Letters to the Editor
Risk Factors for Postoperative Endophthalmitis
Authors' reply
The f ollowing letters concern the lead articlefrom the Decem ber 1991 issue ofthe Journal.
Dear Editor: Dr. Beck notes that the association we observed between haptic materials and endophthalmitis might in fact merely represent an association between haptic materials and some other risk factor , such as the surgeon. This is of course a possibility, although we think it unlikely. The 38 cases of endophthalmitis among patients with posterior chamber lenses involved 32 different surgeons, of whom only 6 surgeons had 2 cases each. No surgeon had more than two cases of endophthalmitis. Therefore, it does not appear that a few "high-risk" surgeons with a preference for using intraocular lenses (IOLs) with polypropylene haptics created the statistical association that we observed. Furthermore, we note that the 6 surgeons who had 2 cases each were responsible for using a disproportionate percentage (60%, or 3 of5) of the all-polymethylmethacrylate (PMMA) IOLs used in patients who developed endophthalmitis. Thus, if one were to hypothesize that these six surgeons somehow were operating in a high-risk manner-a conclusion that we believe is unlikel y, given the small number of cases attributed to each surgeon involved- then these results would further support the protective effect of all-PMMA IOLs: it would have been extremel y rare for a surgeon using an all-PMMA IOL to have produced a case of endophthalmitis unless the surgeon was using a high-risk operating technique. MARK G. SPEAKER, MD , PhD JERRY A. MENIKOFF, MD MICHAEL MARMOR, PhD ELSA M. RASKIN, MD New York, New York
MenikoffJA, Speaker MG , Marmor M, Raskin EM. A Case-control Stud y of Risk Factors for Postoperative Endophthalmitis (Ophthalmology 1991;98:1761-68). Dear Editor: In their article entitled, "A Case-control Study of Risk Factors for Postoperative Endophthalmitis" Menikoff et al conclude that the use of polypropylene haptic intraocular lenses increases the risk of endophthalmitis and project that 700 cases of endophthalmitis annually could be eliminated if use of this type oflens was discontinued. I agree with the authors that a case-control study is the best design for evaluating risk factors for a rare occurrence such as endophthalmitis. However, before concluding that a variable is a true risk factor, it is imperative to minimize chance, bias, and confounding as possible explanations for the association. In view of the magnitude of the relative risk and its 95th percentile confidence interval, the authors appropriatel y concluded that their finding is unlikely to be due to chance. Howe ver, they do not address whether the finding could be the result of confounding, the presence ofanother factor that is associated with both the risk factor (the polypropylene haptic intraocular lens) and the outcome (endophthalmitis). The most obvious factor that must be considered as a potential confounder is the surgeon . Because surgeons tend to use certain lenses, it is necessary to rule out any association between the surgeon and the cases of endophthaI mitis before implicating the lens. A second potential confounding factor to be evaluated could be related to the operating room, if the number of polypropylene haptic intraocular lenses were not implanted with equal frequency in each operating room. A matched case-control stud y may be the best design to control for surgeon and operating room factors. If no significant association is found after the stud y is repeated with matched controls, then polyprop ylene haptic intraocular lenses could still be a risk factor for endophthalmit is, but there would be a strong possibility that the association reported in this article was due to confounding. The authors' conclusions about polypropylene haptic intraocular lenses has serious medical-legal implications. If this type oflens is a true risk factor for endophthalmitis, their conclusion is warranted. However, potential confounding factors must be fully evaluated before this type of intraocular lens is condemned. Roy W. BECK, MD Tampa, Florida
Dear Editor: I read with interest the article by Menikoff et al concerning the risk factors for postoperative endophthalmitis. The increased incidence of endophthalmitis associated with left eye surgery also may shed light on why the haptic material is a risk factor. If the majority of the surgeons in the study were righthanded (and therefore insert the intraocular lens on the " Ifl-o'clock" side of the eye with the right hand) then left eyes may indeed be at increased risk when operated on by right-handed surgeons (and vice versa). The orbital anatomy in many patients makes intraocular lens insertion different depending on whether one is inserting the lens from the nasal side or the temporal side. In this circumstance, insertion in a left eye (for right-handed surgeons) causes more haptic contact with the conjunctiva and episclera. The additional contact might allow greater numbers of offending organisms to be swept or carried into the eye on the haptic. Use ofprolene haptics is associated with an increased incidence of endophthalmitis. Does prolene attract or hold organisms more powerfully than polymethylmethacrylate?
