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Eye Conditions and Scleral Lens Glossary
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Eye Conditions and Scleral Lens Glossary

A Patient-Friendly Guide to Eye and Scleral Lens Terms

This patient-friendly glossary explains eye conditions, vision symptoms, diagnostic tests, and the scleral-lens terms used throughout Eye Freedom. Dr. Edward Boshnick evaluates complex corneal, ocular-surface, post-surgical, and specialty contact-lens cases, but a definition cannot determine what is causing an individual symptom or whether a particular lens is appropriate.

Start with the term you heard during an examination or found in a report, then follow the related Eye Freedom guide for more context. The primary scleral lenses guide explains candidacy, fitting, care, and realistic limitations.

Vision Symptoms and Visual Field Terms

These words describe what a person notices. A symptom is not a diagnosis, and similar symptoms can have different causes.

Blind spot (scotoma)
A scotoma is an area of reduced or missing vision within the visual field. Every eye has a normal physiological blind spot where the optic nerve leaves the retina, but a new, noticeable, or enlarging blind spot needs professional evaluation.
Tunnel vision
A patient description for a narrowed visual field in which central vision remains but side vision is reduced. It is a symptom rather than a diagnosis and may be associated with eye, optic-nerve, retinal, or neurological conditions.
Peripheral vision (side vision)
Vision outside the point you are looking directly at. It supports orientation, mobility, and awareness of movement or objects to either side.
Visual field
The full area visible while the eyes face forward, including central and peripheral vision. A visual field test maps where light is seen or missed.
Central vision
Detailed vision in the center of the visual field. It is important for recognizing faces, reading, and other tasks that require fine detail.
Visual acuity
A measure of how clearly a person can resolve detail, commonly recorded with an eye chart. Acuity does not measure every part of visual quality, such as contrast, glare, peripheral vision, or distortion.
Fluctuating vision
Vision that becomes clearer or blurrier at different times. Tear-film instability, an irregular cornea, lens changes, retinal conditions, blood-glucose changes, and other factors can contribute, so the pattern needs clinical context.
Glare
Difficulty seeing when bright light is present or scattered inside the optical system of the eye. Glare can reduce contrast even when chart acuity appears relatively good.
Halos
Rings or circles seen around lights, often most noticeable at night. Halos can be associated with the tear film, cornea, pupil, crystalline lens, prior surgery, contact lenses, or other optical factors.
Ghosting
A faint secondary or overlapping image that remains when one eye is covered. Irregular astigmatism and higher-order aberrations are among the possible optical causes.
Starbursts
Rays or spikes that appear to extend from a light source. They may be reported with irregular corneal optics, tear-film problems, cataract, large pupils, or prior refractive surgery.
Double vision (diplopia)
Seeing two images of one object. It can be monocular or binocular and has several possible eye, optical, muscular, or neurological causes. Sudden new double vision needs prompt medical assessment.
Photophobia
Unusual light sensitivity or discomfort in light. It is a symptom that can accompany ocular-surface irritation, inflammation, migraine, corneal disease, and other conditions.
Floaters and flashes
Floaters are drifting spots, strands, or cobweb-like shapes; flashes are brief lights that may resemble sparks. A sudden shower of new floaters, flashes, or a dark curtain or shadow can signal a retinal emergency and requires immediate assessment.

Scleral Lens and Specialty Contact Lens Terms

These definitions follow current specialty-lens terminology and explain the words patients may hear during a custom fitting.

