Commentary: Placement of the tube of a glaucoma drainage device: Different horses for different courses.

Glaucoma drainage devices (GDD), both non-valved and valved, provide invaluable management in the refractory and secondary glaucomas. Currently, the only valved device available in India is the Ahmed Glaucoma Valve (AGV) and the only non-valved device available is the indigenously manufactured Aurolab Aqueous Drainage Implant (AADI). The latter is modeled on the Baerveldt implant 350 mm2 plate size. Traditional or conventional training of GDD has been to place the tubes of these types of devices in the anterior chamber (AC). However, more recently, the ciliary sulcus (CS) and the pars plana (PP) placement have been described to overcome the relative disadvantages of the tube in the AC, the most serious of which are endothelial cell loss and corneal decompensation.

Glaucoma secondary to vitreoretinal diseases and procedures, or indeed when combined with it, are extremely challenging to manage. The authors[1] have described a methodical approach to the tube of AADI being placed in the PP along with PP vitrectomy (PPV). Their technique of partial-thickness scleral tunnel obviating a corneal or scleral allograft to cover the tube is noteworthy and has achieved a moderate success of approximately 75% at 1 year. Data published by the same group[2] a few years earlier reported a 47% failure rate at 2 years with PP tube placement whereas Chang et al.[3] reported a sharp decline in the success rate with the same approach at 1.5 years (approximately 60%).

Philip et al.[4] have reported placement of the majority of AADI tubes in the AC and Pathak Ray et al.[56] have reported placement of the tube either in the AC or in the CS. These two studies have similar failure rates at 1 and 2 years of follow-up (approximately 8 and 12%); the failure rate of these two studies at 2 years is approximately half that of the current study at 1 year. Therefore, on the face of it, though PP placement of the tubes is relatively successful in nullifying cornea-related complications, the overall success rate may be less favorable when compared to its placement in the AC or CS. However, on the other hand, in a comparative study, Qin et al.[7] found no difference in the surgical outcomes when they compared PP versus the AC tube placement.

As with any procedure, there are several pros and cons related to PP tube placement. A fundamental requirement in such a procedure is the availability of vitreoretinal skills and related infrastructure for the procedure. Barring a few large organizations in relatively larger cities and towns, this may be a challenge in our country. Second, an indefinite vigil is required for the monitoring of the posterior segment, preferably by a posterior segment colleague. Third, such an approach may minimize corneal complications but does not eliminate it. Parihar et al.[8] reported no statistically significant difference in graft failure in conventional versus PP placement of the tube in penetrating keratoplasty. Also, several other serious posterior segment complications necessitating PP interventions—tube blockage by the vitreous at its base, retinal break, and detachment—have been reported by several authors.[12378] It is surmised that a complete vitrectomy with adequate shaving of the vitreous base can prevent tube occlusion and vitreous traction complications. However, this does not preclude the need for lifelong monitoring against these complications. Furthermore, complications like tube exposure cannot be eliminated by PP tube placement.[9] Lastly, one cannot ignore the cost aspect of such a combined procedure. Therefore, for PP tube placement, both availability and affordability, can be major limiting factors in low-to-middle income countries.

Though it is prudent to use PP tube insertion for all the stated reasons it should be reserved only for those eyes that require simultaneous PPV, viz. in trauma, neovascular glaucoma, tractional retinal detachment, etc., Injudicious use of PPV, merely for the placement of the tube of AADI in the PP, may also override any relative benefit associated with the low cost of this indigenously manufactured GDD. Further, the cost may be compounded if the complications related to it require repeat interventions.

The CS should be the preferred tube destination, especially in pseudophakic or aphakic eyes. This should overcome any perceived challenges of placing the tube in the AC—endothelial compromise, shallow AC, extensive peripheral anterior synechiae, etc. The absence of vitreous is mandatory for such a tube placement too and this may be easily achieved with the cost-effective anterior vitrectomy, performed by the same anterior segment surgeon. Furthermore, in phakic eyes in the presence of cataracts, simultaneous lens extraction can be actively considered, enabling placement of the tube in the CS. Such an approach has been very successful in the management of glaucoma associated with the iridocorneal endothelial (ICE) syndrome achieving dual goals of intraocular pressure control and also aided deferment of any keratoplasty in a series of eyes reported.[10] The consideration of the placement of the tube in the AC should be the last option, and only in phakic eyes with a clear lens, where the loss of accommodation may be an important concern. Pre-operative acquisition of endothelial cell count also, particularly in older individuals, may help prognostication when such a placement is contemplated.

To summarize, the placement of the tube of the GDD in the AC, CS, or PP is a successful approach in the management of refractory glaucomas. While tube placement in the AC may not be desirable when any endothelial compromise is an issue, that in the PP may be inexpedient if no active management of any retinal disease is contemplated. The final destination of the tube should be individualized, not only determined by the clinical situation and probability of complications, but also by the judicious use of available resources.