Introduction
Laser in situ keratomileusis (LASIK) is the most common surgical method for the management of refractive errors. Flap creation is the first and most important step in the LASIK procedure.
Two current techniques used to produce corneal flaps during LASIK surgery are the mechanical microkeratome and the femtosecond laser. Some of the intraoperative complications encountered during LASIK are similar between them, while others are technique specific. This chapter includes a review of the more common complications seen with both methods.
The rates of flap-related intraoperative complications in most studies performed with trained LASIK surgeons are about 0.01% to 1%. An experienced surgeon using the latest technology of microkeratome and/or femtosecond laser may reach a very low incidence of flap-related complications.
A comprehensive awareness of the potential complications of LASIK and the numerous strategies to handle them is fundamental for surgeons performing the procedure.
Intraoperative Complications
Inadequate Exposure
Microkeratome and femtosecond laser head placement is more difficult in sunken globes and in eyes with narrow palpebral fissures and small corneas. By turning the patient’s head slightly to the opposite side or exerting a gentle pull and tilt on the eye through the suction ring handle, these cases can usually be operated on easily. Alternatively, it may be possible to operate without using a speculum. If a speculum is not used, one must ensure that the eyelashes and the eyelids do not overlap and cover the ring. The patient with a prominent brow should be positioned with the chin raised slightly, as this will maximize exposure. When all these measures are unsuccessful, conversion to photorefractive keratectomy (PRK) should be considered.
Suction Loss ( )
Inadequate suction or total loss of suction is a potential source of serious problems during LASIK. Factors that may contribute to lacking or loss of suction include improper technique in applying the suction ring, conjunctival chemosis, flat corneas, narrow palpebral fissures, deep-set eyes, patient movement, eye rotation, and head tilt. Blockage of a microkeratome head progression by the eyelid speculum is also an important cause of suction loss ( Fig. 15.1 ).
Suction loss with mechanical microkeratomes may result in more severe flap complications than with a femtosecond laser. Small, incomplete, or torn flap may result. The management of these cases depends on the available stromal bed for ablation. Usually, however, it is safer to reposition the flap, wait 3 months, on average, and retreat the patient with surface PRK.
Loss of vacuum by femtosecond laser is not as serious a problem as with the mechanical microkeratome. When using the femtosecond laser, the first sign of suction loss is often the appearance of a peripheral asymmetric and incomplete meniscus ( Fig. 15.2 ). Once detected, it is important to discontinue the laser treatment immediately. Loss of suction with the femtosecond laser is resolved by replacing the suction ring and re-docking the same applanation cone to subsequently repeat the treatment at the same depth.
The vertical limbal pocket, typically created to absorb cavitation bubbles, can be deactivated if it was already created in the first pass. If the loss of suction occurs during the side cut, the surgeon must ensure that the subsequent side cut is created within the lamellar cut used to fashion the flap by decreasing the subsequent side cut by 0.5 mm or more in diameter. Surface ablation with mitomycin C (MMC) can also be considered over the incomplete flap after repeated suction attempts prove unsuccessful.
Corneal Epithelial Defect
This uncommon LASIK complication has been shown to be lower with the use of a femtosecond laser than with a microkeratome because there is no microkeratome movement across the epithelium.
There are many precautions that should be taken in an attempt to decrease the risk of epithelial defects. The use of excessive topical medications, especially frequent anesthetics such as proparacaine, should be avoided before flap creation. Topical anesthetic placement should only occur immediately before initiating the procedure to decrease epithelial irregularity and toxicity.
One study recommended using hyperosmotic agents preoperatively in order to reduce the risk for epithelial disruptions.
In the event of detachment, the epithelium is repositioned if possible. A bandage contact lens is placed for 1 or 2 days or until the epithelium is completely healed ( Fig. 15.3 ).
Patients with a history of recurrent erosions and/or anterior basement membrane dystrophy (ABMD) are at higher risk of developing epithelial erosions with LASIK and would be better PRK candidates. Other risk factors for epithelial disturbances are increasing age, diabetes, dry eye, and a long history of contact lens wear.
When severely injured, the epithelium produces high amounts of cytokines, such as interleukin-1α, that stimulate keratocytes to produce chemokines that attract inflammatory cells, leading to diffuse lamellar keratitis (DLK). Epithelial defects have also been associated with postoperative complications, including epithelial ingrowth, recurrent corneal epithelial erosion, and flap melt.
Incomplete or Irregular Cut ( )
Method of flap creation and surgeon experience have both been shown to be the primary reasons for this disparity in complication rates. Of the different types of abnormal flaps, incomplete or short flaps are the most common. Incomplete flaps may be a result of the microkeratome prematurely stopping its course, which may be attributed to a pathway obstruction (e.g., blockage of microkeratome head progression by the speculum) or device malfunction. In addition, it may occur if suction is lost during the flap cut. Irregular cuts are more rare and may be related to defective blades and foreign bodies in the interface ( Fig. 15.4 ).
Femtosecond lasers can also result in incomplete treatments. Impediments to treatment or flap creation include corneal scars, loss of ring suction, irregular corneas, and low intraocular pressure (IOP).