Scleral lens (scleral contact lens)
A large-diameter, gas-permeable lens designed to vault the cornea and limbus and land on tissue over the sclera. The space beneath the lens is filled with preservative-free saline before application. See the Eye Freedom scleral lens guide.
Sclera
The strong white outer coat of the eye. A scleral lens lands on the conjunctival tissue that covers the sclera rather than resting on the central cornea.
Cornea
The clear, curved front surface of the eye that helps focus light. Its shape, transparency, surface health, and relationship to the tear film strongly influence vision.
Limbus
The transition zone where the clear cornea meets the white sclera. Scleral-lens evaluation includes checking that the lens clears this region appropriately.
Vault or corneal clearance
The separation between the back surface of a scleral lens and the cornea. Clinicians assess central, peripheral, and limbal clearance because too little or too much space can affect the fitting relationship.
Fluid reservoir
The layer of preservative-free saline between a properly applied scleral lens and the cornea. It creates a smooth optical interface and keeps fluid over the corneal surface while the lens is worn.
Landing zone (haptic)
The outer portion of a scleral lens that aligns with conjunctival tissue over the sclera. Its shape can be customized to reduce localized pressure, edge lift, or compression in selected areas.
Sagittal depth
The depth of a curved surface measured from a reference plane. In scleral-lens design, sagittal depth helps determine how the lens vaults the cornea and aligns with the ocular surface.
Lens settling
The change in scleral-lens position and measured clearance after the lens has been worn for a period of time. Fit is evaluated after appropriate settling because the initial clearance may decrease.
Application (insertion)
The process of placing a saline-filled scleral lens on the eye without trapping an air bubble. Specialty-lens terminology commonly uses application, while many patients say insertion.
Removal
The process of taking a scleral lens off the eye using the technique and tools taught by the fitting practice. A lens should not be forced off a dry or suctioned eye.
Preservative-free saline
A sterile, nonpreserved solution used to fill a scleral lens before application when recommended by the clinician. Patients should follow product and practice instructions and should not use tap water to fill or rinse a lens.
Gas-permeable lens material
A firm contact-lens material that allows oxygen to pass through it. Oxygen delivery also depends on lens thickness, fluid-reservoir depth, tear exchange, ocular health, and wearing time.
Midday fogging
Clouding that develops when particles collect in the fluid reservoir during wear. Lens fit, tear debris, ocular-surface inflammation, application technique, and other factors may need to be evaluated.
Custom scleral lens
A scleral lens designed around measurements of the individual eye, prescription, surface health, visual needs, and observed fitting response. Customization may include multiple curves, toric or quadrant-specific landing zones, and specialized optics.
Wavefront-guided scleral lens
A custom scleral lens whose optics may be refined using measured higher-order aberrations after a stable fit is established. It is considered for selected visually complex cases, not every scleral-lens wearer. Read about wavefront-guided scleral lenses.
EyePrintPRO
An impression-based process used to design a highly customized ocular surface prosthetic for selected complex eyes. It does not guarantee comfort or vision, and candidacy depends on examination findings. See EyePrintPRO at Eye Freedom.
Hybrid contact lens
A specialty contact lens with a rigid gas-permeable optical center joined to a soft outer skirt. It differs from a scleral lens in design and where it bears on the eye. Learn about hybrid lens technology.
Corneal GP or RGP lens
A smaller rigid gas-permeable lens that rests on the cornea. Unlike a scleral lens, it does not vault the entire cornea and land beyond the limbus.
Contact lens intolerance
Difficulty wearing a contact lens comfortably or for the required amount of time. Dryness, allergy, lens fit, deposits, ocular-surface disease, and other factors may contribute. See scleral lenses for contact lens intolerance.
Scleral crescent
An eye-examination term for a pale crescent beside the optic disc where underlying sclera is more visible, often in association with a tilted or highly myopic disc. It is not a scleral lens and is not a scleral-lens fitting term.

Corneal and Ocular Surface Conditions

These conditions can affect corneal shape, transparency, sensation, healing, or the tear-covered surface of the eye.

