CRT inverts the pattern of peripheral refraction in spherical equivalent refraction, creating a treatment area of myopic reduction within the central 25 degrees of visual field, and a myopic shift beyond the 25 degrees. In peripheral refraction for 30 degrees and 35 degrees, the amount of myopia induced in terms of spherical equivalent has an almost 1:1 relationship with the amount of baseline spherical equivalent refraction to be corrected.

BackgroundTo characterize the axial and off-axis refraction across four meridians of the retina in myopic eyes before and after Orthokeratology (OK) and LASIK surgery.MethodsSixty right eyes with a spherical equivalent (M) between − 0.75 to − 5.25 D (cylinder <− 1.00 D) underwent LASIK (n = 26) or OK (n = 34) to treat myopia. Axial and off-axis refraction were measured with an open-field autorefractometer before and after stabilized treatments. Off-axis measurements were obtained for the horizontal (35° nasal and temporal retina) and vertical (15° superior and inferior retina) meridians, and for two oblique directions (45–225° and 135–315°) up to 20° of eccentricity. The refractive profile was addressed as relative peripheral refractive error (RPRE).ResultsOK and LASIK post-treatment results showed an increase of myopic relative refraction at several eccentric locations. At the four meridians evaluated, the M component of the pre-treatment RPRE values was not statistically different (p > 0.05) from the post-treatment RPRE within 30° and 20° of the central visual field after LASIK and OK, respectively. These results demonstrated that the treatment zone warrants an optimal central field of vision.ConclusionsThe present study gives an overview of RPRE after refractive corneal reshaping treatments (OK and LASIK) across vertical, horizontal and two oblique meridians together. This allows a 3D representation of RPRE at the retina and shows that the myopic shift induced by both treatments is more relevant in horizontal directions.

PURPOSE.To evaluate the changes in spontaneous and stimulusevoked nerve impulse activity of corneal polymodal and mechanonociceptor sensory fibers of the cornea after photorefractive keratectomy (PRK). METHODS.A central corneal ablation 6 mm in diameter and 70 m in depth was performed with an excimer laser in both eyes of three anesthetized cats, after removal of the corneal epithelium. Single nerve fiber activity was recorded in these animals 12 to 48 hours after surgery. Activity in corneal nerve fibers with receptive fields (RFs) within and/or close to the wound, as well as with RFs far from the lesioned area, was studied. Incidence and frequency of spontaneous discharges and nerve impulse firing responses to mechanical (Cochet-Bonet esthesiometer) and chemical (CO 2 gas pulses) stimuli were studied. RESULTS. The incidence of nociceptor fibers exhibiting ongoing activity (15/35 vs. 1/9) and the frequency of their spontaneous firing (0.25 Ϯ 0.09 impulses [imp]/s versus 0.08 Ϯ 0.08 imp/s) was higher in fibers with RFs within and/or bordering the wounded area than in those with RFs far away from the wound. Mechanical responsiveness of fibers with RFs within or nearby the ablated area was often reduced. In these fibers, CO 2 pulses evoked a lower-frequency impulse discharge (0.9 Ϯ 0.2 imp/s inside, 2.3 Ϯ 0.7 imp/s outside the wound). CO 2 -evoked discharges recorded from fibers innervating the intact wound border were similar to those recorded in corneal fibers of intact cats. CONCLUSIONS. The spontaneous impulse activity and the abnormal responsiveness shown by a part of the corneal nerve fibers innervating the injured cornea are presumably the neurophysiological substrate of the pain sensations experienced by human patients hours after PRK surgery. (Invest Ophthalmol Vis Sci. 2007;48:4033-4037) DOI:10.1167/iovs.07-0012 P hotorefractive surgery, and other procedures such as phototherapeutic keratectomy (PTK) performed with excimer laser, has become a widely used procedure for treating myopia, hypermetropia, astigmatism, scars, and other diseases of the cornea.1,2 This type of surgery-in particular photorefractive keratectomy (PRK)-is accompanied by severe ocular pain that becomes strongest 24 hours after PRK and is described as a throbbing, burning, and/or stinging pain usually accompanied by nasal congestion, tearing, and photophobia.3 Postsurgical acute pain is often followed by less-intense discomfort sensations that may persist for weeks or months after surgery. 4,5Photorefractive surgery causes injury to the epithelial and stromal cells of the cornea and to corneal sensory nerve branches running in the lesioned tissues and causes various degrees of local inflammatory reaction.6 -8 The cornea is innervated by sensory fibers that have their origins in different functional types of trigeminal ganglion neurons: mechanonociceptor fibers activated by mechanical forces; polymodal nociceptor fibers that respond to mechanical, thermal, and chemical noxious stimuli; and cold receptor fibers that respond primarily to...

Both, surgical and non-surgical interventions show a mid-peripheral local corneal steepening. However, the narrower optic zone and higher midperipheral steepening with CRT seems to provide the potential to create a more relative peripheralmyopic increase in corneal power than LASIK, which may have implications in slowing down myopia progression.

The 3 techniques increase the wavefront aberrations of the cornea and change the relative contribution of coma-like and spherical-like aberrations. For a large aperture (> 5 mm), corneal refractive therapy induces more spherical-like aberrations than standard and custom LASIK. However, no clinically or statistically significant differences existed for narrower apertures. Standard and custom LASIK did not display statistically significant differences regarding higher-order aberrations.