Once a flap defect has occurred, the procedure should be immediately halted. If the exposed stromal bed is not large enough to allow adequate laser ablation, the flap should be repositioned and the laser procedure postponed. With irregular cuts, the surgeon should not proceed with the ablation, but the flap or fragments should be carefully replaced and realigned to their original position using the gutter width as a landmark. The pieces should fit together like a jigsaw puzzle. Additional waiting/drying time is scheduled, and an overnight bandage contact lens is placed. There is no consensus for a waiting period. Usually, 3 months is enough before reattempting a new procedure. A no-touch transepithelial PRK (or a phototherapeutic keratectomy [PTK] + PRK) has been our preferred technique.
If the created hinge is beyond the visual axis, some surgeons used to manually extend the dissection with a blade. This maneuver is not advised because of the risks of uneven bed creation and flap buttonhole formation. If there is sufficient stroma for laser ablation, the diameter of the available stroma should be measured to allow an adjustment in the optical/ablation zone and the flap should be protected from laser exposure. Placement of a surgical sponge over the flap may prevent inadvertent laser ablation on the hinge and flap ( Fig. 15.5 ). If the flap hinge is beyond the visual axis, laser ablation may be performed.
In regard to prevention, surgeons should ensure a properly functioning and clean device and careful ocular examination to identify a safe path for the microkeratome or femtosecond laser.
Decentered Flaps
If the flap is decentered and the area for ablation is adequate, the surgeon may proceed with laser treatment, possibly with a slight reduction in the optical zone. If this is not possible, the flap is repositioned and the operation repeated in 3 to 4 months with the creation of a new flap in a different depth or by using surface ablation, usually transepithelial PRK. This is particularly important in hyperopic or astigmatic treatments and when a large ablation zone is needed.
Free Cap ( )
A free flap or cap, an uncommon but well recognized complication, occurs when the flap hinge is inadvertently severed during the LASIK procedure. The reported incidence ranges from 0.01% to 1.0% in large sample studies. Common causes of microkeratome-created free flaps include small (typically < 11.5 mm) and flat corneas (typically <41 diopters [D]). It can also occur due to insufficient suction and after suction loss during the microkeratome pass. Other possible causes include incorrect adjustment of the stopper device in the microkeratome head and photoablation of the flap hinge in hyperopic treatments.
In femtosecond LASIK, free flaps may occur with unintended dissection during flap manipulation, as the flap sometimes requires more vigorous manipulation. Appropriate management of a free cap includes realignment and attachment. Fiduciary marks placed prior to flap manipulation usually allow proper alignment and orientation of the flap. However, when the markings are not placed or are effaced during irrigation, proper orientation may be difficult to ascertain. If a free cap is not visible on the surface of the cornea, the microkeratome head should be carefully inspected and, if need be, disassembled because the cap is probably inside the instrument ( Fig. 15.6 ). When the exposed stroma is of appropriate size and quality, the laser ablation proceeds as planned.
It is important to replace the cap with the epithelial side up and to position it properly. The inadvertent rotational positioning of the free flap may induce irregular astigmatism. Such complications have been treated by flap lift and rotation or wavefront or topography-guided customized PRK.
A bandage contact lens may be placed in order to protect the cap, especially if the epithelium has been damaged. Some surgeons prefer to avoid the use of a bandage contact lens if the epithelium is intact because the lens may dislocate the cap. Slit-lamp evaluation of the cap should be performed in this case.
The potential for flap loss is real. Wearing eye shields at night and special protective polycarbonate eyewear for sports activities is helpful. The patient should also be advised against eye rubbing because of the potential of cap dislocation.
Buttonhole ( )
Buttonholes (central perforation of the flap) occurs when the blade surfaces at the middle of the flap and then enters the cornea again to complete the pass. Buttonhole formation is one of the most serious intraoperative LASIK complications, with a prevalence ranging from 0.1% to 0.56%. Distance-corrected visual acuity has the worst outcome when compared with other intraoperative flap complications. Currently, buttonholes rarely occur with all types of modern microkeratomes.
Buttonholed flaps can provide a channel for epithelial cells to infiltrate the flap–stroma interface. There is also an increased risk of subepithelial scar formation ( Fig. 15.7 ). Steep corneas (> 46 D) have been compared with tennis balls that buckle centrally under applanating pressure, resulting in a central dimple missed by the blade, leading to a buttonhole ( Fig. 15.8 ). Blunted blades, poor oscillation, and microflaws of blades have also been described as mechanical microkeratome problems that may lead to buttonholes.
Buttonholes can also occur with the femtosecond laser, specifically during dissection of tissue bridges in the interface. These bridges can be seen after complications such as vertical gas breakthrough (described later) and after femtosecond laser calibration problems ( Fig. 15.9 ).
The therapeutic management of buttonholes is challenging. When a buttonholed flap is encountered, the safest way to proceed is to carefully clean the stromal bed and ensure good alignment between flap buttonhole and the uncut tissue to prevent epithelial ingrowth. Reposition the flap immediately and abort the procedure ( Fig. 15.10 ). Epithelial debris should be gently irrigated out with balanced salt solution (BSS) irrigating solution.
Most patients with buttonholes end up with no significant loss of vision after adequate healing has occurred, especially if uncomplicated by epithelial ingrowth. The most adopted method is waiting 3 to 4 months, followed by transepithelial PRK with adjunctive MMC ( Fig. 15.11 ).
