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Keratoconus Glossary C-D

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Cataract - An age-normal loss of clarity of the natural lens inside of the human eye. Typicaly develops later in life and can reduce vision due to the inability of light to enter the eye.

Center islands - A manageable complication of LASIK related to ablation. The incidence of center islands has been greatly reduced as more ophthalmic research has been devoted to its contributing factors.

CK (Conductive Keratoplasty) - Procedure in which radio waves are used to heat collagen in the cornea's periphery to shrink it.

Collagen - Fibrous protein in bones and connective tissue, it is also present in the eye but in crystalline form. Collagen is the main protein of connective tissue.


Contact lens problem - Contact lens problems can range from minor to sight-threatening, and include protein build-up, debris on the lens, a ripped or nicked lens, infections and more. Symptoms can include frequent blinking, blurred vision, burning, discharge, foreign body sensation, itching, light sensitivity, eye pain or discomfort, a red or pink eye or lid and eyelid swelling. These problems can be avoided and solved by a professional eye care specialist with experience and through education. 


Contrast Sensitivity - The ability to detect subtle differences between details that are not black on white such as shades of gray; important in determining the quality of vision after refractive surgery


Conjunctiva (kahn-junk-TI-vuh) - Transparent mucous membrane covering the outer surface of the eyeball except the cornea, and lining the inner surfaces of the eyelids.

Conjunctivitis (kun-junk-tih-VI-tis), "pink eye. " Inflammation of the conjunctiva. Characterized by discharge, grittiness, redness and swelling. Usually viral in origin, but may be bacterial or allergic; may be contagious.

Cornea - The outer part of the eye that provides 70% of the eye's refractive power. The cornea is approximately 500 microns thick (.5 millimeter) and consists of 5 layers epithelium, Bowman's membrane, stroma, Descemet's membrane and endothelium.


Corneal Abrasion - A loss of the epithelial layer of the cornea, typically due to minor trauma (contact lens trauma, a sports injury, dirt or another foreign body, etc.). Symptoms include blurred vision, foreign body sensation, grittiness, light sensitivity, eye pain or discomfort, a red or pink eye and tearing.


Corneal dystrophy - One of a group of conditions, usually hereditary, in which the cornea loses its transparency. The corneal surface is no longer smooth. Common forms include map-dot-fingerprint dystrophy, Fuch's dystrophy and lattice dystrophy. Symptoms include blurred vision, foreign body sensation, light sensitivity, eye pain or discomfort and vision loss.


Corneal edema - Swelling of the eye's cornea; causes include intraocular surgery, corneal dystrophies, high intraocular pressure and contact lens complications. Symptoms include vision loss, halos around lights, a white or cloudy spot on the eye, photophobia, eye pain and foreign body sensation.


Corneal erosion - Recurrent breakdown of the corneal epithelium, typically caused by a previous corneal abrasion or by map-dot-fingerprint dystrophy. Symptoms include blurred vision, foreign body sensation and eye pain or discomfort.

Corneal opacity - A cloudy spot in the cornea, which is normally transparent. Causes include corneal scar tissue and infection. Symptoms include halos around lights, photophobia, vision loss and a white or cloudy spot on the eye.


Corneal Topography - The advent of computerised videokeratoscopic topographic assessment has vastly improved the early detection, monitoring and management (in terms of contact lens fitting and surgical interventions, such as intacs insertion and post-keratoplasty astigmatism management) of keratoconus.

These systems allow the rapid assessment of thousands of data points from the anterior corneal surface to provide accurate maps of the corneal surface in early and moderate disease. Such maps can give accurate information in terms of height and curvature on the location, morphology and severity of the cone.

In the case of combined scanning slit-lamp and Placido-based systems such as the Orb Scan system (Bausch & Lomb), very valuable pachymetric data and posterior corneal curvature maps can also be obtained. Such systems are vital for the refractive laser surgeon in preventing eyes with early and sub-clinical 'forme fruste' keratoconus from undergoing inadvertent excimer laser ablation with the possibility of exacerbating the ectatic process post-operatively.

In early and moderate cases of keratoconus the videokeratoscopy mires are distorted and typically lie closest together in the inferocentral region at the apex of the cone where the cornea is steepest. Information from these mires can provide:

  • Height maps which allow the localisation and characterisation of the cone and its apex
  • Curvature maps which in early cases may only show an asymmetrical bow-tie pattern, but in more advanced eyes show obvious areas of corneal steepening and ectasia
  • Corneal higher-order wavefront anomalies, which have recently been shown to be valuable in early detection and grading of the condition
  • Statistical indices, which are useful in detection and monitoring progression.

To note, as most commercially available systems rely on the analysis of clear Placido disc and scanning slit-lamp images in advanced disease with corneal scarring and surface irregularities associated with very steep cones, little or no useful data can be obtained.

Cylinde - The amount of lens power necessary to compensate for astigmatism


Cylinder meridian - In ophthalmology, a line that is the symmetrical center of a curved optical surface. Measure of astigmatism.


Cytokine - (sī'tə-kīn') Any of several regulatory proteins, such as the interleukins and lymphokines, that are released by cells of the immune system and act as intercellular mediators in the generation of an immune response.

Decentration - A complication caused by movement of the pupil that can be corrected with an enhancement procedure.


Descemet's membrane - The layer of the cornea between the stroma and endothelium. Five microns thick (.005 millimeters), this membrane provides an adhesion layer for the endothelium.


Deturgescence - The balance of hydration in the eye.


Dilated pupil - Enlarged pupil, resulting from contraction of the dilator muscle or relaxation of the iris sphincter. Occurs normally in dim illumination, or may be produced by certain drugs (mydriatics, cycloplegics) or result from blunt trauma.

Diopters - A measurement of refractive error. Hyperopia is measured in terms of positive diopters. Myopia is measured in terms of negative diopters. The most common refractive errors ranged between +6 to -6 diopters.


Dk/t - The measurement of a contact lens's oxygen transmissibility.

Dry eye - A Syndrome characterized by corneal dryness due to deficient tear production.

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