DOCSLIB.ORG

Treatment of Pellucid Marginal Degeneration 1Abdelsattar N Farrag, 2Ahmed a Hussein, 3Shiji Ummar

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Treatment of Pellucid Marginal Degeneration 1Abdelsattar N Farrag, 2Ahmed a Hussein, 3Shiji Ummar IJKECD Treatment10.5005/jp-journals-10025-1148 of Pellucid Marginal Degeneration REVIEW ARTICLE Treatment of Pellucid Marginal Degeneration 1Abdelsattar N Farrag, 2Ahmed A Hussein, 3Shiji Ummar ABSTRACT Although PMD classically has been affecting the infe- Purpose: To summarize the recent trends in the treatment rior cornea, superior PMD has also been reported, and we of pellucid marginal degeneration (PMD) based on available should consider it in the differential diagnosis of superior published data. corneal ectasia.7 The ectasia in PMD causes progressive Method and literature search: A PubMed search was con- diminution of both uncorrected and corrected visual ducted with combinations not limited to the following search acuity as a result of high against-the-rule astigmatism.1,2 terms: Pellucid marginal degeneration, Corneal ectasia, The condition is most commonly affecting males Corneal collagen cross-linking (CXL), Intracorneal ring seg- ments (ICRS), Contact lens, Keratoplasty in corneal ectasia. and usually presents between the 2nd and 5th decades 3,8 A review of the search results was performed and relevant of life. articles to the topic were included. The PMD can be diagnosed classically by slit-lamp Summary: Ophthalmologists have got a wide array of thera- examination, which shows a clear band of inferior corneal peutic modalities for the management of PMD. However, the key thinning extending from 4 to 8 o’clock. There is typically to optimal treatment is careful clinical assessment of patients a 1 to 2 mm of uninvolved normal cornea. The maximum and their visual requirements and tailoring the treatment to point of protrusion in PMD occurs in the area superior individual patients. to the thinning. This is considered one of the important Keywords: Corneal collagen cross-linking, Corneal ectasia, Intracorneal ring segment, Keratoplasty, Pellucid marginal points in differentiation with keratoconus, in which the 9-11 degeneration. maximum protrusion is within the area of thinning. Corneal topography with classical pattern of crab-claw How to cite this article: Farrag AN, Hussein AA, Ummar S. Treatment of Pellucid Marginal Degeneration. Int J Kerat Ect appearance is not diagnostic for PMD, as it can present Cor Dis 2017;6(2):87-91. with inferior keratoconus. So, it is recommended to Source of support: Nil perform more detailed analysis with slit-lamp micros- copy, topography, anterior and posterior elevation maps Conflict of interest: None by Scheimpflug imaging and full pachymetric maps that shows 12 mm of corneal coverage for improving the INTRODUCTION diagnostic accuracy for PMD.12 Pellucid marginal corneal degeneration is a bilateral, Management modalities of PMD can be broadly clas- noninflammatory, corneal ectasia, most frequently sified as: involving the inferior peripheral cornea in a crescentic • Corneal rehabilitation treatment which includes: pattern. The corneal thinning usually arises between the – Nonsurgical: spectacles, contact lenses 4 and 8 o’clock position and protrusion occurs as a result – Surgical: ICRS and CXL of this thinning.1-3 Schlaeppi4 was the first one who used • Toric intraocular lens (IOL) implantation the term “pellucid,” meaning clear, to signify the clarity • Corneal replacement surgery which includes: of the cornea despite the presence of corneal ectasia. It – Full thickness or penetrating keratoplasty (PKP): is usually bilateral and asymmetrical; nevertheless, uni- oversized central PKP/inferiorly decentered PKP, lateral cases are reported.5,6 lamellar keratoplasty (LK), simultaneous central PKP, and inferior peripheral LK, intrastromal LK, crescentic wedge resection, epikeratophakia, and 1Staff Registrar, 2Resident, 3Cornea Specialist thermokeratoplasty. The aim of this article is to review the current treat- 1Department of Ophthalmology, Singapore National Eye Center Singapore ment modalities of PMD. 2Department of Ophthalmology, Baladi Foundation, Aswan, Egypt NONSURGICAL CORNEAL REHABILITATION 3Department of Ophthalmology, Al Nahdha Hospital, Muscat Oman TREATMENT Corresponding Author: Shiji Ummar, Cornea Specialist Most patients with early and moderate PMD can be visu- Department of Ophthalmology, Al Nahdha Hospital, Muscat ally rehabilitated by spectacles or contact lenses.3,8 Oman, Phone: +96893504023, e-mail: dr.shijiparvez@yahoo. In the early stages of PMD, soft toric lenses can be an co.in option for improvement in visual acuity. International Journal of Keratoconus and Ectatic Corneal Diseases, July-December 2017;6(2):87-91 87 Abdelsattar N Farrag et al In moderate PMD, treatment usually requires the use forward displacement of the anterior surface at this area. of large-diameter rigid, gas-permeable lenses due to the This causes a peripheral steepening and a flattening of the development of irregular astigmatism and progression central part of the anterior cornea due to the morphologic of corneal irregularity.8,13 In addition, specialized contact structure of the corneal lamellae.24 The same principle lenses, such as reverse geometry lenses and “bi-toric” can be applied in cases of PMD resulting in decreasing lenses can be useful in the management of moderate and centralization of the peripheral corneal protrusion, PMD.14,15 inducing a significant change