Pizza Slicing
This complication may occur when a flap is cut in an eye that had radial keratotomy (RK) with the incisions extending beyond the 8- to 9-mm central area. Inadequate healing of the RK incisions causes a part of the flap to separate in a triangular shape. An epithelial plug in the incision almost always precipitates this complication. Refractive errors after RK (usually hyperopia) in most cases are better managed by surface ablation with MMC, usually topography guided.
Limbal Hemorrhage
Bleeding at the corneal limbus is a relatively common complication that may occur when the microkeratome blade or femtosecond laser passes over conjunctival or limbal vessels ( Fig. 15.12 ). The predisposing factors to intraoperative hemorrhages are large diameter flaps, hyperopia, or the use of suction rings that are inappropriately sized or improperly positioned. Eyes with a corneal pannus, such as can be found in chronic contact lens wearers or patients with ocular surface disease, are also more likely to experience hemorrhage during flap creation.
Bleeding can be minimized in the following ways:
- •
Leave the flap in position, apply a dry sponge to the bleeding area, and exert slight pressure for 1 to 2 minutes or until the bleeding has reduced significantly, allowing lifting of the flap and ablation. This is preferred in cases of hyperopia when the ablation is peripheral.
- •
The blood can be wicked away while simultaneously performing the ablation.
- •
A sponge soaked with 2.5% phenylephrine can be applied on the limbal vessels next to the bleeding area. Usually, this acts quickly, but the surgeon should be aware of the rapidity of concomitant segmental pupillary dilation beneath the area that may have a detrimental effect on eye-tracker and treatment centration.
Inadequate Exposure
Microkeratome and femtosecond laser head placement is more difficult in sunken globes and in eyes with narrow palpebral fissures and small corneas. By turning the patient’s head slightly to the opposite side or exerting a gentle pull and tilt on the eye through the suction ring handle, these cases can usually be operated on easily. Alternatively, it may be possible to operate without using a speculum. If a speculum is not used, one must ensure that the eyelashes and the eyelids do not overlap and cover the ring. The patient with a prominent brow should be positioned with the chin raised slightly, as this will maximize exposure. When all these measures are unsuccessful, conversion to photorefractive keratectomy (PRK) should be considered.
Suction Loss ( )
Inadequate suction or total loss of suction is a potential source of serious problems during LASIK. Factors that may contribute to lacking or loss of suction include improper technique in applying the suction ring, conjunctival chemosis, flat corneas, narrow palpebral fissures, deep-set eyes, patient movement, eye rotation, and head tilt. Blockage of a microkeratome head progression by the eyelid speculum is also an important cause of suction loss ( Fig. 15.1 ).
Suction loss with mechanical microkeratomes may result in more severe flap complications than with a femtosecond laser. Small, incomplete, or torn flap may result. The management of these cases depends on the available stromal bed for ablation. Usually, however, it is safer to reposition the flap, wait 3 months, on average, and retreat the patient with surface PRK.
Loss of vacuum by femtosecond laser is not as serious a problem as with the mechanical microkeratome. When using the femtosecond laser, the first sign of suction loss is often the appearance of a peripheral asymmetric and incomplete meniscus ( Fig. 15.2 ). Once detected, it is important to discontinue the laser treatment immediately. Loss of suction with the femtosecond laser is resolved by replacing the suction ring and re-docking the same applanation cone to subsequently repeat the treatment at the same depth.
The vertical limbal pocket, typically created to absorb cavitation bubbles, can be deactivated if it was already created in the first pass. If the loss of suction occurs during the side cut, the surgeon must ensure that the subsequent side cut is created within the lamellar cut used to fashion the flap by decreasing the subsequent side cut by 0.5 mm or more in diameter. Surface ablation with mitomycin C (MMC) can also be considered over the incomplete flap after repeated suction attempts prove unsuccessful.
Corneal Epithelial Defect
This uncommon LASIK complication has been shown to be lower with the use of a femtosecond laser than with a microkeratome because there is no microkeratome movement across the epithelium.
There are many precautions that should be taken in an attempt to decrease the risk of epithelial defects. The use of excessive topical medications, especially frequent anesthetics such as proparacaine, should be avoided before flap creation. Topical anesthetic placement should only occur immediately before initiating the procedure to decrease epithelial irregularity and toxicity.
One study recommended using hyperosmotic agents preoperatively in order to reduce the risk for epithelial disruptions.
In the event of detachment, the epithelium is repositioned if possible. A bandage contact lens is placed for 1 or 2 days or until the epithelium is completely healed ( Fig. 15.3 ).
Patients with a history of recurrent erosions and/or anterior basement membrane dystrophy (ABMD) are at higher risk of developing epithelial erosions with LASIK and would be better PRK candidates. Other risk factors for epithelial disturbances are increasing age, diabetes, dry eye, and a long history of contact lens wear.
When severely injured, the epithelium produces high amounts of cytokines, such as interleukin-1α, that stimulate keratocytes to produce chemokines that attract inflammatory cells, leading to diffuse lamellar keratitis (DLK). Epithelial defects have also been associated with postoperative complications, including epithelial ingrowth, recurrent corneal epithelial erosion, and flap melt.
