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Glossary of VT-Related Terminology
Accommodation is the ability of the eye to change focus to clearly see objects at different distances. This is accomplished through changes in shape of the crystalline lens.
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Amblyopia is reduced visual acuity (poorer than 20/20) which is not correctable by refractive means, is not attributable to obvious structural or pathological anomalies, and is associated with form deprivation, strabismus, anisometropia, or other amblyopiogenic refractive errors.Amblyopia represents a syndrome of compromising deficits (rather than simply reduced visual acuity) including:
- increased sensitivity to contour interaction effects
- abnormal spatial distortions and uncertainty
- unsteady and inaccurate monocular fixation
- poor eye tracking ability
- reduced contrast sensitivity
- inaccurate accommodative response
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Amblyopia ex anopsia, synonymous with form deprivation amblyopia, is secondary to conditions that prevent light from entering the eye. These may include congenital ptosis, corneal opacity, or cataract. This term has also been used to describe amblyopia secondary to suppression, like that resulting from strabismus, anisometropia, or other amblyopiogenic refractive errors.
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Amplitude of accommodation is the difference, expressed in diopters, between the far point of accommodation and the near point of accommodation measured clinically from the spectacle plane.
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Angle E is the magnitude of the angle of eccentric fixation; when testing fixation with afterimage techniques or entoptic phenomena (Maxwell's spot or Haidinger brushes), it is the angular distance between the fixation spot and the afterimage (or entoptic phenomenon).It is represented by the distance on the retina from point 'e' to the center of the fovea.
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The the clinical angle kappa, which is actually angle lambda, is the angle between the pupillary axis and the visual axis. It is "measured" monocularly using the corneal reflexes (reflections).While covering the opposite eye, the position of the corneal reflex is measured.
A *positive* reflex is one located nasal to the center of the pupil.
A *negative* reflex is one located temporal to the center of the pupil.
EXAMPLE
QUESTION The eye with 20/20 vision has an angle kappa measured to be +1/4mm (nasally).
The eye with 20/400 vision has an angle kappa of -1/4mm (temporally).How much is the patient fixating in degrees from the center of the fovea?
How would this type of eccentric fixation be classified?
ANSWER The patient is eccentrically fixating a total of 1/2mm (from +1/4 to -1/4).
EXPLANATION 1.0mm is equal to approximately 22 prism diopters, or 12 degrees.
The patient is eccentrically fixating 11 prism diopters, or 6 degrees.This would be considered peripheral eccentric fixation,
and the patient would be expected to have markedly reduced visual acuity.
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The angle of anomaly (Angle A) is the magnitude of the difference between the objective and subjective angles of deviation.Algebraically, A = H - S. If H equals S, then A equals zero, and correspondence is said to be normal. If A does not equal zero (i.e., H and S are different), then correspondence is said to be anomalous.
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The anomalous point (point 'a') is the point on the retina of the deviating eye which corresponds to the fovea of the nondeviating eye.
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anomalous (retinal) correspondence (A[R]C)
Anomalous retinal correspondence (ARC), an adaptation to strabismus, is a binocular condition in which the two foveae do not correspond in the visual cortex. It occurs when a nonfoveal point in the retina of the deviating eye is "associated" in the visual cortex with the fovea of the fixating eye.It is more correctly referred to as anomalous correspondence because correspondence is cortical, rather than retinal.
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The Brewster stereoscope is an instrument designed to present separate images to each eye, consisting essentially of two parallel viewing tubes, acting to separate the fields of view of the two eyes, each containing convex lenses, usually +5.00 diopters in power, which are adjustable for interpupillary distance and distance from the targets at the base of the instrument. Examples of Brewster-type stereoscopes include the Keystone Telebinocular and the Bernell-O-Scope.
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Concomitancy (comitancy) means that the angle of deviation of the visual axes remains the same in all positions of gaze. There are no abnormal under- or overactions of the extraocular muscles.Perfect concomitancy is rare since the angle of deviation for most people varies slightly from one direction of gaze to another. Clinically, if the angle changes by less than 5 prism diopters, it is considered to be concomitant.
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Confusion occurs when two different objects are seen in the same visual direction, commonly as a result of strabismus.
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Convergence excess (CE) is a condition in which the patient exhibits approximate orthophoria at distance and has an esophoria at near.It is associated with a high AC/A ratio.
Patient symptoms may include visual fatigue, drowsiness, and transient diplopia accompanying prolonged near work.
Effective treatment includes a plus add for near or use of base-out prism for close work to reduce symptoms.
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convergence insufficiency (CI)
Convergence insufficiency (CI) is a condition in which the patient exhibits approximate orthophoria at distance and has a moderate to high exophoria at near.It is associated with a low AC/A ratio. Typically, the NPC is receded, and there is likely to be a low positive fusional vergence reserve.
Patient symptoms may include visual fatigue, drowsiness, and transient diplopia accompanying prolonged near work.
Vision training is an effective way to manage this condition.
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Diplopia occurs when a single object is seen as two objects in different visual directions, commonly as a result of strabismus; a.k.a. double vision.
