Most commonly associated with broad-beam lasers, central islands may cause serious visual disturbances to the patient. Their etiology appears to be multi-factorial, and several theories have attempted to explain their origin. The most commonly accepted theories are the acoustic wave theory of Jeff Machat, MD, the differential hydration theory of David Lin, MD, and the vortex plume theory. The central stroma is more hydrated than the peripheral stroma. During the ablation process, moisture diffuses through the cornea from the anterior chamber and accumulates centrally in the stromal bed.
Moisture absorbs excimer laser energy and results in less corneal ablation.
Therefore, if the laser is applied evenly across the cornea, less ablation will occur centrally because of the moisture, resulting in a central island. Attentiveness to fluid management during the ablation process and central flap compression are surgical techniques that help prevent or reduce central island formation. Also, some laser software programs incorporate a central pretreatment ablation to reduce the incidence of central islands. More fluid accumulates in cases of higher refractive error because of the time and depth of ablation. Pressure is applied centrally as the instrument is rotated in a circular manner.