BritishJournal ofOphthalmology, 1990,74,583-588

How large must an iridotomy be?

B W Fleck

AbstractFour cases of acute angle closure glaucoma ineyes with a small but patent Nd-YAG laserimdotomy are presented, and similar cases inthe literature are reviewed. Theoretically a 15tm diameter iridotomy should be large enoughto prevent angle closure glaucoma due to pupilblock. Mechanisms by which larger irido-tomies fail to prevent angle closure glaucoma,and the role of provocation tests followingiridotomy, are discussed. An iridotomy shouldbe at least 150-200 [tm in diameter if acuteangle closure glaucoma is to be reliablyprevented.

Department ofOphthalmology, RoyalInfirmary, EdinburghEH3 9HABW FleckCorrespondence to:B W Fleck, FRCSEd,Princess Alexandra EyePavilion, Chalmers Street,Edinburgh EH3 9HA.Accepted for publication11 April 1990

Various forms of iris surgery have been used totreat angle closure glaucoma since von Graefe'soriginal description of the broad iridectomy in1857.' Surgical iridotomy was introduced byCurran in 1920.24 Peripheral iridectomy wasdescribed by Chandler in 1952.5 Argon laseriridotomy,6 and more recently Nd-YAG laser

iridotomy,7 have already largely supersededsurgical iridectomy. While laser surgery hasmany advantages over conventional surgery, itmust not be forgotten that laser iridotomyrepresents a return to the use of smaller irisopenings than those obtained by surgicalperipheral iridectomy.' Gifford noted in 1921that very small surgical iridotomies were not aseffective in controlling intraocular pressure aslarger iridotomies.4

Reports are now emerging of acute angleclosure glaucoma developing in the presence of asmall but patent argon90 or Nd-YAG" 12 laseriridotomy. While cases of acute angle closureglaucoma following surgical peripheral iridec-tomy have generally been attributed to plateauiris syndrome,'3"'5 not all cases of acute angleclosure glaucoma following a small laser irido-tomy can be placed in this category."'2 We havetreated four patients who developed acute angleclosure glaucoma despite the presence ofa patentNd-YAG laser iridotomy.The mechanism of failure of very small

iridotomies may be related to obstruction ofaqueous flow at very small iridotomy size. Belowa critical surface area significant iris bombe mayoccur at normal aqueous flow rates. When irisbombe is sufficient to cause iridocorneal contactthroughout a critical extent of the angle,beginning superiorly, acute angle closureglaucoma results. 16 17 The minimum 'safe' size ofiridotomy has not been defined, but an approxi-mate value may be calculated from a simplemathematical model.

Material and methodsWe are undertaking a prospective randomisedcomparison of Nd-YAG laser iridotomy and

surgical peripheral iridectomy in the treatmentof acute angle closure glaucoma. The casespresented belong to the laser treated group inthis study.A Zeiss Visulas Nd-YAG laser and Zeiss

anterior segment YAG contact lens were used inall treatments. Patients were given acetazolamide500 mg orally one hour before treatment, andone pilocarpine eyedrop 2% 30 minutes beforetreatment.A treatment site was chosen in the superior iris

between 10 and 2 o'clock, approximately two-thirds of the distance from the pupil margin tothe base of the iris. An iris crypt was used, whenpresent. A single 5-10 mJ pulse was delivered tothe treatment site. If this pulse did not penetratethe iris, further single pulses were delivered tothe same site until penetration was achieved. Agush ofaqueous fluid from the posterior chambercould be seen at the moment of penetration.Patency was assessed by direct observation of theposterior chamber or anterior lens capsule.The size of iridotomies was measured by

comparison with the 0-2 mm spot produced bya Haag-Streit slit-lamp focused on the irisplane.

