cheiroscope
A haploscopic device used to assess and treat suppression in patients with binocular vision disorders. It consists of a viewing instrument that presents an image to one eye and a drawing pad seen by the other eye. The patient must use both eyes simultaneously to trace or draw the image presented, promoting binocular vision and reducing suppression. Cheiroscopes can also be used to evaluate binocular stability, alignment, and eye-hand coordination.
chiastopic fusion
Also known as cross-fixation fusion, chiastopic fusion is a type of binocular fusion where each eye fixates on a separate target, with the right eye fixating on the left target and the left eye fixating on the right target. This is achieved by voluntarily converging the eyes to align the visual axes on the respective targets. Chiastopic fusion is not a normal viewing condition and is typically used in vision therapy to improve binocular function and reduce suppression.
comitant strabismus
A condition in which the angle of deviation between the eyes remains constant in all directions of gaze and regardless of which eye is fixating. This distinguishes it from incomitant strabismus, where the deviation varies with gaze position or the fixating eye. Comitant deviations are generally not associated with serious underlying neurological pathology. Examples include accommodative esotropia, infantile esotropia, and intermittent exotropia. Treatment options may include glasses, prisms, vision therapy, or surgery.
congenital strabismus
More accurately termed infantile strabismus, congenital strabismus is an ocular misalignment that develops within the first six months of life. It is characterized by a constant horizontal deviation, typically esotropia, with a large angle of deviation (≥30 prism diopters). Associated features may include dissociated vertical deviation, inferior oblique overaction, latent nystagmus, and optokinetic asymmetry. Early surgical intervention is often necessary to promote proper binocular development and prevent amblyopia.
conjugate prisms
Also known as yoked prisms, conjugate prismss are prisms with the bases oriented in the same direction before both eyes. They are sometimes used to treat A- or V-pattern deviations, where the angle of strabismus varies in upgaze and downgaze. For example, in V-pattern esotropia, where the eyes turn inward more in downgaze, base-down yoked prisms can be used to shift the eyes upward, away from the area of lost binocular vision.
consecutive strabismus
A new ocular misalignment that develops after surgical correction of a previous strabismus, differing in direction from the original deviation. It occurs when the surgical procedure overcorrects or undercorrects the initial strabismus, resulting in a consecutive deviation in the opposite direction. Consecutive strabismus may require additional surgery or other interventions to realign the eyes and restore binocular vision.
constant strabismus
A condition where the ocular misalignment is present continuously, regardless of the direction of gaze or the distance of the fixation object. In other words, one eye is constantly deviated inward, outward, upward, or downward relative to the other eye. This is in contrast to intermittent strabismus, where the eye deviation occurs only occasionally or under specific circumstances, such as during periods of fatigue or stress.
contracture
A condition in which an extraocular muscle loses its ability to fully relax, leading to permanent structural changes and inelasticity. It results in a persistent shortening or tightening of the affected muscle, causing a fixed ocular deviation or strabismus. Contractures can develop due to various factors, including trauma, inflammation, neurological disorders, or prolonged muscle overaction. Treatment may involve surgical lengthening or weakening of the contracted muscle to restore ocular alignment and motility.
convergence
The simultaneous inward movement of both eyes toward each other, allowing the eyes to fixate on a near object. It is achieved through the coordinated action of the medial rectus muscles, which adduct the eyes, and the horizontal vergence system. Convergence is an essential component of binocular vision, enabling the fusion of images from both eyes onto corresponding retinal points for clear, single vision at near distances.
convergence excess
A condition characterized by a significantly greater degree of esophoria (inward deviation of the eyes) at near viewing distances compared to distance viewing, typically associated with a high accommodative convergence/accommodation (AC/A) ratio.
convergence insufficiency
Convergence insufficiency is a binocular vision disorder characterized by the inability to achieve or sustain proper eye alignment and convergence when focusing on near objects. It occurs due to a deficiency in the neural control or musculature responsible for converging the eyes. Symptoms may include double vision, eye strain, headaches, blurred vision, and difficulty with prolonged near work such as reading. The condition is typically diagnosed through clinical tests evaluating near point of convergence, positive fusional vergence, and the degree of exophoria at near compared to distance.
covariation
In strabismus, covariation refers to a condition where the objective angle (the actual angle of ocular misalignment) and the angle of anomaly (the angle of abnormal binocular visual direction) change simultaneously, while the subjective angle (the perceived angle of misalignment) remains zero. In other words, the patient’s eyes are misaligned, but they perceive the visual world as being properly aligned due to the brain’s adaptation mechanisms. This phenomenon is often observed in long-standing strabismus cases and can pose challenges in treatment and management.
cross fixation
A condition where a patient with a large, alternating esotropia uses the adducted eye to fixate on objects in the opposite temporal field. It frequently occurs in cases of infantile esotropia. The patient alternates fixation between the two eyes, with one eye fixating on objects in the nasal field while the other eye crosses over to fixate on objects in the temporal field. This adaptation allows the patient to maintain some degree of binocularity despite the misalignment of the eyes.
crowding phenomenon
Refers to the amblyopic eye’s difficulty in discriminating small visual acuity targets when presented together in a row, despite being able to resolve the same size targets when presented individually. This phenomenon is characterized by the amblyopic eye’s reduced ability to perceive and distinguish closely spaced optotypes or letters, even though the individual optotypes can be identified when presented in isolation. It is a common feature observed in amblyopia and can impact visual acuity measurements and reading ability.
cycloplegia
Temporary paralysis of the ciliary muscle, preventing the lens from changing shape and eliminating the eye’s ability to accommodate or focus on near objects. It is typically induced by using cycloplegic eye drops containing medications that temporarily paralyze the ciliary muscle. Cycloplegia is employed during comprehensive eye examinations to obtain an accurate measurement of refractive error by eliminating accommodation’s influence.
cyclovertical muscles
The extraocular muscles responsible for vertical eye movements, including the superior and inferior rectus muscles, as well as the superior and inferior oblique muscles. These muscles work together to elevate, depress, and rotate the eyes, enabling vertical and torsional eye movements necessary for proper binocular vision and gaze stabilization.