We identified isolated, non-epidemic cases of DLK, and found that some degree of inflammation appeared in both eyes in a similar proportion of cases, whether or not surgery was performed on the same day or one day apart. This observation is consistent with the hypothesis that in a certain proportion of cases, DLK can be attributed to intrinsic rather than environmental causes. It might be expected that observed DLK in one eye might lead to more careful attention to potential causes of DLK in the second eye and, as noted, patients were advised to use topical corticosteroids more frequently on the first night following surgery in the second eye. In spite of this, inflammation in the second eye occurred at least as frequently with these measures as it did in simultaneous cases. Thus, a better understanding of the causes of DLK is necessary to effectively reduce its incidence.
In order to exclude epidemic cases of DLK that could produce multiple cases of DLK in both eyes of a number of patients over many days whether surgery was sequential or simultaneous, we excluded data from all sessions in which any degree of inflammation was noted in more that one patient. In so doing, we have characterised the incidence of bilateral inflammation specifically for sporadic and isolated cases of DLK. Our data suggest that such sporadic cases represent the majority of DLK events (88%), and the majority of such isolated cases are mild (94% grade 1 or 2). Our data specifically examining sequential surgery suggest that if inflammation develops in one eye, approximately 18% of patients will develop inflammation in the other. Inflammation in the second eye can very probably be attributed to factors intrinsic to the patient, and in a majority of cases is not caused by environmental factors that are associated with epidemic inflammation.
This study is certainly limited by its retrospective design. The data we recorded do not afford us an opportunity to reliably identify those features unique to the patient that may increase their risk for DLK, in one or both eyes. As a follow up study, we feel it would be worthwhile to prospectively examine the role that preoperative ocular symptoms, medications, meibomian gland health and function, lid margin dysfunction or conjunctival and lid flora might have in this condition. If associations were identified it would be reasonable to further investigate the role that specific intervention might have on DLK incidence.
This study might also be interpreted as suggesting that since bilateral DLK seems to occur with similar frequency whether surgery is simultaneous or sequential, with regard to the risk of inflammation there is no advantage to be gained by sequential rather than simultaneous surgery. However, because of the relatively low degree of inflammation (grade 1 to 2) in the majority of cases, we were unable to evaluate the effectiveness of the more aggressive corticosteroid treatment that was applied to the contralateral eye when surgery was performed in a sequential fashion. If more aggressive corticosteroid treatment does indeed limit the severity of the DLK that is more likely to occur in the second eye if seen in the first, then there would indeed be an advantage to sequential rather than simultaneous surgery.
In summary, there was no difference in the incidence of non-epidemic, isolated bilateral DLK in simultaneous and sequential LASIK, suggesting an underlying intrinsic cause for isolated DLK. Further study is required to better define specific risk factors for the development of DLK following LASIK, and the effectiveness of various treatment strategies.