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Divergence excess (DE) is characterized by high exophoria at distance and low exophoria or orthophoria at near.It is associated with a high AC/A ratio.
Divergence excess cases can be treated with vision training, BI prism for distance, or a decrease in plus power or an increase in minus power for distance.
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Divergence insufficiency (DI) is characterized by an esophoria at distance and an approximate orthophoria at near.It is associated with a low AC/A ratio.
Effective treatment includes base-out prism for distance and vision training to increase divergence ability.
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Eccentric fixation is an adaptation to strabismus, and occurs when a nonfoveal retinal point is used for fixation. That is, the deviating eye does not use the central foveal area for fixation. This explains why all patients with eccentric fixation are amblyopic.. Eccentric fixation is tested under monocular conditions only, because it is a monocular phenomena.In esotropia, the eccentrically located retinal point used for fixation is usually in the nasal retina. In exotropia, the eccentrically located retinal point used for fixation is usually in the temporal retina.
In documenting eccentric fixation, variables such as steadiness (steady or unsteady) and direction (nasal, temporal, superior, inferior) of fixation, as well as the magnitude of the deviation must be noted.
- Foveal off-center: The fixation point used is 1 degree or less from the fovea.
- Parafoveal: The fixation point used is 1 to 3 degrees from the center of the fovea.
- Paramacular: The fixation point used is 3 to 5 degrees from the center of the fovea.
- Peripheral: The fixation point used is more than 5 degrees from the center of the fovea.
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The voluntary act of using a different area of the retina because the fovea is not funtional. This action is often used by patients with ARMD, toxo, or other diseases affecting the macular area and destroying vision.
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Esophoria occurs when the visual axes of the eyes converge at a point between the patient and the object of regard when fusion is suspended.Esotropia occurs when the visual axes of the eyes converge at a point between the patient and the object of regard under normal, binocular viewing conditions (i.e. a convergent strabismus).
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Exophoria occurs when the visual axes of the eyes converge at a point beyond the object of regard (or diverge in the case of an object of regard located at optical infinity) when fusion is suspended.Exotropia occurs when the visual axes of the eyes converge at a point beyond the object of regard (or diverge in the case of an object of regard located at optical infinity) under normal, binocular viewing conditions (i.e., a divergent strabismus).
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Facility of accommodation is a measure of the ease and speed of accommodative changes of the eye.
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Flat fusion (second-degree fusion) is the integration of two similar ocular images into a single percept. The target must be two-dimensional and identical in form for each eye.
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harmonious anomalous correspondence
Harmonious anomalous correspondence (HAC) occurs when the objective angle (angle H) and the angle of anomaly (angle A) are equal (and, thus, the subjective angle(angle S) is zero).The anomalous correspondence can be considered to be "successful" in allowing visual direction to be similar for the two eyes with the strabismic eye in its deviating position. It is the most common and the most logical type of ARC.
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Heterophoria, also referred to as "phoria" or "dissociated phoria," is a latent deviation of the visual axes of the eyes.It is manifested by eliminating all stimuli to fusion, such as in the cover test or with a stereoscope. The magnitude of the heterophoria is a result of the combined effects of tonic, accommodative, and proximal vergences.
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Heterotropia (or "tropia") is a manifest deviation of the visual axes of the two eyes. The deviation occurs even when stimulus to fusion exists. It is also referred to as strabismus.
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In horror fusionis, superimposition and second degree fusion are not possible, and simultaneous perception is the best form of binocularity that can be obtained.The usual recommendation is no treatment since the prognosis is poor.
Horror fusionis has been defined as the inability to obtain binocular fusion or superimposition of binocular retinal images, occurring frequently as a characteristic in long-standing strabismus. The images approaching superimposition seem to slide or jump past each other without fusion or suppression.
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Lag of accommodation is the dioptric difference between the accommodative response and the stimulus to accommodation.It can be thought of as a measure of accommodative accuracy.
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Latent phoria is the magnitude of the measured heterophoria following a prolonged period of absence of fusion, typically with extended unilateral occlusion; after several hours or overnight, for example.
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Malingering is a voluntary or intentional reduction in visual acuity or other examination data for the purpose of gaining an objective.
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Manifest phoria is the magnitude of the measured heterophoria under standard testing conditions, immediately after elimination of fusion.
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Nonconcomitancy occurs when the deviation changes with different positions of gaze. Therefore, there is either abnormal restriction to movement or overaction of one or more of the extraocular muscles.Clinically, if the change in deviation is more than 5 prism diopters, the deviation is considered to be nonconcomitant.
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normal (retinal) correspondence (N[R]C)
Normal retinal correspondence (NRC) occurs when the foveae of the two eyes are corresponding neural points in the visual cortex. Thus, normal retinal correspondence is actually a misnomer, since this correspondence occurs in the cortex and not in the retina.
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The objective angle (angle H) of deviation is the magnitude of the horizontal angle of heterophoria or heterotropia as measured by objective testing methods, such as with the cover test or corneal reflex testing.