Incomplete or Irregular Cut ( )
Method of flap creation and surgeon experience have both been shown to be the primary reasons for this disparity in complication rates. Of the different types of abnormal flaps, incomplete or short flaps are the most common. Incomplete flaps may be a result of the microkeratome prematurely stopping its course, which may be attributed to a pathway obstruction (e.g., blockage of microkeratome head progression by the speculum) or device malfunction. In addition, it may occur if suction is lost during the flap cut. Irregular cuts are more rare and may be related to defective blades and foreign bodies in the interface ( Fig. 15.4 ).
Femtosecond lasers can also result in incomplete treatments. Impediments to treatment or flap creation include corneal scars, loss of ring suction, irregular corneas, and low intraocular pressure (IOP).
Once a flap defect has occurred, the procedure should be immediately halted. If the exposed stromal bed is not large enough to allow adequate laser ablation, the flap should be repositioned and the laser procedure postponed. With irregular cuts, the surgeon should not proceed with the ablation, but the flap or fragments should be carefully replaced and realigned to their original position using the gutter width as a landmark. The pieces should fit together like a jigsaw puzzle. Additional waiting/drying time is scheduled, and an overnight bandage contact lens is placed. There is no consensus for a waiting period. Usually, 3 months is enough before reattempting a new procedure. A no-touch transepithelial PRK (or a phototherapeutic keratectomy [PTK] + PRK) has been our preferred technique.
If the created hinge is beyond the visual axis, some surgeons used to manually extend the dissection with a blade. This maneuver is not advised because of the risks of uneven bed creation and flap buttonhole formation. If there is sufficient stroma for laser ablation, the diameter of the available stroma should be measured to allow an adjustment in the optical/ablation zone and the flap should be protected from laser exposure. Placement of a surgical sponge over the flap may prevent inadvertent laser ablation on the hinge and flap ( Fig. 15.5 ). If the flap hinge is beyond the visual axis, laser ablation may be performed.
In regard to prevention, surgeons should ensure a properly functioning and clean device and careful ocular examination to identify a safe path for the microkeratome or femtosecond laser.
Decentered Flaps
If the flap is decentered and the area for ablation is adequate, the surgeon may proceed with laser treatment, possibly with a slight reduction in the optical zone. If this is not possible, the flap is repositioned and the operation repeated in 3 to 4 months with the creation of a new flap in a different depth or by using surface ablation, usually transepithelial PRK. This is particularly important in hyperopic or astigmatic treatments and when a large ablation zone is needed.
Free Cap ( )
A free flap or cap, an uncommon but well recognized complication, occurs when the flap hinge is inadvertently severed during the LASIK procedure. The reported incidence ranges from 0.01% to 1.0% in large sample studies. Common causes of microkeratome-created free flaps include small (typically < 11.5 mm) and flat corneas (typically <41 diopters [D]). It can also occur due to insufficient suction and after suction loss during the microkeratome pass. Other possible causes include incorrect adjustment of the stopper device in the microkeratome head and photoablation of the flap hinge in hyperopic treatments.
In femtosecond LASIK, free flaps may occur with unintended dissection during flap manipulation, as the flap sometimes requires more vigorous manipulation. Appropriate management of a free cap includes realignment and attachment. Fiduciary marks placed prior to flap manipulation usually allow proper alignment and orientation of the flap. However, when the markings are not placed or are effaced during irrigation, proper orientation may be difficult to ascertain. If a free cap is not visible on the surface of the cornea, the microkeratome head should be carefully inspected and, if need be, disassembled because the cap is probably inside the instrument ( Fig. 15.6 ). When the exposed stroma is of appropriate size and quality, the laser ablation proceeds as planned.
It is important to replace the cap with the epithelial side up and to position it properly. The inadvertent rotational positioning of the free flap may induce irregular astigmatism. Such complications have been treated by flap lift and rotation or wavefront or topography-guided customized PRK.
A bandage contact lens may be placed in order to protect the cap, especially if the epithelium has been damaged. Some surgeons prefer to avoid the use of a bandage contact lens if the epithelium is intact because the lens may dislocate the cap. Slit-lamp evaluation of the cap should be performed in this case.
The potential for flap loss is real. Wearing eye shields at night and special protective polycarbonate eyewear for sports activities is helpful. The patient should also be advised against eye rubbing because of the potential of cap dislocation.
Buttonhole ( )
Buttonholes (central perforation of the flap) occurs when the blade surfaces at the middle of the flap and then enters the cornea again to complete the pass. Buttonhole formation is one of the most serious intraoperative LASIK complications, with a prevalence ranging from 0.1% to 0.56%. Distance-corrected visual acuity has the worst outcome when compared with other intraoperative flap complications. Currently, buttonholes rarely occur with all types of modern microkeratomes.
Buttonholed flaps can provide a channel for epithelial cells to infiltrate the flap–stroma interface. There is also an increased risk of subepithelial scar formation ( Fig. 15.7 ). Steep corneas (> 46 D) have been compared with tennis balls that buckle centrally under applanating pressure, resulting in a central dimple missed by the blade, leading to a buttonhole ( Fig. 15.8 ). Blunted blades, poor oscillation, and microflaws of blades have also been described as mechanical microkeratome problems that may lead to buttonholes.