Results: case reports

CASE IA 75-year-old hypermetropic man presentedwith right acute angle closure glaucoma of 48hours' duration. The intraocular pressure was 78mmHg, the anterior chamber was shallow, andthe angle was closed throughout 3600. The lefteye had a very narrow, open angle. Treatmentwith acetazolamide 500 mg intravenously andpilocarpine eyedrops 2% hourly resulted in a fallin intraocular pressure to 12 mmHg, with anopen angle throughout 3600. Nd-YAG laser irido-tomy was performed three days later. A treatmentsite 1 mm from the limbus at 10 o'clock waschosen. The iris was a light grey colour, with nocrypts. Three 9 0 mJ pulses resulted in a small(200x100 [tm) patent iridotomy. Minimal irisbleeding occurred at the time of treatment, butthere were no other complications. One weekafter treatment the iridotomy was patent, thoughsmall. The intraocular pressure was 10 mmHg.No further eye medication was prescribed.Two weeks after treatment the patient com-

plained of blurred vision in the right eye. Theintraocular pressure was 68 mmHg, and gonio-scopy revealed 300° of angle closure superiorly.Medical treatment with topical pilocarpine 2%and timolol 0 5% reduced the intraocularpressure to 13 mmHg, with an open angle. Theiridotomy was small (50x 100 [tm) but patent.Trabeculectomy with peripheral iridectomy wasperformed, leading to an intraocular pressure of10 mmHg, and 6/5 visual acuity.

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Figure I Case 2. Initial iridotomy size too small. Acute angle closure glaucoma developed.Iridotomy diameter 50 pm. The arrow indicates the very small area offull thickness opening atthe centre ofthe treatment site.

CASE 2A 65-year-old woman presented with left acuteangle closure glaucoma of 24 hours' duration.The intraocular pressure was 58 mmHg, theanterior chamber was shallow, and the angle wasclosed throughout 3600. The right anteriorchamber was shallow, with 1200 of angle closuresuperiorly, and the intraocular pressure was 15mmHg.

After initial medical control left trabeculec-tomy was performed.

Further examination of the right eye aftertreatment with pilocarpine eyedrops 2% revealedan open angle throughout 3600. Right Nd-YAGlaser iridotomy was performed. The treatmentsite was 1 mm from the limbus, at 10 o'clock.The iris was brown, with no crypts. Four 9-7 mJpulses were used to produce a patent iridotomy,and a further single 9-7 mJ pulse was used toenlarge the opening.One week later the iridotomy was patent,

though small. The intraocular pressure was 14mmHg. At review three weeks later the rightintraocular pressure was found to be 41 mmHg,and the pupil was dilated. Gonioscopy was not

Figure 2 Case 2. Following laser enlargement all symptoms resolved and provocation testingwas negative. Iridotomy diameter 200 tim.

performed. The patient had been using cyclo-pentolate 1% drops to the left eye, and may haveinadvertently contaminated the right eye. Aftertreatment with pilocarpine eyedrops 2% theintraocular pressure returned to 16 mmHg. -Theiridotomy was patent, but small - 50x50 jim(Fig 1). Further laser treatment was given toenlarge the opening. Five 2-0 mJ Nd-YAGpulses were delivered to the sides of the existingiridotomy, enlarging the opening to 100x 100im. After this treatment, eyedrops of Beta-methasone 0 1% four times a day and tropicamide1% once daily were prescribed. The patientreturned four days later complaining of pain andreduced vision in the right eye. The intraocularpressure was 50 mmHg, and the pupil was mid-dilated and unreactive. The iridotomy was nowsituated very peripherally, with iris strandsrunning forwards to the cornea, but appeared tobe patent. Gonioscopy was not performed. Theintraocular pressure returned to normal aftertreatment with acetazolamide 500 mg intra-venously, and pilocarpine eyedrops 2% hourly.The iridotomy was patent, but small (I00x 100jm). The angle was open on gonioscopy. Treat-ment with pilocarpine eyedrops 2% four times aday was prescribed.A pilocarpine/phenylephrine provocation test

as described by Mapstone'8 was performed onthe right eye one week later and was negative.

Intermittent aching pain above the right eyecontinued, and further Nd-YAG laser enlarge-ment of the iridotomy was performed six weekslater. Twenty-one 1 1 mJ pulses were deliveredby means of a Wise 103-dioptre contact lens,resulting in some oozing of blood. A further 101-0 mJ pulses were delivered 30 minutes later.The resulting iridotomy measured 200x 200 jim.One week later all symptoms had resolved,

and the iridotomy measured l00x200 jim.Pilocarpine/phenylephrine testing was againnegative. However, provocation with Tropic-amide eyedrops 1% produced a rise of 9 mmHgin IOP, with folds of iris blocking the trabecularmeshwork over 2700 on gonioscopy. At follow-up three weeks later the intraocular pressure was17 mmHg, with no medication (Fig 2).

