the study design was approved by the ethics committee of iran university of medical sciences .
patients with > 6 months of follow - up after the second agv surgery were enrolled .
demographic data as well as best - corrected visual acuity ( bcva ) , iop ( measured by calibrated goldmann applanation tonometry ) , slit lamp biomicroscopy , funduscopic findings ( using 78 diopter lens ) , and number of glaucoma medications ( including topical and oral agents preoperatively ) and complications ( if any ) at the preoperative and every postoperative follow - up visits were recorded .
the primary outcome measure was surgical success defined in terms of iop control using two criteria : ( 1 ) 5 iop 21 mmhg with at least 20% reduction in iop without glaucoma medication ( complete success ) or with no more than 2 medications ( qualified success ) and ( 2 ) similar to previous criterion with the exception of 16 mmhg being the iop cut - off point . according to criterion 1 ,
the surgery was classified as failure when iop was < 5 mmhg or more than 21 mmhg and according to criterion 2 , when iop was < 5 mmhg or more than 16 mmhg in at least two consecutive visits 3 months after the surgery .
incidence of hypertensive phase ( hp ) , bcva , number of medications , and postoperative complications were analyzed as secondary outcome measures . in this study , hp was defined as iop measurement > 21 mmhg during the first 3 months after surgery not as a result of tube obstruction , retraction , or valve malfunction . in all cases ,
a single surgeon ( naveed nilforushan ) performed all second operations with a standard technique .
a fornix - based conjunctival flap was fashioned in superonasal or inferotemporal quadrant , based on the availability of healthy conjunctiva and preference of the surgeon .
mitomycin c ( 0.2 mg / ml ) was then applied by weck - cel sponges under the conjunctiva for 2 min . the agv ( model fp7 , new world medical , rancho cucamonga , ca , usa ) was primed , and its plate was secured to the sclera with 8 - 0 nylon suture , 10 - 11 mm behind the limbus .
the anterior part of the tube was secured to the sclera using 10.0 nylon suture and was subsequently covered with donor scleral patch graft .
postoperatively , all patients were prescribed ciprofloxacin eye drops 4 times a day for approximately 2 weeks and betamethasone eye drops every 2 h for 2 weeks which was then tapered over the next 68 weeks .
patients were examined on the postoperative day 1 , then at least weekly for 4 weeks , and then every 13 months , based on the clinical judgment .
iop lowering medications were initiated on discretion of the surgeon when the preset target pressure was not reached .
meier survival analysis was used to determine the probability of surgical success based on two criteria .
statistical analysis was performed using spss software version 20 for windows ( ibm corp , armonk , new york , usa ) . for analysis ,
paired t - test and wilcoxon test were used to compare baseline and outcome variables .
statistical significance was determined using two - tailed t - test ( p  0.05 ) .
in all cases , the first agv had been implanted in superotemporal quadrant . a single surgeon ( naveed nilforushan ) performed all second operations with a standard technique .
a fornix - based conjunctival flap was fashioned in superonasal or inferotemporal quadrant , based on the availability of healthy conjunctiva and preference of the surgeon .
mitomycin c ( 0.2 mg / ml ) was then applied by weck - cel sponges under the conjunctiva for 2 min . the agv ( model fp7 , new world medical , rancho cucamonga , ca , usa ) was primed , and its plate was secured to the sclera with 8 - 0 nylon suture , 10 - 11 mm behind the limbus .
the anterior part of the tube was secured to the sclera using 10.0 nylon suture and was subsequently covered with donor scleral patch graft .
postoperatively , all patients were prescribed ciprofloxacin eye drops 4 times a day for approximately 2 weeks and betamethasone eye drops every 2 h for 2 weeks which was then tapered over the next 68 weeks .
patients were examined on the postoperative day 1 , then at least weekly for 4 weeks , and then every 13 months , based on the clinical judgment .
iop lowering medications were initiated on discretion of the surgeon when the preset target pressure was not reached .
kaplan  meier survival analysis was used to determine the probability of surgical success based on two criteria .
statistical analysis was performed using spss software version 20 for windows ( ibm corp , armonk , new york , usa ) . for analysis ,
paired t - test and wilcoxon test were used to compare baseline and outcome variables .
statistical significance was determined using two - tailed t - test ( p  0.05 ) .
thirty - six eyes of 34 patients ( 15 men ) who had undergone the second agv implantation in the same eye with follow - up of at least 6 months were enrolled in this study .
