Dear Editor

Regarding the editorial by Khaw et al.[1] we are surprised that after quite a few years now non- penetrating filtering surgery (NPFS) remains only partly understood by many ophthalmologists. There are at present two main NPFS: viscocanalostomy as described by Stegmann, in which outflow filtration is at least in theory not subconjunctival, and deep sclerectomy with or without an implant or even with viscoelastics, the success of which depends on several outflow routes - an important one being subconjunctival. Careful postoperative follow up becomes therefore mandatory and is at least as important as the procedure itself. If needed subconjunctival injection of antimetabolites, needling or use of lasers for goniopuncture, iris desincarceration and attempts of possible bleb reduction or closure of possible seidel may be required. It also becomes evident that antimetabolites play an important role in high risk cases for filtration failure undergoing NPFS (apart from viscocanolostomy as describd by Stegmann). Furthermore in our hospital we have been using antimetabolites also in cases requiring low postoperative IOP such as normal tension glaucoma since 1994. We do not understand the comment made stating that greater care should be taken with antimetabolites used during NPFS, since we intraoperatively use these agents before the deeper scleral flap is being excised or even created. Thus at this stage this does not differ with trabeculectomy. Later on the anterior chamber is not entered and furthermore the deep scleral flap which has been exposed to antimetabolites is being excised making the danger of intraoperative intraocular penetration considerably less than with trabeculectomy (even with unintended macroperforation during NPFS).

Additional sutures are added in cases of accidental macroperforaton so that the incidence of early significant hypotony in the hands of experienced surgeons with NPFS is not high. Moreover postoperative suturelysis may then be required in these cases if the sutures have been made too tight or too numerous.

References are being made to the article by Brart et al. However how reliable is it to compare the efficacy of two procedures without giving the same chances of success to both? Intraoperative antimetabolites were used for all trabeculectomies and yet never with NPFS. Yet strangely enough, postoperative antimetabolites as well as needling with an attempt to lift the scleral flap in some, were used in both groups. The author also writes in the discussion that patients with successful drainage at 6 and 12 months following viscocanalostomy had evidence of subconjunctival drainage of aqueous as opposed to eyes without successful drainage. Later on he further states that 'with our viscocanalostomy technique, the subconjunctival route is the main pathway' and 'observation of the disappearance of subconjunctival blebs in our patients with drainage failure after viscocanalostomy appears to suggest that subconjunctival fibrosis is responsible'. Clearly, if antimetabolites are being used in trabeculectomies, it should be used in NPFS before any reliable conclusions can be drawn. Goniopunture was also only done after 18 months which of course will not be of great help if the outflow route after the trabeculodescemet's membrane has scarred down. Thus to promote good filtration in addition to intraoperative and postoperative use of antimetabolites, goniopuncture can help in enhancing and thus in maintaining a flow under the scleral flap. Using lasers for suturelysis in trabeculectomies or for goniopuncture in NPFS is part of the armementarium we have in glaucoma surgery. The final aim for the patient is not to know whether the procedure is penetrating or not, but rather how effective it is so that the discussion on whether or not to use goniopunture is futile.

For ophthalmologists performing NPFS, the later is compared to trabeculectomy what phacoemulsification was to extracapsular cataract extraction. They will never go back to it unless obliged to do so. However it is clear that there is a learning curve to this surgery and that it is not as forgiveful as is trabeculectomy.

E. Ravinet, MD
A. Mermoud, MD

Reference

(1) Khaw PT, Wells AP, Lim KS. Surgery for glaucoma in the 21st century. Br J Ophthalmol 2002;86: 710-711.

Dear Editor

I read with interest the article by Hiti et al.[1] on the susceptibility of Acanthamoeba to a multipurpose disinfecting contact lens solution and two hydrogen peroxide systems. The association between acanthamoeba keratitis and contact lens wear is now firmly established. Thus, the use of contact lens disinfecting solutions effective at killing Acanthamoeba organisms is important in preventing corneal infections. I would like to make a few comments on this article.

The lack of standard methods for testing disinfecting solutions against Acanthamoeba represents a critical problem when sensitivity assays are set up.[2] The authors say that 1 ml of disinfectant per well was applied in 24-well plates, but the amount of acanthamoeba suspension distributed in each well is missing. PHMB (0.0005 %) was found to be ineffective against A.hatchetti and A. lenticulta cysts after 8 hours' exposure. This is not surprising. Indeed, at concentrations (0.5 to 15 µg/ml) used in commercial contact lens solutions, PHMB is almost ineffective against amoeba cysts.[3-6] PHMB concentrations ranging from 45 to 90 µg/ml are needed to kill 99.9 % of Acanthamoeba cysts in under 1 hour of exposure.[7]

The authors observed that a one-step 3 % H₂O₂ system with catalase was ineffective against Acanthamoeba cysts after 8 hours' exposure. Silvany et al.[4] reported similar results. Because the catalyst is present from the very beginning of the disinfection step, the H₂O₂ is neutralised long before any disinfection can occur. Therefore, adequate exposure time before neutralisation is crucial.

