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.
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.