in corneal asymmetry and In advanced PMD, the scleral contact lens is a helpful refraction. nonsurgical treatment when the fitting with corneal The implantation of ICRS in PMD proved to be effec- lenses is extremely difficult because of the peripheral tive in the improvement of visual outcome either by location of the ectasia.16 manual or femtosecond surgery.25-31 Scleral lenses are large-diameter gas-permeable A case report showed improvement of visual acuity lenses that completely vault the cornea and rest on the after femtosecond implantation of a single ICRS in supe- sclera. Many studies reported the successful use of rior PMD. However, follow-up was only for 3 months.32 scleral lenses in the treatment of ocular surface disorders, The ICRS implantation may be associated with post- postkeratoplasty patients, and advanced corneal ectatic operative complications, which include segment extru- disorders.16-18 sion, corneal neovascularization, infectious keratitis, In a recent study evaluating the success of prosthetic channel deposits around the ring, ring segment migra- replacement of the ocular surface ecosystem (PROSE) tion, epithelial plug at the incision site, corneal melting, in PMD patients, a significant improvement in visual and night halos.33 acuity with PROSE (Boston Foundation for Sight, USA) Limited number of case reports or case series of ICRS is reported in all patients with PMD after failure of all implantation in PMD may be due to rare presentation in other nonsurgical options. However, we should be careful clinical practice and that ICRS implantation is limited to while prescribing these lenses at the advanced stage of the specific criteria, which may not be fit for most patients. disease as the risk of hydrops is high in these patients.19 Future studies are recommended with larger number of Also, another study reported success in the use of patients. scleral Misa1 lenses (Microlens Contact lens Techno­ logy, Arnhem, Netherlands) to improve visual acuity Collagen Cross-linking in long-term follow-up in patients with PMD, when any Wollensak et al34 demonstrated the effects of CXL on other optical modalities failed, and they recommend it the biomechanical properties of human and porcine for moderate advanced PMD when other types of contact cornea. Riboflavin (vitamin B2) is a photosensitizer lens cannot be fitted or in cases of poor operative prog- with an absorption peak for ultraviolet (UV) A at a nosis and for patients who refuse surgical intervention.20 wavelength of 370 nm. The molecule fluoresces and SURGICAL CORNEAL REHABILITATION excites to a triplet state when exposed to UV radiation of this wavelength, with subsequent release of singlet TREATMENT oxygen and superoxide radicals. In a cornea saturated Intracorneal Ring Segment with riboflavin, there will be a formation of new cova- Intracorneal ring segments are small pieces made of lent bonds between collagen molecules by oxidative polymethylmethacrylate, which are implanted in the deamination. It has proved its efficacy in arresting 35-37 deep corneal stroma, aiming for modifications of corneal progression of keratoconus. curvature and refractive changes. This notion was pro- A limited number of studies and case reports posed by Reynolds in 1978. The first implants were full have been published describing the effect of CXL in rings implanted into a circumferential corneal channel PMD. All showed improvement and/or stabilization 38-41 through a peripheral single corneal incision. Then to of visual acuity and keratometric parameters. The decrease the rate of incision-related complications and CXL was also combined with topography-guided tran- make it surgically easier, the rings were changed to the sepithelial surface ablation and showed improvement C-shape range, and they were renamed as ICRS.21-23 of visual outcome. However, the follow-up period was The ICRS is used in early-to-moderate PMD for 12 months.42 deferring corneal transplant and improving tolerance of Koller et al43 examined the peripheral effect of CXL contact lens. The principle of using this originated from by evaluation of the stromal demarcation line by OCT, what was demonstrated by the addition of extramaterial and they concluded that the targeted depth of CXL using at the normal
Load more

Recommended publications
  • Binocular Vision Disorders  Prescribing Guidelines
    Prescribing for Preverbal Children Valerie M. Kattouf O.D. FAAO, FCOVD Illinois College of Optometry Associate Professor Prescribing for Preverbal Children Issues to consider: Age Visual Function Refractive Error Norms Amblyogenic Risk Factors Birth History Family History Developmental History Emmetropization A process presumed to be operative in producing a greater frequency of occurrence of emmetropia than would be expected in terms of chance distribution, as may be explained by postulating that a mechanism coordinates the formation and the development of the various components of the human eye which contribute to the total refractive power Emmetropization Passive process = nature and genetics 60% chance of myopia if 2 parents myopic (Ciuffrieda) Active process = mediated by blur and visual system compensates for blur Refractive Error Norms Highest rate of emmetropization – 1st 12-17 months Hyperopia Average refractive error in infants = +2 D > 1.50 diopters hyperopia at 5 years old – often remain hyperopic Refractive Error Norms Myopia 25% of infants are myopic Myopic Newborns (Scharf) @ 7 years 54% still myopic @ 7 years 46% emmetropic @ 7 years no hyperopia Refractive Error Norms Astigmatism Against the rule astigmatism more prevalent switches to with-the-rule with development At 3 1/2 years old astigmatism is at adult levels INFANT REFRACTION NORMS AGE SPHERE CYL 0-1mo -0.90+/-3.17 -2.02+/-1.43 2-3mo -0.47+/-2.28 -2.02+/-1.17 4-6mo -0.00+/-1.31 -2.20+/-1.15 6-9mo +0.50+/-0.99 -2.20+/-1.15 9-12mo +0.60+/-1.30 -1.64+/-0.62
