Minimum stromal thickness for corneal collagen crosslinking

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  • 250 LETTERSREFERENCES1. Samuelson TW. Microinvasive glaucoma surgerydcoming of

    age [editorial]. J Cataract Refract Surg 2014; 40:12531254

    2. Swaminathan SS, Oh D-J, Kang MH, Rhee DJ. Aqueous outflow:

    segmental and distal flow. J Cataract Refract Surg 2014;


    3. Smit BA, Johnstone MA. Effects of viscoelastic injection into

    Schlemms canal in primate and human eyes; potential relevance

    to viscocanalostomy. Ophthalmology 2002; 109:786792Reply : I thank Dr. Morgan for his thoughtful,articulate, and well-reasoned letter. He makesmany excellent points with regard to canalo-

    plasty and its role in glaucoma management. He isalso accurate in his observation that canaloplasty,although referenced, did not receive detailed discus-sion in my editorial concerning microinvasive glau-coma surgery (MIGS).

    While I share most of Dr. Morgan's views on canal-oplasty, my editorial and the special section withinJCRSwas aboutMIGS surgery. Although canaloplastyhas well-established efficacy and a very loyal cadre ofproponents, it is not a MIGS procedure by any defini-tion. Canaloplasty requires extensive dissection andtissue manipulation at the level of conjunctiva, Tenon,sclera, and the cornealscleral interface. Even so,canaloplasty has played a vital role in the evolutionof canal-based surgery and I have been an enthusiasticcanaloplasty surgeon and proponent since inception,including joint authorship of the original manuscriptsreporting the results on the seminal internationalcanaloplasty trial.1,2 I share Dr. Morgan's belief thatcanaloplasty has an important role in glaucoma man-agement. However, I do not believe that it is a MIGSprocedure, nor is it as synergistic as ab interno canalprocedures for use as a combined procedure withphacoemulsification. While a more comprehensivereview of contemporary glaucoma surgery wouldhave included a detailed discussion on canaloplasty,its lack of emphasis in the MIGS supplement was notintended to slight the procedure, but rather to focuson the new MIGS surgeries. For those interested inlearning more about canaloplasty, Dr. Morgan's videoon the 5 Steps of Canaloplasty Surgery is one of thebest that I have seen on this topic and is worth a look.A

    Again, I thank Dr. Morgan for his enthusiasticsupport of canaloplasty surgery. Glaucoma needsmore like Dr. Morgan; passionate surgeons workingto optimize patient care.dThomas W. Samuelson, MDREFERENCES1. Lewis RA, von Wolff K, Tetz M, Koerber N, Kearney JR,

    Shingleton BJ, Samuelson TW. Canaloplasty: circumferential

    viscodilation and tensioning of Schlemm canal using a flexibleJ CATARACT REFRACT SURG -microcatheter for the treatment of open-angle glaucoma in adults;

    two-year interim clinical study results. J Cataract Refract Surg

    2009; 35:814824

    2. Lewis RA, von Wolff K, Tetz M, Koerber N, Kearney JR,

    Shingleton BJ, Samuelson TW. Canaloplasty: three-year results

    of circumferential viscodilation and tensioning of Schlemm canal

    using a microcatheter to treat open-angle glaucoma. J Cataract

    Refract Surg 2011; 37:682690

    OTHER CITED MATERIALA. Morgan M. Five Steps to Canaloplasty Success, April 2011. Avail-

    able on You Tube at: Accessed November 11, 2014Minimum stromal thickness for cornealcollagen crosslinkingWe read the article by Sakla et al.1 with interest andsome concern. Corneal collagen crosslinking (CXL)ensures corneal stabilization in diseases such askeratoconus and corneal ectasias that severely affectvisual prognosis. Although it is regarded as a safeprocedure when it is applied to corneal stroma thickerthan 400 mm,2 the authors determined 350 mmas amin-imal stromal thickness to apply a topography-guidedlaser in some corneas. Even though the hypotonic so-lutions are used in stromal beds between 350 mm and400 mm, there are debates about the effectiveness ofthis method.3,4 Applying CXL in corneas that arethinner than 400 mm increases the risk of the procedurerelatively.