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Direct occlusion means occluding the non-amblyopic eye.Indirect, or inverse, occlusion means occluding the amblyopic eye.
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Orthophoria occurs when the two visual axes are directed toward the point of binocular fixation in the absence of an adequate stimulus to fusion. That is, the magnitude of the heterophoria is zero.Orthotropia occurs when the two visual axes are directed toward the point of binocular fixation while stimulus to fusion exists.
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paradoxical anomalous correspondence
Paradoxical anomalous correspondence (PAC) can be classified as either Type 1 or Type 2. PAC is typically found in the postoperative strabismus case.
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Past pointing is misjudging the location of an object due to eccentric fixation.
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Prism adaptation is the process by which the eyes return to their original condition of habitual heterophoria after introduction of prisms or other fusional stimuli before the two eyes.Prism adaptation is thought to occur as a result of changes in tonic vergence over time.
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Sensory fusion is the process by which stimuli seen separately by the two eyes are combined into a single percept.Under normal binocular conditions, this occurs when corresponding retinal areas are stimulated by the same object, or objects of similar content.
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Simultaneous perception occurs when the images of the two eyes are seen simultaneously, with or without sensory fusion.This perception is termed 'bi-ocular' if fusion is absent and 'binocular' if fusion is present.
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Squint is synonymous with heterotropia and strabismus.
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Stereopsis (third-degree fusion) is the binocular perception of depth based on small amounts of retinal disparity.Stereopsis is not synonymous with depth perception and is only one cue to depth.
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Strabismus is synonymous with heterotropia and squint. Clinically, strabismus must be properly characterized, using the following categories:
- Unilateral strabismus: Strabismus in which only one eye deviates.
- Alternating strabismus: Strabismus in which the deviating eye can change.
- Intermittent strabismus: Strabismus which is not present at all times.
- Constant strabismus: Strabismus which is present at all times.
- Periodic strabismus: Strabismus which occurs at one testing distance but not at another.
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The subjective angle (angle S) of deviation is the magnitude of the horizontal angle of heterophoria or heterotropia as measured by subjective testing methods, requiring the patient to align the stimuli of the two eyes.
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Superimposition of two dissimilar targets is also known as first degree fusion. When this occurs, confusion exists because similar targets are not being integrated and true sensory fusion does not exist.
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Suppression is a binocular phenomenon resulting in the inhibition at the cortical level of the image from one eye. Suppression typically occurs as an adaptation to an anomaly of binocular vision, like strabismus.
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unharmonious anomalous correspondence
Unharmonious anomalous correspondence (UAC) occurs when the angle of anomaly is less than the objective angle of strabismus.
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Vergence eye movements are dysjunctive eye movements used to maintain fusion. Convergence occurs when the visual axes move towards one another. Divergence occurs when the visual axes move apart.
- tonic vergence
Tonic vergence is the passive state of vergence of the eyes in the absence of a stimulus to fusion while viewing a distant target. This position is maintained primarily by the tonus of the extraocular muscles and the persisting activity of converging information.
Tonic convergence is responsible for moving the eyes from the position of anatomical rest to a more convergent position, and is responsible for the magnitude of the heterophoria at distance.
Deficient tonic convergence would result in exophoria and excessive tonic convergence results in esophoria.
- proximal vergence
Proximal vergence is the component of convergence resulting from the "awareness of nearness" of the object of regard. It is an unpredictable and nonlinear quantity.
- accommodative vergence
Accommodative vergence is the component of convergence which occurs as a direct result of accommodation. Accommodation and convergence are two components of the "near triad," the third being pupillary miosis.
- fusional vergence
Fusional vergence is the component of convergence which serves to maintain fusion of the two retinal images. It is that amount of vergence that is required in addition to tonic, accommodative, and proximal vergence to maintain fusion of a target stimulus.
- fast (phasic) vergence
A type of fusional vergence that acts quickly to reduce retinal disparity--about 1 second. Once the image is single and clear, the fast vergence begins to decay, and slow vergence is needed to reduce stress on the system.
- slow (tonic) vergence
Slow vergence, a type of fusional vergence, is driven by output of the fast vergence system. It is slower acting (30 seconds) and does not decay rapidly. The effects produce changes in tonic vergences that supplement the fast vergence system as it decays.
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The Wheatstone stereoscope is an instrument designed to present separate images to each eye using two plane mirrors joined at one edge at a 90-degree angle and two target holders, one opposite one mirror and the other, mounted on a screw base which, when turned, synchronously moves the targets toward or away from each other.Devised by Charles Wheatstone in 1838, it is constructed so that each eye can see independently of the other.
Examples of Wheatstone-type stereoscopes include the major amblyoscope, the Troposcope, the Synoptophore and the Bernell Mirror Stereoscope.
The Synoptophore is one of the most elaborate of all Wheatstone stereoscopes.
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The zero point (also known as the "zero measure point", or point 'z') is the point on the retina of the deviating eye representing no vergence demand (if there is bifixation, point 'z' is at point 'f', the center of the fovea).
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