P
Pachymetry
An procedure used to measure the thickness of the cornea.
PARK
See Photoastigmatic Refractive Keratectomy
Papilledema
Non-inflammatory swelling/elevation of the optic nerve often due to increased intracranial pressure or the presence of a tumor.
Pellucid Marginal Degeneration
Pellucid marginal degeneration (PMD) is a rare, non-ulcerative thinning disorder involving the outer edge of the lower portion of the cornea. PMD causes a portion of the cornea to bulge forward, often first presenting as astigmatism. In contrast to keratoconus, where the greatest corneal protrusion is in the area of maximal thinning, the greatest corneal protrusion with PMD is above the area of maximal thinning.
Penetrating Keratoplasty
Abbreviated as PK or PKP. A surgical procedure when a button-like full thickness segment of the cornea is removed and replaced with a donor cornea from another person; a corneal transplant.
Peripheral Vision
The ability to see objects and movement outside of the direct line of vision.
Perioperative
Pertaining to the period extending from the time of hospitalization or admittance for surgery to the time of discharge.
PERK Study
A multi-center study of radial keratotomy (RK) outcomes funded by the National Eye Institute. Ten-year results of RK patients who had the surgery in 1983 were published in Archives of Ophthalmology in October 1994.
The study found that seven out of ten patients who had the operation on both eyes no longer wore or required corrective lenses ten years after surgery, and 85% of patients saw 20/40 or better without glasses. It concluded that RK is a "reasonably safe operation" that can "effectively reduce but not eliminate myopia" (nearsighted, shortsighted vision).
The study also found that 43% of patients experienced a hyperopic shift following surgery over the ten-year period. These patients typically experienced an improvement in their vision as their refractive error moved closer to 20/20. It is for this reason that ophthalmic doctors will often intentionally undercorrect their patients, then monitor the rate of healing before performing a follow-up enhancement procedure.
Patients whose vision had achieved full correction or near full correction through surgery and who experience a hyperopic shift could become farsighted and need to wear glasses.
Phacoemulsification
A cataract surgical procedure which uses an ultrasonic vibration to shatter and break up a cataractous lens, making it easier to remove. The vibration is delivered by an irrigation-aspiration instrument. In a survey of ASCRS members in 1994, 86% preferred the phacoemulsification cataract removal technique over the extracapsular cataract extraction technique. The technique was invented by Charles D. Kelman, MD, and was first published in 1967.
Phakic
Meaning that the natural crystalline lens of the eye is present.
Phakic Intraocular Lens
Abbreviated as P-IOL. Also known as Intraocular Contact Lens and abbreviated as P-IOL. This is a tiny plastic lens that is placed inside the eye in front of the natural crystalline lens to provide additional refractive change. A Phakic Intraocular Lens is placed either immediately behind or in front of the iris. Detailed Verisyse and Visian ICL Phakic Intraocular Lens (P-IOL) Information
P-IOL
See Phakic Intraocular Lens
Phi-motion Angiography
Phi-motion angiography employs a scanning laser to capture rapid-sequence images of blood vessels underlying the retina.
Phoropter
An optical instrument containing many lenses which is used to determine the required power of glasses or refractive error.
Phototherapeutic Keratectomy
Abbreviated as PTK. The use of an excimer laser to remove surface tissue of the cornea for medical or optical treatment reasons. Very similar to PRK, but for therapeutic rather than refractive purposes. See Phototherapeutic Keratectomy Details
Photoablation
The "cold" process of tissue removal which occurs with excimer laser radiation in the 200nm wavelength range. This far-ultraviolet wavelength possess light photons so powerful that the molecular bonds of the target tissue both break down and have sufficient extra kinetic energy to fly off the surface; hence ablation. Microscopic pictures show incredibly precise cuts with no evidence of tissue burning in adjacent tissue.
Photocoagulation
The process of tissue destruction accomplished by visible light radiation. Tissue is broken down by the light and "clots" as if it were cooked.
