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I read with great interest the article by Okada et al,1 reporting the efficacy and complications of trans-Tenon’s retrobulbar infusion of triamcinolone acetonide for posterior uveitic inflammation. The authors have to be commended for the excellent description of this novel technique.
The efficacy of various methods of corticosteroid injection has always been a matter of debate with different studies giving different results. McCartney et al2 showed that the major route of penetration of steroids after subconjunctival injection was directly through the adjacent sclera, choroid, and retina. In addition, the authors described methods to inject steroids in the sub-Tenon’s space and concluded that the injections should be placed immediately adjacent to the site of intraocular inflammation that was under treatment. In contrast, in a study on rabbit eyes, Wilson et al3 have elegantly demonstrated that injection of corticosteroids by the sub-Tenon’s route does not show a significant effect on the blood-retinal barrier owing to inadequate penetration. The authors analysed the severity of blood-retinal barrier breakdown following panretinal photocoagulation, using rapid sequential magnetic resonance image with contrast. Of note, in this study the authors have taken particular care to ensure the accurate placement of the needle in the sub-Tenon’s space. A similar result was obtained in a study Jennings et al4 on the efficacy of posterior sub-Tenon’s injections in patients with cystoid macular oedema caused by uveitis. The authors found that the injection of steroids by the sub-Tenon’s route did not consistently affect the blood-retinal barrier permeability in such patients and that there was no diffusion of the steroids into the eye in therapeutically meaningful concentrations. This is of particular concern since it is the breakdown in the blood-retinal barrier that leads to influx of serum/serum components leading to macular oedema, epiretinal membrane, and other sequelae.
Sub-Tenon’s injections when compared to intravitreal injections have the disadvantage of probably a decreased and difficult drug penetration through the sclera and choroid and a rapid removal of the drug by the choroidal circulation after penetration with the resultant shortened duration of action. This is probably the reason why the sub-Tenon’s route of injection of steroids has not become popular in diabetic macular oedema in contrast with the increased popularity of intravitreal steroid injections.
Interestingly, Freeman et al5 have postulated that the lack of therapeutic response to sub-Tenon’s corticosteroids may be because of placement at a site relatively far from the target zone. They determined the location of repository corticosteroid after sub-Tenon’s injection by echography and showed that the steroid was deposited within the sub-Tenon’s space over the macula in only 11 of 24 cases. They hence concluded that the therapeutic response manifested by improvement in macular function may be related to the proximity of the corticosteroid to the macular area. The impressive efficacy reported in the study by Okada et al1 could probably be the result of reliable drug placement thanks to the visual confirmation of cannula entry into the sub-Tenon’s space, as the authors speculate. However, it is important to note that most of the patients in this study continued to receive topical steroid drops. Whether these drops had an additive effect is unclear.
It would probably be worthwhile to consider a planned, primary intravitreal injection of corticosteroids under aseptic conditions that has the distinct advantage of getting distributed into a much larger volume for selected conditions. There would be no cases of “therapeutic failures” that are seen after injection of steroids into the sub-Tenon’s space and the resultant confusion as to whether the unsatisfactory response is secondary to the disease process or failure to inject the steroid into the sub-Tenon’s space or the debated lower efficacy of this route of injection. The procedure is simpler than the described trans-Tenon’s retrobulbar infusion (no special cannula is required), but the risk of endophthalmitis is daunting.6
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