Nocturnal bright halos & spherical aberration
The occurrence of halos light night is a side effect reported by corneal refractive surgery LASIK or PKR. They match the perception of halos more or less extended around light sources, and are generally seen in the following circumstances: fall of day, night, dimly lit room, subway platform, night road... In these circumstances, the IRIS pupil dilates to collect more light: the light rays that pass through the pupil on the outskirts are less well refracted and contribute to the increase in the rate of optical aberrations, and the degradation of the quality of the retinal image.
In addition to the dilation of the pupil, the high dynamics of the Visual scenes contributes to the increase in the perception of halos: bright light standing well out a darker environment. headlights of vehicles, street lights, neon lights, etc.
Review by OPD allows to confirm the optical source of halos, and point the cause, which is often linked to a rise in the optical aberrations of high degree (ex: spherical aberration). The spherical aberration of positive sign is particularly involved in the genesis of halos felt after surgery for nearsightedness.
Currently, most of the halos felt after surgery dissipate in a few days or weeks: their incidence is low because treatment issued by the current platforms are optimized so as to preserve the aspheric profile of the cornea (prolate), and issued on more wide optical areas than a decade ago.
Causes of halos
Halos can be caused by two types of optical anomalies:
(1) the reduction of the transparency of the eye circles: corneal edema is a classic source of halos (halos are typically colored or iridescent in this context). The occurrence of a cataract can be accompanied by halos.
(2) the increase in the rate of optical aberrations of high degree, on the first plan of which the spherical aberration.
These causes can be associated: nuclear cataracts typically causes an elevation of the spherical aberrations of negative sign, and is accompanied by definition of reduction of transparency of the lens.
Elevation of the eye spherical aberration
The increase of the spherical aberration causes are either corneal (refractive surgery, corneal Pathology), or cristalliniennes (nuclear cataract).
Corneal surgery (LASIK, PKR) myopia may increase the rate of spherical aberrations positive: even when the asphericity of the corneal profile is controlled by aspheric treatment, the peripheral camber induced by the area of fitting with the raw edge (geometry oblate) systematically causes an increase in the vergence to the edges of the optical zone.
Conversely, corneal surgery to the hyperopia (this surgery is performed through the LASIK technique essentially) can increase the rate of negative spherical aberrations for opposite reasons: flattening marked toward the periphery of the optical zone (prolate geometry) is a reduction of the vergence to the periphery of the optical zone. The rays refracted by the edges of the cornea are focused beyond the rays focused by the centre. This modulation may be used to compensate for presbyopia: in this case, to be the Central rays 'short-sighted' (near vision), the reduction of the vergence to the periphery so ensuring an emmetropisation (for distance vision). The photoablations laser to induce a cornea hyperprolate are based on a "Customizing the Asphericity factor (Q-factor)..
The appearance of a nuclear cataract is a classic case of elevation of the negative spherical aberrations. In case of nuclear cataracts, there is an increase in the refractive index of the core of the lens, causing a "index myopia": the center of the lens has optical power (vergence) more than the peripheral cortex. This difference induces a negative spherical aberration.
Halos after corneal surgery risk factors
The risk factors of halos of myopia (LASIK or PKR) excimer laser corneal surgery are:
-The degree of correction issued: halos are rare below 5 diopters of correction, because the connection between the central area (Flattening) and the peripheral zone induces no major increase in corneal curvature.
-The type of correction issued: the aspherical, optimized or guided by the Wavefront treatments are designed to reduce the risk of nocturnal light halos
-Pupillary diameter: halos are nocturnal because pupillary dilation increases the proportion of light rays refracted by the outskirts of the optical zone, or the area of transition where the corneal curvature is undergoing a rapid increase (connection between the 'decambree' central and peripheral area untreated).
Clinical example: halos after surgery for myopia
The following clinical example is particularly illustrative: a patient was operated of myopia (-5 diopters) in the early 2000s, by a laser treatment of surface (PKR) issued on a small (5 mm) optical area. Since the intervention, the patient reported disturbing nocturnal light halos, that disrupt it for driving at night.
In these circumstances, the halos are related to excessive refraction of light devices:
Topo aberrometrique (OPD SCAN III, Nidek, Japan) examination to measure:
-a profile corneal oblate (positive Q factor), generator of positive spherical aberration of corneal origin.
-a marked elevation of optical aberrations of high degree of type positive spherical aberration.
The map of vergence in the pupillary area reflects the importance of changes in vergence between the Center and the edges of the pupillary area: greater anterior corneal camber induced peripheral excessive refraction of rays, is a peripheral "annular myopia".
The maps of ocular wave-front full, corneal and internal to quantify the elevation of spherical aberrations for the various contingents of the eye:
The origin of spherical aberrations rise is to seat the geometry of the anterior corneal dome that can be view in Cup:
The decomposition of the anterior corneal profile in Zernike polynomials found unsurprisingly increased with symmetrical terms related to the "spherical aberration":
To remedy these halos, it is possible to advocate for lighting the Interior of the vehicle when driving at night (by turning on the Interior lights as a ceiling light). Some eye drops such as brimonidine (Alphagan) can also reduce the increase in pupillary diameter in terms mesopiques. In some case, surgery may be suggested (extension of the optical zone).
Halos after correction of myopia in Orthokeratology
The Orthokeratology can temporarily correct low myopia and average: this technique is based on the port during the night of rigid contact lenses, which cause a reshaping of the epithelium of the cornea in his serving central and paracentrale. The epithelium of the cornea regenerates quickly, and induction of prolonged Central compression out of the night cause a thinning of central and peripheral hyperplasia. The Central curvature decreases (Flattening) and allows to correct nearsightedness (reduction of the corneal vergence) during the 24 to 36 h after removal of lenses in the morning.
The Orthokeratology does not directly control the asphericity of the corneal profile, and it always takes a geometry oblate, causing a rise in positive spherical aberration. Flattening the night port-induced is maximum at the center of the cornea but decreases rapidly (it causes a reversal of the corneal Asphericity which becomes oblate).
The example following matches that of a thirty-something, initially nearsighted of-2.50 patient D, corrected for several months by the port orthokeratologique (night) of rigid lenses (Menicon Z-night) and who couldn't drive because of the induction of important nocturnal halos. In the 24 hours following the withdrawal of the lenses, her visual acuity was 12/10th without correction. The comparative review topo-aberrometrique (OPDscan III) to illustrate the consequences of a rise in positive spherical aberration related to a profile corneal excessively oblate. Functional optical area (allowing from light rays from distant sources to be focused on the retina) is too narrow, the rays refracted by the corneal periphery to mesopic condition (and who are not stopped by the iris because they go through a dilated pupil) are focused in front of the retina. This explains the occurrence of halos in conditions mesopiques.