7 Femtosecond Laser–Assisted In Situ Keratomileusis: Complications and Management

J. Bradley Randleman and Heather M. Weissman

Keywords: Femtosecond, LASIK, flap, rainbow glare, opaque bubble layer, vertical gas breakthrough

7.1 Introduction

Laser in situ keratomileusis (LASIK) is one of the most successful, elective surgical procedures performed in the world. It has become the mainstay of treatment for most forms of refractive error. The first critical step of the procedure involves the creation of the corneal flap, which can be performed using either a mechanical microkeratome blade (MK) or a femtosecond laser (FS). The MK creates a flap using an oscillating blade that traverses through the corneal stroma in a controlled fashion. ¹ The FS laser uses infrared light (1,053 nm) to produce microplasma and microcavitation bubbles within the corneal stroma to functionally create a flap interface that can be manually opened with minimal effort. ²

Since the introduction of the FS in 2001, its technology has continued to evolve making it the preferred method for LASIK flap creation. Many studies have been done comparing outcomes of femtosecond LASIK versus microkeratome LASIK. The data from these studies have been inconsistent, but the overall consensus is that femtosecond flap creation may offer more accuracy, reproducibility, and uniformity than microkeratome flap creation. ^{2,​ 3,​ 4,​ 5}

The use of the FS in flap creation has allowed for a more customizable LASIK flap. A unique feature of FS is the ability to create thinner, smoother flaps with more planar or uniform architecture. ⁶ Theoretically, thinner flaps may prevent most cases of post-LASIK ectasia by providing more biomechanical stability and have also been associated with a faster visual recovery. ^{7,​ 8,​ 9} The ability to program a uniform flap thickness and angulation of the periphery with the FS has allowed for very good accuracy and precision. Stahl et al used anterior segment optical coherence tomography (OCT) to examine 25 eyes with flaps created with the IntraLase FS prior to laser ablation. The study found only a 4 µm standard deviation of thickness within each flap. ¹⁰

Femtosecond-assisted LASIK (femtoLASIK) shares similar complications with microkeratome LASIK; however, there are unique complications associated only with FS use. These complications are rare making diagnosis and management challenging even for the most experienced refractive surgeon. Complications unique to femtosecond LASIK surgery can be divided into optical issues, flap-related complications, and interface-related complications. In addition to complications unique to femtoLASIK, ocular surface issues, residual ametropia, and ectasia are still a concern with this procedure and warrant discussion. In this chapter, we describe the most common complications associated with femtoLASIK surgery and provide management strategies for them.

7.2 Optical Issues

First described by Krueger ¹¹ in 2008, rainbow glare is a mild optical side effect associated with femtosecond-assisted LASIK flap creation. This phenomenon is poorly understood and described in some cases of otherwise uneventful LASIK. The etiology of rainbow glare is thought to be due to diffraction of light off the grating pattern on the back surface of the LASIK flap. ¹² Gatinel and colleagues recently demonstrated the induced grating pattern with confocal microscopy at the level of the flap interface in the right eye (▶ Fig. 7.1). ¹³ Often immediately after surgery, patients describe 4 to 12 lines of rainbow-colored light radiating from a white-light source viewed on a dark background. Bamba et al found a bimodal incidence of rainbow glare after LASIK. ¹² The first group experienced rainbow glare immediately after instillation of the FS and this was thought to be due to inadequate alignment of the laser and higher raster energy. The second group of patients experienced rainbow glare immediately preceding a service call for the laser. The authors propose that the quality of the laser beam is the most important factor to reduce the incidence of rainbow glare. Rainbow glare was not correlated with degree of refractive error, age, or sex. ¹² Rainbow glare is hard to treat given its poorly understood etiology and is self-limiting in many cases.

Recently, Gatinel et al reported successful resolution of rainbow glare symptoms with flap undersurface ablation. The authors describe a patient with unilateral rainbow glare and minimal residual myopic astigmatism following uncomplicated LASIK. The LASIK flap was lifted and laser correction was delivered to the stromal side of the flap. The patient’s symptoms resolved immediately after the procedure. ¹⁴

7.3 Flap-Related Complications

During flap creation with the FS, microcavitation bubbles are created to enable a lamellar dissection. During the bubble formation, three unique complications may occur: vertical gas breakthrough, an opaque bubble layer (OBL), and bubbles in the anterior chamber. In addition, the ability to customize flap thickness has led to the creation of a flap that is too thin, which may increase the risk of interface haze.

7.3.1 Vertical Gas Breakthrough

Analogous to the buttonhole flap with MK use, vertical gas breakthrough occurs when small gas bubbles break through and lodge within the dissection plane and the subepithelial space (▶ Fig. 7.2). If these bubbles then escape through the epithelium, a buttonhole in the flap is created. Caution should be taken if this occurs and the flap should not be lifted as the buttonholed area can lead to scarring and epithelial ingrowth. Corneal scarring, microscopic breaks in Bowman’s membrane, and thin flaps may contribute to the occurrence of vertical gas breakthrough; therefore, a detailed slit lamp exam is imperative prior to surgery. ^{1,​ 15}

7.3.2 Opaque Bubble Layer

An opaque bubble layer (OBL) occurs when gas bubbles accumulate within the superficial stromal layers (▶ Fig. 7.3). ¹⁶ This creates a diffuse opacity that usually goes away with flap lifting but may interfere with laser tracking, measurement of the residual stromal bed, and flap creation. There are two types of OBL: hard and soft. The hard type is denser and the soft type is more diffuse. Liu et al recently examined 40 eyes with femtosecond flap creation and found that thicker corneas tended to develop an OBL. The authors propose that to minimize the possibility of OBL creation, surgeons can use a higher pulse rate and less line spacing to ensure an adequate cleavage plane and minimize the retention of stromal bridges and gas buildup. An OBL rarely has an effect on postoperative visual acuity but has not been extensively studied. ¹⁷

7.3.3 Anterior Chamber Bubbles

Anterior chamber bubbles occur when gas bubbles created by the laser exit through the trabecular meshwork into the anterior chamber of the eye (▶ Fig. 7.4). Anterior chamber bubbles often have minimal impact on visual outcome but may impede pupil tracking during the laser ablation. ¹ If pupil tracking is impeded, the surgeon can wait for the bubbles to dissipate and proceed with surgery oftentimes on the same day.