Buttonholes can also occur with the femtosecond laser, specifically during dissection of tissue bridges in the interface. These bridges can be seen after complications such as vertical gas breakthrough (described later) and after femtosecond laser calibration problems ( Fig. 15.9 ).
The therapeutic management of buttonholes is challenging. When a buttonholed flap is encountered, the safest way to proceed is to carefully clean the stromal bed and ensure good alignment between flap buttonhole and the uncut tissue to prevent epithelial ingrowth. Reposition the flap immediately and abort the procedure ( Fig. 15.10 ). Epithelial debris should be gently irrigated out with balanced salt solution (BSS) irrigating solution.
Most patients with buttonholes end up with no significant loss of vision after adequate healing has occurred, especially if uncomplicated by epithelial ingrowth. The most adopted method is waiting 3 to 4 months, followed by transepithelial PRK with adjunctive MMC ( Fig. 15.11 ).
Pizza Slicing
This complication may occur when a flap is cut in an eye that had radial keratotomy (RK) with the incisions extending beyond the 8- to 9-mm central area. Inadequate healing of the RK incisions causes a part of the flap to separate in a triangular shape. An epithelial plug in the incision almost always precipitates this complication. Refractive errors after RK (usually hyperopia) in most cases are better managed by surface ablation with MMC, usually topography guided.
Limbal Hemorrhage
Bleeding at the corneal limbus is a relatively common complication that may occur when the microkeratome blade or femtosecond laser passes over conjunctival or limbal vessels ( Fig. 15.12 ). The predisposing factors to intraoperative hemorrhages are large diameter flaps, hyperopia, or the use of suction rings that are inappropriately sized or improperly positioned. Eyes with a corneal pannus, such as can be found in chronic contact lens wearers or patients with ocular surface disease, are also more likely to experience hemorrhage during flap creation.
Bleeding can be minimized in the following ways:
- •
Leave the flap in position, apply a dry sponge to the bleeding area, and exert slight pressure for 1 to 2 minutes or until the bleeding has reduced significantly, allowing lifting of the flap and ablation. This is preferred in cases of hyperopia when the ablation is peripheral.
- •
The blood can be wicked away while simultaneously performing the ablation.
- •
A sponge soaked with 2.5% phenylephrine can be applied on the limbal vessels next to the bleeding area. Usually, this acts quickly, but the surgeon should be aware of the rapidity of concomitant segmental pupillary dilation beneath the area that may have a detrimental effect on eye-tracker and treatment centration.
Intraoperative Complications Specific to Femtosecond Laser LASIK
Vertical Gas Breakthrough
Cavitation bubbles created by the femtosecond (FS) laser can dissect upwards toward the epithelium and are called vertical bubble breaks . The bubbles may either stay below the Bowman membrane or break through the epithelium. When the bubbles stay under the Bowman layer, a focal thinning in the flap is noted. The gas breakthrough can mislead the surgeon into leaving a piece of stroma intact when creating the flap. As the flap is lifted, the intact stroma will tear, causing a hole. If the break is through the epithelium, this is considered a buttonhole. The bubble that escapes to the surface may prevent additional bubbles from forming, resulting in a focal area that cannot be separated during flap reflection.
Risk factors include previous RK surgery, corneal scars, microscopic breaks in the Bowman membrane, and thin flaps (programmed at 90 µm).
The morbidity associated with this complication varies and may range from minimal corneal damage to significant corneal tearing, requiring abortion of the LASIK procedure.
Vertical gas breakthrough (VGB) is an intraoperative complication that may not permit same-day flap creation because a second attempt will likely result in repeat breakthrough. Once VGB occurs, the surgeon should stop immediately and assess the situation. The vertical cut should not be performed to provide extra support that prevents flap mobility.
If a significant VGB is seen between the glass cone and the epithelium, then the surgeon must stop the procedure and not wait for the side cut to finish ( Fig. 15.13 ). Focal thinning in the flap may be lifted without complications. A true buttonholed flap should not be lifted because it can lead to scarring or epithelial ingrowth. If the side cut is completed, then it is recommended that the flap not be lifted and that the surgeon treat the patient several months later either with PRK with MMC, mechanical microkeratome, or cut with the FS laser at least 40 µm deeper than the original flap intended depth. It will also be prudent to save the cone used and return to the manufacturer as well as have the FS laser system serviced to check the z-calibration. This comprises a complete investigation of the probable cause of the incident.
Anterior Chamber Bubbles ( )
Presence of gas bubbles in the anterior chamber (AC) is another complication specific to the FS laser, with an incidence of 1% ( Fig. 15.14 ).
Postulated risk factors for the development of AC bubbles include the use of large-diameter flaps and the treatment of smaller-diameter corneas. Both of these factors lead to FS dissection and gas formation closer to the limbus, thus facilitating migration of bubbles to the trabecular meshwork via the Schlemm canal.
The bubbles accumulate centrally over the pupil and, although self-limiting, they can interfere with pupillary tracking. In this situation, the surgeon may delay excimer laser application for a few hours or longer until the bubbles are reabsorbed or perform ablation with manual centration.