CASE 3An 83-year-old woman presented with inter-mittent pain in the left eye. The intraocularpressure was 44 mmHg, the anterior chamberwas deep, and Goldmann gonioscopy revealed90° ofangle closure superotemporally. The planeof the iris was flat, with an anteriorly placed irisinsertion. The right eye had a similar angleconfiguration, with normal intraocular pressure.Plateau iris configuration was diagnosed. Intra-ocular pressure in the left eye quickly returned tonormal after treatment with acetazolamide 500mg orally and pilocarpine eyedrops 2% hourly.Gonioscopy revealed 180° of angle closuresuperiorly. Nd-YAG laser iridotomy was per-formed three days later. The iris was light grey,with no crypts. A treatment site 1 mm from thelimbus at 2 o'clock was selected. Four 10-9 mJpulses produced a moderately large (200x400jm) patent iridotomy. There was minimal irisbleeding at the time of treatment.

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After this treatment, eyedrops of Timolol0 5% twice daily, cyclopentolate 1% twice daily,and betamethasone 0-1% four hourly were

prescribed. Fourteen days later the patientreturned with pain and reduced vision of 24hours' duration in the left eye. The intraocularpressure was 50 mmHg. After treatment withPilocarpine eyedrops 2% and timolol eyedrops0 5% the intraocular pressure fell to 7 mmHg.The iridotomy was patent and moderately large(200x400 iim). Identation gonioscopy revealedgoniosynechiae over 1500 superiorly, with plateauiris configuration. A sector iridectomy wasperformed. On further follow-up the intraocularpressure in the left eye was found to be 28 mmHgon no treatment, falling to 21 mmHg on additionoftimolol eyedrops 0 5% twice daily. Gonioscopyrevealed an open angle throughout 3600.

CASE 4A 71-year-old hypermetropic woman presentedwith left acute angle closure glaucoma of eighthours' duration. The intraocular pressure was46mmHg and the anterior chamber was shallow.The right eye had a normal intraocular pressureand a shallow anterior chamber. After medicaltreatment with acetazolamide 500 mg intra-venously and pilocarpine eyedrops 2% hourlythe left intraocular pressure fell to 9 mmHgwithin two hours. Goldmann gonioscopyrevealed 1500 of angle closure superiorly. Theright eye also had a very narrow angle, with 1500of angle closure superiorly.Nd-YAG laser iridotomy was attempted in the

left eye the next day. The iris was brown, with nocrypts. Three 9.9 mJ pulses were delivered totwo sites, with no penetration. Three days laterfour 10-2 mJ pulses were delivered, two to eachof the previous treatment sites. The final pulseproduced a very small, patent iridotomy at 2o'clock, in the mid iris position. A singleenlarging pulse of 9-2 mJ was delivered to thissite on the following day. A very small (50 pm)iridotomy resulted. The patient was dischargedhome with betamethasone eyedrops 0-1% twicedaily and cyclopentolate eyedrops 1% once dailyto the left eye.

She returned six days later complaining ofsevere pain in the left eye. The intraocularpressure was 45 mmHg, and the anteriorchamber was very shallow. The iridotomy was

patent but very small. The intraocular pressurequickly fell to 8 mmHg after treatment with

50

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S. 10]

S. IV0 1 0 20 30 40 50

Irldotomy Size (gom)Figure 3 Pressure difference across the iris as afunction ofiridotomy size.

acetazolamide 500 mg intravenously and pilo-carpine eyedrops 2% hourly. A surgical peri-pheral iridectomy was performed the next day.

