Aniseikonia
Encyclopedia
Aniseikonia is an ocular condition where there is a significant difference in the perceived size
of images. It can occur as an overall difference between the two eyes, or as a difference in a particular meridian.
es affect these characteristics and are used commonly to correct refractive error
. The optics of the eye including its refractive power and axial length also play a major role in retinal image size.
Aniseikonia can occur naturally or be induced by the correction of a refractive error
, usually anisometropia
(having significantly different refractive errors between each eye) or antimetropia (being myopic
(nearsighted) in one eye and hyperopic
(farsighted) in the other.) Meridional aniseikonia occurs when these refractive differences only occur in one meridian (see astigmatism
). Refractive surgery
can cause aniseikonia in much the same way that it is caused by glasses and contacts
.
One cause of significant anisometropia and subsequent aniseikonia has been aphakia
. Aphakic patients do not have a crystalline lens
. The crystalline lens is often removed because of opacities called cataract
s. The absence of this lens left the patient highly hyperopic (farsighted) in that eye. For some patients the removal was only performed on one eye, resulting in the anisometropia / aniseikonia. Today, this is rarely a problem because when the lens is removed in cataract surgery
, an intraocular lens
, or IOL is left in its place.
, disorientation, eyestrain, headache
, and dizziness
and balance disorder
s.
es). The optical magnification properties of spectacle lenses can be adjusted by changing parameters like the base curve, vertex distance
, and center thickness. Contact lenses may also provide a better optical magnification to reduce the difference in image size. The difference in magnification can also be eliminated by a combination of contact lenses and glasses (creating a weak telescope system). The optimum design solution will depend on different parameters like cost, cosmetic implications, and if the patient can tolerate wearing a contact lens.
Note however that before the optics can be designed, first the aniseikonia should be known=measured. When the image disparity is astigmatic (cylindrical) and not uniform, images can appear wider, taller, or diagonally different. When the disparity appears to vary across the visual field (field-dependent aniseikonia), as may be the case with an epiretinal membrane
or retinal detachment
, the aniseikonia cannot fully be corrected with traditional optical techniques like standard corrective lenses. However, partial correction often improves the patient's vision comfort significantly. Little is known yet about the possibilities of using surgical intervention to correct aniseikonia.
Magnification
Magnification is the process of enlarging something only in appearance, not in physical size. This enlargement is quantified by a calculated number also called "magnification"...
of images. It can occur as an overall difference between the two eyes, or as a difference in a particular meridian.
Causes
Retinal image size is determined by many factors. The size and position of the object being viewed affects the characteristics of the light entering the system. Corrective lensCorrective lens
A corrective lens is a lens worn in front of the eye, mainly used to treat myopia, hyperopia, astigmatism, and presbyopia. Glasses or "spectacles" are worn on the face a short distance in front of the eye. Contact lenses are worn directly on the surface of the eye...
es affect these characteristics and are used commonly to correct refractive error
Refractive error
A refractive error, or refraction error, is an error in the focusing of light by the eye and a frequent reason for reduced visual acuity.-Classification:...
. The optics of the eye including its refractive power and axial length also play a major role in retinal image size.
Aniseikonia can occur naturally or be induced by the correction of a refractive error
Refractive error
A refractive error, or refraction error, is an error in the focusing of light by the eye and a frequent reason for reduced visual acuity.-Classification:...
, usually anisometropia
Anisometropia
Anisometropia is the condition in which the two eyes have unequal refractive power; that is, are in different states of myopia , hyperopia or in the extreme, antimetropia , the unequal refractive states cause unequal rotations thus leading to diplopia and asthenopia.Anisometropia can adversely...
(having significantly different refractive errors between each eye) or antimetropia (being myopic
Myopia
Myopia , "shortsightedness" ) is a refractive defect of the eye in which collimated light produces image focus in front of the retina under conditions of accommodation. In simpler terms, myopia is a condition of the eye where the light that comes in does not directly focus on the retina but in...
(nearsighted) in one eye and hyperopic
Hyperopia
Hyperopia, also known as farsightedness, longsightedness or hypermetropia, is a defect of vision caused by an imperfection in the eye , causing difficulty focusing on near objects, and in extreme cases causing a sufferer to be unable to focus on objects at any distance...
(farsighted) in the other.) Meridional aniseikonia occurs when these refractive differences only occur in one meridian (see astigmatism
Astigmatism
An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances...
). Refractive surgery
Refractive surgery
Refractive eye surgery is any eye surgery used to improve the refractive state of the eye and decrease or eliminate dependency on glasses or contact lenses. This can include various methods of surgical remodeling of the cornea or cataract surgery. The most common methods today use excimer lasers to...
can cause aniseikonia in much the same way that it is caused by glasses and contacts
Corrective lens
A corrective lens is a lens worn in front of the eye, mainly used to treat myopia, hyperopia, astigmatism, and presbyopia. Glasses or "spectacles" are worn on the face a short distance in front of the eye. Contact lenses are worn directly on the surface of the eye...
