Duane’s syndrome

A congenital disorder involving abnormal innervation of the lateral rectus muscle, resulting in limited eye movements. It is characterized by retraction of the eye into the orbit, associated with either limited abduction (outward movement), limited adduction (inward movement), or limited abduction and adduction. On attempted adduction, there is globe retraction and narrowing of the palpebral fissure. The condition arises from miswiring of the cranial nerves controlling eye movements during development.

ductions

The ability of each eye to demonstrate a full range of motion in all directions (up, down, left, right) when tested individually under monocular viewing conditions. Evaluating ductions assesses the mechanical integrity and function of the extraocular muscles and cranial nerves controlling eye movements. Limitations in ductions can indicate muscle or nerve dysfunction, restrictive forces, or mechanical restrictions.

dynamic retinoscopy

A technique where retinoscopy is performed while the patient fixates on a near target, allowing assessment of the accommodative response. The examiner observes the retinoscopic reflex and neutralizes it by adding or removing lenses, measuring the accommodative lag or lead compared to the expected response. It provides an objective evaluation of accommodative ability and aids in diagnosing accommodative disorders.

eccentric fixation

Eccentric fixation is the use of a non-foveal retinal area, known as the preferred retinal locus (PRL), for fixation instead of the central fovea. It occurs in conditions where the fovea is damaged or not being used effectively, such as macular degeneration, macular holes, or amblyopia. The PRL location can vary but is often in the upper or upper-left retinal area near the scotoma. Eccentric fixation allows the patient to utilize a healthier retinal region for fixation and maintain some visual function, though the PRL used may differ for different visual tasks.

electro-oculogram (EOG)

A technique that records eye movements and eye position by measuring the difference in electrical potential between electrodes placed on the skin around the eyes. Eye movements generate a corneo-retinal potential difference detected by the electrodes, with the EOG signal corresponding to the direction and amplitude of eye movements. EOG is used to study eye movement disorders, nystagmus, optokinetic responses, and can also assess retinal function. While non-invasive with high temporal resolution, it has limitations of low spatial resolution and potential signal artifacts.

epicanthus

A condition where a fold of skin partially covers the inner corner (canthus) of the eye. It is normal and common in infants but may give the appearance of an inward eye deviation (esotropia). In some cases, epicanthus can persist beyond infancy and be associated with craniofacial syndromes or genetic disorders. It can occur in isolation or combined with other features like increased distance between the inner canthi (telecanthus) or narrow palpebral fissures (blepharophimosis). Surgical correction may be considered if epicanthus causes functional or cosmetic concerns.

esophoria

Esophoria is a binocular vision condition where the eyes have a tendency to turn inward, especially when fusion is disrupted.

esotropia

A manifest inward turning of one or both eyes towards the nose. The deviation may be constant or intermittent, present from birth or acquired later in life, and can vary in angle depending on the direction of gaze.

exophoria

A binocular vision condition where the eyes have a tendency to turn outward, especially when fusion is disrupted.

Exotropia

A manifest turning of one or both eyes away from the nose. The deviation may be constant or intermittent, present from birth or acquired later in life, and can vary in angle depending on the direction of gaze.

extorsion (excyclodeviation)

The outward wheel-like rotation or torsional movement of the eye around its anteroposterior axis, causing the superior pole of the vertical meridian to rotate temporally. It often occurs due to imbalances in the actions of the oblique extraocular muscles.

Faden procedure

A strabismus surgery where the belly of a rectus muscle is sutured to the sclera posterior to the globe’s equator. This fixates the muscle in a posteriorly displaced position, reducing its ability to rotate the eye when contracting without altering the resting eye alignment. The Faden procedure selectively weakens a muscle’s pulling force while preserving its resting tone. It is often used for the medial rectus muscles in convergence excess esotropia.

first-degree fusion

The ability to superimpose or fuse dissimilar targets or images seen by the two eyes into a single, unified percept. It involves cortical processing of binocular visual input to combine the different views from each eye into one coherent 3D image. First-degree fusion is a fundamental aspect of binocular vision and stereopsis, allowing depth perception. Disruptions can lead to diplopia or suppression, and it is tested by presenting different targets to each eye.

fixating eye

In strabismus or eye misalignment, the fixating eye refers to the non-deviating eye that is fixating on the target and has normal alignment. It is the eye being used for fixation, while the non-fixating eye is the deviating eye that is turned inwards, outwards, upwards, or downwards.

fixation disparity

Fixation disparity is a subtle misalignment of the eyes that occurs during binocular viewing, where each eye slightly deviates from perfect alignment while still maintaining single vision. It can cause symptoms such as eyestrain and headaches, particularly during tasks that require sustained visual attention.

forced duction test

A procedure performed under local or general anesthesia to assess mechanical restrictions of eye movements. It involves grasping the conjunctiva with forceps and attempting to rotate the eye in the direction of the restricted movement. A positive forced duction test, where rotation is limited, indicates a tight muscle or other mechanical restriction causing the strabismus. In contrast, a negative test suggests paralysis of the extraocular muscle.

Fresnel press-on prisms

A Fresnel lens is a compact lens made from a series of concentric grooves that allows the lens to capture more oblique light from a large area while maintaining a thin, flat profile. Fresnel press-on prisms are low cost, temporary means of providing prismatic correction.

full-muscle transposition

A surgical procedure used to treat complete extraocular muscle palsies like third or sixth nerve palsies where the entire tendon of a rectus muscle is disinserted from its original scleral insertion. The disinserted rectus muscle is then transposed and sutured adjacent to the insertion of the paretic rectus muscle. This augments the function of the paralyzed muscle by adding vector forces from the transposed muscle.

functional amblyopia

fusional convergence

The inward movement of the eyes to maintain binocular single vision when fusion is disrupted, without a change in accommodation. It occurs in response to disparity cues and is driven by the fusional vergence system to realign the visual axes onto the object of regard. Fusional convergence helps compensate for phoric deviations and maintain comfortable binocular vision at a given viewing distance.