Aniseikonia

Description, Causes and Risk Factors:

An ocular condition in which the image of an object in one eye differs in size or shape from the image of the same object in the fellow eye.

Types:

Static aniseikonia or aniseikonia in short means that in a static situation where the eyes are gazing in a certain direction, the perceived (peripheral) images are different in size.

• Dynamic aniseikonia means that the eyes have to rotate a different amount to gaze (i.e. look with the sharpest vision) at the same point in space. This is especially difficult for eye rotations in the vertical direction.

The cause of the unequal image size may be optical, that is, be due to differences in refractive errors of the two eyes; or it may be congenital, due to unequal disposition of the rods and cones in the retina, or it may still occasionally be cerebral.

The other cause of aniseikonia is anisometropia and the wearing of glasses that differ in magnifying power for the two eyes. The next most common cause of aniseikonia is asymmetrical convergence. When an object is brought near the eyes, its image increases in size. If it is in the midline, so that the eyes converge symmetrically upon it, the increase is equal in the two eyes. But if the object is far to the right or the left, it is obviously not equally near each eye, but much nearer one eye.

As many of 10-15% of the population may have some aniseikonia; not everyone becomes symptomatic from it. It becomes clinically significant (about 4% of the population based on the Dartsmouth Study usually if (1) it is of late-onset, i.e. the brain has a hard time readjusting to the size difference, (2) the patient is very sensitive to small visual changes, and/or (3) the patient's work of life-style places great demand on the visual system.

The consequences of aniseikonia may be considered from two points of view: (1) How it affects seeing, and (2) what symptoms it causes.

Symptoms:

People with aniseikonia often develop eye strain, dizziness, disorientation, and balance problems. They can have trouble with depth perception and manipulating objects in their environment. They may also have difficulty processing visual scenes, something that can be potentially hazardous while engaging in activities like driving or operating heavy machinery. When the difference in image size is very small, people may not realize what is going on until they are evaluated by a doctor.

General symptoms may include:

Headaches.

• Asthenopia.

• Photophobia.

• Reading difficulty.

• Nausea.

• Diplopia.

• Distorted space perception.

• Vertigo and dizziness.

Diagnosis:

Diagnosis of this condition usually requires a workup by an ophthalmologist. The doctor will examine the eyes, run a series of vision tests to learn more about the patient's visual acuity, and interview the patient about any vision or health problems noted. This information can be pulled together to develop a diagnosis, determine how severe the problem is, and work on a treatment plan.

In older optometric/ophthalmic textbooks, rules of thumb have been defined to correct aniseikonia, without actually testing for the amount of aniseikonia. These rules are based on Knapp's law, which deals with an image size difference as projected onto the retina in anisometropia (i.e., only optical effects are taken into account). Eye care professionals using these rules will base treatment on the patient's prescription and perhaps the difference in corneal curvature or eye length between the eyes. However, in the more recent literature it has been well established that even in anisometropia, the retinal receptor distribution may also play a role,and therefore these rules of thumb should not be used. Instead the aniseikonia should be measured.

There are basically two methods to test for aniseikonia: the space eikonometric method and the direct comparison method.The space eikonometric method is based on binocular space perception, while the direct comparison method is based on directly comparing perceived image sizes between the two eyes.

The aniseikonia testing mentioned above has all been 'objective' testing. However, an eye care provider serious about aniseikonia management should also have a set of size lenses for 'subjective' testing.

Treatment:

Treatment is done by changing the optical magnification properties of the auxiliary optics (corrective lenses). 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.

NOTE: The above information is educational purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.

DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.