OP1.1-5 | Visual Foundations and Refraction — Glossary

The process by which the eye increases its refracting power by increasing the curvature of the crystalline lens, mediated by contraction of the ciliary muscle and relaxation of the zonules.

Reduction in best-corrected visual acuity in one or both eyes not attributable to any structural abnormality of the eye or visual pathway; caused by abnormal visual experience during the critical period of visual development; the most common cause of monocular visual loss in children.

Any refractive error — myopia, hypermetropia, or astigmatism — in which parallel light from infinity does not focus exactly on the retina in the unaccommodated state.

Bilateral amblyopia from high uncorrected refractive error in both eyes (e.g. bilateral high hypermetropia or significant astigmatism); both eyes receive a blurred image; neither eye dominates; treated with bilateral optical correction.

The difference in vergence power between the far point and near point of the eye; approximately 10–12 D in a young adult, declining to near zero by age 60 (presbyopia).

A significant difference in refractive error between the two eyes (clinically relevant above ~2–3 D); a major risk factor for amblyopia in children because the brain suppresses the image from the more defocused eye.

Amblyopia caused by a significant difference in refractive error between the two eyes (typically ≥2–3 D); the more ametropic eye receives a chronically defocused retinal image and is cortically suppressed; often presents without visible strabismus.

A refractive error in which the refracting power differs in different meridians, preventing formation of a point focus; regular astigmatism is corrected with cylindrical lenses.

The pharmacological treatment of amblyopia using atropine 1% eye drops instilled in the fellow (dominant) eye to blur its near vision, forcing use of the amblyopic eye for near tasks; an alternative to patching, useful when compliance with patching is poor.

Myopia caused by an abnormally long axial length of the eye (>24 mm), with normal refracting power; the commonest form; every 1 mm increase in axial length adds approximately 3 D of myopia.

The highest Snellen VA achievable with optimal spectacle or contact lens correction; the standard endpoint for clinical trials and the reference VA for assessing organic visual loss.

Loss of the temporal (outer) visual field in both eyes, caused by a lesion at the optic chiasm (typically pituitary adenoma) compressing the crossing nasal retinal fibres.

A reflex blink elicited by touching the cornea; afferent = CN V1 (nasociliary branch, corneal sensation); efferent = CN VII bilaterally (direct and consensual blink); tests integrity of the trigemino-facial reflex arc.

The blink response in the EYE NOT being tested when the cornea of the other eye is touched; present because the efferent CN VII signals both orbicularis muscles; its presence when the direct response is absent indicates CN VII palsy (efferent gone) rather than CN V1 palsy (afferent gone).

A treatment for progressive keratoconus (and post-LASIK ectasia) using riboflavin eye drops and UV-A light to create new covalent bonds between stromal collagen fibrils, stiffening the cornea and halting progression.

Progressive corneal bulging and irregular astigmatism developing after LASIK, resembling keratoconus; caused by insufficient residual stromal bed or unrecognised pre-existing corneal ectatic disease; the most serious late complication of laser refractive surgery.

Cross-sectional imaging of the cornea using a rotating Scheimpflug camera (e.g. Pentacam); measures anterior AND posterior corneal surfaces and corneal thickness (pachymetry); superior to topography alone for detecting subclinical/posterior keratoconus.

A non-invasive imaging method that maps the curvature (anterior surface) of the cornea using Placido disc reflection; essential for detecting regular/irregular astigmatism, asymmetric patterns, and screening for keratoconus before refractive surgery.

A level of visual acuity below 6/60 in which the patient can count the examiner's fingers at a specified distance (e.g. CF at 2 m); documented with the distance at which counting is possible.

The developmental window (birth to approximately 7–8 years in humans, with maximal plasticity in the first 2–3 years) during which the visual cortex requires appropriate visual input to establish normal spatial acuity; abnormal input during this period causes permanent cortical changes.

A feature of amblyopia in which a child reads isolated single letters better than letters presented in a row; caused by cortical contour interaction (adjacent letters suppress each other in the amblyopic visual cortex); absent in organic visual loss.

Measurement of refractive error after pharmacological paralysis of accommodation (cycloplegia) using drops such as cyclopentolate 1%; essential in children to reveal the total (including latent) refractive error.

The most severe form of amblyopia; caused by a physical obstacle (congenital cataract, dense ptosis, corneal opacity, vitreous haemorrhage) blocking formed visual input to one or both eyes during the critical period; congenital cataract requires urgent surgical treatment.

An inflammatory reaction within the LASIK flap interface, presenting as a diffuse haze with a 'Sands of the Sahara' appearance on slit-lamp examination; occurs in the first weeks post-operatively; treated with intense topical steroids; severe cases require flap lift and irrigation.

