used in the procedure, though several thousand procedures using the Trabectome technique have been undertaken since its introduction in 2006.
Glaucoma implant drainage devices
One family of glaucoma implant devices effectively provides an outflow channel from the anterior chamber of the eye to the surface of the conjunctiva. Table 1 summarises a selection of available devices.
The Ahmed glaucoma product range operate on a principle of Venturi pressure differential established by flow of fluid at higher pressure in the anterior chamber. The rate of drainage is proportional to the cross sectional area of the drainage device in contact with the eye surface and where double plate structures can be configured to increase the drainage rate. The Ahmed Glaucoma Valve Flexible Plate (AGV-FP7) is shown in Figure 1, note the draining tube (lower left) and the area of the plate which contacts the eye surface. The circular holes are for sites of suturing.
A characteristic of conventional glaucoma drainage devices, however, is the lack of control over flow rates after implant, where lack of flow can give rise to elevated IOPs (ocular hypertony) which can damage the optic nerve and excessive flow can give rise to excessively low IOP (ocular hypotony), with attendant complications of retinal detachment, maculopathy or haemorrhages.
Specific designs of ‘glaucoma valve’ have specific design characteristics. The venturi effect is utilised in the Ahmed Glaucoma Valve (AGV) product as stated where fluid enters a tapered trapezoidal chamber where the outflowing fluid establishes a pressure differential across the chamber which in turn encourages the outwards flow of fluid. The Ahmed glaucoma valve, for example, has a characteristic variable resistance where a pressure drop of 7 to 12 mm Hg is maintained over a wide range of flow rates of aqueous humour. The behaviour of such valves has been modelled by Stay et al.3 Studies of specific drainage implant devices provide insight into the relative effectiveness of specific products. A recent study4 of the Ahmed group product and the Baerveldt group product indicated that after one year levels of IOP were 16.5 ±5 .3mm Hg in the Ahmed group and 13.6 ± 4.8mmHg in the Baerveldt group (P < 0.001). It was identified, however, that the Baerveldt group devices required more clinical interventions.
Schwartz, Lee and Gedde5 compare the clinical suitability of a range of commonly used glaucoma drainage implants and indicate the range of problems typically encountered by such devices. Bleb encapsulation – raised portions of the conjunctiva at the site of drainage implant can leak due to thinning of conjunctival tissue. Further complications can arise when blebs become infected. Diplopia or double vision can occur due lack of symmetry between the function of the extraocular muscles due to the introduction of the drainage implant.
Developments at École Polytechnique Fédérale de Lausanne
A novel technique to control flow rate after implantation has been developed by a team of scientists at École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland. Initial development work was carried out at the Laboratory of Hemodynamics and Cardiovascular Technology (LHTC) under the direction of Nikolaos Stergiopulos where an external magnetic field was used to change the spin rate of an eccentric rotating disk which established the aspiration rate of the device. The initial concept was patented in 2008. In a recent news feature on the project, plans were revealed for a clinical study on a group of 30 patients in the spring of 2013 and with a CE mark anticipated for the product during 2014. Development of the product is being undertaken by Rheon Medical, an EPFL start-up company.
Subsequent to implant, such as at routine follow up visit, it would be possible for the clinician to adjust the flow rate to establish the desired IOP level. This is a feature not available with comparable designs. In addition, the EPFL device is more compact than previous glaucoma implant devices. Since many of the complications from drainage implants arise because of lack of pressure control once the drainage implant has been installed, the proposed shunt device from Rheon Medical is anticipated to give rise to fewer complications.
Micro stents and micro drainage ducts
Developments at Rheon Medical are taking place against development of ‘micro stent’ devices such as the Hydrus Microstent which is fabricated from super-elastic, biocompatible alloy (nitinol) which is a widely used implant material. The miniature implant which functions as a micro scaffold is designed for insertion to revitalise the function of the canal of Schlemm. This can be undertaken at the end of cataract surgery using the incisions already made for the cataract procedure. The action of the implant is to restore the function of the canal of Schlemm and does not involve the establishment of a clearance channel to drain into the conjuctival space. Clinical trials evaluating the effectiveness of the Hydrus implant are currently on going. This is therefore a less significant intervention than a conventional glaucoma drainage device.
The MIDI Arrow micro drainage device being developed by Innovia as shown in Figure 3 is currently being developed as a procedure to be undertaken ab-interno (from the inside) in association with cataract extraction/lens implantation. The principle is essentially the same as that of the conventional glaucoma ‘plate’ devices, but without the significant trauma of plate insertion. Details of other devices are described by Gedde.6
Discussion
As an increasing array of technologies are becoming available for glaucoma management through surgical procedures, it will become increasingly relevant for such techniques to be reviewed by bodies such as NICE to indentify where, for example, the inclusion of procedures such as micro stents or the Trabectome technique can be included for patients with high levels of IOP which cannot be effectively, nor cost-effectively, managed by conventional means. Such estimations would, for example, take into consideration the relative costs of approaches and where surgical intervention is likely to result in reductions in costs of medication. Many ophthalmologists, however, will be cautious in implementing emerging surgical techniques to actively reduce IOP or act as a preventive measure against development of raised IOP at some time in the future.
While there have been some noted developments in the challenge to regulate eye pressure through surgical intervention, the ‘perfect’ device remains to be developed. There would seem to a preference for systems which can recover/repair the intrinsic function of the trabecular meshwork rather than establish drainage channels which can give rise to blebs under the conjunctiva.?
References
1 Francis BA, See RF, Rao NA, Minckler DS, Baerveldt G. Ab interno trabeculectomy: Development of a Novel Device (Trabectome) and Surgery for Open-Angle Glaucoma. J Glaucoma, 2006; 15:68-73.
2 Ting JL, Damji KF, Stiles MC; Trabectome Study Group. Ab interno trabeculectomy: outcomes in exfoliation versus primary open-angle glaucoma. J Cataract Refract Surg, 2012 ;38(2):315-23.
3 Stay MS, Pan T, Brown JD, Ziaie B, Barocas VH. Thin-film coupled fluid-solid analysis of flow through the Ahmed glaucoma drainage device. J Biomech Eng, 2005;127(5):776-81.
4 Christakis PG, Kalenak JW, Zurakowski D, Tsai JC, Kammer JA, Harasymowycz PJ, Ahmed II. The Ahmed Versus Baerveldt study: one-year treatment outcomes. Ophthalmology. 2011;118(11):2180-9.
5 Schwartz KS, Lee RK, Gedde SJ. Glaucoma drainage implants: a critical comparison of types. Curr Opin Ophthalmol, 2006 ;(2):181-9.
6 Gedde SJ. The Tube Trials: New Information to Guide Clinicians, Ophthamology Management, 2012;16: 52 – 58.
? Dr Douglas Clarkson is development and quality manager at the department of clinical physics and bio-engineering, Coventry and Warwickshire University Hospital Trust