The authors also found that a two-step 0.6 % H₂O₂ system was effective against A. castellanii and A. hatchetti cysts after 8 hours' exposure. However, Zanetti et al.[6] observed that an equivalent dilution of 3 % H₂O₂ was ineffective against the cysts of a corneal isolate of A. castellanii after 9 hours' exposure. These variations in susceptibility may depend on inherent strain differences.[2] Therefore, unlike the authors, I would not recommend the two-step 0.6 % H₂O₂ system as a safe disinfectant against Acanthamoeba. From previous data,[6,8-9] I suggest that the following measures should result in less contact lens case and contact lens contamination, thereby possibly reducing the risk of microbial keratitis.

-Naturally: wash hands before handling contact lenses.
-Use "one day" disposable contact lenses. If other types are preferred:
(a)Use a two-step 3% H2O2 system and neutralize after 9 hours' exposure (overnight).
(b)Replace the contact lens case regularly (preferably fortnightly).

References

(1) Hiti K, Walochnik J, Haller-Schober EM, et al. Viability of Acanthamoeba after exposure to a multipurpose disinfecting contact lens solution and two hydrogen peroxide systems. Br J Ophthalmol 2002;86:144-6.

(2) Meisler MD, Rutherford I. Acanthamoeba and disinfection of soft contact lenses. Rev Infect Dis 1991;13:S410-2.

(3) Hugo ER, McLaughlin WR, Oh K, et al. Quantitative enumeration of Acanthamoeba for evaluation of cyst inactivation in contact lens care solutions. Invest Ophthalmol Vis Sci 1991;32:655-7.

(4) Silvany RE, Dougherty JM, McCulley JP. Effect of currently available contact lens disinfection systems on Acanthamoeba castellanii and Acanthamoeba polyphaga. Ophthalmology 1990;97:286-90.

(5) Silvany RE, Dougherty JM, McCulley JP. Effect of contact lens preservatives on Acanthamoeba. Ophthalmology 1991;98:854-7.

(6) Zanetti S, Fiori PL, Pinna A, et al. Susceptibility of Acanthamoeba castellanii to contact lens disinfecting solutions. Antimicrob Agents Chemother 1995;39:1596-8.

(7) Burger RM, Franco RJ, Drlica K. Killing Acanthamoebae with polyaminopropyl biguanide:quantitation and kinetics. Antimicrob Agents Chemother 1994;38:886-8.

(8) Grey TB, Cursons RTM, Sherwan JF, et al. Acanthamoeba, bacterial, and fungal contamination of contact lens storage cases. Br J Ophthalmol 1995;79:601-5.

(9) Pinna A, Sechi LA, Zanetti S, et al.Bacillus cereus keratitis associated with contact lens wear. Ophthalmology 2001;108:1830-4.

Dear Editor

Poon et al.[1] in their excellent article have reiterated the efficacy of autologous serum in the treatment of persistent epithelial defects (PEDs) of the cornea. We would like to invite the attention of the authors to certain aspects of the study. The authors have considered a period of one week for labeling an epithelial defect to be persistent. However, most studies on a similar subject have taken the criterion to be two weeks.[2] It is generally recommended that a 'wash out' period of a least two weeks be given with preservative free artificial tears3 and only those epithelial defects that remain either static or demonstrate an increase in size in this period be included in the study. The authors have not mentioned such a 'wash out' period being included in the protocol. When using autologous serum drops most investigators have not used any other therapeutic modality at the same time to enhance epithelialization apart from preservative free lubricants. The use of therapeutic contact lenses in five cases by the authors makes it difficult to evaluate the actual contribution of serum drops in the healing of the epithelial defect in these cases. Further, the use of serum drops in the immediate post operative period in two patients with poor ocular surface undergoing keratoplasty without waiting for the corneal epithelial defect to heal by itself cannot be extrapolated to making a comment on the beneficial effect of autologous serum. Also the rationale behind the use of 100 % serum when previous studies have proved the efficacy of a 20 % solution[2] is not clear. The 100 % concentration of serum can cause stickiness, which would be inconvenient to the patients and may reduce compliance. The use of the slit lamp micrometer by the authors for measuring the epithelial defects may not be accurate because of its inherent inter-observer and intra-observer variations. A better method would be measuring the area of the defect instead of the greatest dimensions by the use of digital photographs and area measuring software such as Image Pro PlusTM available from Media Cybernetics.

References

(1) Poon AC, Geerling G, Dart JKG, et al. Autologous Serum eye drops for dry eyes and epithelial defects: clinical and in vitro toxicity studies. Br J Ophthalmol 2001;85:1188-97.

(2) Tsubota K, Goto E, Shimmura S, et al. Treatment of persistent epithelial defect by autologous serum application. Ophthalmology 1999;106:1984-9. (3) Gordon JF, Johnson P, Musch DC. Topical fibronectin ophthalmic solution in the treatment of persistent defects of the corneal epithelium. Am J Ophthalmol 1995;119:281-7.