Keratoconus
A condition in which the cornea becomes thinner and develops an irregular, cone-like shape. It can cause blur, ghosting, glare, and irregular astigmatism. Learn about scleral lenses for keratoconus.
Corneal ectasia
Progressive corneal thinning and steepening that creates an irregular shape. It may occur with keratoconus or after refractive surgery and requires medical evaluation. See scleral lenses for corneal ectasia.
Irregular astigmatism
Uneven optical power across the cornea that cannot always be fully corrected with ordinary glasses. It may result from keratoconus, ectasia, scarring, transplant surgery, trauma, LASIK, RK, or another corneal condition.
Dry eye disease
A condition involving tear-film instability and ocular-surface symptoms or signs. Burning, grittiness, redness, fluctuating vision, watering, or contact-lens difficulty may occur. See the guide to scleral lenses for dry eye.
Meibomian gland dysfunction (MGD)
Abnormal function of the oil-producing glands along the eyelids. Reduced or altered oil can increase tear evaporation and contribute to dry-eye symptoms.
Ocular surface disease
A broad term for disorders affecting the tear film, eyelids, conjunctiva, corneal epithelium, or related tissues. The underlying cause and severity determine whether medical treatment, protective lenses, or referral is appropriate.
Sjogren's syndrome
An autoimmune disease that can affect tear-producing and saliva-producing glands, causing dry eyes and dry mouth among other systemic effects. Ocular management is individualized. Read the Sjogren’s syndrome scleral-lens guide.
Graft-versus-host disease (GVHD)
A complication that can occur after an allogeneic stem-cell transplant when donor immune cells attack the recipient’s tissues. Ocular GVHD can cause severe dryness and surface inflammation or scarring.
Stevens-Johnson syndrome (SJS)
A rare, serious reaction that affects skin and mucous membranes and can cause acute and long-term ocular-surface injury. Eye involvement requires medical care. See the complex ocular-surface guide.
Neurotrophic keratitis
A corneal disease caused by reduced corneal nerve function and sensation, which can impair epithelial healing. Symptoms may be less severe than the examination findings, so monitoring is important.
Corneal scar
A loss of normal corneal transparency after injury, infection, inflammation, surgery, or another condition. Its location, depth, and effect on corneal shape determine how much it influences vision.
Corneal transplant (keratoplasty)
Surgery that replaces selected corneal tissue with donor tissue. After healing, irregular astigmatism or surface disease may still affect vision. Read about scleral lenses after corneal transplant.
Fuchs endothelial corneal dystrophy
A progressive disorder of corneal endothelial cells. When these cells no longer manage fluid effectively, the cornea may swell and become hazy. See the Fuchs dystrophy patient guide.
Corneal dystrophy
One of several usually inherited disorders in which abnormal material or cellular changes affect a layer of the cornea. Different dystrophies have different symptoms, progression, and management. Explore corneal dystrophy and degeneration.
Persistent corneal epithelial defect
An area of the corneal surface that does not close within the expected healing period. It can increase the risk of infection, scarring, thinning, and other complications and requires timely medical management.

Diagnostic Tests and Eye Technology

A test contributes information to an examination. No single map, scan, pressure reading, or software color code establishes every diagnosis by itself.

Corneal topography
A noncontact map of the front corneal surface and its curvature. It helps document corneal shape and supports keratoconus, ectasia, irregular-astigmatism, and specialty-lens evaluation. See computerized corneal topography.
Corneal tomography
Three-dimensional imaging that can analyze the front and back corneal surfaces, elevation, and thickness distribution. It provides different information from front-surface topography.
Optical coherence tomography (OCT)
A noncontact imaging method that uses reflected light to create cross-sectional views of selected eye structures. The scan type and device determine whether the retina, optic nerve, cornea, or anterior segment is evaluated.
Anterior-segment OCT
OCT configured to image structures at the front of the eye. It can document corneal thickness, anterior-chamber anatomy, and selected scleral-lens fitting relationships. Learn about Visante anterior-segment OCT.
Retinal and optic-nerve OCT
Cross-sectional imaging of retinal layers, the macula, or structures around the optic nerve. Results must be interpreted with image quality, symptoms, examination findings, and other testing. See the Cirrus OCT patient guide.
Wavefront aberrometry
Measurement of how light travels through the eye’s optical system. It can quantify lower- and higher-order aberrations and may support advanced optical design in selected cases. Learn about wavefront aberrometry.
Corneal and scleral profilometry
Three-dimensional measurement of corneal and surrounding ocular-surface shape. The data can support customized landing-zone and overall scleral-lens design.
Specular microscopy and endothelial cell count
Noncontact imaging of corneal endothelial cells. Clinicians assess cell density and appearance with the history and examination; the count alone does not determine whether a contact lens or procedure is appropriate. See specular microscopy.
Pachymetry
Measurement of corneal thickness. Thickness is relevant to corneal evaluation and to interpretation of some other findings, including eye-pressure measurements.
Slit-lamp examination
A magnified examination using a focused beam of light to inspect the eyelids, tear film, conjunctiva, cornea, anterior chamber, iris, lens, and contact-lens relationship.
Fluorescein dye
A temporary diagnostic dye used on the eye to highlight tear-film patterns, epithelial defects, and some contact-lens fitting relationships. It is different from fluorescein angiography, which uses injected dye for retinal imaging.
Visual field test (perimetry)
A test that maps sensitivity across central and peripheral vision. It may help evaluate glaucoma, retinal disease, optic-nerve disorders, neurological conditions, or a reported blind spot.
Tonometry
Measurement of pressure inside the eye. Eye pressure is one part of a glaucoma evaluation; a single pressure reading does not diagnose or exclude glaucoma. Learn more about glaucoma evaluation.
Digital retinal photography
Photography of the visible retina, optic nerve, and retinal blood vessels. It documents surface appearance, while OCT provides cross-sectional information. Read the retinal imaging guide.
Dilated eye examination
An examination performed after drops widen the pupils so the clinician can inspect more of the retina, optic nerve, macula, and other internal structures. Temporary light sensitivity and near blur are common after dilation.

