ABO Advanced key terms

A single number that expresses a lens power by adding half the cylindrical power to the spherical power, used when a toric correction cannot be given or when summarizing refractive error.

A refractive condition in which the eye's optical power differs across meridians, so light from a point does not focus at a single point and vision is blurred in one direction.

Measurement of the curvature of the anterior corneal surface, usually reported as radii and powers in the two principal meridians, used to quantify corneal astigmatism and fit contact lenses.

A mapped, color-coded measurement of curvature and elevation across the whole cornea, used to detect irregular astigmatism, ectasia, and to guide contact lens fitting.

A progressive noninflammatory thinning and steepening of the cornea that produces irregular astigmatism and reduced best-corrected vision, often managed with rigid lenses or corneal cross-linking.

Astigmatism in which the principal meridians are not perpendicular or the surface is optically uneven, so ordinary spectacle cylinders cannot fully correct the blur.

Corneal or refractive astigmatism in which the steeper meridian is closer to horizontal, commonly appearing in older adults as lid tone and corneal shape change with age.

Astigmatism in which the steeper meridian is closer to vertical, the usual pattern in young eyes and often associated with tighter eyelids and steeper vertical corneal curvature.

An objective technique in which a streak of light is swept across the pupil and the movement of the reflected glow is neutralized with lenses to estimate the eye's refractive error.

Refraction performed after paralyzing the ciliary muscle with medication, revealing the full hyperopic correction and preventing accommodation from masking latent refractive error.

The eye's ability to increase optical power by changing the shape of the crystalline lens, allowing clear focus on near objects.

The maximum increase in optical power the eye can produce through accommodation, measured as the difference between far and near points of focus.

The age-related loss of accommodative ability that makes near work difficult without added plus power, typically becoming noticeable in the fourth and fifth decades.

Reduced vision in one or both eyes caused by abnormal visual experience during development, not explained by structural eye disease and often treatable if detected early.

A misalignment of the eyes in which the visual axes do not point at the same target, which can cause diplopia or, in children, amblyopia and loss of binocular vision.

A condition in which the refractive error of the two eyes differs enough that the retinal images are unequal in size or clarity, sometimes leading to amblyopia or poor binocular function.

A perceived difference in the size or shape of the images seen by the two eyes, often caused by unequal spectacle magnification and capable of causing headache or discomfort.

A binocular assessment in which one eye is covered while the other fixates, and the uncovered eye's movement on removal reveals the presence and direction of a deviation.

Depth perception produced by the brain's fusion of slightly different images from the two eyes, serving as a sensitive indicator of binocular alignment and cooperation.

Treatment for amblyopia in which the better-seeing eye is patched for prescribed periods to force use of the weaker eye, with monitoring for compliance and reverse amblyopia.

A treatment for amblyopia that blurs the better eye with a cycloplegic drug or an over-plus lens instead of a patch, encouraging use of the amblyopic eye.

The sharpness of central vision, usually measured as the smallest letters or symbols a person can identify at a standard distance under controlled lighting.

The ability to detect differences in luminance between an object and its background, revealing visual loss that letter acuity testing may miss.

Assessment of vision while a bright light source is placed in the field, used to reveal disability from stray light scatter in conditions such as cataract or corneal haze.

The full area a person can perceive while the eye is fixed on a central target, mapped by perimetry to detect defects from glaucoma, neurological, or retinal disease.

A grid pattern viewed monocularly at near that helps patients detect central or paracentral distortion and scotomas, often used to monitor macular disease at home.

A visual distortion in which straight lines appear bent or wavy, characteristically caused by retinal elevation or macular disease such as epiretinal membrane or macular edema.

The fluid pressure inside the eye, measured with tonometry and influenced by aqueous production, outflow, and ocular rigidity; elevated values raise glaucoma risk.

A method that estimates intraocular pressure from the force needed to flatten a fixed corneal area, considered a clinical reference standard when technique and calibration are correct.

The thickness of the cornea at its center, which influences how applanation tonometry readings relate to true intraocular pressure and is a risk factor in glaucoma assessment.

Examination of the anterior chamber angle with a special lens and mirror or prism, used to distinguish open from narrow or closed angles and to look for abnormal angle structures.

A form of glaucoma in which the iris blocks the drainage angle, causing sudden or chronic pressure rise and requiring urgent pressure-lowering and definitive angle treatment.

An opacification of the crystalline lens that scatters light and reduces clarity, glare tolerance, and contrast, treated surgically when vision loss interferes with daily life.

The condition of an eye in which the natural crystalline lens has been replaced by an intraocular lens implant, changing refractive power, accommodation, and lens fitting considerations.

The separation between the back surface of a spectacle lens and the front of the cornea, which changes the effective power of stronger lenses when the lens is moved.

The change in retinal image size produced by a spectacle lens, depending on lens power, thickness, position of wear, and the eye's refractive state.

A contact lens made of a firm, oxygen-permeable material that maintains its shape on the eye, often giving sharp vision for irregular corneas but requiring adaptation.

Clinical decision-making that integrates the best available research evidence, the clinician's expertise, and the patient's values and circumstances.

A systematically developed statement that helps clinicians and patients make decisions about appropriate care for specific clinical circumstances, ideally based on graded evidence.

The level and type of care that a reasonably competent practitioner in a similar setting would provide under comparable circumstances, used as a benchmark in quality and legal review.

The range of clinical activities, procedures, and prescribing authority a licensed professional is legally permitted to perform within a given jurisdiction.

Ongoing learning activities that maintain and update professional knowledge and skills, usually required in set amounts for license renewal and often tracked by credit hours.

Evaluation of a clinician's records or care by professional colleagues against accepted standards, used to improve quality and support accountability within the profession.

A patient's voluntary agreement to a test or treatment after receiving and understanding information about its purpose, risks, benefits, and reasonable alternatives.

The documented history, findings, diagnoses, and care plan for a patient, serving as a clinical communication tool and a legal record of the care provided.

United States legislation that sets national standards for protecting the privacy and security of identifiable health information and for handling electronic health transactions.