The functional duality of the retina using two types of photoreceptors: rods for scotopic (dim-light, low-resolution) vision and cones for photopic (bright-light, high-resolution, colour) vision.

The refractive state in which parallel light from infinity is brought to focus exactly on the retina with the accommodation fully relaxed; neither myopia nor hypermetropia.

An argon-fluoride ultraviolet laser (193 nm wavelength) used in corneal refractive surgery; photoablates corneal stromal collagen peptide bonds without generating heat ('cold' ablation); each pulse removes approximately 0.25 µm of stroma.

A small central depression in the retina (~1.5 mm diameter) that contains only cones at highest density and provides the greatest visual acuity; its centre (foveola, ~0.35 mm) is cones-only with a 1:1:1 photoreceptor-to-ganglion-cell ratio.

A dark pigmented macular lesion seen in pathological myopia, caused by subretinal choroidal neovascularisation and haemorrhage in the macular area; causes central visual loss.

Loss of vision in the same visual field half (e.g. left) in both eyes, caused by a post-chiasmal lesion (optic tract, radiation, or cortex) involving the pathway representing the contralateral visual field.

A refractive error in which parallel light from infinity would focus posterior to the retina; partially compensated by accommodation in young patients; corrected with convex (plus) lenses.

A colour vision test consisting of plates of coloured dots in which numbers (or paths) are embedded; designed to detect red-green colour deficiency; persons with normal colour vision see one number, those with red-green deficiency see a different number or none.

A near-vision test chart using graded print sizes; J1 = finest print (equivalent to N5); held at 33 cm from the eye; used to document near visual acuity.

A progressive, non-inflammatory ectatic disorder of the cornea in which the central corneal stroma thins and protrudes forward in a cone shape, producing high irregular astigmatism; corrected with rigid gas-permeable contact lenses or corneal transplantation.

A corneal refractive surgery in which a thin flap is created in the anterior corneal stroma (microkeratome or femtosecond laser), folded back, and excimer laser ablation reshapes the stromal bed; the flap is then repositioned. Most widely performed refractive surgery globally.

A thalamic relay centre for visual signals; 6-layered structure with magnocellular (M) layers 1–2 processing motion/contrast and parvocellular (P) layers 3–6 processing colour/fine detail.

Visual acuity less than 6/60 in the better eye with best correction, as defined by the National Programme for Control of Blindness and Visual Impairment (NPCBVI) for blindness certification purposes in India.

Preservation of central (macular) vision despite contralateral homonymous hemianopia from an occipital cortex lesion; attributed to the dual blood supply (MCA + PCA) of the macular cortical representation.

A reflex blink elicited by a rapid threatening gesture toward the eye; afferent = CN II (visual perception of threat); efferent = CN VII (orbicularis oculi contraction); normally absent in infants until ~3 months of age.

The inferior loop of the optic radiation that carries fibres representing the superior visual field (lower visual quadrant) through the temporal lobe before reaching the occipital cortex; lesion produces superior quadrantanopia.

The smallest angular separation between two points that the eye can distinguish as separate; normally 1 arcminute; the denominator of the Snellen fraction is the distance at which the letter subtends 5× MAR = 5 arcminutes.

A refractive error in which the far point is at a finite distance in front of the eye; parallel light from infinity is brought to focus anterior to the retina; corrected with concave (minus) lenses.

Complete absence of light perception in an eye; indicates total non-function of the retina, optic nerve, or visual pathway; confirmed in a darkened room with a bright light source.

The treatment of amblyopia by covering the dominant (fellow) eye with an adhesive patch, forcing the brain to use and develop the amblyopic eye; the PEDIG trials demonstrated 2 hours/day with near activities is effective for moderate amblyopia.

The cortical neuroplastic process in which the visual cortex representation expands for the dominant (normally stimulated) eye and contracts for the deprived/suppressed eye; the underlying mechanism of amblyopia at the neural level.

The X-shaped structure at the base of the brain where the optic nerves of both eyes partially decussate: nasal retinal fibres cross to the contralateral side, temporal fibres remain ipsilateral.

Inflammation/demyelination of the optic nerve, presenting with subacute unilateral visual loss, periocular pain on eye movement, colour desaturation, and RAPD; strongly associated with multiple sclerosis.

High myopia (>−6 D) associated with progressive elongation of the globe and structural complications including posterior staphyloma, lattice degeneration, retinal detachment, and Fuchs' spot.