Photophobia
Sensitivity to light.
Photorefractive Keratectomy
Abbreviated as PRK. A surgical procedure using an excimer laser to reshape the central cornea to a flattened shape for people who are myopic and a more curved surface for people who are hyperopic. Photorefractive Keratectomy techniques may also be used to correct astigmatism. See Photorefractive Keratectomy Details.
Photovaporization
The process of tissue destruction as occurs with infra red light radiation.
Phthisis Bulbi
Shrinking of eyeball following injury, infection, or disease.
Pinguecula
A yellowish spot seen on the white of the eye at the junction of the clear cornea and white sclera of the eye. These lesions are usually caused by ultraviolet radiation. The white surface of the eye cannot "tan" and therefore cannot protect itself from sunburn.
Pinhole Glasses
Used in a diagnostic setting to differentiate whether reduced vision is secondary to uncorrected refractive error or to disease/amblyopia. By eliminating the peripheral light rays and concentrating the central bundles of light, the vision can be "sharper". But they have no therapeutic value. Pinhole glasses can't permanently improve one's vision.
PK
See Penetrating Keratoplasty.
Plano
No refractive error. Normal vision. A diopter of 0.
Point Spread Function
Often used in wavefront diagnostic analysis. A single round spot of light becomes diffused when it passes through an aberrated element such as a human eye. Aberrations cause some of the light to spread beyond the boundaries of the light source, causing the dot of light to appear fuzzy. Analysis of the diffusion of light (spread) helps doctors determine what changes may be made to remove aberrations causing the diffusion.
Polynomial
See Zernike Polynomial
Polyopia
Two or more images. Monocular polyopia would be when a person sees multiple images in one eye.  Monocular polyopia is commonly described as ghosting or providing a ghost image. Binocular polyopia is multiple images only when both eyes are used, indicating strabismus, a misalignment of the eyes. Polyopia may be caused by neurological dysfunction, disease, or medication.
Posterior
Back surface or behind.
Posterior Capsular Opacification
Opacification of the posterior lens capsule. Sometimes called a "secondary cataract". Often a consequence of modern cataract surgery. It occurs when a thin membrane of tissue grows over the remaining capsule following cataract surgery, and can develop in as many as half of all cases between several months and several years after surgery. Most often treated using the YAG laser to ablate a hole in the capsule.
Posterior Chamber
The space between the back of the iris and the front face of the vitreous; filled with aqueous fluid.
Posterior Optical Segment
The part of the eye behind to the crystalline lens, including the vitreous, choroid, retina, and optic nerve.
Posterior Vitreous Detachment
The separation of the vitreous body from the retinal surface due to shrinkage from degenerative or inflammatory conditions or trauma. Often an age-related condition.
PPO
See Preferred Provider Organization
Preferred Provider Organization
A medical or vision insurance plan that entices beneficiaries to use only the facilities and physicians who have contracted with the plan to provide services to beneficiaries by creating a financial disincentive if the beneficiary seeks care outside the plan's panel of providers. Care provided to a patient by a facility or physician that is not contracted by the Preferred Provider Organization is normally a covered benefit of the plan, however at a significantly lower rate than the same care provided by a facility or physician who is a contracted provider. The exception is in some emergency situations or with prior authorization from the plan.
PRELEX
See Presbyopic Lens Exchange
Preoperative
Pertaining to the period of time before surgery.
Presbyopia
Part of the normal process of aging. As a person becomes older, we begin to lose the flexibility of the natural crystalline lens of the eye and weakness of the ciliary muscle. Presbyopia actually starts at about age ten. Most people do not begin to experience the effects of presbyopia until their forties. This loss of flexibility limits the ability of the eye to change its point of focus from distance to near. Because of this normal process, people begin to wear bifocals or other reading correction. The one advantage of mild myopia (nearsighted, shortsighted vision)is the ability to remove your glasses after the onset of presbyopia and continue to read. Mild myopia (nearsighted, shortsighted vision)effectively counteracts presbyopia. Refractive surgery does not change effects of presbyopia.
