Background

Aniseikonia is a visual condition characterized by a significant difference in the perceived size or shape of images between the two eyes. This difference in retinal image size can result in visual discomfort, altered depth perception, and other visual disturbances. Aniseikonia can occur as a primary condition or as a secondary consequence of various factors, including differences in refractive error, unequal magnification of eyeglass lenses, or ocular surgery.

When the size or shape of the retinal images differs between the eyes, the brain processes the information from each eye independently, leading to a perceptual mismatch. This mismatch can result in symptoms such as eyestrain, headache, blurred or distorted vision, difficulty with depth perception, and reduced visual acuity. Individuals with aniseikonia may also experience visual disturbances when viewing stereoscopic images or when trying to fuse the images from both eyes into a single, clear image.

Epidemiology

Anatomy

Pathophysiology

Aniseikonia refers to the condition in which there is a significant difference in the perceived size or shape of images between the two eyes. The pathophysiology of aniseikonia can vary depending on the underlying cause or contributing factors. Here are some key factors involved in the pathophysiology of aniseikonia:

• Differences in Retinal Image Size: Aniseikonia typically arises due to a discrepancy in the size of the retinal images between the eyes. This difference can be caused by various factors, such as differences in refractive error (anisometropia), corneal irregularities, lens opacities, or changes in ocular shape induced by surgery.
• Binocular Vision Processing: In normal binocular vision, the brain combines the visual inputs from both eyes to create a single, fused image. However, in the presence of aniseikonia, the brain processes the images from each eye separately due to the size or shape discrepancy. This results in a perceptual mismatch and can lead to visual disturbances.
• Magnification Differences: In cases of aniseikonia with magnification differences, such as anisometropia, the discrepancy in refractive error between the eyes causes differences in retinal image size. The eye with a higher refractive error may have a smaller retinal image compared to the other eye, leading to a perceived size difference between the eyes.
• Shape Distortion: Aniseikonia with shape distortion occurs when there are irregularities in the shape or curvature of the cornea, lens, or retina. These irregularities can cause distortions in the retinal images between the eyes, leading to a difference in perceived shape or distortion of objects.
• Neural Adaptation: The visual system has some capacity for neural adaptation to aniseikonia, particularly in cases of mild or chronic aniseikonia. The brain may attempt to adjust or compensate for the size or shape discrepancy to minimize the perceptual differences. However, in cases of larger or acute aniseikonia, neural adaptation may be limited, and visual disturbances may persist.

The exact mechanisms of aniseikonia and its impact on visual processing are still not fully understood. Further research is needed to elucidate the precise pathophysiological processes involved in aniseikonia and the neural mechanisms underlying the perception of size and shape differences between the eyes.

Etiology

Aniseikonia can have various underlying causes, contributing factors, or associated conditions that can lead to a significant difference in the perceived size or shape of images between the two eyes. Here are some common etiological factors of aniseikonia:

• Refractive Errors: Anisometropia, which refers to a significant difference in refractive error between the two eyes, is a common cause of aniseikonia. This occurs when one eye has a different level of nearsightedness (myopia), farsightedness (hyperopia), or astigmatism compared to the other eye. The difference in the optical properties of the eyes can result in a difference in retinal image size, leading to aniseikonia.
• Ocular Surgery: Certain ocular surgeries can induce changes in the shape or size of the eye, resulting in aniseikonia. For example, cataract extraction with intraocular lens implantation can lead to aniseikonia if there is a discrepancy in the lens power or size between the two eyes. Similarly, refractive surgeries such as LASIK or PRK (photorefractive keratectomy) can alter the corneal shape and lead to aniseikonia if there is asymmetry in the treatment between the eyes.
• Corneal Irregularities: Conditions that cause irregularities in the shape or curvature of the cornea, such as keratoconus or corneal scars, can contribute to aniseikonia. These irregularities can cause differences in the size or shape of the retinal images between the eyes, resulting in perceptual differences.
• Lens Opacities: Opacities or irregularities in the natural crystalline lens, such as cataracts, can lead to aniseikonia. Changes in the transparency or shape of the lens can cause variations in the retinal image size or shape between the eyes.
• Retinal Abnormalities: Certain retinal conditions, such as macular degeneration or retinal detachment, can cause aniseikonia. These conditions can affect the structure or function of the retina, resulting in differences in retinal image size or shape between the eyes.
• Binocular Vision Disorders: Some binocular vision disorders, such as strabismus (eye misalignment) or amblyopia (lazy eye), can contribute to aniseikonia. These conditions may cause differences in the position or alignment of the eyes, leading to variations in retinal image size or shape.