A multi-centre research network that has conducted landmark RCTs on amblyopia treatment; demonstrated that 2 hours/day patching is as effective as 6 hours/day for moderate amblyopia, and that weekend atropine dosing equals daily dosing; also showed treatment benefit in children up to 12 years.

A level of visual acuity in which the patient can detect the presence of light but cannot count fingers; further qualified by projection (PR = perception with projection, indicating the direction of the light source can be identified) or no projection.

A flexible posterior-chamber intraocular lens implanted between the iris and the natural crystalline lens without removing the natural lens; used for high myopia where corneal surgery is unsafe; preserves accommodation; can be removed if needed.

The process by which the 193 nm excimer laser breaks molecular bonds in corneal stromal tissue without generating thermal damage; highly precise and predictable removal of corneal tissue in increments of ~0.25 µm per pulse.

The conversion of light energy into a neural (graded) electrical signal in the photoreceptors; involves G-protein (transducin) activation, cGMP hydrolysis by phosphodiesterase, and closure of Na+ channels causing photoreceptor hyperpolarisation.

A clinical test using a small-aperture disc (~1–2 mm) placed before the eye; restricts light to the central optical axis, minimising refractive blur; improvement with pinhole indicates a refractive cause; no improvement suggests organic (retinal, optic nerve) pathology.

Age-related physiological reduction in the amplitude of accommodation due primarily to loss of elasticity of the crystalline lens; symptomatic around age 40; corrected with plus-add lenses.

A surface corneal refractive surgery in which the epithelium is removed before excimer laser ablation of Bowman's layer and anterior stroma; no flap is created; recovery is slower than LASIK but the procedure is safer for thin corneas and active occupations.

On the swinging flashlight test, the pupil dilates rather than constricts when light swings from the fellow eye to the affected eye; indicates organic optic nerve or retinal pathology; ABSENT in amblyopia even when amblyopic VA is very poor — its presence rules out pure amblyopia.

The orange-red reflection seen through a direct ophthalmoscope aimed at the pupil; absence or white discolouration (leukocoria) suggests media opacity (congenital cataract, corneal opacity) or retinoblastoma; an abnormal red reflex in a neonate requires urgent ophthalmology referral.

The commonest hereditary colour vision defect; X-linked recessive; affects ~8% of males and ~0.5% of females; caused by absence or mutation of the red (L) or green (M) cone photopigment gene; detected by Ishihara plates.

A measure of the ability of a lens or refracting surface to converge or diverge light; 1 dioptre = the reciprocal of the focal length in metres.

Clear lens extraction followed by IOL implantation; uses the same technique as cataract surgery but on a non-cataractous lens; corrects high hypermetropia or extreme myopia; permanently eliminates accommodation — not appropriate for young patients who have not yet developed presbyopia.

On the swinging-flashlight test, the pupil of the affected eye dilates (rather than constricts) when light is swung from the normal to the affected eye, indicating reduced afferent input (optic nerve or retinal disease) in that eye.

The thickness of corneal stroma remaining beneath the LASIK flap after excimer laser ablation; must be ≥250 µm to maintain corneal biomechanical integrity and prevent post-operative ectasia.

An objective method of refraction in which light is projected into the eye and the movement of the retinoscopic reflex in the pupil guides the selection of correcting lenses; requires no verbal patient response.

The visual photopigment of rod photoreceptors, composed of the apoprotein opsin and the chromophore 11-cis-retinal (a vitamin A derivative); bleached by light during phototransduction.

A flapless corneal refractive surgery in which a femtosecond laser creates an intrastromal lenticule of the appropriate shape, which is manually extracted through a 2–4 mm incision; approved for myopia and myopic astigmatism; less dry eye than LASIK.

The notation used to express visual acuity: numerator = actual test distance (standard 6 m); denominator = distance at which the smallest letter correctly read subtends 5 arcminutes for a normal eye. 6/6 = normal; values >6 in the denominator indicate worse than normal acuity.

A prerequisite for elective refractive surgery: refraction must not have changed by more than 0.50 D per year over at least 12 months (many surgeons require ≥24 months); unstable refraction contraindicates surgery as correction would become inaccurate as the error progresses.

Amblyopia caused by a constant unilateral strabismus; the brain suppresses the image from the deviating eye to avoid diplopia; only constant (not alternating) strabismus causes amblyopia.

A measure of the spatial resolving power of the visual system — specifically, the ability to discriminate fine detail; quantified using the Snellen chart as the ratio of test distance to the distance at which a letter subtends 5 arcminutes for a normal eye.

Astigmatism in which the vertical corneal meridian is steeper (greater refracting power) than the horizontal; common in young individuals; corrected with minus cylinder at axis 180°.