Presbyopic Lens Exchange
The term Presbyopic Lens Exchange (PRELEX) was created to describe a Refractive Lens Exchange with an intraocular lens designed to accommodate presbyopia.
Prescription Terms
Medical prescription terms were originally all written in Latin. Many of the Latin terms continue to be used today. Eyeglass prescriptions are somewhat different, and you can learn about them at Read Your Eyeglass and Contact Prescription. Common medical prescription terms are:
PresVIEW
Surgical treatment for presbyopia owned by Refocus Group, Inc., and partnered with CIBA Vision for distribution. PresVIEW has received European CE Mark approval for the PresVIEW Scleral Incision System in August 2003. The company has re-engineered the system for its PresVIEW Scleral Spacing Procedure, which surgically treats presbyopia, primary open angle glaucoma and ocular hypertension. The updated procedure incorporates automated components to simplify and enhance placement of Refocus Group's PresVIEW scleral implants, resulting in greater reproducibility of the procedure by clinicians. Refocus Group's strategic partner, CIBA Vision, plans a commercial launch of the PresVIEW Scleral Spacing Procedure in selected European markets by the end of 2003. Refocus Group also aims to begin FDA Phase II clinical trials of the procedure for presbyopia in late 2003, subject to FDA approval.
Prism
A prism is a lens that will bends light towards its base. Prisms are used to measure the angle of deviation in strabismus to correcting binocular polyopia when the eyes become misaligned in adulthood. A prism may be included in spectacle correction to help align images.
PRK
See Photorefractive Keratectomy.
PSF
See Point Spread Function
Pseudophakic
When the natural crystalline lens of the eye has been replaced with an artificial lens during cataract surgery or Refractive Lens Exchange.
PTK
See Phototherapeutic Keratectomy.
Ptygeria
A growth of scar tissue on the cornea. These lesions like pinguecula are caused by an ultraviolet burn to the surface layer or epithelium.
Punctal Occlusion
Closing off the punctum, either with plugs, cauterization, or other natural or artificial methods
Punctal Plugs
Small silicone or plastic plugs that are inserted into any or all of the punctum.
Punctum
Tear drains around the eye. There are two in the upper lid and two in the lower lid.
Pupil
Appears as a small black dot in the center of the iris. The pupil changes its diameter in response to changes in ambient lighting. The pupil varies the amount of light reaching the retina and the depth of focus of the eye.
Pupillary Response
The constriction and dilation of the pupil due to stimulation by light or accommodation.
Q R
Readers
See Reading Glasses.
Really Simple Syndication
Really Simple Syndication (RSS) is an XML-based format for news distribution that includes headlines, summaries and links back to a publisher website for the full article. You load RSS news feeds into a reader or visit them on a personalized web page. RSS keeps you up-to-the-moment on your favorite news sources, providing an indicator when news breaks. You may use any of the popular RSS readers to organize your own feeds, including feeds from USAEyes. Learn more about Lasik RSS-XML Newsfeed
Reading Glasses
Spectacles that induce a small amount of myopia (nearsighted, shortsighted vision), also called plus lenses. Reading glasses, often called readers, are used temporarily by individuals who are presbyopic to assist with near vision. See also bifocals, trifocals, and monovision.
Radial
Pertaining to the radius or line from a circle center to the circumference of the circle.
Radial Keratotomy
Abbreviated as RK. A surgical procedure that permanently alters the shape of the cornea by placing microscopically thin relaxing incisions in the peripheral cornea. The incisions cause the central portion of the cornea to flatten, thereby reducing the power of the cornea.
Refractec
Manufacturer of ophthalmic equipment for Conductive Keratoplasty. 
