L.7-Cover Test

Introduction to Binocular Vision Measurement

  • Various methods to assess binocular vision are discussed, including:

    • Prism bar method: Utilized to measure the magnitude of a deviation (phorias or tropias) by neutralizing the eye movement observed during cover testing. Prisms are placed in front of one eye until no movement is observed, indicating the amount of deviation in prism diopters.(\Delta).

    • Maddox rod: A device consisting of a series of red (or clear) cylinders that convert a spot of light into a line. It dissociates the eyes, preventing fusion, and is used to measure phorias and tropias, especially vertical deviations (hyper/hypo) and cyclodeviations.

    • Techniques utilizing prisms without an occluder: These methods, such as the Von Graefe phoria test, also dissociate the eyes without entirely blocking vision, assessing the inherent tendency of the eyes to deviate under monocular conditions while maintaining a binocular stimulus.

Importance of Anatomy in Binocular Vision

  • Anatomy of the Face

    • Eyes are positioned for overlapping fields, a characteristic of forward-facing eyes, which is crucial for enabling stereopsis (fine depth perception) by allowing the brain to compare slightly different images from each eye.

    • Adaptation seen in predators with forward-facing eyes, providing a large binocular field of view necessary for judging distance and tracking prey.

    • Comparison with prey animals that have side-positioned eyes, offering a wider monocular field of view for detecting predators from various directions, but with limited overlapping fields and therefore reduced stereoscopic depth perception.

  • Image Quality

    • Binocular vision relies heavily on:

      • Quality of retinal images: Clear, focused images from both eyes are fundamental. Any blur, opacity, or refractive error can degrade image quality and disrupt fusion.

      • Equivalence in size and shape of images presented to the eyes: Significant differences (aniseikonia) can make fusion difficult or impossible, often leading to suppression of the eye with the less desirable image.

      • Correct alignment on foveas of both eyes: Precise alignment of the images on the fovea (the area of sharpest vision) of each eye is essential for single, clear binocular vision. Misalignment results in sensory conflict.

    • Poor image quality, such as that caused by uncorrected refractive errors, amblyopia, or significant aniseikonia, may lead to suppression of one eye by the brain to avoid diplopia (double vision) and visual confusion.

Local Signs in Binocular Vision

  • Definition of Local Signs

    • Each individual point on the retina (specifically, bundles of rods and cones) has a unique 'local sign,' which is a neurological tag informing the brain of its precise position in visual space. This allows the brain to map out the visual field and localize objects.

    • External stimulation (e.g., pressing on the eye) also evokes perception relevant to the stimulated area of the retina, demonstrating the fixed spatial localization associated with each retinal point.

  • Principal Visual Direction

    • Defined as the primary line of fixation emanating from the fovea of the dominant eye (or both foveas when aligned) to an object perceived as being in the straight-ahead position. This serves as the egocentric reference for spatial localization.

  • Corresponding Retinal Points

    • These are points in the retinas of both eyes that, when activated by stimuli from the same point in physical space, project to the same location in the visual cortex. Activation of corresponding points leads to single vision.

    • Important for fusing images and eliminating double vision when they align precisely on the foveas or other corresponding points on the retina.

    • Misalignment of the eyes causes stimuli from a single object to fall on noncorresponding points, leading to disparate images, which can result in diplopia or suppression.

  • Sensory Fusion

    • The neural process where the brain combines input from both eyes into a single, coherent, three-dimensional image. This process is crucial for preventing diplopia and for achieving stereoscopic depth perception.

    • Sensory fusion is often tested using a Wirth four dot test (or similar fusion tests), which assesses how many dots a patient can perceive when viewing through red/green filters:

      • Four dots indicate successful fusion, where the images from both eyes are processed together.

      • Two or three dots indicate suppression of one eye (e.g., seeing only green indicates the red eye is suppressed; seeing only red/green combinations suggests partial suppression or a lack of fusion).

      • Five dots (2 red, 3 green) indicates diplopia or confusion, where the patient simultaneously perceives separate images from each eye without fusion. This often points towards a manifest deviation (tropia).

      • This test can help determine the presence and degree of suppression, as well as the presence of a tropia.

Conditions Affecting Binocular Vision

  • Suppression Mechanism

    • Typically a neurological response by the brain to avoid the confusing and uncomfortable experience of diplopia or visual confusion that arises from conflicting images.

    • Occurs due due to various factors such as significant disparity in image quality, ocular misalignment (strabismus), unequal image sizes (aniseikonia), or differences in refractive error (anisometropia).

    • The brain may actively inhibit or 'turn off' the visual input from one eye that has lower acuity, poorer image quality, or is significantly misaligned, for smoother and single visual perception.

  • Types of Suppression

    • Central suppression: Involves the active inhibition of vision from the fovea and parafoveal region, the area responsible for sharp, detailed central vision. This is common in strabismic amblyopia.

    • Peripheral suppression: Affects the peripheral visual field, where the brain suppresses input from outer retinal areas. This is often an adaptive mechanism in larger-angle strabismus to maintain some degree of central fusion or to avoid peripheral diplopia.

  • Binocular Visions Testing Techniques

    • Testing for stereoscopic vision (stereopsis) as the highest level indicator of depth perception. This is commonly performed using polarized tests like the Randot stereotest forms or Titmus Fly stereotest.

      • The Randot test presents different images to each eye, and the brain's ability to fuse these disparate images creates the perception of depth. It typically measures stereopsis in seconds of arc (''), with lower values indicating better depth perception.