the mean age of the patients was 32.7 years ( sd 18.7 , range 465 ) , and the mean duration of follow - up was 21.4 months ( sd 21.7 , median 12 , range 696 ) .
the time interval between the first and second agv implantation was on average 4.05 years ( range 110 ) .
preoperatively , the mean iop was 26.94 mmhg ( sd 7.03 ) , and the patients were using 2.8 glaucoma medications on average ( sd 0.9 ) . the mean iop and number of medications decreased significantly to 13.28 mmhg ( sd 3.59 ) and 1.4 ( sd 1.1 ) at the last postoperative visit , respectively ( p = 0.00 in both ) .
tonometric failure detected at last follow - up was occurred in 9 ( 25% ) and 11 ( 30% ) eyes according to criteria 1 and 2 , respectively [ table 2 ] . however , with regard to incomplete follow - up for many of the individuals in this study , kaplan 
meier approach was utilized to estimate the cumulative incidence of surgical failure [ figs . 2 and 3 ] . according to criterion 1 , 6 , 12 , 18 , and 42 months success rate ( 95% confidence intervals ) was 94% ( 87 , 100 ) , 85% ( 72 , 99 ) , 80% ( 63 , 96 ) , and 53% ( 21 , 85 ) , respectively .
median time to failure was 60 ( 26 , 94 ) months . according to criterion 2 , 6 , 12 , 18 , and 42 months success ( 95% confidence intervals ) was 94% ( 87 , 100 ) , 85% ( 72 , 99 ) , 75% ( 57 , 93 ) , and 45% ( 16 , 75 ) , respectively .
success at last follow - up based on criterion 1 and 2 ( n=36 ) intraocular pressure changes during the postoperative 3-year period .
+ p values on the basis of comparison with preoperative values using paired t - test , # p values on the basis of comparison with preoperative values using wilcoxon test .
agm : the number of antiglaucoma medication ; iop : intraocular pressure success of the second ahmad glaucoma valve in refractory glaucoma based on criterion 1 .
meier analysis , where the dotted lines indicate the 95% confidence intervals and the hatch marks indicate censored subjects ( eyes ) .
the 6 , 12 , 18 , and 42 months success ( 95% confidence intervals ) was 94% ( 87 , 100 ) , 85% ( 72 , 99 ) , 80% ( 63 , 96 ) , and 53% ( 21 , 85 ) , respectively .
median time to failure was 60 ( 26 , 94 ) months success of second ahmad glaucoma valve in refractory glaucoma based on criterion 2 .
success was calculated using kaplan  meier life table analysis , where the dotted lines indicate the 95% confidence intervals and the hatch marks indicate censored subjects ( eyes ) .
the 6 , 12 , 18 , and 42 months success ( 95% confidence intervals ) was 94% ( 87 , 100 ) , 85% ( 72 , 99 ) , 75% ( 57 , 93 ) , and 45% ( 16 , 75 ) , respectively .
median time to failure was 42 ( 14 , 70 ) months the right eye of a patient with the second ahmed glaucoma valve implantation in superonasal quadrant of the conjunctiva .
the first ahmed glaucoma valve was implanted in superotemporal quadrant no significant intraoperative complication was determined and also , there was no case of endophthalmitis , or retinal detachment postoperatively .
in addition , three patients underwent needling of the encapsulated bleb around the plate ( one with adjunctive mitomycin c ) 310 weeks after surgery .
. it did not change significantly 6 months postoperatively ( 0.94 , p = 0.19 ) and at the last visit ( 0.93 , p = 0.18 ) .
hp was detected in 22 patients ( 61.11% ) during the first 3 months of surgery for whom iop - lowering medication was started . in 13 cases ( 59.09% ) , hp occurred within 1 month after surgery ( 4 of them in the first 2 weeks ) .
we also divided the patients to those with preoperative iop 21 mmhg and preoperative iop > 21 mmhg for comparison .
this showed that out of 30 eyes with preoperative iop > 21 mmhg , according to criteria 1 and 2 , 7 ( 23% ) and 9 ( 30% ) eyes were failed , respectively .
moreover , out of 6 eyes with preoperative iop 21 , 2 ( 33% ) eyes were failed according to both criteria .
no significant difference was found between subgroups with preoperative iop 21 and preoperative iop > 21 mmhg ( p > 0.99 , fisher exact test ) . of 11 cases with failure according to criterion 2 , in three cases , medical treatment was continued ; in two cases , cyclophotocoagulation was performed ; and in the rest , we did not have further follow - up records to report herein .