Refraction
(1) A test to determine the best eyeglasses or contact lenses to correct a refractive error including myopia (nearsighted, shortsighted vision), hyperopia (farsighted, longsighted vision), or astigmatism (irregular cornea).
(2) The bending of light by the use of lens or other material.
See also Manifest Refraction and Cycloplegic Refraction.
Refractive Error
The degree to which images received by the eyes are not focused on the retina (myopia, hyperopia, or astigmatism) measured in diopters. Refractive error is commonly presented in an prescription for spectacles or contact lenses.
Refractive Lens Exchange
Refractive Lens Exchange (RLE), also known as Clear Lens Exchange (CLE), is essentially cataract surgery without the cataract. The crystalline lens is removed from the eye and replaced with a clear plastic intraocular lens (IOL). The new IOL will be of a power to correct refractive error. Cataract surgery is performed when the crystalline lens has become cloudy. RLE is performed for purely refractive purposes. Refractive Lens Exchange (RLE) Details
Refractive Surgery
Any surgical procedure which permanently alters the focusing power of the eye in order to change refractive errors. Refractive surgery may include corneal surgery such as Lasik, LASEK, PRK, Intacs, CK, LTK or lens surgery such as RLE or P-IOL
Regression
A return by the cornea toward the original refractive state. See Regression Details
Restasis
An prescription eye drop with diluted cyclosporine anti-inflammatory used for the treatment of dry eye and other ocular conditions.
ReSTOR
The brand name for the AcrySof ReSTOR apodized multifocal intraocular lens. The ReSTOR lens is used with cataract surgery and Refractive Lens Exchange (RLE). The potential advantage of any multifocal lens is the ability to see clearly objects both near and distant. ReSTOR attempts to resolve presbyopia.  Details about RLE Details about Presbyopia Surgery.
Retina
The light sensitive layer of cells (rods and cones) on the inner back surface of the eye that converts light images into nervous impulses sent along the optic nerve for transmission to the brain.
Retinal Axons
The nerve fiber extensions of the retinal photoreceptors that form the nerve bundle called the optic nerve.
Retinal Detachment
A separation of the neural tissue of the retina from the pigmented epithelium layer and therefore the blood supply. Results in loss of vision in the detached area. Repairable with fair to good prognosis for vision if diagnosed in the early stages. See Retinal Detachment Details
Retinal Pigment Epithelium
The pigment cell layer that nourishes the retinal cells; located just outside the retina and attached to the choroid.
Retinitis Pigmentosa
The name given to a group of disorders of the Retina, all of which result in a progressive reduction in vision. See Retinitis Pigmentosa Details.
Retreatment
See enhancement.
ReZoom
The brand name for the ReZoom multifocal intraocular lens. The ReZoom lens is used with cataract surgery and Refractive Lens Exchange (RLE). The potential advantage of any multifocal lens is the ability to see clearly objects both near and distant. ReZoom attempts to resolve presbyopia. Details about RLE  Details about Presbyopia Surgery.
RGP
See Rigid Gas Permeable Contact Lens.
Rigid Gas Permeable Contact Lens
A hard contact lens that allows significant amount of air to pass through. This type of lens often provides superior refractive correction to a soft contact lens.
RK
See Radial Keratotomy.
RLE
See Refractive Lens Exchange
RMS
See Root Mean Square
RSS
See Really Simple Syndication
Rods, Rod Cells
One type of specialized light sensitive cells (photoreceptors) in the retina that provide side vision and the ability to see objects in dim light (night vision). Also, see cones.
Root Mean Square
Actually means the root of the mean of the squares and is an evaluative measurement of a wavefront diagnostic. It describes the smoothness of the surface in terms of deviation from the ideal spherical wavefront and is calculated using the same technique that is used to derive standard deviation. As a general rule of thumb, a Root Mean Square value of .30 microns or greater is an indication that a wavefront-guided ablation is necessary to provide a patient a good refractive surgery outcome.
Rosacea
See Ocular Rosacea.