1007
Risk Factors for Postoperative Endophthalmitis
Authors' reply
The f ollowing letters concern the lead articlefrom the Decem ber 1991 issue ofthe Journal.
Dear Editor: Dr. Beck notes that the association we observed between haptic materials and endophthalmitis might in fact merely represent an association between haptic materials and some other risk factor , such as the surgeon. This is of course a possibility, although we think it unlikely. The 38 cases of endophthalmitis among patients with posterior chamber lenses involved 32 different surgeons, of whom only 6 surgeons had 2 cases each. No surgeon had more than two cases of endophthalmitis. Therefore, it does not appear that a few "high-risk" surgeons with a preference for using intraocular lenses (IOLs) with polypropylene haptics created the statistical association that we observed. Furthermore, we note that the 6 surgeons who had 2 cases each were responsible for using a disproportionate percentage (60%, or 3 of5) of the all-polymethylmethacrylate (PMMA) IOLs used in patients who developed endophthalmitis. Thus, if one were to hypothesize that these six surgeons somehow were operating in a high-risk manner-a conclusion that we believe is unlikel y, given the small number of cases attributed to each surgeon involved- then these results would further support the protective effect of all-PMMA IOLs: it would have been extremel y rare for a surgeon using an all-PMMA IOL to have produced a case of endophthalmitis unless the surgeon was using a high-risk operating technique. MARK G. SPEAKER, MD , PhD JERRY A. MENIKOFF, MD MICHAEL MARMOR, PhD ELSA M. RASKIN, MD New York, New York
MenikoffJA, Speaker MG , Marmor M, Raskin EM. A Case-control Stud y of Risk Factors for Postoperative Endophthalmitis (Ophthalmology 1991;98:1761-68). Dear Editor: In their article entitled, "A Case-control Study of Risk Factors for Postoperative Endophthalmitis" Menikoff et al conclude that the use of polypropylene haptic intraocular lenses increases the risk of endophthalmitis and project that 700 cases of endophthalmitis annually could be eliminated if use of this type oflens was discontinued. I agree with the authors that a case-control study is the best design for evaluating risk factors for a rare occurrence such as endophthalmitis. However, before concluding that a variable is a true risk factor, it is imperative to minimize chance, bias, and confounding as possible explanations for the association. In view of the magnitude of the relative risk and its 95th percentile confidence interval, the authors appropriatel y concluded that their finding is unlikely to be due to chance. Howe ver, they do not address whether the finding could be the result of confounding, the presence ofanother factor that is associated with both the risk factor (the polypropylene haptic intraocular lens) and the outcome (endophthalmitis). The most obvious factor that must be considered as a potential confounder is the surgeon . Because surgeons tend to use certain lenses, it is necessary to rule out any association between the surgeon and the cases of endophthaI mitis before implicating the lens. A second potential confounding factor to be evaluated could be related to the operating room, if the number of polypropylene haptic intraocular lenses were not implanted with equal frequency in each operating room. A matched case-control stud y may be the best design to control for surgeon and operating room factors. If no significant association is found after the stud y is repeated with matched controls, then polyprop ylene haptic intraocular lenses could still be a risk factor for endophthalmit is, but there would be a strong possibility that the association reported in this article was due to confounding. The authors' conclusions about polypropylene haptic intraocular lenses has serious medical-legal implications. If this type oflens is a true risk factor for endophthalmitis, their conclusion is warranted. However, potential confounding factors must be fully evaluated before this type of intraocular lens is condemned. Roy W. BECK, MD Tampa, Florida
Dear Editor: I read with interest the article by Menikoff et al concerning the risk factors for postoperative endophthalmitis. The increased incidence of endophthalmitis associated with left eye surgery also may shed light on why the haptic material is a risk factor. If the majority of the surgeons in the study were righthanded (and therefore insert the intraocular lens on the " Ifl-o'clock" side of the eye with the right hand) then left eyes may indeed be at increased risk when operated on by right-handed surgeons (and vice versa). The orbital anatomy in many patients makes intraocular lens insertion different depending on whether one is inserting the lens from the nasal side or the temporal side. In this circumstance, insertion in a left eye (for right-handed surgeons) causes more haptic contact with the conjunctiva and episclera. The additional contact might allow greater numbers of offending organisms to be swept or carried into the eye on the haptic. Use ofprolene haptics is associated with an increased incidence of endophthalmitis. Does prolene attract or hold organisms more powerfully than polymethylmethacrylate?
1007
