Out of a population of 4,000 eyes treated during the study period, there were 20 cases of suction loss (0.50%). The types of eye movement that were identified are described below. Table 1 shows the breakdown of suction loss according to the type of eye movement.
Eye Movement Classification
Fixation Light Tracking. Once docking is achieved, the patient is instructed not to follow the green light if it moves. However, if the fixation light is slightly displaced on the retina, the patient may perceive it to be off to one side, causing the eye to turn if he or she tries to focus on it. This acts to shift the light further and ends up in a negative feedback loop, hence moving the eye under suction and potentially causing suction loss (Video 1, available in the online version of this article). This is usually related to anxiety-mediated lack of cooperation where the patient is not focused on the surgeon's instructions.
Bell's Reflex. There is an involuntary reflex upward movement of the eye to “protect the cornea” by the patient attempting to close the eye (Video 2, available in the online version of this article). As with fixation light tracking, this usually occurs in highly anxious patients not listening closely to the surgeon.
Saccades. If there is a sudden movement of the eye in one direction, this can be strong enough to slightly move the cornea without breaking suction. These movements are often not large enough to create a visually significant discontinuity in the femtosecond laser cut interface, but are visible through the microscope view (Video 3, available in the online version of this article). However, a large saccade can cause a lateral shift large enough to create a discontinuity in the femtosecond laser interface, which may lead to an irregular interface (Figure 1).
Oscillations. Adrenaline-driven tremor can cause small involuntary eye movements in some extremely anxious patients. In some cases, this can be strong enough to make small oscillatory movements of the cornea under suction due to cardiac balloting of the upper body and head (Video 4, available in the online version of this article).
Anxiety/Uncooperative Patient. Anxious patients may not be able to listen attentively to the surgeon or manage to keep their muscles relaxed and flaccid. Involuntary muscle tension can lead to triggering of the natural protective mechanisms, including pulling or turning the head away or squeezing the eyelids, which can cause the eye to move (Video 5, available in the online version of this article). In extreme cases, such eye movement can break suction by forcing the speculum onto the treatment cone.
Nociceptive Reflex Movement. Although uncommon, there can be a sharp pain sensation during photodisruption despite topical anesthetic (Video 6, available in the online version of this article).
False Suction. Although the suction ports are applied onto the cornea itself, in almost all cases the vacuum ports extend partially onto the conjunctiva in some areas. In extremely rare cases, this can result in false suction, so the eye is not actually immobilized, leading to eye movements and a cascade of conjunctival intrusion and loss of suction (Video 7, available in the online version of this article).
Nystagmus. Patients with nystagmus can be treated by SMILE without consequence because the immobilization provided by the suction nullifies the nystagmus; involuntary eye movements associated with nystagmus are not strong enough to overcome the suction force and shift the cornea. The accurvation of the cornea into the contact glass produces enough force and friction to neutralize the oscillations (Video 8, available in the online version of this article).
Suction Stability Management Protocol
The full suction stability management protocol is presented as a decision tree in Figure 2. The reasoning behind the protocol is described below.
Type I Eye Movements. As indicated in the top section of the flow chart, if there are small eye movements, regardless of when they occur during the femtosecond laser cutting, the treatment can be continued without stopping. If small movements persist, the surgeon may decide to pause to verbally regain control of the patient before continuing. We refer to this as pause, regain control, continue (P/R/C) (Videos 1 and 3).
Before Starting the Femtosecond Laser Cutting. For type II, if an eye movement occurs before the femtosecond laser cutting has started, the surgeon can delay initiating laser cutting to evaluate whether centration has been compromised. If centration is deemed to still be suitable and suction appears stable, then treatment can be continued. Otherwise, the surgeon can release suction and restart the docking process.
For type III, if a patient-generated suction loss occurs before the femtosecond laser cutting has started, there is no risk of creating tissue slivers and the procedure can be safely restarted without affecting the final outcome. If there are residual meibomian oil droplets or debris from the tear film on the contact glass, it may be worth replacing the contact glass to avoid the formation of black spots.9,10
During the Lenticule Interface (First 10%). For type II (well centered), if there is a large eye movement, the first aspect to consider is whether the eye is still centered in the contact glass. It is possible for the eye to move back and forth and end up approximately where it started. P/R/C may be used to regain control of the patient before continuing. When the femtosecond laser cutting restarts, the bubble pattern will be regular and continuous if the centration was close enough to the original position. If not, there will be a visible discontinuity in the bubble pattern, indicating that the centration had been altered, in which case the surgeon should initiate a suction loss. Because more than 10% of the lenticule interface would then have been created, the restart treatment module will load the option to convert to thin flap LASIK. Alternatively, if the original cap thickness was 135 μm or greater, a new SMILE treatment can be planned using a thinner cap. Performing SMILE at least 20 to 25 μm more superficially will avoid the new lenticule interface crossing the aborted interface.