Genetics

Prognostic Factors

The prognosis of aniseikonia depends on several factors, including the underlying cause, the severity of aniseikonia, and the effectiveness of treatment. In general, if aniseikonia is correctly diagnosed and managed early, the prognosis for reducing or eliminating the perceptual differences in image size or shape between the eyes is favorable. However, it is important to note that the complete elimination of aniseikonia may not always be achievable in certain cases.

Here are some factors that can influence the prognosis of aniseikonia:

• Underlying Cause: The prognosis may vary depending on the underlying cause of aniseikonia. For example, aniseikonia caused by refractive errors (anisometropia) can often be effectively managed with appropriate optical correction, leading to significant improvement in symptoms. On the other hand, aniseikonia resulting from structural abnormalities or surgical interventions may have a more complex prognosis and may require specialized treatment approaches.
• Treatment Approach: The choice and effectiveness of the treatment approach can impact the prognosis. Optical correction, prism glasses, vision therapy, or surgical interventions may be used alone or in combination, depending on the specific case. The success of these treatments in reducing aniseikonia and improving visual comfort varies among individuals.
• Severity of Aniseikonia: The severity of aniseikonia can influence the prognosis. Mild cases of aniseikonia may respond well to treatment, resulting in significant reduction or elimination of symptoms. However, in more severe cases, complete correction of aniseikonia may not be achievable, and the goal of treatment may be to minimize symptoms and improve visual function to the greatest extent possible.
• Patient Compliance and Adaptation: Patient compliance with the prescribed treatment and their ability to adapt to the treatment methods can impact the prognosis. Vision therapy, for example, may require regular and consistent participation to achieve optimal results. Compliance with wearing corrective lenses or prism glasses as prescribed is crucial for successful management of aniseikonia.
• Associated Conditions: The presence of any associated ocular or systemic conditions can influence the prognosis of aniseikonia. Some underlying conditions, such as macular degeneration or optic nerve disorders, may have additional visual impairments that can affect the overall outcome.

Clinical History

Clinical history

When taking the clinical history of a patient with suspected aniseikonia, it is important to gather information about their symptoms, medical history, and any relevant ocular or systemic conditions. Here are some key aspects to consider during the clinical history:

• Presenting Complaints: Determine the patient’s chief complaints and specific visual symptoms. Ask about any difficulties with vision, such as blurred or distorted vision, differences in image size or shape between the eyes, eyestrain, headache, or reduced depth perception. Inquire about any specific situations or activities that exacerbate or alleviate the symptoms.
• Duration and Progression: Determine the onset and duration of symptoms. Inquire about whether the symptoms have been present since childhood or have developed recently. Note any progression or changes in symptoms over time.
• Medical History: Obtain a comprehensive medical history, including any pre-existing ocular conditions, ocular surgeries, or systemic conditions that may be associated with aniseikonia, such as refractive surgeries, cataract surgery, or corneal abnormalities. Inquire about any known refractive errors, history of strabismus or amblyopia, or other ocular disorders.
• Medications and Allergies: Ask about current medications, including any ophthalmic medications or systemic drugs that may have an impact on vision or ocular health. Inquire about any known allergies, particularly related to medications or contact lens wear.
• Ocular Symptoms: In addition to aniseikonia-related symptoms, ask about other ocular symptoms, such as eye redness, irritation, tearing, or foreign body sensation. Inquire about any history of dry eye, eye infections, or eye trauma.
• Family History: Ask about any family history of ocular conditions, including anisometropia, strabismus, or other vision-related disorders. A family history of aniseikonia or related conditions can provide valuable information about potential underlying genetic factors.
• Lifestyle and Occupation: Inquire about the patient’s occupation, hobbies, and daily activities that may be affected by the visual symptoms. Certain activities that require precise depth perception or visual coordination, such as driving, reading, or using computers, may be particularly affected by aniseikonia.
• Impact on Quality of Life: Assess the impact of aniseikonia on the patient’s daily life, including their ability to perform tasks, engage in social activities, or pursue hobbies. Inquire about any psychological or emotional impact, such as frustration, anxiety, or reduced self-confidence related to their visual symptoms.