Postoperatively the patient was treated withbetamethasone eyedrops 0-1% four times dailyand tropicamide eyedrops 1% once daily. Twodays later she complained of pain over the lefteye. The pupil was dilated, and the peripheraliridectomy was patent. The intraocular pressurewas 41 mmHg. Gonioscopy was not tolerated.The intraocular pressure fell to 10 mmHg aftertreatment with acetazolamide 250 mg orallytwice daily, pilocarpine eyedrops 2% four timesdaily, and timolol eyedrops 0 5% twice daily.Long term treatment with pilocarpine eyedrops2% four hourly and timolol eyedrops 0 5% twicedaily has been necessary to maintain normalintraocular pressure in the left eye.

MATHEMATICAL MODELA very small iridotomy may be of inadequatephysical size to by-pass pupil block adequately.Given a constant rate of aqueous flow, thepressure difference across an iridotomy increasesas the size of the iridotomy decreases. Increasedposterior chamber pressure relative to anteriorchamber pressure will result in iris bombe. Thedegree of bombe is dependent on the rigidity ofthe iris and the pressure difference across theiris. At a critical level, dependent on peripheralanterior chamber depth, iris bombe may becomesufficient to cause iridocorneal contact over alarge enough extent of the angle to precipitateacute angle closure glaucoma.1617 19 20The minimum 'safe' size of an iridotomy may

be calculated from a simple mathematical model.Firstly, an assessment is made as to whetheraqueous flow is laminar or turbulent through aniridotomy aperture of 10 lm. On the assumptionthat aqueous viscosity equals the viscosity ofwater (nr= 10- Newton seconds per squaremetre), aqueous density equals the density ofwater (e= 1000 kilograms per cubic metre), andaqueous flow, Q, is approximately 2 ,d perminute (3 3x 10-" cubic metres per second), asmeasured by Bloom et al,2' then Reynolds'number for the system may be calculated.

If the iridotomy size (d) is 10 ,um diameter,the aqueous velocity

v=flow/Tr2=4-2x 10-1 ms-I.Reynolds' number for the system

R=evd/lr, R=42.If R is considerably less than 1000, flow is

laminar. Aqueous flow through an iridotomy ofdiameter 10 Itm or larger will be laminar.

Secondly, the iridotomy channel may beconsidered to be a pipe of length 50 ,um on theassumption that the peripheral iris thickness is50 ,um. The pressure drop (pd) along a pipe oflength 1=50 Rm, diameter d=100 [tm, at flowrate Q is expressed by the formula:pd= 128l1Q/hd4

=128x10x-3x 10-5x3 3x10 "/3142x10-16=0-672 Nm-2=0 00494 mmHg, as 1 mmHg= 136 Nm-2.

An iridotomy of 100 ptm diameter will have apressure drop of 0 00494 mmHg across it on theassumption of total pupil block.

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However, as pd is proportional to l/d4, if thediameter of the iridotomy were 50 [tm pd wouldbe 0 079 mmHg, under the same conditions. Ifthe iridotomy were 25 pm, the pd would be 1-27mmHg. If the iridotomy were 12-5 rn, the pdwould be 20-2 mmHg, and if the iridotomy were10 [rn the pd would be 49-4 mmHg. It is notdifficult to conceive that a pressure difference of10-20 mmHg across the iris could result insignificant iris bombe, with iridocorneal contactand angle closure. Such a pressure differencecould develop if the size of the iridotomy wasbetween 10 and 15 pin in diameter (Fig 3).

DiscussionCurran observed in 1920 that in some patientswith glaucoma the passage of aqueous throughthe pupil appeared to be impeded by the iris'hugging the lens over too great a surfaceextent'.2 Surgical iridotomy allowed free drain-age of fluid from the posterior chamber to theanterior chamber, with posterior movement ofthe iris and a reduction in intraocular pressure.Barkan elaborated the concept of pupil block asthe causative mechanism of iris bombe, withresulting angle closure.22 A peripheraliridectomy causes 'collapse' of iris bombed,resulting in widening of the 'entrance' to theangle.22 Peripheral anterior chamber depth, butnot central anterior chamber depth, increasesfollowing peripheral iridectomy'72324 owing toresolution of iris bombed.