7.3.4 Thin Flap Haze

The ability to create thin flaps with the FS has proven to be beneficial with the creation of a more stable flap; however, there is some evidence that ultra-thin flaps may increase the risk of postoperative interface haze. Rocha et al recently evaluated the threshold for interface haze formation with thin flap LASIK. ⁸ The study looked at 199 eyes undergoing myopic LASIK with thin femtosecond flap creation. The authors found a higher risk of interface haze formation in young individuals with ultrathin flaps (<90 µm) undergoing LASIK for myopia. The exact cause of interface haze with ultrathin flaps is not known but is thought to be related to epithelial cell and Bowman’s membrane injury. Injury to Bowman’s membrane may initiate an inflammatory cascade leading to interface haze. ¹⁸ The authors propose a threshold of flap thickness of 100 µm or more to prevent interface haze formation. ⁸

7.4 Interface-Related Complications

Interface-related complications can occur with both microkeratome and femtosecond LASIK. ¹⁹ There are a few unique complications strongly associated with FS use, including diffuse lamellar keratitis (DLK), transient light sensitivity syndrome (TLSS), and blood in the interface, while others, such as infectious keratitis, pressure-induced stromal keratopathy (PISK), and central toxic keratopathy, are equally likely from microkeratome or femtosecond flap creation. A good understanding of all interface-related complications associated with LASIK is necessary to effectively achieve the correct diagnosis and provide proper management.

7.4.1 Diffuse Lamellar Keratitis

Diffuse lamellar keratitis (DLK) is characterized by a noninfectious, white and granular inflammation within the flap interface that presents within the first week postoperatively. DLK can occur with both microkeratome and femtosecond-assisted LASIK; however, an increased incidence has been reported in cases with femtosecond flap creation. ^{1,​ 20,​ 21} It has been theorized that the formation of gas bubbles and use of femtosecond energy at the flap interface increase the inflammatory response at the flap interface. ¹

Symptoms of DLK include foreign body sensation and mildly decreased visual acuity. Most cases resolve with a short course of steroids and the process rarely has a significant effect on vision. The exact etiology of DLK is unknown; however, it has been linked to bacterial endotoxins, debris or blood within the interface, and povidone–iodine solutions. ^{19,​ 20,​ 21,​ 22}

Linebarger et al ²³ described four stages of DLK. Stage 1 is defined by the presence of white blood cells located at the periphery of the flap edge, out of the visual axis. Stage 2 is defined by the presence of white cells in the center of the flap. This stage can involve the visual axis and the periphery of the flap. Stage 3 is defined by a denser, clumped aggregate of white cells in the center of the flap with clearing in the periphery. Vision can decline in this stage due to the inflammation in the central visual axis. Stage 4 is defined by lamellar keratitis associated with stromal melting, scaring, and permanent visual loss. Fortunately, stage 4 is rare and has been estimated to occur in 1 in 5,000 patients with DLK. ^{19,​ 23}

DLK is exquisitely sensitive to topical steroids. It is important to look for DLK in the flap interface as early as postoperative day 1. Stages 1 and 2 are usually managed primarily with topical steroids as frequently as every hour. Stages 3 and 4 can initially be managed with topical steroids but may need a flap lift and irrigation if there is no resolution at day 2 or 3. ²³

7.4.2 Transient Light Sensitivity Syndrome

Transient light sensitivity syndrome (TLSS) is a complication specific to FS use. TLSS is described by increased photosensitivity 3 to 6 weeks postoperatively after an uncomplicated LASIK procedure with good visual acuity and no obvious inflammation. Stonecipher et al first described TLSS in 2006 and found a direct correlation with higher energy settings. Management strategies to prevent TLSS include using the lowest possible laser energy settings to create a flap dissection. Patients with TLSS generally improve with a course of topical steroids and have no long-term visual consequences. ^{1,​ 4,​ 24}

7.4.3 Interface Heme

A unique feature of the Wavelight FS200 FS (Alcon Laboratories, Inc., Fort Worth, TX) is the creation of a gas evacuation at the onset of flap creation. This evacuation canal allows for the escape of gas created with the FS via a linear pattern at the flap–bed interface and extends to the limbus to prevent OBL. Limbal blood vessels may become injured by laser pulses and bleed into the canal, which can track all the way to the flap–bed interface (▶ Fig. 7.5). Blood in the interface can cause a decrease in visual acuity and induction of irregular astigmatism. Au and Krueger recently described three cases of interface blood that required flap lifting and irrigation. ²⁵ The authors suggest that when using this laser, it is imperative to view the canal throughout the entire procedure and watch for blood. If a large degree of blood is present, one can limit the flap dissection to prevent a connection between the canal and the interface. As soon as interface blood is recognized on exam, it is advisable to perform a flap lift immediately. ²⁵

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