Physical Examination

Physical examination

The physical examination of a patient with suspected aniseikonia involves a comprehensive evaluation of their visual function, eye alignment, and binocular vision. Here are the key components of the physical examination for aniseikonia:

• Visual Acuity: Measure the visual acuity of each eye separately using a Snellen chart or other appropriate visual acuity testing methods. Assess both distance and near visual acuity to determine any significant differences between the eyes.
• Refraction: Perform a thorough refraction to determine the refractive error of each eye. Note any significant differences in the prescription between the eyes, which may contribute to aniseikonia.
• Ocular Alignment: Assess the alignment of the eyes to detect any misalignment or strabismus. Evaluate both the primary gaze and alternate gaze positions, including both distance and near fixation. Use methods such as the cover-uncover test, the alternate cover test, or the Hirschberg test to assess eye alignment.
• Ocular Motility: Evaluate the range of eye movements, including pursuits and saccades, to assess the coordination and symmetry of eye movements. Any limitations or asymmetries in eye movements can suggest underlying binocular vision disorders.
• Stereopsis: Perform a stereopsis test to evaluate the patient’s depth perception. Stereopsis assesses the ability to perceive the depth and spatial relationships of objects using binocular vision. Reduced or absent stereopsis may indicate a disruption in binocular vision due to aniseikonia or other factors.
• Cover Test: Perform a cover test to assess for any latent or manifest strabismus. This test involves alternately covering one eye while observing for any eye movement or misalignment when the cover is switched between eyes.
• Retinal Image Size Assessment: Specialized tests, such as the New Aniseikonia Test (NAT) or the Aniseikonia Inspector, can be used to quantitatively measure the perceived size difference between the retinal images of the two eyes. These tests employ optical devices or image comparison techniques to determine the aniseikonia magnitude.
• Fundus Examination: Perform a dilated fundus examination to evaluate the health of the retina, optic nerve, and macula. Look for any signs of retinal abnormalities or optic nerve conditions that may contribute to aniseikonia.

In addition to these specific examinations, a general ocular examination should be conducted, including evaluation of ocular structures, assessment of intraocular pressure, and examination of the anterior and posterior segments of the eye. The physical examination findings, along with the patient’s clinical history and symptoms, help in confirming the presence of aniseikonia and determining its underlying cause. The results of the examination guide appropriate management strategies, such as refractive correction, vision therapy, or surgical interventions if necessary.

Age group

Associated comorbidity

Associated activity

Acuity of presentation

Differential Diagnoses

Differential diagnosis

When evaluating a patient with suspected aniseikonia, it is important to consider other conditions that can cause similar symptoms or visual disturbances. The following are some of the differential diagnoses to consider:

• Anisometropia: Anisometropia refers to a significant difference in refractive error between the two eyes, resulting in unequal focusing abilities. Like aniseikonia, anisometropia can lead to differences in retinal image size and may cause similar symptoms such as blurred or distorted vision. However, in anisometropia, the primary issue is the difference in refractive error, whereas aniseikonia involves a difference in perceived size or shape of images.
• Binocular Vision Disorders: Conditions affecting binocular vision, such as strabismus (misalignment of the eyes) or amblyopia (lazy eye), can cause visual disturbances that may resemble aniseikonia. These conditions can result in differences in the position or alignment of the eyes, leading to variations in retinal image size or shape. Proper assessment of eye alignment and binocular vision function is necessary to differentiate these conditions from aniseikonia.
• Visual Field Defects: Visual field defects, such as homonymous hemianopia (loss of vision on one side of the visual field) or scotomas (blind spots), can cause visual disturbances that may be mistaken for aniseikonia. These conditions result from damage to the visual pathway or specific areas of the visual cortex, and they can lead to perceptual differences or distortions in the visual field.
• Optic Nerve Disorders: Disorders affecting the optic nerve, such as optic neuritis or optic neuropathy, can cause visual disturbances that may mimic aniseikonia. These conditions can lead to variations in visual acuity, contrast sensitivity, or color perception, which may result in perceived differences in image size or shape.
• Macular Disorders: Conditions affecting the macula, such as macular degeneration or macular edema, can cause central vision disturbances that can be mistaken for aniseikonia. These conditions typically result in reduced visual acuity, central scotomas, or distorted vision in the central visual field.
• Psychophysical Factors: Some visual disturbances, including those related to perceptual differences, may have a psychophysical or psychogenic basis. Psychological factors, such as anxiety, stress, or somatization disorders, can manifest as visual symptoms that resemble aniseikonia. A comprehensive evaluation is necessary to differentiate these factors from organic causes of aniseikonia.

Accurate diagnosis of aniseikonia requires a thorough assessment of the patient’s visual symptoms, measurement of retinal image size differences, evaluation of eye alignment and binocular vision function, and consideration of other potential causes. Ophthalmological examination, including visual acuity testing, refraction, and specialized tests like aniseikonia testing, may be conducted to differentiate aniseikonia from other conditions.

Laboratory Studies

Imaging Studies

Procedures

Histologic Findings

Staging

Treatment Paradigm

The treatment of aniseikonia aims to reduce or eliminate the perceptual differences in image size or shape between the eyes, thereby improving visual comfort and function. The choice of treatment depends on the underlying cause of aniseikonia and the severity of symptoms. Here are several approaches that may be employed:

• Optical Correction: Anisometropia, which is a common cause of aniseikonia, can often be managed through the use of corrective lenses. Prescription glasses or contact lenses that compensate for the refractive error in each eye can help minimize the size discrepancy in retinal images. Customized lenses designed specifically for aniseikonia correction, known as aniseikonic lenses, may be prescribed in certain cases.
• Prism Glasses: Prism glasses are another option for managing aniseikonia, especially when there is a significant misalignment or ocular deviation. Prisms can be incorporated into the lenses to shift the visual field and align the retinal images, reducing the perceptual differences. Prism glasses should be prescribed by an eye care professional with expertise in aniseikonia management.
• Vision Therapy: Vision therapy involves a series of exercises and activities designed to improve binocular vision, eye coordination, and visual processing. It can be beneficial for patients with aniseikonia, particularly when the underlying cause is related to binocular vision disorders, such as strabismus or amblyopia. Vision therapy may include techniques like eye patching, eye tracking exercises, convergence exercises, or stereopsis training.
• Surgical Interventions: In some cases, surgical interventions may be considered for the management of aniseikonia. This is more commonly applicable when aniseikonia is a result of ocular surgery or structural abnormalities that can be corrected surgically, such as refractive surgery enhancements, lens exchange, or corneal procedures. Surgical options should be evaluated and discussed with an ophthalmologist specialized in the relevant field.
• Magnification/Minification Devices: In certain situations where aniseikonia cannot be fully corrected optically or surgically, the use of magnification or minification devices may be beneficial. These devices alter the perceived size of images seen by one eye to match the other eye, reducing the aniseikonia-related discrepancies. Examples include aniseikonic filters, prismatic reading glasses, or electronic devices that modify the image size.

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References

https://www.ncbi.nlm.nih.gov/books/NBK585108/