Ghosting, double vision
The visual perception of "ghost" images is a source of concern and visual discomfort for patients who sometimes unexpectedly discover these symptoms through the appearance of a duplication of a line of movie subtitles, a second crescent moon shifted in the night sky, etc. These symptoms, when they are monocular (they persist when the other eye is closed: we speak of monocular diplopia) are generally (always?) of optical origin. Ghost images are thus a indication for carrying out an aberrometric examinationand ideally topo-aberrometric (aberrometric study of the entire eye coupled with a study of the cornea alone by corneal topography).
Symptoms of double vision, Ghost images.
A ghost image can be defined as any "parasitic" image whose pattern perceived by the patient has a certain fidelity to the main image. The patient realizes that he sees double with one eye: this perception is maximum in certain circumstances, such as the viewing of subtitled films: to detach himself from the images of the film, the subtitles are particularly bright, and their duplication clearly visible. The dark atmosphere of the projection rooms causes an expansion of the pupiliris (mydriasis) which also contributes to increasing the effect of aberrations at the origin of visual parasites.
Even if they are sometimes associated with ghosting, bright halos correspond to a concentric circular spreading of light around the source. they often have an optical origin, but do not exactly match the symptomatology of the ghost image. The "starbusts" anglicism dedicated for particularly starry appearance or "spicule' of a point source of light are also distinct images split, even if they are also related to a reduction in the optical quality of the eye. The famous Van Gogh canvas titled 'Starry night '.provides an artistic representation of the subjective perception of these luminous "halos".
As pointed out earlier, the Visual symptoms characteristic of ghosting are «» monoculars ": when you close one eye, then the other, this double or shifted vision persists for at least one of the eyes (double vision can be observed on both eyes separately). On the other hand, if the occlusion of an eye suppresses double vision, the origin is not optical but orthoptic ("failing" ocular parallelism).
The ghost images are essentially perceived when the ambient brightness is reduced, and when the eye looks particularly bright light sources, which easily stand out against a dark background (ex: under titles, urban displays bright, neon, LEDs etc.). In other words, the observed object is mixed, and greater sense of ghosts (s) image (s) is marked. The double vision is related to the existence of a "parasite" bright home, or a directional light spread (unlike the halos where bright sprawl is concentric). It is logical that more a light source is bright, more secondary home receives energy and stimulates the retinal photoreceptors concerned effectively.
Observing a line of subtitles, a traffic light lamp, street light in the distance, or crescent moonlight by the eye concerned generally allows the patient to characterize the intensity and location of the ghost image(s) (above, below, etc. of the main image).
The distance and the size of the light sources (their apparent diameter) also affect ghosting: the ghost images are often more pronounced when the source is far, and occupies a small Visual angle.
Optical mechanism causing ghosting
The cornea and the lens to focus the light emitted or reflected from the surrounding objects and captured by the eye. Central vision is Foveal: the light captured by the targets must be focused on the mosaic of the retinal photoreceptors of the fovea. If the image of an object is double, it is a particular distribution of incident light energy focused on the retinal screen (fovea) happens.
We can break down a scene set in a set of points basic sources: these sources points should be imaged punctually also ' ' as possible on the retina to "reconstruct" a faithful image of the source: this property is referred to as: "stigma". Thus, a good stigma (a property that allows a point source to be imaged almost punctually in terms of collection of image - here the retina) is an important condition for obtaining a good performance in Visual optics. In certain circumstances, the image of a point source may give rise to a one-time main image and a one-time "satellite." Some of the light energy emitted by the source and captured by the eye is focused in a main focus, and a secondary home (where there is a sufficient 'concentration' of light energy emitted by the source to give rise to a "perceived home", by effective stimulation of photoreceptors. By assimilation to a set of point sources, light information transported from the observed source will be so split into two homes. The management of the secondary home gives the direction that the offbeat image is perceived. Because of the retinal inversion, an offset anatomically noted "to the right" secondary home by an observer who despite the retina of the patient within the eye would in fact be perceived as a duplication "to the left" by the patient.
In the case of multiple ghosting (polyplopie), there is not one but several bright secondary outbreaks, where incident light energy is sufficiently concentrated to make an image appear 'ghost' by juxtaposition of these homes.
Exploration aberrometrique can confirm the optical source of symptoms and, facing the eye exam, specifying the cause.
Confirmation of the optical source: calculation of the PSF and convolution
Calculation of the PSF
To study the stigma of the eye, the aberrometer measures the ocular Wavefront then performs a calculation of the PSF (Point Spread Function) or FEP (function of spreading of the Point). This calculation can be done for the whole of the optical defects of the eye (aberrations of low and high level) or simply aberrations of high degree; in this case, we simulate the retinal image from a source point seen by one eye corrected for the defocus (myopia or hyperopia) and theastigmatism. The appearance of the PSF is an element of importance; a split-aspect, where the light is concentrated in at least a secondary zone located near a main focus to support optical originally by a feeling of double or multiple vision.
From the PSF, one can generate an image called "convoluee" of a Board reading for measurement of Visual acuity. The resulting image can be compared with the sensation described by the patient; as for the PSF, homology in terms of direction and spatial distribution of ghosting (up, down, left, right, etc.) allows to accredit the optical assumption of visual disturbances. It is often instructive to draw the patient the picture he perceives a distant point light source, or a letter (Snellen) and compare it to the PSF calculated by the aberrometric or image convoluee of the optotype for the same conditions of correction (for aberrations of high and low degree for an eye uncorrected, for the only aberrations of high degree if the patient is corrected in glasses).