Opaque Bubble Layer
An opaque bubble layer (OBL) is produced by gas bubbles that accumulate in the superficial layers of the stromal bed during FS laser flap creation, producing a diffuse tissue opacity ( Fig. 15.15 ). The buildup of trapped microplasma gas bubbles that are unable to vent through the pocket, under high vacuum and corneal compression, travel in errant directions and push apart collagen fibrils to infiltrate the stroma. It was most commonly seen on earlier FS laser platforms, when higher energies and lower frequencies were used. Excessive OBL may interfere with flap creation and separation and with excimer laser tracking systems, which can delay the surgical procedure. Identified risk factors include thick corneas hard-docking technique, steep corneal curvature, and small flap diameter. When the OBL is not intense, the usual approach is to ignore it and immediately proceed with excimer laser ablation. Another option is to wait (typically 30–45 minutes) for the OBL to spontaneously dissipate within the stroma prior to lifting the flap and applying excimer laser ablation. Two distinct types of OBL were already reported: the hard OBLs that look denser and the soft ones that are more transparent.
Early or Hard Opaque Bubble Layer
This occurs when bubbles spread into the stromal tissue anterior and posterior to the plane at which the laser pulses are applied. Early or hard OBL can block subsequent pulses and lead to uncut or poorly cut tissue, making flap lifts more difficult and increasing the risk for flap tears.
Late or Soft Opaque Bubble Layer
The produced gases can also travel into the intralamellar spaces after laser dissection has passed through an area of the stroma. The main cause of this type of OBL is poor separation of the corneal tissue, which appears more transparent and patchy. Again, lifts can be more difficult with late OBL. Also, eye trackers and iris recognition may be temporarily impaired with thick and central OBL.
Vertical Gas Breakthrough
Cavitation bubbles created by the femtosecond (FS) laser can dissect upwards toward the epithelium and are called vertical bubble breaks . The bubbles may either stay below the Bowman membrane or break through the epithelium. When the bubbles stay under the Bowman layer, a focal thinning in the flap is noted. The gas breakthrough can mislead the surgeon into leaving a piece of stroma intact when creating the flap. As the flap is lifted, the intact stroma will tear, causing a hole. If the break is through the epithelium, this is considered a buttonhole. The bubble that escapes to the surface may prevent additional bubbles from forming, resulting in a focal area that cannot be separated during flap reflection.
Risk factors include previous RK surgery, corneal scars, microscopic breaks in the Bowman membrane, and thin flaps (programmed at 90 µm).
The morbidity associated with this complication varies and may range from minimal corneal damage to significant corneal tearing, requiring abortion of the LASIK procedure.
Vertical gas breakthrough (VGB) is an intraoperative complication that may not permit same-day flap creation because a second attempt will likely result in repeat breakthrough. Once VGB occurs, the surgeon should stop immediately and assess the situation. The vertical cut should not be performed to provide extra support that prevents flap mobility.
If a significant VGB is seen between the glass cone and the epithelium, then the surgeon must stop the procedure and not wait for the side cut to finish ( Fig. 15.13 ). Focal thinning in the flap may be lifted without complications. A true buttonholed flap should not be lifted because it can lead to scarring or epithelial ingrowth. If the side cut is completed, then it is recommended that the flap not be lifted and that the surgeon treat the patient several months later either with PRK with MMC, mechanical microkeratome, or cut with the FS laser at least 40 µm deeper than the original flap intended depth. It will also be prudent to save the cone used and return to the manufacturer as well as have the FS laser system serviced to check the z-calibration. This comprises a complete investigation of the probable cause of the incident.
Anterior Chamber Bubbles ( )
Presence of gas bubbles in the anterior chamber (AC) is another complication specific to the FS laser, with an incidence of 1% ( Fig. 15.14 ).
Postulated risk factors for the development of AC bubbles include the use of large-diameter flaps and the treatment of smaller-diameter corneas. Both of these factors lead to FS dissection and gas formation closer to the limbus, thus facilitating migration of bubbles to the trabecular meshwork via the Schlemm canal.
The bubbles accumulate centrally over the pupil and, although self-limiting, they can interfere with pupillary tracking. In this situation, the surgeon may delay excimer laser application for a few hours or longer until the bubbles are reabsorbed or perform ablation with manual centration.
Opaque Bubble Layer
An opaque bubble layer (OBL) is produced by gas bubbles that accumulate in the superficial layers of the stromal bed during FS laser flap creation, producing a diffuse tissue opacity ( Fig. 15.15 ). The buildup of trapped microplasma gas bubbles that are unable to vent through the pocket, under high vacuum and corneal compression, travel in errant directions and push apart collagen fibrils to infiltrate the stroma. It was most commonly seen on earlier FS laser platforms, when higher energies and lower frequencies were used. Excessive OBL may interfere with flap creation and separation and with excimer laser tracking systems, which can delay the surgical procedure. Identified risk factors include thick corneas hard-docking technique, steep corneal curvature, and small flap diameter. When the OBL is not intense, the usual approach is to ignore it and immediately proceed with excimer laser ablation. Another option is to wait (typically 30–45 minutes) for the OBL to spontaneously dissipate within the stroma prior to lifting the flap and applying excimer laser ablation. Two distinct types of OBL were already reported: the hard OBLs that look denser and the soft ones that are more transparent.
