A pattern

A type of incomitant strabismus characterized by a difference in the horizontal deviation between upgaze and downgaze of at least 10 prism diopters. In A pattern exotropia, the outward deviation is greater in downgaze than in upgaze. In A pattern esotropia, the inward deviation is greater in upgaze than in downgaze. Etiologies may include superior oblique overaction, orbital pulley abnormalities, or ocular torsion.

abducens nerve

Cranial Nerve VI is a somatic efferent nerve that innervates the lateral rectus muscle of the eye. It originates from the abducens nucleus in the pons and emerges at the pontomedullary junction. The nerve travels through the subarachnoid space, cavernous sinus, and superior orbital fissure to reach the lateral rectus muscle, enabling abduction of the eye. Damage to the abducens nerve results in impaired lateral eye movement and esotropia.

abduction

Abduction is the outward movement of the eye towards the temple. It is controlled by the lateral rectus muscle, which is innervated by the abducens nerve (cranial nerve VI).

aberrant regeneration

Aberrant regeneration is a possible sequela of an oculomotor nerve palsy, resulting in abnormal lid, pupillary, and ocular motility. It occurs when regenerating axons are misdirected to anomalous connections within the oculomotor nerve. Common signs include lid elevation on adduction (pseudo-Graefe’s sign) or infraduction, and pupillary constriction on adduction. The most frequent causes are trauma, tumors, and aneurysms. Onset typically appears within 6-12 months after the initial injury.

accommodation

The eye’s ability to adjust its refractive power by changing the shape of the crystalline lens, allowing objects at various distances to be focused on the retina. This process is mediated by the ciliary muscle and zonular fibers, which control lens curvature. Accommodation is a dynamic response to blurred retinal images, with the goal of maximizing image sharpness. The accommodative range decreases with age due to lens stiffening, resulting in presbyopia.

accommodative convergence

The inward movement of the eyes that occurs in conjunction with the accommodation reflex when viewing near objects. As the crystalline lens changes shape to focus on a closer target, a neurological link triggers the medial rectus muscles to converge the eyes. This convergence response is proportional to the amount of accommodation and helps maintain binocular fusion and clear single vision at near distances.

accommodative convergence/accommodation (AC/A) ratio

The convergence response of an individual to a unit stimulus of accommodation, expressed as the quotient of accommodative convergence in prism diopters divided by the accommodative stimulus in diopters.

accommodative esotropia

accommodative insufficiency

adduction

Adduction is the inward movement of the eye towards the nose. It is controlled by the medial rectus muscle, which is innervated by the oculomotor nerve (cranial nerve III).

Adie’s tonic pupil

adjustable suture

A technique used in strabismus surgery where the extraocular muscle is reattached to the sclera using sutures with temporary knots. After the patient recovers from anesthesia, the knots can be adjusted to modify the final muscle position and eye alignment.

afocal lens (size lens)

afterimage test

A method for assessing retinal correspondence by projecting afterimages of two flashing lights, one vertical and one horizontal, onto each eye. In normal retinal correspondence (NRC), the afterimages form a cross. In anomalous retinal correspondence (ARC), the afterimages are offset, indicating a mismatch between the fovea of one eye and an extra-foveal point in the other eye.

alternate cover test

A method for measuring the total ocular deviation, including both the manifest (tropia) and latent (phoria) components. The examiner alternately occludes each eye, preventing binocular fusion. The deviation is quantified by placing prisms of increasing power before one eye until no fixation movement is observed when the cover is switched. The prism strength at neutralization represents the total deviation, measured in prism diopters.

amblyopia

amblyopiagenic

Any condition that can cause amblyopia if left untreated during early childhood. Common amblyopiagenic factors include anisometropia (unequal refractive errors between the eyes), strabismus (misalignment of the eyes), and visual deprivation (obstruction of vision in one eye due to conditions like congenital cataract or ptosis). These factors disrupt normal visual development, leading to the brain favoring one eye over the other, resulting in amblyopia if not addressed promptly.

angle kappa

The angle between the pupillary axis, which passes through the center of the entrance pupil and is perpendicular to the cornea, and the visual axis, which connects the fixation point with the fovea. A positive angle kappa occurs when the pupillary axis is nasal to the visual axis, which is the typical configuration. Angle kappa is often confused with angle lambda, which is the angle between the pupillary axis and line of sight.

angle of anomaly

The difference between the objective angle of deviation, which is the actual misalignment of the eyes, and the subjective angle of deviation, which is the perceived misalignment based on the anomalous retinal correspondence. In harmonious anomalous retinal correspondence, the angle of anomaly equals the objective angle, while in unharmonious anomalous retinal correspondence, the angle of anomaly is less than the objective angle. The angle of anomaly can be measured using the synoptophore with special slides.

angle of deviation

The angle formed between the visual axes of the fixating and the deviated eye when one eye is misaligned in strabismus. It can be measured subjectively using a synoptophore, where the patient superimposes first-degree targets, or objectively using the alternate cover test with prisms. The angle of deviation quantifies the magnitude of ocular misalignment and is essential for diagnosing and monitoring strabismus. Larger angles generally correlate with poorer binocular vision.