Panum's Fusional Area

  • Refers to a small, dynamic region surrounding the corresponding retinal points, within which slight misalignments or disparities in the images falling on the retinas of both eyes can still be fused into a single percept without causing diplopia.

  • This area allows for some physiological retinal disparity that is essential for stereoscopic depth perception. Its size varies, being smaller centrally and larger peripherally.

  • Retinal Disparity

    • Occurs when objects in space stimulate slightly noncorresponding retinal points in each eye. There are two types:

      • Crossed (or temporal) disparity: Occurs when an object is closer than the point of fixation (i.e., in front of the horopter), causing its image to fall on the temporal retina of each eye.

      • Uncrossed (or nasal) disparity: Occurs when an object is farther than the point of fixation (i.e., behind the horopter), causing its image to fall on the nasal retina of each eye.

    • When object images fall significantly outside Panum's fusional area, it leads to perceived diplopia (double vision).

    • Normal, physiological retinal disparity within Panum's area is not only tolerated but initiates motor movements (fusional vergence) necessary for achieving sensory fusion and is the primary stimulus for fine depth perception (stereopsis).

Eye Movements and Motor Fusion

  • Conjugate Movements

    • Both eyes move simultaneously in the same direction, allowing the fovea to track objects or shift fixation accurately. Examples include:

      • Saccades: Rapid, ballistic eye movements that quickly shift the line of sight from one object to another.

      • Pursuits: Smooth, slow eye movements that allow the eyes to follow a moving object, keeping its image on the fovea.

      • Versions: General term for conjugate movements in specific directions (e.g., dextroversion for right gaze, levoversion for left gaze).

  • Disjunctive Movements

    • Eyes move simultaneously in opposite directions, primarily to maintain binocular alignment and fusion as viewing distance changes.

    • Convergence: Inward movement of both eyes towards each other, assisting in focusing on close objects. It is part of the near triad (convergence, accommodation, miosis).

    • Divergence: Outward movement of both eyes away from each other, helping with distant objects or when shifting gaze from near to far.

    • Both convergence and divergence are fusional vergence movements, driven by retinal disparity, to ensure images fall on corresponding retinal points.

Types of Deviations in Binocular Vision

  • Phoria

    • A latent deviation of the eyes where the visual axes tend to misalign only when binocular vision is disrupted (e.g., when one eye is covered). It is a tendency for the eyes to drift, but fusion mechanisms keep them aligned under normal viewing conditions. Phorias are compensated.

  • Tropia

    • A manifest (present) deviation of the eyes, visible even under binocular conditions. One eye is noticeably misaligned with the other, and the deviation cannot be overcome by the patient's fusional reserves. Tropias are uncompensated and often lead to suppression or diplopia.

  • Measurement Techniques

    • Use of cover tests (unilateral and alternating) are the primary clinical methods to differentiate between phoria and tropia and to measure the magnitude of the deviation.

      • Unilateral cover test (cover-uncover test): Performed by covering one eye and then uncovering it while observing the uncovered eye. If the uncovered eye moves to take up fixation, it indicates a tropia (manifest deviation) in the eye that was just uncovered.

      • Alternating cover test (cross-cover test): Involves alternately covering each eye, thereby disrupting fusion and revealing the full magnitude of any deviation (phorias + tropias). The total movement observed represents the phoric and tropic components. Prisms are then used to neutralize this movement to determine the prism diopter value.

      • These tests allow for identification of the direction (eso, exo, hyper, hypo) and magnitude of the deviations.

Documenting Deviations

  • Recording methods include the direction (eso- for inward, exo- for outward, hyper- for upward, hypo- for downward), frequency (constant or intermittent), and laterality (right (R), left (L), or alternating (A)). The magnitude is measured in prism diopters (\Delta).

  • Examples:

    • "10\Delta eso 4" might more accurately be written as "10\Delta esophoria at distance without movement in unilateral tests," indicating a latent inward deviation. (The '4' likely refers to distance, which is often implicit).

    • "5\Delta intermittent left exotropia" reflects an outward deviation of 5 prism diopters, occurring sometimes, with the left eye being the one that deviates overtly. If 'alternating' were added, it would mean the deviation shifts between the left and right eyes.

    • "8\Delta R hypertropia" signifies a constant upward deviation of the right eye by 8 prism diopters.

    • Reports must adhere to standard conventions for clarity and consistency in clinical documentation, ensuring other practitioners understand the patient's binocular status.

Summary of Procedures and Tests

  • Cover Testing

    • The unilateral cover test specifically checks for the presence of tropias (manifest deviations) by observing if an eye moves to refixate upon uncovering the other eye.

    • The alternating cover test is used to reveal the patient's full phoric posture (latent deviation components) or total deviation (phorias + tropias if present) by breaking fusion. This test helps determine the magnitude and direction of all deviations, which are neutralized by prism bars until no movement is seen.

    • Adjustments with prism bars determine the corresponding deviations by inducing movements back to neutral positions, quantifying the amount of prism needed to correct the deviation.

  • Normal Ranges for Cover Tests

    • Distance cover test (at 6 meters/20 feet): Normal findings typically fall within 1\Delta exophoria to 3\Delta exophoria. A small amount of exophoria at distance is considered physiological due to the slight inherent divergence of the visual axes in parallel gaze.

    • Near cover test (at 40 cm/16 inches): Normal findings are generally 3\Delta exophoria to 6\Delta exophoria. A greater degree of exophoria at near compared to distance is expected, reflecting the active convergence required for near tasks. Emphasis is placed on carefully assessing esophoric conditions during near vision tasks, as even small esophorias at near can indicate a risk for eye strain or accommodative issues.