    The sentence in which the authors reported correc-tion of up to 70% of astigmatism and some of the spher-ical component without exceeding a 50 mm ablation isnot clear to us. In keratoconus, as higher-order aberra-tions (HOAs) increase, corrected visual acuitydecreases.5 Therefore, only the HOAs should be cor-rected; other methods should be used for lower-orderaberrations.

    Isilay Kavadarli, MDVedat Kaya, MDIstanbul, TurkeyREFERENCES1. Sakla H, Altroudi W, Mu~noz G, Albarran-Diego C. Simultaneous

    topography-guided partial photorefractive keratectomy and

    corneal collagen crosslinking for keratoconus. J Cataract

    Refract Surg 2014; 40:14301438

    2. Spoerl E, Mrochen M, Sliney D, Trokel S, Seiler T. Safety of

    UVAriboflavin cross-linking of the cornea. Cornea 2007;


    3. Nassaralla BA, Vieira DM, Machado ML, Figueiredo MN,

    Nassaralla JJ Jr. Corneal thickness changes during corneal

    collagen cross-linking with UV-A irradiation and hypo-osmolar

    riboflavin in thin corneas. Arq Bras Oftalmol 2013; 76:155

    158. Available at:

    3a05.pdf. Accessed October 30, 2014VOL 41, JANUARY 2015

  • 251LETTERS4. Kaya V, Utine CA, Ylmaz OF. Intraoperative corneal thickness

    measurements during corneal collagen cross-linking with hypo-

    osmolar riboflavin solution in thin corneas. Cornea 2012;


    5. Pantanelli S, MacRae S, Jeong TM, Yoon G. Characterizing the

    wave aberration in eyes with keratoconus or penetrating kerato-

    plasty using a high-dynamic range wavefront sensor. Ophthal-

    mology 2007; 114:20132021Reply : The CXL standard protocol (Dresden)applies to eyes with a minimum stromal thick-ness of 400 mm. This would limit the

    ultraviolet-A irradiance at the corneal endotheliumto well below the damage threshold for this tissue.However, during standard CXL, there is a significantdecrease in central corneal thickness caused by the on-cotic effect of dextran 20% concentration contained inisoosmolar riboflavin and the evaporative water lossfrom the deepithelialized cornea.1 A simple measuresuch as avoiding the eyelid speculum during ribo-flavin instillation results in a significantly smallerdecrease in stromal thickness during standard CXL.2

    Collagen crosslinking with isoosmolar riboflavinhas been shown to be toxic to corneal endothelium,and there is overall agreement that it should beavoided in eyes with a stromal thickness less than400 mm.3 However, patients with keratoconus oftenpresentwith corneal thicknesses less than that, makingmodifications of the standard procedure necessary.Options include transepithelial CXL and the use of hy-poosmolar riboflavin. In our study, we used hypoos-molar riboflavin 0.1% solution every 2 minutes for60 minutes to swell the cornea before and during irra-diation in eyes with a stromal thickness between350 mm and 400 mm. This protocol has been used ineyes with thin corneas and has stabilized the ectasiaand had no side effects for the corneal endothelium.4

    Repeated application of hypoosmolar riboflavin dur-ing the irradiation process seems to prevent intraoper-ative corneal stromal thinning.

    We agree that HOAs are the main source of visualloss in keratoconic eyes, and this is the reason to usea topography-guided ablation algorithm to regularizethe corneal shape. As tissue sparing is a prime consid-eration, stromal ablation is limited to 50 mm.However,lower-order aberrations (LOAs) and HOAs cannot beseen as separate entities, since variations in LOAsalso affect HOAs. Reduction in defocus and astigma-tism in an asymmetrically decentered system such asthe keratoconic cornea reduces spherical aberrationand coma, the latter being the most prominent aberra-tion in the keratoconic eye.5 Finally, a significant num-ber of patients in our study requested refractivesurgery to reduce the need for optical aids and thusa global strategy was considered to improve theJ CATARACT REFRACT SURG -visual function, both corrected and uncorrected.dGonzalo Mu~noz, MD, PhD,


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