Dear Editor

Thank you for the opportunity to reply to the letter from Dr Alpins concerning our recent article "Astigmatism and the analysis of its surgical correction".[1] Noel Alpins is a widely respected contributor to many international meetings, having written comprehensively on the use of astigmatism vector analysis. His software program ASSORT ^TM is widely used for the planning of refractive surgery and provides many derived indices (transformations) from the vector analysis of both refractive and topographic astigmatism. Although the derived indices are summary measures, we have argued that their usefulness for statistical analysis is limited. This is because the perception of astigmatism is a psycho-physical phenomenon altered by the orientation of the axis of astigmatism (the power meridians of the cornea and crystalline lens). Unfortunately the perceptual response means that the measurement of the axis of astigmatism (which is with an arbitrary 180° scale) is non-linear in outcome terms, as related to visual acuity outcome. Astigmatism obliquity is the least desirable outcome but this is separated into two on the scale by with-rule-astigmatism (WTR), which is generally the most desirable outcome. Oblique astigmatism also separates the two groups of against-the-rule astigmatism (ATR) from the WTR astigmatism. Developments of vector analysis so far have not resolved this issue of non-linearity of the axis of astigmatism compared with the visual outcome. Dr Alpins recognised the relative value of WTR astigmatism and described how to plan refractive corrections using this principle in his January 1997 article (Figure 10a, J Cataract Refract Surg 1997;23:65-75, reference 33 in our article). We suggested the "by-the-rule" transformation would help eliminate the problem of divided oblique and WTR astigmatism, but this makes the use of vector analysis difficult and does result in data compression.

I agree that my understanding of astigmatism is incomplete. With over 4000 responses to a search for astigmatism on PubMed, there is much to know and yet more still unknown. The references cited in the article were simply representative or illustrative of the arguments discussed in the article. By way of apology, the correct reference for the "surgical error" (originally given as 34) in Figure 7, equation 20, and the relevant text page 1131 should in fact be reference 70, Noel Alpins' first article on vector analysis. The surgical error is the arithmetic result of the preoperative vector combined with the surgically induced vector (SIA), less the target induced vector (TIA), which is analogous to neutralising a lens with another of the opposite sign (hence the reverse direction arrow). This produces two outcome measures, the difference in magnitude and axis. However, as our article's discussion on obliquely crossed cylinders described, misalignment due to rotation of the corrective cylinder produces not only an error in cylinder magnitude and axis, but also a spherical power error. Because of this interdependence all three should be analysed together, but this produces statistical difficulties. One way around this problem is to use an appropriate summary measure of the outcome instead.

The surgical error may be applied to treatments targeting non-zero goals despite not addressing changes in corneal shape. As an outcome measure of the surgical process it is equally applicable to the arithmetic result of the SIA with the TIA as it is with the SIA and the preoperative astigmatism vector. The transformation of the error between the SIA and the preoperative astigmatism vector into the "difference vector" (which is a mathematically precise and absolute measure of the surgical error) unfortunately does not address the problem of non-linearity (i.e. the relative value in terms of visual acuity outcome) so is not useful as a summary measure of the outcome. However the difference may be useful in understanding the effects of the surgery (i.e. as a process measure) and for deriving the "index of success".

Dr Alpins describes the SIA "torque" effect with reference to the preoperative axis of astigmatism (or the TIA) in his December 1997 article not cited in our Perspective article (J Cataract Rafract Surg 1997;23:1503-14). Torque needs to be distinguished from the effect of rotation of the corrective cylinder that is derived from the post-operative astigmatism value. Unfortunately, our discussion on the optical decomposition did not clearly state that the 45° polar value is derived from the postoperative result, thus correctly describes the rotation effect (as discussed with the obliquely crossed cylinder effects). We apologise for creating some confusion with the "torque" effect.

We agree that the healing response is connected to the surgical process, however healing is a very individual response. Vector analysis in terms of the SIA can only reflect the surgical process. Although a "vector" could be used to represent the measurement of the healing response at any point in time, it may not be representative of the healing responses at other times because the healing process is continuous. Furthermore an individual's response may not be well represented by the aggregate or mean vectorial response, which as discussed, is compounded by the non-linearity problem of the separation of the oblique and ATR astigmatism axis values (see reference 104 from our article).

In his early 1997 article (reference 33) Noel Alpins discusses surgical treatment planning combining the topographic astigmatism values with the refractive values to produce an optimal corneal curvature. Dr Alpins suggests that the surgical emphasis is best directed towards a WTR result when there is a disparity between the values requiring some residual astigmatism after surgery. Without recognising Javal's rule, Dr Alpins nonetheless has ascribed a better relative value to ATR astigmatism suggesting that optimal treatment planning be based on this psycho-physical phenomenon. As we stated "only using keratometric data for the planning of refractive surgery" would create a problem otherwise.

It is understandable that Dr Alpins feels that the concepts presented in our article are in conflict with some of his own, but these do not diminish the value of vector analysis as a process measure, particularly for individual cases. It is the use of vector analysis as an outcome measure relative to the visual acuity that was critically evaluated by our article.

Reference

(1) Morlet N, Minassian D, Dart J. Astigmatism and the analysis of its surgical correction. Brit J Ophthalmol 2001;85:1127-38.