Early or Hard Opaque Bubble Layer
This occurs when bubbles spread into the stromal tissue anterior and posterior to the plane at which the laser pulses are applied. Early or hard OBL can block subsequent pulses and lead to uncut or poorly cut tissue, making flap lifts more difficult and increasing the risk for flap tears.
Late or Soft Opaque Bubble Layer
The produced gases can also travel into the intralamellar spaces after laser dissection has passed through an area of the stroma. The main cause of this type of OBL is poor separation of the corneal tissue, which appears more transparent and patchy. Again, lifts can be more difficult with late OBL. Also, eye trackers and iris recognition may be temporarily impaired with thick and central OBL.
Early or Hard Opaque Bubble Layer
This occurs when bubbles spread into the stromal tissue anterior and posterior to the plane at which the laser pulses are applied. Early or hard OBL can block subsequent pulses and lead to uncut or poorly cut tissue, making flap lifts more difficult and increasing the risk for flap tears.
Late or Soft Opaque Bubble Layer
The produced gases can also travel into the intralamellar spaces after laser dissection has passed through an area of the stroma. The main cause of this type of OBL is poor separation of the corneal tissue, which appears more transparent and patchy. Again, lifts can be more difficult with late OBL. Also, eye trackers and iris recognition may be temporarily impaired with thick and central OBL.
General Photoablation-Related Complications
Decentration
Accurate centration during the excimer laser procedure is critical in optimizing visual results. Centration is even more crucial for hyperopic than myopic treatments. A decentered ablation zone may go unrecognized during surgery and result in irregular astigmatism.
A decentered ablation may occur if the patient’s eye slowly begins to drift and loses fixation or if the surgeon initially positions the patient’s head improperly; if the patient’s eye is not perpendicular to the laser treatment, parallax can result. This decentration may cause visual symptoms such as halo, glare, and monocular diplopia as well as a decrease in distance-corrected visual acuity.
Decentration can be characterized as mild (0–0.5 mm), moderate (0.5–1.0 mm), or severe (> 1 mm). With modern excimer lasers that incorporate ultrafast real-time tracking systems, this complication is very rare. These systems detect changes in fixation and respond by moving the laser beam to the new location. When fixation changes excessively, the system stops. Decentration may be reduced by ensuring that the patient’s head remains in the correct plane throughout the treatment—that is, perpendicular to the laser (parallel to the ground)—and that there is no head tilt.
Once decentration has occurred, it may be difficult to treat. The most adopted method is topography-guided ablation, with variable results ( Fig. 15.16 ).
Overcorrection and Undercorrection
Overcorrection may occur if substantial stromal dehydration develops before the laser treatment is initiated because more stromal tissue will be ablated per pulse. A long delay before beginning ablation after lifting the flap in LASIK allows for excessive dehydration of the stroma and increases the risk of overcorrection. Controlling the humidity and temperature in the laser suite within the recommended guidelines should standardize the surgery and ideally improve refractive outcomes. Overcorrection tends to occur more often in older individuals with moderate to high myopia. A laser retreatment is usually performed after 3 to 6 months in LASIK-treated eyes and after 1 year in PRK.
Undercorrection is also a frequent complication of primary LASIK. It is usually diagnosed in the first few weeks postoperatively, and the refractive error stabilizes early thereafter. Undercorrection after LASIK typically requires flap lift and laser treatment of the residual refractive error after the refraction has remained stable for at least 3 months.
Undercorrection and overcorrection are related to the ablation algorithm, no accurate nomograms, age, and the amount of myopia, astigmatism, or hyperopia to be corrected.
Decentration
Accurate centration during the excimer laser procedure is critical in optimizing visual results. Centration is even more crucial for hyperopic than myopic treatments. A decentered ablation zone may go unrecognized during surgery and result in irregular astigmatism.
A decentered ablation may occur if the patient’s eye slowly begins to drift and loses fixation or if the surgeon initially positions the patient’s head improperly; if the patient’s eye is not perpendicular to the laser treatment, parallax can result. This decentration may cause visual symptoms such as halo, glare, and monocular diplopia as well as a decrease in distance-corrected visual acuity.
Decentration can be characterized as mild (0–0.5 mm), moderate (0.5–1.0 mm), or severe (> 1 mm). With modern excimer lasers that incorporate ultrafast real-time tracking systems, this complication is very rare. These systems detect changes in fixation and respond by moving the laser beam to the new location. When fixation changes excessively, the system stops. Decentration may be reduced by ensuring that the patient’s head remains in the correct plane throughout the treatment—that is, perpendicular to the laser (parallel to the ground)—and that there is no head tilt.
Once decentration has occurred, it may be difficult to treat. The most adopted method is topography-guided ablation, with variable results ( Fig. 15.16 ).
Overcorrection and Undercorrection
Overcorrection may occur if substantial stromal dehydration develops before the laser treatment is initiated because more stromal tissue will be ablated per pulse. A long delay before beginning ablation after lifting the flap in LASIK allows for excessive dehydration of the stroma and increases the risk of overcorrection. Controlling the humidity and temperature in the laser suite within the recommended guidelines should standardize the surgery and ideally improve refractive outcomes. Overcorrection tends to occur more often in older individuals with moderate to high myopia. A laser retreatment is usually performed after 3 to 6 months in LASIK-treated eyes and after 1 year in PRK.
