Methods: A retrospective non-comparative case series of 1632 eyes that had undergone bilateral, simultaneous or sequential LASIK between April 1998 and February 2001 at a university based refractive centre by three surgeons. All cases that developed clinically evident DLK were identified and reviewed. In order to identify isolated cases and exclude those caused by environmental factors, when more than one patient in a given session developed DLK, the session was excluded. The main outcome measure was the incidence of unilateral and bilateral isolated, non-epidemic DLK.

Results: Of 1632 eyes, 126 eyes (7.7%) of 107 patients developed at least grade 1 DLK. In six operating sessions, DLK was observed in more than one patient per session, and on this basis 13 patients were excluded. 16 of the 94 remaining patients developed DLK in both eyes (17.0%). Six of 41 patients (14.6%) in the simultaneous group, versus 10 of 53 patients (18.9%) in the sequential group developed bilateral DLK (p >0.5).

Since the initial descriptions of cases of DLK, multiple aetiologies have been hypothesised. However, it is unlikely that a single causative agent might explain all cases, as it most likely represents a non-specific inflammatory response. A potential plane that persists indefinitely is created by the microkeratome, allowing inflammatory cells to migrate along this interlamellar space in response to an inflammatory event. Potential stimulants may include debris deposited by surgical instruments including the microkeratome,^5–7 powder from gloves,⁵ intralamellar haemoglobin,⁸ particles from the drape,⁹ endotoxins,^10–12 meibomian gland secretions,¹³ epithelial defects,^14,15 and povidone-iodine solution.¹

It was our opinion that DLK seemed to represent a non-specific response to a wide variety of potential insults. As it has been emphasised in previous descriptions and studies, clustered epidemic cases can often be attributed to identifiable environmental causes such as endotoxins released from steriliser biofilms.^10,12 However, more frequently we have observed isolated, non-epidemic cases. Although a bilateral case may still represent an environmental source for DLK, it may in fact be the result of an endogenous factor common to both eyes of the same patient—for example, lid flora, meibomian secretions or host immune system response. If this were true, we would expect isolated DLK to be bilateral in a certain number of sequential as well as simultaneous cases, even if other patients undergoing surgery at the same time were unaffected. We therefore elected to examine whether non-epidemic, isolated bilateral DLK occurred as frequently in simultaneous as in sequential cases.

Sequential surgery was defined as surgery performed on each eye on two consecutive days, while simultaneous surgery was defined as surgery performed on both eyes on the same day. In simultaneous surgery, the second eye was draped separately after completing the treatment of the first eye, and a second microkeratome and new blade used for the second eye. Before surgery, the eyelids were prepared with Betadine 5% (Purdue Frederick, Norwalk, CT, USA). The eyelashes and eyelids were isolated with a drape. A lid speculum was placed. Preoperative medication included oxybuprocaine (proparacaine) 0.5% and ofloxacin 0.3%. The surgeons and assistants wore powder-free gloves during the instrument set up and surgery. The automated corneal shaper (ACS) (Bausch and Lomb, Irvine, CA, USA) or the Moria Carriazo-Barraquer (Microtech, Doylestown, PA, USA) were used to make corneal flaps. All patients were prescribed a topical fluoroquinolone, ofloxacin 0.3% (Allergan, Irvine, CA, USA) four times a day and fluoromethalone 0.1% (FML, Allergan, Irvine, CA, USA) four times a day starting immediately postoperatively. For surgery performed sequentially, if DLK was noted in the first eye, for the second eye the frequency of topical corticosteroid application following surgery was prophylactically increased to every hour while awake until the degree of inflammation could be assessed on the first day after surgery.

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.