.
One cause of significant anisometropia and subsequent aniseikonia has been aphakia
Aphakia
Aphakia is the absence of the lens of the eye, due to surgical removal, a perforating wound or ulcer, or congenital anomaly. It causes a loss of accommodation, hyperopia, and a deep anterior chamber. Complications include detachment of the vitreous or retina, and glaucoma.Aphakic people are...
. Aphakic patients do not have a crystalline lens
Lens (anatomy)
The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...
. The crystalline lens is often removed because of opacities called cataract
Cataract
A cataract is a clouding that develops in the crystalline lens of the eye or in its envelope, varying in degree from slight to complete opacity and obstructing the passage of light...
s. The absence of this lens left the patient highly hyperopic (farsighted) in that eye. For some patients the removal was only performed on one eye, resulting in the anisometropia / aniseikonia. Today, this is rarely a problem because when the lens is removed in cataract surgery
Cataract surgery
Cataract surgery is the removal of the natural lens of the eye that has developed an opacification, which is referred to as a cataract. Metabolic changes of the crystalline lens fibers over time lead to the development of the cataract and loss of transparency, causing impairment or loss of vision...
, an intraocular lens
Intraocular lens
An intraocular lens is an implanted lens in the eye, usually replacing the existing crystalline lens because it has been clouded over by a cataract, or as a form of refractive surgery to change the eye's optical power. It usually consists of a small plastic lens with plastic side struts, called...
, or IOL is left in its place.
Demonstration
A way to demonstrate aniseikonia is to hold a near target (ex. pen or finger) approximately 6 inches directly in front of one eye. The person then closes one eye, and then the other. The person should notice that the target appears larger to the eye that it is directly in front of. When this object is viewed with both eyes, it is seen with a small amount of aniseikonia. The principles behind this demonstration are relative distance magnification (closer objects appear larger) and asymmetrical convergence (the target is not an equal distance from each eye).Symptoms
When this magnification difference becomes excessive the effect can cause diplopiaDiplopia
Diplopia, commonly known as double vision, is the simultaneous perception of two images of a single object that may be displaced horizontally, vertically, or diagonally in relation to each other...
, disorientation, eyestrain, headache
Headache
A headache or cephalalgia is pain anywhere in the region of the head or neck. It can be a symptom of a number of different conditions of the head and neck. The brain tissue itself is not sensitive to pain because it lacks pain receptors. Rather, the pain is caused by disturbance of the...
, and dizziness
Dizziness
Dizziness refers to an impairment in spatial perception and stability. The term is somewhat imprecise. It can be used to mean vertigo, presyncope, disequilibrium, or a non-specific feeling such as giddiness or foolishness....
and balance disorder
Balance disorder
A balance disorder is a disturbance that causes an individual to feel unsteady, for example when standing or walking. It may be accompanied by feelings of giddiness or wooziness, or having a sensation of movement, spinning, or floating...
s.
Treatment
Treatment is done by changing the optical magnification properties of the auxiliary optics (corrective lensCorrective lens
A corrective lens is a lens worn in front of the eye, mainly used to treat myopia, hyperopia, astigmatism, and presbyopia. Glasses or "spectacles" are worn on the face a short distance in front of the eye. Contact lenses are worn directly on the surface of the eye...
es). The optical magnification properties of spectacle lenses can be adjusted by changing parameters like the base curve, vertex distance
Vertex distance
Vertex distance the distance between the back surface of a corrective lens, i.e. glasses or contact lenses, and the front of the cornea. Increasing or decreasing the vertex distance changes the optical properties of the system, by moving the focal point forward or backward, effectively changing...
, and center thickness. Contact lenses may also provide a better optical magnification to reduce the difference in image size. The difference in magnification can also be eliminated by a combination of contact lenses and glasses (creating a weak telescope system). The optimum design solution will depend on different parameters like cost, cosmetic implications, and if the patient can tolerate wearing a contact lens.
Note however that before the optics can be designed, first the aniseikonia should be known=measured. When the image disparity is astigmatic (cylindrical) and not uniform, images can appear wider, taller, or diagonally different. When the disparity appears to vary across the visual field (field-dependent aniseikonia), as may be the case with an epiretinal membrane
Epiretinal membrane
Epiretinal membrane is a disease of the eye in response to changes in the vitreous humor or more rarely, diabetes. It is also called macular pucker. Sometimes, as a result of immune system response to protect the retina, cells converge in the macular area as the vitreous ages and pulls away in...
or retinal detachment
Retinal detachment
Retinal detachment is a disorder of the eye in which the retina peels away from its underlying layer of support tissue. Initial detachment may be localized, but without rapid treatment the entire retina may detach, leading to vision loss and blindness. It is a medical emergency.The retina is a...
, the aniseikonia cannot fully be corrected with traditional optical techniques like standard corrective lenses. However, partial correction often improves the patient's vision comfort significantly. Little is known yet about the possibilities of using surgical intervention to correct aniseikonia.
External links
- Aniseikonia information at Optical Diagnostics