Failure of surgical peripheral iridectomy, orlaser iridotomy, as evidenced by development ofacute angle closure glaucoma, may occur by oneof two mechanisms. A small number of cases ofangle closure glaucoma are related to 'plateau irisconfiguration' - an anteriorly placed iris root, flatiris plane, and a normal or near normal anteriorchamber depth. Pupil dilatation results in peri-pheral iris bunching, with occlusion of thetrabecular meshwork.'325 As pupil block is notcausative, surgical peripheral iridectomy or laseriridotomy are relatively ineffective in preventingfurther attacks of acute angle closure glaucoma.One ofour patients (case 3) probably had plateauiris syndrome, and developed acute angle closureglaucoma following mydriasis despite thepresence of a relatively large, patent iridotomy.

Case 4 developed acute angle closure glaucomafollowing mydriasis despite the presence of asmall patent iridotomy and subsequently a largersurgical iridectomy. The mechanism of acuteintraocular pressure rise may have been iriscrowding in the angle. Plateau iris configurationwas not present. Perhaps the term 'angle crowd-ing' angle closure glaucoma would be a moreappropriate term to use for eyes which developacute angle closure glaucoma followingmydriasis in the presence of a patent peripheraliridectomy. The cases of two other patients withmoderately large peripheral iridectomies whodeveloped acute angle closure glaucoma whileusing mydriatic drops have been reported. 14 15

Mydriatic or dark room provocation testsfollowing peripheral iridectomy have been foundto be positive in 3-19% of cases.25-28 Gonioscopyat the end of a positive test in eyes with patentperipheral iridectomies has revealed angle

closure in some cases, due to angle crowding.25 29However, in other eyes the angle has beenopen.21-27 Raised intraocular pressure in theseeyes may be explained by the outflow facilitylowering effect of anticholinergic drugs.`0Surgical peripheral iridectomy appears entirelyto overcome pupil block. No cases of acute angleclosure glaucoma due to iris bombe have beenreported in the presence of a patent surgicaliridectomy.

Mydriatic provocation tests following argonlaser iridotomy3' 32 and Nd-YAG laseriridotomy33 have given similar results to thoseperformed following surgical peripheraliridectomy. However, pilocarpine/pheny-lephrine tests have been negative in all eyestested following argon laser iridotomy34 or Nd-YAG laser iridotomy." 12 The test produces onlymoderate mydriasis, and increased tone in boththe sphincter and dilator muscles of the iris mayprevent angle crowding. The test induces maxi-mal pupil block,35 and failure to obtain anypositive results suggests that pupil block wasfully overcome in all cases tested.Acute angle closure glaucoma in the presence

of a small, patent argon laser iridotomy haspreviously been reported in two patients.9'Mandelkorn et al reported a case of acute angleclosure glaucoma following a 50 [tm diameterargon laser iridotomy.9 Further argon laser treat-ment was applied to the site, resulting in satis-factory long term pressure control. Brainardreported a similar case, in an eye with a 75 [imdiameter argon laser iridotomy. "' Further argonlaser treatment to enlarge the iridotomy to125 [im resulted in satisfactory long term intra-ocular pressure control, suggesting inadequateiridotomy size was responsible for initial failure.

Acute angle closure glaucoma has also beenreported in the presence of a small, but patentNd-YAG laser iridotomy." 12 33 Brazier reportedtwo cases. One was treated by surgical peripheraliridectomy, with no further complications. Thesecond was treated with miotics." Wishart andHitchings reported two cases of acute angleclosure glaucoma and two cases of subacuteangle closure glaucoma in the presence of patentNd-YAG laser iridotomies.33 In three cases theiridotomy was 100 jim diameter and in thefourth 150 pim. Gray et al reported two cases ofacute angle closure glaucoma in the presence of asmall, patent iridotomy.'2 One developed acuteangle closure glaucoma while using pilocarpineeye drops 4%, and was treated by trabeculec-tomy. The second was treated by a furtheriridotomy. Intraocular pressure subsequentlyremained normal in both cases. Two of our cases(cases 1 and 2) behaved in a very similar way tothe two cases reported by Gray et al. Case 1responded to trabeculectomy, and case 2 res-ponded to laser enlgargement of the iridotomy.The occurrence of acute angle closure glaucomain the presence of a small, but patent, iridotomywhich responds to surgical peripheral iridectomyor laser enlargement, suggests the size of theinitial iridotomy was inadequate.

