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.
decompensated heterophoria
A condition where a previously controlled misalignment of the eyes (heterophoria) progresses to an uncontrolled, manifest misalignment (strabismus). This occurs when the fusional vergence mechanisms, responsible for maintaining binocular alignment, become overwhelmed and can no longer compensate for the heterophoria, resulting in a constant or intermittent strabismus.
deviating eye
In strabismus, the eye that is misaligned or deviated from the straight-ahead position, while the other eye fixates on the object of regard. It is the eye that is not properly aligned with the fixating eye, resulting in a lack of binocular vision and potential double vision or suppression of the deviating eye’s image.
dextroversion
The conjugate movement of both eyes to the right, a type of version. It occurs due to simultaneous contraction of the right lateral rectus and left medial rectus muscles.
diagnostic positions of gaze
The nine cardinal positions of ocular fixation used to assess normal or defective eye movements. These include the primary position (straight ahead), four secondary positions (up, down, left, and right), and four tertiary positions (up and left, up and right, down and left, down and right). Evaluating eye movements in these positions helps diagnose disorders of ocular motility and cranial nerve function.
diplopia
A condition where a single object is perceived as two separate images instead of one. Homonymous diplopia occurs when the image seen by the right eye is to the right of the image seen by the left eye. Heteronymous diplopia occurs when the image seen by the right eye is to the left of the image seen by the left eye. It results from a disruption in binocular vision due to misalignment of the eyes or impaired fusion mechanisms.
disparometer
An instrument used to measure fixation disparity, which is a small misalignment of the visual axes when attempting to bifixate on an object. It quantifies the degree of residual deviation between the lines of sight of the two eyes when fusion is disrupted. Evaluating fixation disparity helps assess binocular vision status and guide treatment decisions for conditions like strabismus and amblyopia.
dissociated vertical deviation
A condition where, upon occlusion or dissociation of one eye, the occluded eye deviates upward, while the viewing eye maintains fixation. When the right eye fixates, the left eye deviates upward (left hyperdeviation). Conversely, when the left eye fixates, the right eye deviates upward (right hyperdeviation). This vertical misalignment of the occluded eye is only present during monocular viewing conditions.
divergence
The outward turning of the eyes, causing the lines of sight to intersect behind the eyes. It allows the eyes to shift focus from near to far objects by relaxing the medial rectus muscles and contracting the lateral rectus muscles. The neural control of divergence involves pathways in the midbrain, pons, and cerebellum. Unlike convergence, the dynamics of divergence eye movements depend on the initial vergence position.
divergence excess
A binocular vision disorder characterized by a significantly higher exophoria or intermittent exotropia at distance compared to near fixation. It involves an excessive outward deviation of the eyes when viewing distant objects, while the exophoria is much lower or absent at near. This condition can lead to diplopia or suppression and is often associated with convergence insufficiency or high AC/A ratio.
divergence insufficiency
A binocular vision disorder where there is a higher esophoric or intermittent esotropic deviation at distance compared to near fixation. The eyes have difficulty diverging or turning outward to view distant objects, while the deviation is much lower or absent at near. This condition can lead to diplopia or suppression when viewing distant targets. It is frequently associated with convergence excess or high accommodative convergence/accommodation ratio and may require treatment with prism, vision therapy exercises, or surgical intervention depending on the severity.
doll’s-head maneuver
An involuntary eye movement where the eyes rotate in the opposite direction to a sudden head rotation, through an equal angle, before slowly returning to the original position. This reflex is mediated by the vestibulo-ocular reflex pathways and is normally suppressed during voluntary gaze. However, in cases of supranuclear lesions affecting voluntary eye movement control, the oculocephalic reflex may be exaggerated or persist. Also known as the oculocephalic reflex.