For type II (shift in centration), if the position of the cornea has shifted, then the surgeon should release suction to avoid a decentered treatment. This will initiate the restart treatment module, which will offer the option to restart SMILE with the same settings.
For type III, if there is a patient-generated suction loss, the restart treatment module will be loaded with the recommended option to restart SMILE with the same settings.
For type III (tracked interface), if a patient-generated suction loss occurs but the femtosecond laser cutting was continued leading to the interface tracking upward through the cornea (Figure 3, Video 6), the default option provided by the restart treatment module to restart SMILE with the same settings should be ignored in preference for converting to thin-flap LASIK.
During the Lenticule Interface (10% to 100%). For type II, the management in this scenario is exactly the same as that described for 10% of the lenticule interface: pause the femtosecond laser cutting, regain control, consider the centration, and check for any discontinuity. The surgeon should initiate a suction loss if either the centration or femtosecond laser bubble pattern is unsatisfactory (Video 5). The options are then to restart SMILE with a thinner cap (if the original cap thickness was 135 μm or greater) or use the restart treatment module to convert to thin-flap LASIK. If there is no discontinuity, it might be worth obtaining an optical coherence tomography (OCT) scan to review the geometry of the interfaces before proceeding to extract the lenticule, particularly if the eye movement occurred when the bubble pattern creation was nearing the visual axis. However, micro-irregularities in the stromal surface will be at least partially, and sometimes completely, masked by epithelial remodeling.11–15
For type III, if there is a patient-generated suction loss, the restart treatment module will be loaded with the recommended option to convert to LASIK with the flap thickness set to the programmed cap thickness (Video 7). However, it is usually preferable to reduce the flap thickness to maximize the residual stromal thickness. If the cap thickness was 135 μm or greater, the alternative option is to restart SMILE with a thinner cap (Figure A, available in the online version of this article).
For type III (tracked interface), if a patient-generated suction loss occurs but the femtosecond laser cutting was continued, leading to the interface tracking upward through the cornea, this would usually rule out the possibility of restarting SMILE with a thinner cap. Therefore, the restart treatment module should be used to convert to LASIK, again after adjusting the flap thickness to the preferred thickness. An OCT scan is recommended to verify the extent of the interface deviation away from the intended plane.
During the Lenticule Side Cut. For type II, the treatment can be continued and completed even if an eye movement is large enough to induce a misalignment between the lenticule interface and the lenticule side cut. The location of the lenticule side cut will not affect the refractive power of the lenticule and the gap can usually be punched through manually by the surgical instrument. The cap interface will also consequently be slightly decentered. This, in principle, will not affect the lenticule geometry because the cap interface is planar to the surface. P/R/C may also be used to regain control of the patient before continuing.
For type III, if a patient-generated suction loss occurs, the restart treatment module should be used to restart SMILE from the lenticule side cut.
During the Cap Interface. For type II, because the cap interface is programmed to be parallel to the surface, it is somewhat tolerant to a shift in position, so the treatment can be continued after P/R/C. If there is a visible discontinuity in the bubble pattern, an OCT scan would be useful to confirm that the interface is regular before extracting the lenticule. If the discontinuity is deemed large enough, a surgeon-initiated suction loss can be considered to bring up the restart treatment option of restarting the cap interface.
For type III, if a patient-generated suction loss occurs, the restart treatment module can be used to restart SMILE from the cap interface using the same thickness.
For type III (outside lenticule diameter), if a patient-generated suction loss occurs but the cap interface progress has passed the lenticule diameter, there is no need to recreate the whole cap interface as recommended by the restart treatment module. One option is to create only a small incision. Alternatively, an incision might be created manually using a diamond knife.
For type III (tracked interface), if a patient-generated suction loss occurs but the femtosecond laser cutting was continued, leading to the interface tracking upward through the cornea, continuing with SMILE, as indicated by the restart treatment module, may not be appropriate due to the risk of tissue slivers, the creation of false planes, and an irregular lenticule. Instead, the restart treatment module can be aborted to convert the procedure to thin-flap LASIK after waiting for the bubbles to dissipate. An OCT scan can be used to verify the extent of the interface deviation away from the intended plane.
During the Small Incision. For type II, there is no real need to stop the femtosecond laser cutting if there is an eye movement during the small incision because it will always be possible to manually bridge any gap.
For type III, the restart treatment module can be used to restart only the small incision. The surgeon may choose to slightly reduce the cap diameter and incision depth to ensure that the incision intersects the cap interface. If more than 50% of the incision has been created, then it is possible to manually complete the incision by blunt dissection or using a surgical knife.