saccades

 Abrupt, voluntary shifts in fixation from one target to another, rapidly moving the fovea to a new object of interest. Saccades are very fast, ballistic, conjugate eye movements that alternate with smooth pursuit to scan a visual scene. Their velocity depends on the amplitude of the movement.

saccadic velocity

The speed of saccadic eye movements, typically measured using techniques like electro-oculography that record the electrical potentials corresponding to eye movements. Saccadic velocity follows a predictable relationship with saccade amplitude, peaking around 20-30 degrees. Abnormally slow velocities can indicate muscle paresis, helping differentiate it from mechanical restrictions.

second-degree fusion

The ability to combine portions of two slightly dissimilar images into a single percept by fusing areas with common borders or contours. It represents an advanced level of binocular fusion beyond fusing identical images, occurring over Panum’s fusional area where some horizontal disparity is tolerated. Second-degree fusion allows stable single binocular vision and stereopsis by integrating binocular inputs with minor differences.

secondary angle of deviation

In incomitant strabismus, the secondary angle of deviation is the angle of misalignment of the normally fixating eye from the point of fixation when the affected eye with limited motility is fixating. It is typically smaller than the primary deviation measured when the normal eye fixates.

secondary position

Any position of the eye represented by a vertical or horizontal deviation of the line of sight from the primary position of straight ahead gaze.

sensory fusion

The process by which the brain combines the slightly different images from each eye into a single coherent visual percept, allowing for depth perception and stereopsis. It occurs over Panum’s fusional area, where some horizontal disparity between the retinal images is tolerated by the visual system’s binocular neurons.

sensory strabismus

Misalignment of the eyes caused by poor vision in one eye. The brain suppresses the image from the eye with poorer vision to avoid diplopia, resulting in a drift of that eye outward (exotropia) or inward (esotropia) due to lack of fusion. Common causes include amblyopia, congenital cataracts, corneal opacities, and optic nerve disorders.

shape magnification

Shape magnification refers to the distortion of image size and shape caused by variations in the curvature and thickness across a spectacle lens surface. It results in objects appearing stretched or compressed, especially towards the lens periphery due to increasing oblique astigmatism. Shape magnification is more pronounced in high minus lenses and lenses made of higher refractive index materials.

Sheard’s criterion

The amount of heterophoria should not exceed half of the opposing fusional vergence reserves, or the patient is likely to experience eyestrain.

Exophoria creates a demand for positive fusional vergence (convergence) to overcome the outward deviation of the eyes. Esophoria requires negative fusional vergence (divergence) to compensate for the inward deviation.

skew deviation

A vertical misalignment of the eyes that does not follow the pattern of an isolated muscle paresis. It results from a supranuclear lesion, often in the brainstem or cerebellum, disrupting pathways that integrate vestibular and visual inputs for ocular alignment. Skew deviation is frequently associated with ocular torsion and pathologic head tilt.

spasm of accommodation

An involuntary, sustained contraction of the ciliary muscle, producing excess accommodation resulting in blurred distance vision but clear near vision (pseudomyopia).

spasm of the near reflex

Spasm of the near reflex refers to intermittent episodes characterized by involuntary action of the entire near vision triad—sustained convergence of the eyes, accommodative spasm producing excess near focus, and pupillary miosis.

stereopsis

The binocular visual perception of three-dimensional space and depth based on the horizontal retinal disparity between the two eyes’ slightly different viewpoints. The brain processes and combines these disparate retinal images to extract precise depth information, allowing for accurate judgment of relative distances.

Strabismus

Heterotropia, or strabismus is a condition where the eyes are misaligned such that binocular fixation is not achieved under normal viewing conditions. It occurs when the foveal line of sight of one eye fails to intersect the object of fixation, resulting in the inability to fuse images from both eyes. Strabismus can be inward, outward, upward, or downward, and may be constant or intermittent.

superior oblique tuck

A surgical procedure that strengthens the superior oblique muscle by doubling over its loose tendon and securing it with a suture woven through the tendon base in a double figure-eight fashion. This shortens the effective tendon length, enhancing the muscle’s actions of depression, intorsion, and abduction. The superior oblique tuck is commonly performed to treat superior oblique palsy or paresis, correcting the associated hypertropia, excyclotorsion, and limited depression.

suppression

The inability to perceive normally visible objects in part or all of the visual field of one eye, occurring only when both eyes are open and receiving simultaneous visual input. It is attributed to cortical inhibition and serves as a sensory adaptation to prevent diplopia when binocular fusion cannot be achieved.

supraduction

Supraduction is the upward movement of the eye, controlled by the superior rectus and inferior oblique muscles. Limitation in supraduction can indicate dysfunction of these muscles or their innervating cranial nerves (oculomotor and trochlear nerves).

supranuclear

Structures or pathways located above the level of the oculomotor, trochlear, and abducens cranial nerve nuclei. Supranuclear control centers coordinate and modulate different types of eye movements soLesions in these supranuclear areas can cause nystagmus, gaze palsies, saccadic pursuit dissociation, etc.

supravergence

The conjugate movement of both eyes upward, a type of version. It occurs due to simultaneous contraction of the superior rectus muscles in each eye.

synergist

A muscle  that acts cooperatively with another muscle (the agonist) to produce a movement. It has a similar primary action as the agonist for that specific movement. For example, the inferior oblique is a synergist to the superior rectus for elevation of a given eye.

synoptophore

Also know as the major amblyoscope, the synoptophore is a reflecting mirror haploscopic device consisting of two angled tubes that present separate images to each eye. The tubes can be adjusted to vary the convergence or divergence angle, allowing measurement of the angle of ocular deviation at different gaze positions. The synoptophore disrupts sensory fusion to evaluate binocular vision and is also used for orthoptic exercises to treat conditions like strabismus and amblyopia.