Optic Nerve Physician Resource Center Glossary

Apoptosis – programmed cell death.^1a
Artifact – anything made or introduced artificially which misleads the results or interpretation of an investigation; in the case of imaging, a visible defect in an image caused by limitations in the software or hardware.^2a
A-scan – single scanning line that forms part of the overall Optical Coherence Tomography (OCT) image.^3a (when these individual scanning lines are placed sequentially, they form the image)
Astigmatism – a defect of vision in which the image of an object is distorted because not all the light rays come to a single point of focus in the eye; rather, there are multiple points where the image is formed. Usually due to abnormal curvature of the cornea and/or lens.^1b
Birefringent – referring to a medium that doubly refracts a beam of light, i.e. bends it in two different directions.^1c
Cataract – partial or complete loss of transparency of the crystalline lens substance or its capsule.^2b
Corneal epitheliopathy – disease involving the cornea’s outermost layer. ^4,5a
Confocal Scanning Laser Ophthalmoscopy (CSLO) - An imaging technology available to aid and inform diagnosis and monitoring of glaucoma through measurements of optic nerve and retinal nerve fiber layer parameters.^6a Available commercially as Heidelberg Retinal Tomograph (HRT).^6b
Cup – a funnel shaped depression at or near the center of the optic disc through which pass the central retinal vessels.^2c
Cup-to-disc ratio – the ratio of horizontal and vertical diameter of the rims of the physiological cup to that of the horizontal and vertical diameter of the optic disc. It is usually less than 0.5. If there is a difference in ratio between the two eyes, or if there is progressive enlargement of the cup, glaucoma may be suspected.^2d
Disc hemorrhage – bleeding from the vessels of the optic disc usually creating a small fan-like hemorrhage on the surface (“haemorrhage”).^1d
Drainage angle – angle at the periphery of the anterior chamber formed by the root of the iris, the front surface of the ciliary body and the peripheral posterior surface of the cornea and includes the trabecular meshwork. (Also known as “angle of filtration”).^2e,7
Elschnig’s ring – the appearance of a white patch of sclera adjacent to the optic disc where the retinal pigment epithelium and the choroid do not extend to the optic disc.^2f (Also known as the “scleral ring”).⁸
Frequency-doubling perimetry – a method of testing the visual field based on the frequency doubling illusion and thus assessing the integrity of the large-diameter retinal ganglion M cells which are susceptible to early glaucomatous damage.^2g
Fundus – the interior of the eye, including the vitreous and retina (as may be seen with the aid of an ophthalmoscope).^2h
GDxVCC – brand name of a Scanning Laser Polarimeter,^9a used to aid diagnosis and monitoring of glaucoma.^9b Useful in monitoring changes in the retinal nerve fiber layer.^9a
Glaucoma Probability Score (GPS) – an output of Heidelberg Retinal Tomograph (HRT). GPS is an approach to analyzing scan data using artificial intelligence methods, producing an easy numerical way to identify probability of disease.¹⁰
Gonioscopy – observation of the angle of the anterior chamber of the eye with a gonioscope or with a contact or prism lens.^5b
Henle fiber layer – layer located in the macular region and formed by cone and rod fibers which run parallel to the retinal surface within the outer molecular layer of the retina.²ⁱ
Heidelberg Retinal Tomograph (HRT) – brand name of most widely available Confocal Scanning Laser Ophthalmoscope (CSLO), used to aid diagnosis and monitoring of glaucoma.¹¹ (also see definition of CSLO)
Intraocular pressure – the pressure within the eyeball occurring as a result of the constant formation and drainage of the aqueous humor.^2j
Lamina cribrosa – the perforated part of the sclera through which passes the retinal ganglion cell axons and central retinal vessels as they pass through the optic nerve head.¹²
Macular pucker – distortion of the macular reflex caused by formation of glial cells over the surface of the internal limiting membrane of the macular region of the retina. (also known as “preretinal macular fibrosis”).^2k
Moorfields Regression Analysis (MRA) – an output of Heidelberg Retinal Tomograph (HRT). MRA compares data obtained from the patient against a normative database to classify eyes and identify probable glaucoma based upon the relationship between the cup and rim area, among other parameters.^3b (also see HRT)
Myopia – refractive condition of the eye in which the images of distant objects are focused in front of the retina when the accommodation is relaxed. Thus, distance vision is blurred.^2l
