Central Islands After Laser Refractive Surgery
A central island (also called steep central island or SCI) is a complication of excimer laser refractive surgery, in which the laser fails to uniformly reshape a small portion of the cornea. The non-uniform corneal tissue can result in a small, raised area in the center of the cornea with a higher refractive power than the surrounding tissue. This causes light to bend erratically and can lead to reduced visual acuity and other visual abnormalities.
A central island is a type of irregular astigmatism that occurs after laser refractive surgery. It is generally defined as a central area of steeper corneal tissue having increased refractive power, as seen on topography, which is surrounded by a flattened corneal region with reduced refractive power,,,.
In refractive surgery, uniform ablation is necessary to achieve clear, undistorted vision. When topographical irregularities of the cornea are created, the refractive power and resulting irregular astigmatism of the cornea results in multifocality of the cornea and impaired vision.
Symptoms of a central island in post-refractive surgery patients are variable, but are reported to include the following: monocular diplopia, ghost imaging, halos, starbursts, night-driving disability, reduced best-corrected visual acuity, and loss of contrast sensitivity,,,.
These symptoms are most noticeable in the immediate postoperative period. Evidence shows that many central island defects resolve naturally, especially those that occur after PRK,,. Central islands after LASIK are slower to resolve, and have a higher residual incidence at one year. Additionally, though many central islands resolve over time, they are associated with slow visual rehabilitation, and therefore should be avoided.
With current laser technology, as well as preoperative prevention measures, central islands are now a rare complication of refractive surgery; however patients treated with early generation laser systems may still need corrective treatment.
Incidence varies, and is dependent on the type of refractive surgery, defining criteria, post-procedure elapsed time, type of excimer laser used, ablation zone diameter, and videokeratography device,,,,.
PRK vs LASIK
PRK has a higher reported incidence of central islands than does LASIK. The reported incidence of central islands after PRK is between 0% - 84%, depending on the defining criteria and elapsed postoperative evaluation time. The reported incidence of central islands after LASIK is between 0% - 24%, depending on the same variables.
Incidence of central island formation is much higher with broad-based laser systems versus scanning-slit and flying-spot systems.
Of the broad-base lasers, the VisX laser has a higher reported incidence than the Summit laser. This is thought to be due to the Gaussian energy profile of the Summit system, which concentrates more energy at the center of the beam versus the VisX laser, which has a flatter beam profile,,.
The smaller the defining criteria, the higher the reported incidence of central islands. In studies where 1.0 mm diameter was used as the defining criteria, incidence was significantly higher than in studies were 3.0mm was used. The same trend applies for dioptric power.
Central island defects, especially those occurring after PRK procedures, have a tendency to spontaneously resolve over time. Therefore, studies with an extended evaluation window report a lower incidence of central islands than those studies with a brief postoperative assessment,.
Precise etiology is unknown and is likely multifactorial. A number of theories have been put forward to explain the development of central islands after refractive surgery.
Large Ablation Zones
Larger ablation zones are correlated with steeper central island height.
Corneal Tissue Hydration
Well hydrated corneal tissue absorbs less of the laser beam’s energy and thus ablates relatively slower; whereas drier corneal tissue absorbs more energy and ablates faster. Corneal tissue tends to be more hydrated centrally and drier peripherally. This can lead to differences in topography post-operatively,.
Type of Laser
Central island formation is almost exclusively linked to broad-beam laser treatment, and is rare with scanning-slit or flying-spot laser systems,. This may be because a broad-beam laser can lead to fluid accumulation centrally by inducing an acoustic shock wave.
Studies suggest that ablated products can absorb the laser beam’s energy, creating a non-homogeneous energy beam,,,.It has also been observed that the ablative products form a ring vortex, allowing for particles to be redeposited on the cornea, leading to central island formation,,.
Inherent Laser Defects or Degraded Optics
A number of preoperative precautions and interventions have been shown to reduce the incidence of central island formation.
Blowing Nitrogen Gas
Blowing nitrogen gas has consistently proven to be very effective at reducing the incidence of central islands,,,. Blowing nitrogen gas dries the cornea and creates a more uniform surface; it may also remove ablation byproducts. There is conflicting data regarding whether nitrogen gas increased occurrence of post-operative corneal haze,,,.
Blowing aerosol also reduces the formation of central islands, although not to the extent of blowing nitrogen gas. One study showed incidence of central island formation using aerosol is roughly equal to incidence when using anti-central island software. Aerosol does not dry the corneal surface, but it is thought to create a uniformly hydrated work surface,.
One study found cleaning the cornea of fluid every 35 pulses reduced the height of central islands, although the effect of this reduction was not clinically significant.
Frequent Replacement of Optics
Regular replacement of optics helps insure a consistent, non-degraded energy beam.
Central Island Prevention Software
These programs attempt to compensate for the inhomogeneous quality of ablation around the central cornea. The prevention programs are effective in reducing, but not eliminating central islands,.
In patients with visual complaints after PRK or LASIK, central islands can easily be identified with topography, however it is important to note that topographical islands in patients without visual complaints should not be treated.
After PRK, many central islands resolve by one month, and the vast majority resolve within one year,. McGhee et al found a 90% resolution at six months and 100% resolution at one year. Central islands after LASIK are less likely to spontaneous resolve than those that follow PRK, several studies have found that post-LASIK central islands persist for more than 6 months,,.
Conservative management is recommended in the initial postoperative months. Central islands can be monitored regularly with topography to determine if they are resolving or not. Visual complaints during this time can be corrected with glasses or contact lenses. Persistent central islands may need surgical intervention.
Re-treatment using phototheraputic keratectomy (PTK) is recommended in patients with visually disruptive central islands persisting more than six months. Prior to retreatment, central islands should be regularly monitored to ensure stability.
Correction of the central island should be guided by topographical data,,. Some debate exists about whether using Munnerlyn’s formula to calculate the height of the island is appropriate, or if more precise method for calculating height should be used. Wavefront sensing devices, although effective in other forms of astigmatism, have so far proven difficult to use in the treatment of central islands. This may be due to the significant change in curvature across a small area, which does not allow for accurate data acquisition.
As yet, no consensus has been reached regarding the best ablation algorithm to treat central islands.
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