Undercorrection is also a frequent complication of primary LASIK. It is usually diagnosed in the first few weeks postoperatively, and the refractive error stabilizes early thereafter. Undercorrection after LASIK typically requires flap lift and laser treatment of the residual refractive error after the refraction has remained stable for at least 3 months.
Undercorrection and overcorrection are related to the ablation algorithm, no accurate nomograms, age, and the amount of myopia, astigmatism, or hyperopia to be corrected.
Early Postoperative Complications
Interface Debris
Interface debris may arise from conjunctival or skin epithelial cells swept onto the interface by excessive irrigation or excessive tearing. The debris can also be caused by meibomian secretions, powder from the gloves or swabs used to clean the interface, metal fragments from the microkeratome blade, mucus from the ocular surface, or blood from cut pannus ( Fig. 15.17 ). It is important to distinguish debris from an inflammatory or infectious reaction. Whereas most substances are biodegradable and cause no lasting harm to the patient, metallic or plastic material in the interface may induce an inflammatory reaction consistent with a corneal foreign body and ultimately has the ability to produce permanent corneal scarring. DLK is another possible postoperative sequela that has been observed with retention of interface debris.
Keeping the flap environment free of debris is challenging. Several methods can be employed to minimize lint and debris that may contribute to interface debris. Scrub suits for the surgeon, cover over patients’ clothes, use of polyvinyl alcohol sponge (PVA) or cellulose sponge and powder-free gloves are all useful measures. Care should be taken to carefully clean thoroughly around the skin, eyelids, and eyelashes. In general, the operative room should be examined, cleaned, and steps taken to ensure that the room air is purified, filtered, and circulating horizontally.
If the debris is outside the visual axis and not visually significant, it can be left intact. However, debris observed in the visual axis should be removed if deemed visually significant. If it is noted during or immediately after the procedure, flap elevation and repositioning after irrigation are helpful, and perhaps both surfaces should be wiped with a moist LASIK sponge. If the debris is thought to be contributing to a significant inflammatory reaction, it is best to lift the flap and irrigate copiously. Generous irrigation is the most effective approach to prevent debris in the flap interface. An examination at the slit lamp afterwards to ensure interface clarity should be done as well.
Flap Displacement and Flap Folds ( )
Flap displacement occurs most commonly in the first 24 hours after LASIK, before the epithelium has had time to heal over the lamellar entry site ( Fig. 15.18 ). In contrast with traumatic dislocation, which can be caused by an injury many years after LASIK, displacements occurring early on in the postoperative course usually have no obvious precipitating event. It may also follow lid action, eyelid rubbing, or squeezing. Usual symptoms include acute pain and decreased vision. Studies have shown a significant lower incidence of flap displacements in LASIK flaps created with FS lasers when compared to mechanical microkeratome, probably due to better flap stability associated with the angulation of the side cut, resulting in increased flap adhesion strength.
Some risk factors have been related to the presence of folds in the flap, including initial intraoperative misalignment, deep ablations with flap–bed mismatch, persistent interface fluid, excessive flap dehydration, trauma while removing the eyelid speculum, eye rubbing, and early alteration of lubrication, which can cause adherence of the flap to the tarsal conjunctiva. Larger diameter and thinner flaps may be more prone to be displaced, especially if the hinge is small.
A dislodged flap is an emergency. It should be repositioned as soon as possible to prevent fixed folds and epithelial ingrowth. Failure to act promptly increases the likelihood of permanent fold formation with decreased visual quality ( Fig. 15.19 ). The flap should first be reflected and the interface (stromal bed and stromal aspect of the flap) carefully examined for epithelial cells or other debris. They should be scraped prior to repositioning the flap. Additional time should be taken in smoothing and drying the flap. A contact lens can be applied to provide added protection from further displacement. Recalcitrant folds may require removal of the central epithelium as it may prevent flattening of the folds owing to epithelial hyperplasia in the crevices formed by the folds.
An eye shield may be suggested for an extended period of time. The surgeon should be aware that striae initially remain visible but disappear over 24 to 48 hours if the flap has been fully distended.
Flap Striae and Folds ( )
Striae and folds on the flap can lead to symptoms such as halos, diplopia, glare, and starbursts. Folds, often called macrostriae , represent full-thickness, undulating stromal folds and occur because of initial flap malposition or postoperative flap slippage. Folds can resemble fingerprint lines on biomicroscopy (see Fig. 15.19 ). Early intervention is often crucial as they induce irregular astigmatism with optical aberrations and loss of best-corrected visual acuity (BCVA), especially if they involve the visual axis.
Striae or microstriae are fine, hair-like optical irregularities that are best viewed on red reflex illumination or by light reflected off the iris, often have normal fluorescein patterns, and do not interfere with BCVA ( Fig. 15.20 ). They result from the adaptation of the original posterior curvature of the flap to the modified curvature of the ablated stroma. Microstriae are more likely to be seen in patients undergoing higher myopic LASIK treatments. This is due to the reduced central convexity and stromal support, resulting in flap redundancy that may be quite difficult to flatten. The latter is referred to as the tenting effect. Monitoring is usually the treatment of choice.
