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
A type of strabismus in which one or both eyes cross inward due to excessive accommodation in uncorrected hyperopia or a high accommodative convergence to accommodation (AC/A) ratio. The eyes converge excessively when focusing on near objects, leading to esotropia. Accommodative esotropia typically manifests in children aged 2-3 years and is treated with full hyperopic correction using glasses or contact lenses. In some cases, bifocals or surgery may be necessary.
accommodative insufficiency
A condition in which the eye’s ability to focus on near objects is below the expected amplitude for an individual’s age. It is characterized by difficulty sustaining near vision focus, leading to blurred vision, eye strain, and headaches. Accommodative insufficiency may be caused by various factors, including illness, medications, or visual stress. It is commonly associated with convergence insufficiency and can be managed with reading glasses or vision therapy.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
A neurological disorder characterized by a pupil that reacts poorly to light but better to accommodation. The affected pupil is initially dilated and responds sluggishly to light. Pupillary constriction is more noticeable with near focus and remains tonically constricted with slow re-dilation. Adie’s pupil is usually unilateral, with an average onset age of 32 years, and may be associated with absent deep tendon reflexes in Holmes-Adie syndrome.
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)
An optical system with zero net focal power, consisting of two or more lenses separated by the sum of their focal lengths. Parallel light rays entering an afocal lens emerge parallel, with their size and orientation changed. Afocal lenses are used in laser beam expanders, telescopes, and as size lenses in aniseikonia management to minify or magnify images without altering vergence.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
A decrease in best-corrected visual acuity, typically in one eye, that cannot be directly attributed to any structural abnormality of the eye or visual pathway. It results from abnormal visual experience during early childhood, leading to cortical suppression of vision in the affected eye. Common causes include strabismus, anisometropia, and visual deprivation. Early detection and treatment with glasses, patching, or atropine can improve visual outcomes.
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.