panoramic vision
An expansion of the peripheral visual field that can occur in some patients with exotropia, particularly those with intermittent or divergence excess types. Due to the misalignment of the eyes, each eye sees a different portion of the visual field. The brain combines these two separate images, resulting in a wider field of view compared to normal binocular vision.
Panum’s area
A region in the retina of one eye that corresponds to a specific point in the other eye. Stimulation of any point within Panum’s area simultaneously with the corresponding point in the fellow eye results in a single fused percept, allowing for a small degree of retinal disparity while maintaining single binocular vision. The size of Panum’s area varies, being smallest at the fovea and increasing peripherally. It plays a crucial role in sensory fusion and stereopsis.
paresis
A partial paralysis (palsy) of an extraocular muscle, resulting in weakness and reduced movement in its field of action. This results in an incomitant deviation, where the misalignment is more pronounced in the direction of the paretic muscle.
paresis of accommodation
A reduction or complete loss of accommodative ability due to paralysis of the ciliary muscle. It can occur unilaterally or bilaterally and may result from damage to the oculomotor nerve or ciliary ganglion. Symptoms include blurred near vision and reduced accommodative amplitude.
penalization
A treatment method for amblyopia that involves decreasing vision in the fixating eye with pharmacologic agents or optical lenses to encourage use of the amblyopic eye.
Percival’s criterion
Asthenopia is likely to occur when the vergence demand falls outside the middle third of the total fusional vergence range—the fusional vergence system should operate within a comfortable range to avoid excessive strain.
period of visual immaturity
The critical period in early childhood, typically from birth to 7-8 years old, when the visual system is highly sensitive to abnormal visual experiences. Disruption of normal visual development during this time can lead to suppression, anomalous retinal correspondence, and amblyopia.
peripheral fusion
Sensory fusion in which images in the extrafoveal peripheral portions of the retinae are combined, excluding the foveae. It has a wider fusional range than bifoveal fusion but cannot support high-acuity vision or fine stereopsis alone. Peripheral fusion contributes to the overall binocular visual field and perception of visual direction.
photophobia
An abnormal sensitivity to light, causing ocular discomfort or pain. It can be associated with various eye conditions, such as corneal abrasions, uveitis, and retinal disorders, as well as neurological conditions like migraine, meningitis, and traumatic brain injury.
physiologic diplopia
A normal phenomenon in binocular vision where objects not being fixated are seen as double. It occurs when images of non-fixated objects fall on non-corresponding retinal points outside of Panum’s fusional area. Objects closer than fixation produce crossed diplopia, while objects farther than fixation cause uncrossed diplopia.
pleoptics
A dated and ineffective method of treating amblyopia, pleoptics involves intensive stimulation of the fovea in the amblyopic eye using techniques such as bright lights to dazzle the eccentric fixation point while protecting the fovea, and using after-images to encourage foveal fixation. The goal is to eliminate eccentric fixation and improve visual acuity.
Pleoptics has largely fallen out of favor, with studies finding mixed results compared to conventional occlusion therapy.
positive relative accommodation
The maximum amount of accommodation that can be stimulated while maintaining clear, single binocular vision at a fixed distance, typically 40 cm. It is measured by adding minus spherical lenses binocularly until the patient reports sustained blur, with the total minus lens power representing the PRA value. High PRA (>2.5D) may indicate accommodative excess or uncorrected myopia, while low PRA (<1.5D) suggests accommodative insufficiency or an over-minused refraction.
preferential looking
A method used to assess visual acuity in pre-verbal infants and young children. It involves presenting two side-by-side stimuli: one with black and white stripes (gratings) of varying spatial frequencies, and the other a homogeneous gray field matched for luminance. An observer judges which stimulus the infant preferentially looks at by monitoring their eye and head movements. The finest grating the infant can resolve provides an estimate of their visual acuity.
primary angle of deviation
In incomitant strabismus, the primary angle of deviation is the angle of misalignment of the non-fixating eye from the point of fixation when the normal, unaffected eye is fixating. It represents the maximum angle of deviation when the affected eye is not fixating.
primary position (of gaze)
The orientation of the eyes when looking straight ahead with the head erect and still. It refers to the position where the visual axes are parallel, aligned with the head, and the eyes are in their most relaxed state with minimal extraocular muscle activity.
prism diopter
A unit that quantifies the deviating power of a prism. 1 prism diopter causes a 1 cm deflection of a light ray at a distance of 1 meter from the prism. Prism diopters differ from lens diopters and are used clinically to measure the angle of ocular misalignment and prescribe prism correction for strabismus.
progressive-addition lens
A type of multifocal spectacle lens with a gradual change in power from the distance portion at the top to the near portion at the bottom. This smooth transition allows wearers to see clearly at all distances without abrupt image jumps. PALs have an aspheric, progressive surface, but may cause peripheral distortion due to varying power across the lens.
proximal convergence
Proximal convergence is the inward movement of the eyes triggered by the perception of an object’s proximity, independent of accommodation or fusional stimuli. It is a reflexive convergence response elicited by the awareness of a nearby target, even in the absence of blur or diplopia cues. Proximal convergence contributes to the overall convergence response, along with accommodative and fusional components, to maintain binocular single vision at near distances.
pseudostrabismus
Pseudostrabismus is the false appearance of misaligned or crossed eyes when the visual axes are actually properly aligned. It is caused by facial features like a wide nasal bridge, prominent epicanthal folds, or angle kappa abnormalities that create an illusion of inward or outward eye deviation.
psychogenic amblyopia
Also known as functional or non-organic vision loss, psychogenic amblyopia is a decrease in visual acuity or visual field without an identifiable organic cause. It is a diagnosis of exclusion, made after ruling out other ocular or neurological disorders. Patients may present with bilateral vision loss, tubular visual fields, or spiral visual fields. Psychogenic factors such as stress, anxiety, or depression are often associated with this condition. Treatment involves a multidisciplinary approach, including reassurance, psychotherapy, and addressing underlying psychological issues.
psychometric visual acuity
A method of measuring visual acuity that uses psychophysical techniques and forced-choice responses. It involves presenting optotypes like Tumbling E or Landolt C in varying orientations, and the patient indicates the perceived orientation. Psychometric acuity testing reduces crowding and contour interaction effects, providing more reliable measure of acuity for amblyopia.
pursuits
Voluntary, smooth tracking movements that allow the eyes to fixate on a slowly moving target by matching eye velocity to target velocity. They are initiated by visual motion signals to maintain the image of the moving object on the fovea.
Pursuits differs from rapid re-fixating saccades and have an inherent delay before initiation due to visual processing time. At higher speeds, catch-up saccades aid pursuit in maintaining foveation.