aniseikonia

A condition where the perceived size and/or shape of images differs between the two eyes. It can be caused by anisometropia (unequal refractive errors), retinal pathology, or optical factors such as anisophoria. Symptoms may include headaches, eye strain, dizziness, and diplopia. Treatment options depend on the underlying cause and may involve corrective lenses, contact lenses, or surgery. Measuring aniseikonia is important for determining the appropriate treatment approach.

anisometropia

A condition in which the refractive error of one eye differs significantly from the other, usually by one diopter or more. This imbalance can cause unequal focus between the eyes, leading to blurred vision, eyestrain, and headaches. Anisometropia may involve myopia, hyperopia, or astigmatism. If left untreated in children, it can result in amblyopia. Treatment options include corrective lenses, contact lenses, or refractive surgery, depending on the severity and type of anisometropia.

anisophoria

A type of heterophoria in which the magnitude of the eye misalignment varies depending on the direction of gaze. It can be caused by paresis or spasm of one or more extraocular muscles or by anisometropic spectacle correction, where different prismatic effects induce different phorias in different gaze positions (optical anisophoria). Anisophoria is a form of incomitance, as the angular relationship between the visual axes changes with the fixating eye.

anomalous retinal correspondence

A sensory adaptation in strabismus where the fovea of the fixating eye corresponds with an eccentric retinal area in the deviated eye, allowing the eyes to work together without diplopia. In harmonious ARC, the angle of anomaly equals the objective angle of deviation. In unharmonious ARC, the angle of anomaly is smaller than the objective angle. ARC can be confirmed using afterimage testing or the synoptophore.

antagonist (muscle)

A muscle that has the opposite primary action to another muscle in the same eye. For example, the lateral rectus abducts the eye while its antagonist, the medial rectus, adducts the eye. Antagonist muscles work in pairs to produce smooth, coordinated eye movements. When one muscle contracts, its antagonist relaxes, allowing the eye to move in the desired direction. This reciprocal innervation is essential for precise ocular motility.

associated phoria

The amount of prism required to eliminate a fixation disparity. It represents the prism power needed to compensate for the misalignment of the visual axes under binocular viewing conditions. Associated phoria is typically measured using a fixation disparity test, such as the Mallett unit or the Bernell lantern with a polarized fixation disparity target. The associated phoria helps guide the prescription of prismatic corrections for symptomatic patients with binocular vision disorders.

asthenopia

Also known as eye strain, asthenopia is a condition characterized by nonspecific symptoms that occur after prolonged use of the eyes. These symptoms may include eye fatigue, discomfort, burning, itching, watering, blurred vision, headache, and occasionally diplopia. Asthenopia can be caused by various factors such as uncorrected refractive errors, accommodative or binocular vision disorders, prolonged near work, inappropriate lighting conditions, and dry eyes. Treatment involves addressing the underlying cause and implementing ergonomic measures to reduce visual stress.

attenuation

A form of occlusion therapy in which the transmission of light to the non-amblyopic eye is reduced using translucent filters or materials. This approach aims to stimulate the amblyopic eye while maintaining some binocular input, potentially improving treatment compliance and outcomes compared to traditional patching. Attenuation can be achieved using Bangerter filters, translucent tape, or other materials that blur the vision in the non-amblyopic eye, forcing the amblyopic eye to be used for visual tasks.

Bagolini striated lenses

A diagnostic tool used to assess binocular vision, retinal correspondence, and suppression. The lenses have fine, parallel striations that cause a point light source to appear as a line perpendicular to the striations. When placed with the striations at 135° before the right eye and 45° before the left eye, a patient with normal binocular vision will perceive two lines crossing to form an “X.” Variations in the perceived image can help diagnose conditions such as suppression and anomalous retinal correspondence.

bifoveal fixation

Bifixation, or bifoveal fixation refers to sensory fusion in which only the foveal images from each eye are combined into a single percept. It allows for clear, single binocular vision by uniting the high-resolution foveal images onto corresponding points in the visual cortex and enables fine stereopsis.

blind-spot esotropia

A type of strabismus where the angle of inward eye deviation is approximately 15 degrees, causing the image of the fixation point to fall on the physiological blind spot of the deviating eye. This eliminates the need for a suppression scotoma to prevent diplopia, as the blind spot naturally obscures the second image. The condition is relatively rare and typically occurs in older children and adults with a later onset of esotropia.

Brewster stereoscope

An optical device that uses two converging lenses and a septum to present a separate image to each eye, creating a three-dimensional effect when viewing stereoscopic photographs. Invented by Sir David Brewster in 1849, it improved upon Charles Wheatstone’s earlier mirror-based stereoscope. Brewster’s lenticular design was more compact and portable, contributing to its commercial success in the mid-19th century.

Brown’s syndrome

An ocular motility disorder characterized by a limitation of elevation in adduction, causing a divergence in straight upgaze. The restriction is typically of the same degree on version and duction testing, differentiating it from inferior oblique palsy. It can be congenital or acquired, with the congenital form being more common. Etiologies include a tight or inelastic superior oblique tendon-trochlea complex, inferior oblique adhesions, or a displaced lateral rectus pulley. Treatment depends on the severity of the limitation and includes surgery to weaken or lengthen the superior oblique tendon.

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.