Wishart studied iris bombe using slit-lampphotographic assessment of peripheral anteriorchamber depth. In four eyes with very smalliridotomies and progressive narrowing of the

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angle, repeat laser iridotomy resulted in deepen-ing of the peripheral anterior chamber.33 Thisstudy provides further evidence that very smalliridotomies may not fully relieve iris bombe.We have calculated that the minimum func-

tional iridotomy size should be approximately 15pm, as previously suggested by Wheeler.36However, our clinical experience, and that ofothers,'33 shows that iridotomies in the rangeof 50-150 pm diameter may fail to prevent acuteangle closure glaucoma.

Several factors may contribute to the reduc-tion in size of an iridotomy of theoretically 'safe'size below that needed to prevent acute angleclosure glaucoma. Most cases of acute angleclosure glaucoma, following Nd-YAG laseriridotomy have occurred within one month oftreatment. Naveh et al observed rapid diminu-tion of Nd-YAG laser iridotomy size within anhour of treatment in six eyes.37 In each case areturn to original treatment size occurred withintwo weeks. Localised iris oedema round thetreatment site could be responsible.

Re-closure of Nd-YAG laser iridotomiesdue to iris pigment epithelium proliferation isunusual, and tends to occur 1-4 months aftertreatment." 12333738 Re-closure resulted inacute angle closure glaucoma in two eyesreported on by Wishart and Hitchings33 andone eye reported on by Gray et al."2

Case 2 developed acute angle closure glaucomafollowing mydriasis. Pupil dilatation mightreduce the surface area of some iridotomies, andfolds of iris tissue might functionally block aniridotomy when the pupil is dilated. Mydriaticsmight therefore produce acute angle closureglaucoma in the presence of a patent iridotomyby two mechanisms - pupil block due toreduction ofiridotomy size, and angle crowding.To answer the question, How large must an

iridotomy be to prevent acute angle closureglaucoma? several factors must be considered. Informs ofangle crowding angle closure glaucoma,such as plateau iris syndrome, a large iridotomywill not prevent the development of acute angleclosure glaucoma following mydriasis, as pupilblock is not causative.

Theoretical modelling suggests an iridotomyof 10-15 im diameter will prevent angle closureglaucoma due to pupil block. However, aniridotomy of greater size may fall below thiscritical level due to early post-treatment oedema,late pigment epithelium proliferation, or pupildilatation. Wishart and Hitchings reported sub-acute angle closure glaucoma in an eye with a 150im diameter iridotomy.33 All other cases ofacute angle closure glaucoma in the presence of apatent iridotomy have occurred in eyes with aniridotomy of less than 100 ptm in diameter. Thesmallest potentially safe size of an iridotomywould therefore appear to be 100 [tm. However,in order to incorporate a safety margin, we wouldrecommend that iridotomies should be at least150-200 [Lm in diameter.Provocation tests have not been helpful in

identifying eyes at risk, as eyes developing acuteangle closure glaucoma following Nd-YAG laseriridotomy have done so before a provocation testhad been performed.8 12 33 If provocation tests areto be employed, we suggest the use of tropi-

camide eyedrops 1% soon after the iridotomy has.been performed, in order to detect cases at risk ofdeveloping acute angle closure glaucoma.Routine pupil dilatation to prevent posteriorsynechiae formation following laser iridotomymay be more safely achieved with phenylephrineeyedrops 10%, with measurement of intraocularpressure following dilatation. Lowe has shownthat phenylephrine very rarely results in a rise inintraocular pressure following peripheraliridectomy,' and the effect may be quicklyreversed using thymoxamine eyedrops 0'5% if apressure rise does occur.

Dr N A Fleck, Department of Engineering, University ofCambridge, developed the mathematical model. Professor C IPhillips and Dr J F Cullen, Department of Ophthalmology,Edinburgh Royal Infirmary, made helpful comments duringpreparation of the manuscript. Mr Stuart Gairns, ophthalmicphotographer, assisted in preparation of the illustrations.

This paper forms part of an ongoing research programmefunded by theWH Ross Foundation.

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