Common Eye, Refraction, and Surgery Terms

These terms provide context for diagnoses, prior procedures, and visual symptoms discussed elsewhere on Eye Freedom.

Myopia (nearsightedness)
A refractive condition in which distant objects are blurred because light focuses in front of the retina when accommodation is relaxed. The prescription is usually written with minus power.
Hyperopia (farsightedness)
A refractive condition in which the eye must use focusing effort because light would otherwise focus behind the retina. Near tasks may become more difficult, although symptoms vary with age and prescription.
Astigmatism
A refractive condition in which optical power differs by meridian, commonly because of corneal or lens shape. Regular astigmatism is often corrected with glasses or contact lenses; irregular astigmatism may require a different optical approach.
Presbyopia
The age-related reduction in the eye’s ability to focus at near. Reading glasses, multifocal or monovision contact lenses, and other options may be considered based on the individual.
Cataract
Clouding of the natural lens inside the eye. Cataract can reduce acuity or contrast and increase glare; an ophthalmic examination determines whether surgery or monitoring is appropriate.
Glaucoma
A group of diseases that damage the optic nerve and can cause visual-field loss. Eye pressure is an important risk factor, but glaucoma can occur at different pressure levels and is assessed with multiple findings.
Retina
The light-sensitive tissue lining the back of the eye. It converts light into signals that travel through the optic nerve to the brain.
Optic nerve
The bundle of nerve fibers that carries visual information from the retina toward the brain. Glaucoma and other optic neuropathies can damage these fibers.
Macula
The central area of the retina responsible for detailed central vision. The fovea is the small central region where visual acuity is highest.
Low vision
Vision impairment that continues to limit daily activities despite ordinary glasses, contact lenses, medication, or surgery. Rehabilitation may use magnification, lighting, contrast, technology, and task-specific strategies.
Color vision deficiency
Reduced ability to distinguish certain colors. Many inherited forms affect red-green discrimination, while acquired changes can have other patterns and may warrant evaluation. See color vision evaluation.
LASIK
A refractive procedure that reshapes the cornea with a laser. Some people later report dryness, glare, halos, fluctuating vision, irregular astigmatism, or ectasia. See scleral lenses after LASIK and RK.
Radial keratotomy (RK)
An older refractive procedure that used radial corneal incisions to reduce myopia. Years later, some eyes develop prescription shifts, fluctuation, glare, irregular astigmatism, or corneal instability.
Higher-order aberrations
Optical distortions more complex than ordinary nearsightedness, farsightedness, or regular astigmatism. They may contribute to glare, halos, starbursts, ghosting, or reduced contrast and can be measured with wavefront aberrometry.
Corneal cross-linking (CXL)
A medical procedure using riboflavin and ultraviolet light to increase corneal biomechanical strength in selected progressive ectasia cases. It is intended to reduce progression risk, not to replace vision correction or guarantee a specific visual result.

Medical and Terminology Sources

Definitions were checked against patient education and standardized terminology from the following sources. Eye Freedom links are included for practice-specific context.

This glossary is educational and does not diagnose a condition, interpret a test, or recommend treatment. For an individualized evaluation, contact Eye Freedom or call 305-271-8206. The original consultation form appears directly below this page content.

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