tonic convergence

The baseline convergence maintained by the tonic contraction of the extraocular muscles, primarily the medial rectus muscles, which contributes to the distance phoria.

torsion

Also known as cyclodeviation, torsion refers to the rotation or wheel-like movement of the eye around its anteroposterior axis, which corresponds approximately to the line of sight when looking straight ahead. It allows the eyes to rotate in a clockwise or counterclockwise direction and plays a role in maintaining proper binocular alignment across different gaze positions and head tilts.

tranaglyphs

From transparent anaglyph, tranaglyphs are a variety of training and test targets consisting of transparencies with various red and green details or images. They are viewed while wearing anaglyph glasses with complementary red and green filters over each eye. This dissociates the images so the brain must combine them into a single stereoscopic 3D percept through sensory fusion.

trochlear nerve

Also known as cranial nerve IV, the trochlear nerve is a purely motor nerve that innervates the superior oblique muscle of the eye. The “Trochlea” refers to the pulley-like structure the superior oblique tendon passes through, giving the nerve its name.

Anatomically, it emerges from the dorsal aspect of the brainstem, decussates to the opposite side, and innervates the contralateral superior oblique muscle—no other cranial nerves innervate a muscle contralateral of their nucleus.

TV trainer

A vision therapy device consisting of a plastic filter attached to a television screen. The patient views the screen through red-green or polarized filters, dissociating the images seen by each eye. It is used to treat suppression and develop binocular vision skills by presenting separate targets to each eye.

unilateral cover test

The unilateral (cover-uncover) cover test is an objective method used to determine the presence of a strabismus and differentiate it from a heterophoria. One eye is covered with an opaque occluder while the patient fixates on a target. The uncovered eye is observed for movement during the cover phase, indicating a manifest deviation. If movement is not observed from either eye, move on to the alternate cover test to quantify any heterophoria.

unilateral strabismus

A condition where only the non-strabismic eye can maintain fixation while both eyes are open. The deviated eye is typically suppressed to avoid diplopia. In young children, constant unilateral strabismus is amblyopiagenic if left untreated.

V pattern

A vertically incomitant horizontal strabismus where the deviation increases in a specific vertical gaze direction. In V-pattern exotropia, the outward deviation is greater in upgaze compared to downgaze or primary position. In V-pattern esotropia, the inward deviation is greater in downgaze compared to upgaze or primary position. The difference between upgaze and downgaze deviations must be at least 15 prism diopters to be considered clinically significant.

vectogram

A polarized stereogram that presents a different image to each eye, allowing for the perception of depth. It consists of two photographic images printed on opposite sides of a transparent film, with their axes of polarization oriented at right angles to each other. When viewed through polarized filters, each eye sees only one image. Vectograms are commonly used in vision therapy to improve binocular vision skills, including convergence, divergence, and stereopsis.

vergence

The simultaneous movement of both eyes in opposite directions to maintain single binocular vision. Convergence is an inward rotation of the eyes, while divergence is an outward rotation. Driven by retinal disparity, blur, and proximal cues, vergence works in conjunction with saccades to quickly align the eyes. .

vergence facility

A measure of how quickly and accurately the fast fusional vergence system responds to rapidly changing vergence demands. Typically tested using prism flippers, the patient alternates fusion through 3Δ base-in and 12Δ base-out prisms at near, and the cycles per minute (cpm) are recorded. Reduced vergence facility (<15 cpm) can indicate binocular vision disorders. This dynamic test reflects vergence automaticity, as opposed to slow fusional vergence ranges.

vertical offset-setting (of the horizontal recti)

Reattaching the medial and lateral rectus muscles at a new scleral insertion point that is vertically displaced from the original insertion, usually between 1/2 to 1 full tendon width higher or lower. Moving the medial rectus insertions downward decreases adduction in downgaze, treating A-pattern esotropia. Moving the lateral rectus insertions upward decreases abduction in upgaze, treating A-pattern exotropia. The opposite vertical transpositions treat V-pattern strabismus by increasing the horizontal deviation in the opposite field of gaze.

visual confusion

A phenomenon that can occur in strabismus when the foveas of the two misaligned eyes are fixating on different objects. The brain perceives the two dissimilar objects as occupying the same visual direction, causing them to appear superimposed. Visual confusion is more likely to be experienced in acute strabismus before suppression mechanisms develop to avoid double vision.

visual evoked potentials

Electrophysiological signals recorded from the visual cortex in response to visual stimulation. VEPs assess the functional integrity of the visual pathways from the retina via the optic nerves to the occipital cortex. Electrodes placed on the scalp measure the electrical activity elicited by flash, pattern reversal, or pattern onset/offset stimuli. Abnormalities in VEP waveform latencies or amplitudes can indicate dysfunction within the visual system.

visuoscope

An ophthalmoscope adapted with a small, central fixation target that projects a shadow onto the retina to assess monocular fixation in amblyopia. The position of the shadow relative to the fovea indicates the type of fixation. Visuoscopy allows direct observation of the retinal fixation point, requiring patients to voluntarily fixate the target. It assumes the foveal reflex accurately demarcates the foveal center.

Wheatstone stereoscope

The first device to artificially produce the perception of depth from two flat pictures. It consists of two angled mirrors that reflect images from adjustable side panels to each eye separately. The brain fuses the two images into a single stereoscopic percept. Charles Wheatstone used it to demonstrate that stereopsis arises from binocular disparity processed in the visual cortex, not the eyes themselves.

Worth four dot test

The Worth four dot test is a binocular vision test that assesses fusion and suppression at near and distance. It uses a flashlight with four dots (one red, two green, one white) and red-green anaglyph glasses.  Patients report the number of dots seen, indicating fusion, suppression, or diplopia.

Seeing 4 dots (1 red, 2 green, 1 white/yellow):
– This indicates normal binocular fusion and flat fusion.

Seeing 2 dots (2 red):
– Indicates suppression of the left eye

Seeing 3 dots (3 green):
– Indicates suppression of the right eye

Seeing 5 dots (2 red on one side, 3 green on other side):
– Indicates diplopia
– If red dots are on the right, it suggests an exotropia (outward deviation)
– If red dots are on the left, it suggests an esotropia (inward deviation)

X pattern

A form of incomitant exotropia where the outward deviation of the eyes increases in both upward and downward gaze compared to primary gaze. It is commonly seen in long-standing exotropias due to the leash effect of tight lateral rectus muscles restricting abduction in up and down gaze. X pattern can also occur in type 3 Duane retraction syndrome. Overaction of all four oblique muscles has been proposed as an underlying mechanism.

Y-splitting procedure

The Y-splitting procedure divides the lateral rectus muscle tendon lengthwise into two halves that are then reinserted 10-15 mm apart on the sclera in a “Y” configuration. This stabilizes the muscle during co-contraction by preventing the “slipping leash” effect where the tendon slides over the sclera, producing vertical deviations. Y-splitting reduces incomitant vertical deviations that increase with lateral gaze, as seen in Duane syndrome, while preserving some abduction.

yoke muscles

Pairs of extraocular muscles that move the two eyes conjugately in the same direction through equal and simultaneous innervation. Examples include the lateral rectus of one eye and the medial rectus of the other eye for horizontal movements.