mean iop at final exam was 13.28 mmhg , a reduction of 13.66 mm hg or 50% from preoperative values even though patients were on fewer glaucoma medications .
no major complication occurred and the mean best corrected visual acuity did not show a statistically significant reduction . during the last decade ,
several studies have addressed the success rate of various secondary shunt implantation such as agv , molteno , baerveldt , krupin and schocket in adults , and children [ table 3 ] with reported success rates of 50 - 93% . but a comparison of the overall success rate in these case series is not literally feasible not only because of different mix of glaucoma types studied , various lengths of follow - up and different definitions of surgical success ; but even more notably because these studies are all relatively small in sample size and consequently a relatively wide confidence interval straddles their point estimates of surgical success .
evaluated patients who had undergone 2 glaucoma drainage device surgeries ( exclusively agv ) in a retrospective case series and found 84.2% overall success ( comparable with success definition of our study according to criterion 1 ) in a follow - up of at least 1 year .
the reported success rate was apparently more than that of the present study ( 75% ) .
this difference may be accounted for by longer follow up in the present study ( up to 96 months ) in comparison with smith 's study ( up to 80 months ) .
surgical results of secondary shunt implantation in eyes with failed prior shunts in our study age , gender , laterality of studied eyes , preoperative iops , preoperative number of medications and glaucoma type of individual patients did not differ significantly between success and failure groups of patients ( results of t - test and fisher 's exact test not shown here ) .
21 participants showed that patients with preoperative iop 21 mm hg benefited more from second agv implantation than those with lower iops but in our study we did not found a significant difference in this regard .
hp in this study occurred in relatively similar rates as reported in the literature ( 59.4 % at first 3 month after surgery ) that is close to the findings of smith et al . with the rate of 61% .
wu sc et al . reported occurrence of hp in 63.2% of their patients , 49.3% of them experienced hp at 1 month and the rest at 2 months after the operation .
it occurred after a mean of 5.0 weeks ( median , 4 weeks ; range , 1 - 13 weeks ) . besides the retrospective and noncomparative design of this study and including nonuniform types of glaucoma , the other limitation of the present study is a relatively small sample of patients for analyzing the success rate after 3 years post - agv re - implantation .
although the wide confidence interval for measured success rate after 3 years of the surgery makes it difficult to predict the long - term prognosis , a practical conclusion can be extracted from the present body of evidence , i.e. , until a better option emerges , glaucoma specialists can offer secondary shunt implantation instead of cyclodestructive procedures to patients with failed primary shunts with a considerable hope of tonometric success with or without medications .
repeated agv implantation seems to be a safe modality of treatment with acceptable success rate in cases of failed previous agv surgery .
 purpose : to evaluate the outcome of the second ahmed glaucoma valve ( agv ) surgery in eyes with failed previous agv surgery.design:retrospective case series.patients and methods : following chart review , 36 eyes of 34 patients with second agv implantation were enrolled in this study . 
 the primary outcome measure was surgical success defined in terms of intraocular pressure ( iop ) control using two criteria : success was defined as iop 21 mmhg ( criterion 1 ) and iop 16 mmhg ( criterion 2 ) , with at least 20% reduction in iop , either with no medication ( complete success ) or with no more than two medications ( qualified success ) . 
 kaplan  
 meier survival analysis was used to determine the probability of surgical success.results:the average age of the patients was 32.7 years ( range 465 ) , and the mean duration of follow - up was 21.4 months ( range 696 ) . 
 preoperatively , the mean iop was 26.94 mmhg ( standard deviation [ sd ] 7.03 ) , and the patients were using 2.8 glaucoma medications on average ( sd 0.9 ) . 
 the mean iop decreased significantly to 13.28 mmhg ( sd 3.59 ) at the last postoperative visit ( p = 0.00 ) while the patients needed even fewer glaucoma medications on average ( 1.4  1.1 , p = 0.00 ) . 
 surgical success of second glaucoma drainage devices ( kaplan  meier analysis ) , according to criterion 1 , at 6 , 12 , 18 , and 42 months was 94% , 85% , 80% , and 53% respectively , and according to criterion 2 , was 94% , 85% , 75% , and 45% , respectively.conclusion:repeated agv implantation seems to be a safe modality of treatment with acceptable success rate in cases with failed previous agv surgery . 
Patients and Methods
Surgical technique
Statistical analysis
Results
Discussion
Conclusion
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