Neural pathways – tracts of nerve cells connecting one part of the nervous system with another.¹³
Nystagmus – a regular, repetitive, involuntary movement of the eye whose direction, amplitude and frequency are variable.^2m
Optical Coherence Tomography (OCT) - An imaging technology available to aid and inform diagnosis and monitoring of glaucoma. Available commercially as Stratus OCT and Cirrus HD-OCT.^14,15 (also see A-scan)
Optic disc – region of the fundus of the eye corresponding to the optic nerve head.²ⁿ
Optic nerve head – common name used for the optic disc.¹⁶
Penetrating keratoplasty – surgical procedure involving the removal of the central cornea and replacing it with a human-donor cornea (corneal transplant). This full thickness corneal donor tissue is then sutured to the remaining peripheral cornea of the patient/recipient.^5c
Perimetry – a method of mapping the extent of a patient’s area of vision (visual field) by displaying light images of different size and intensity; automated perimeters are commonly used in the diagnosis and follow-up of glaucoma.^1e
Peripapillary atrophy – the absence of pigmented retinal tissue and the thinning or absence of the choroid blood vessels adjacent to the optic disc.¹⁷
Polarized light - light waves restricted to certain directions of vibration, such as vertically as opposed to both vertically and horizontally. ¹⁸
Posterior subcapsular cataract – cataract involving the cortex at the posterior pole of the lens, usually causing blurring within the center of vision.^5d
Retinal ganglion cell – retinal cell which connects the bipolars and other cells in the inner plexiform layer with the lateral geniculate body in the brain. These cells are responsible for conducting the visual information from the retina to the brain where visual interpretation occurs. The axons of the ganglion cells constitute the optic nerve fibers.^2o
Retinal nerve fiber layer (RNFL) – layer of the retina composed of the unmyelinated axons of the ganglion cells which converge towards the optic disc where they exit the eye and form the optic nerve.^2p
Short Wavelength Automated Perimetry – a procedure for detecting and monitoring visual field defects in patients suspected of having early glaucomatous visual field loss and/or ocular hypertension. It is said to be more sensitive in detecting glaucomatous change than Standard Automated Perimetry (SAP, white on white perimetry). It uses a blue stimulus on a yellow background.^2q
Scanning Laser Polarimetry (SLP) - An imaging technology available to aid and inform diagnosis and monitoring of glaucoma.^9b Available commercially as GDxVCC.^9a (also see GDxVCC)
Stereometric parameters – a set of three-dimensional measurable factors used for detecting abnormalities within the optic nerve head.^6c The most important parameters are disc area, cup and disc rim area, cup and disc rim volume, mean and maximum cup depth, a measure for the three-dimensional shape of the cup, and for the mean thickness of the retinal nerve fiber layer along the contour line.^6d
Synechiae – adhesions between the iris and other parts of the eye, usually the lens, cornea or trabecular meshwork.^1f
Tonometry – measurement of intraocular pressure with an instrument usually applied to the corneal surface, called a tonometer.^2r
Topography – a detailed representation of the surface features of a region.¹⁹ Also, the description of any part of the body, especially in relation to a definite and limited area of the surface.^5e
Trabeculoplasty (laser) – a procedure aimed at improving the outflow of aqueous humor in open-angle glaucoma by producing a series of laser burns or applications to the trabecular meshwork.^2s
TSNIT – referring to a temporal-superior-nasal-inferior-temporal scan pattern used by GDxVCC; displays the retinal nerve fiber layer (RNFL) thickness values along the calculation circle starting temporally and moving superiorly, nasally, inferiorly, and ending temporally.²⁰
Variable Corneal Compensation (VCC) - A software feature of GDxVCC to correct for corneal variability that can influence polarimetry.^9a
Vitreous – gel contained in the eye, consisting of 99% water and 1% protein, collagen filaments and hyaluronic acid molecules.²¹
Vitreous floaters – heterogeneities in the vitreous humor which may be of embryonic origin or pathological (e.g., in vitreous detachment, retinal detachment). The patient sees spots which float as the eye moves. This may be most visible to a patient under sunlight and bright light used for reading. (Also known as “floaters”).^2t

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Kabat AG and Sowka JW. A clinician’s guide to flashes and floaters. Optometry Today. 2001. Available at: http://www.optometry.co.uk/articles/docs/640c1a5b606a7d89320511ca29ccc288_kabat20010323.pdf. Accessed April 21, 2008.