Even if the parallelism is perfect, the presence of qualitative similarities to generally corroborate the symptoms of the patient to the aspect of the theoretical PSF, or the convoluee image. The differences between the PSF or image) calculated and the PSF (or image) drawn ('subjective' visual perception) take many factors such as the fact that the PSF is a calculation of physical optics, which ignores the transduction mechanisms, coding and transmission of information captured at the retinal level toward the Visual brain areas, and that this calculation is done for a correction to be complete or is zero aberrations of low degree (while the correction in glasses can be imperfect). Finally, the effect of diffusantes opacities (frequent in case cataract, or inflammation corneal laser post) is generally not well made at the level of the calculation of the PSF. Similarly, the presence of folds or micro pleats at the level of a flap of LASIK is a potential source of duplication of image, but the very high levels generated optical aberrations sometimes exceed the dynamic range of the aberrometric. The use of the OPD double-pass (OQAS) is an interesting option when we suspect the presence of an optical broadcast and/or the presence of aberrations of high degree.
Causes and origin of ghosting.
The cornea and the lens are the main optical tunics of the human eye. It is at their level that may sit optical phenomena responsible for the appearance of ghost images.
Ghosting of corneal origin
Corneal topography combined with the aberrometrique review is the test of choice when exploring the pathologies listed after. In particular, it must focus on the topographic instruments analysis software allows the specific calculation of corneal aberrations, as well as the calculation of the corneal PSF.
The presence of a corneal asymmetry is a frequent cause of ghost images, and resists correction in spectacle lenses. The Keratoconuswhich is associated with progressive corneal deformation and thinning, is a common etiology (cause) of double vision. Keratoconus is a corneal disease caused by the occurrence of eye rubbingrepeated and intense (allergy, dryness and eye irritation, fatigue, etc.). These frictions cause the appearance of a progressive deformation of the cornea; this deformation is irregular and induces a loss of the optical quality of the eye, characterized by the elevation of certain optical aberrations such as coma, trefoil, etc.
Diplopia appears or increases when it pupilexpands. Thus, the exploration of a ghost image sensation often leads to the discovery of a keratoconus beginning infraclinically. This is related to asymmetry and/or corneal irregularity. Classically, the rise in the rate of coma-type high degree optical aberrations is responsible for directional "spreading" of the PSF, well correlated with the sensation of the ghost image.
Any corneal trauma (SCAR) inducing a deformity marked and asymmetrical of the cornea at the level of the pupillary area is also potentially induced a feeling of ghost image.
Irregular astigmatism after corneal transplant (keratoplasty) is potentially inducing a feeling of duplication single or multiple of the image, despite an often marked regular astigmatism correction. The deformation of the graft and Chin-ups imposed by sutures points are responsible for pronounced asymmetries.
The radiaire keratotomii technique now obsolete (superseded by techniques laser since the end of the 1990s) was based on the realization of deep corneal incisions in 'Star' around the pupil. The necessary imperfections related to this manual gesture were source of an important irregularity of the surface of the cornea
Finally, the folds of flap in LASIK may be responsible for the appearance of a troublesome image split. If there is in general a good spatial correlation between the direction of duplication and the orientation of the fold, the mechanisms to the acquisition of data allowing the calculation of the ocular wavefront by the aberrometric do not always allow to characterize the optical effect of these folds.
Internal origin (crystalline, implant)
The occurrence of a partial or broadcast of the lens opacification may be responsible for a monocular diplopia, or even of a polyplopie. The existence of local fluctuations of the refractive index of the clouded lens can create significant variations in the refractive power of the lens. some advanced nuclear cataracts cause sometimes the perception of a 'triplopie' (triple vision).
A subluxe implant and/or tilted (tilt) is also provider of a lateral light spreading (in the sense of the rocker or the displacement of the implant, retinal reversal near the point of view of subjective perception). The calculation of the internal optical aberrations (corneal extra) to accredit the internal origin of these visual disorders. The disappearance of the symptoms after surgery of the eye and/or the cristallinien implant reaffiliation to finally confirm ex post the internal cause of these symptoms.
Multifocal implants, refractive individuals with asymmetric distribution of the additional power, may be responsible for perception of ghosting, when they are inserted into eyes including the optical quality of the corneal diopter is altered. Refractive multifocal implants induce the multifocalite thanks to a 'cocktail' of optical aberrations of high degree (provided by the implant depth of field is increased through the stigma reduction provided by these aberrations). If the cornea generates an additional rate of aberrations (alterations of the corneal surface, infra clinical Keratoconus unknown, irregularity of the corneal curvature, scar, astigmatism uncorrected, etc.), the addition of a refractive implant exposes to a significant reduction in the quality of the retinal image.
The presence of pronounced folds in the capsule of theCrystalline lens light trails around the sources of bright light. The orientation of the perceived trains is similar to that of the folds, with the exception of retinal inversion.
A simple test is to show the patient a Basic Visual pattern as a source disk, then ask him to draw the orientation and the appearance of the Visual streak. This is what observes the previous patient with his eye surgery cataract but with folds of the posterior capsule:
A special case: the rainbow glare
The 'rainbow glare' (= Rainbow rainbow) is a feeling of duplication of bright images, with a gradation in "Rainbow". This complication occurs in the immediate course of LASIK (with femtosecond laser). One page is specifically dedicated to him.