    [Show full text]
  • Analysis of Tear Film Spatial Instability for Pediatric Myopia Under Treatment
    www.nature.com/scientificreports OPEN Analysis of tear flm spatial instability for pediatric myopia under treatment Wan‑Hua Cho, Po‑Chiung Fang, Hun‑Ju Yu, Pei‑Wen Lin, Hsiu‑Mei Huang & Ming‑Tse Kuo * In Taiwan, the prevalence of myopia in children between 6 and 18 years old is over 80%, and high myopia accounts for over 20%, which turned out to be in the leading place worldwide. Orthokeratology and low-dose atropine are proven treatments to reduce myopia progression, though the potential corneal disturbances remain an issue in young populations. The alteration of the tear flm is widely discussed but there is no consensus to date, so we aim to investigate the tear flm spatial instability in children with myopia control using atropine or orthokeratology. Thirty-eight treatment-naïve participants and 126 myopic children under treatments were enrolled. The ocular surface homeostasis, spatial distribution of tear break-up, and high-order aberrations (HOAs) of the corneal surface were assessed. We found out that myopic children treated with either atropine or orthokeratology showed ocular surface homeostasis similar to that in treatment-naïve children. Nevertheless, children treated with orthokeratology presented higher HOAs (p < 0.00001) and a tendency of the frst tear break-up zone at the inner half of the cornea (p = 0.04). This unique spatial instability of the tear flm associated with myopia treatment might provide a more focused way of monitoring the pediatric tear flm instability. Many studies have revealed diferences in the prevalence of myopia across diferent regions and ethnicities, and the increased rate of myopia is most prominent in Asian/Pacifc children1,2.
    [Show full text]
  • Refractive Errors a Closer Look
    2011-2012 refractive errors a closer look WHAT ARE REFRACTIVE ERRORS? WHAT ARE THE DIFFERENT TYPES OF REFRACTIVE ERRORS? In order for our eyes to be able to see, light rays must be bent or refracted by the cornea and the lens MYOPIA (NEARSIGHTEDNESS) so they can focus on the retina, the layer of light- sensitive cells lining the back of the eye. A myopic eye is longer than normal or has a cornea that is too steep. As a result, light rays focus in front of The retina receives the picture formed by these light the retina instead of on it. Close objects look clear but rays and sends the image to the brain through the distant objects appear blurred. optic nerve. Myopia is inherited and is often discovered in children A refractive error means that due to its shape, your when they are between ages eight and 12 years old. eye doesn’t refract the light properly, so the image you During the teenage years, when the body grows see is blurred. Although refractive errors are called rapidly, myopia may become worse. Between the eye disorders, they are not diseases. ages of 20 and 40, there is usually little change. If the myopia is mild, it is called low myopia. Severe myopia is known as high myopia. Lens Retina Cornea Lens Retina Cornea Light rays Light is focused onto the retina Light rays Light is focused In a normal eye, the cornea and lens focus light rays on in front of the retina the retina. In myopia, the eye is too long or the cornea is too steep.
    [Show full text]
  • CHAMP Brochure
    To learn more about this study, please contact: Myopia can keep your child from seeing the full picture. Who is eligible to participate in the CHAMP study? To pre-qualify for this study, your child must: • Be 3 to 17 years of age • Have been diagnosed with myopia Further screening questions will be asked prior to scheduling an appointment. Learn more about CHAMP – the study of an investigational eye drop being evaluated to slow the progression of nearsightedness (myopia) in children. 21Dec2017_V1_CP-NVK002-0001_Brochure_English What will happen during the CHAMP study? • Your child will receive one drop of study medication into each eye once daily at bedtime for 4 years. • Your child’s total study participation will last approximately 4 years. • During this time, you will attend clinic visits every 3 months to receive study medication. • Every 6 months the doctor will monitor your child’s myopia closely. Your child’s eyewear prescription, the length of his or her eyes, visual function, and eye What is myopia? health will be assessed. Why should my child participate in the CHAMP study? Myopia, commonly known as “nearsightedness,” is when the eye grows too long and light does not focus accurately Myopia is increasing at an alarming rate worldwide. on the retina. This causes distant objects to appear blurry. Identifying a way to control myopia progression is a key step Typically, myopia increases during school years. Higher towards preserving eye sight and preventing serious eye myopia results in the need for thicker glasses and increases disease. By participating in this study, you and your child the risk of certain eye diseases, such as glaucoma and retinal become an important part of this effort.
    [Show full text]
  • Practical Tips for Managing Myopia
    MYOPIA MANAGEMENT Practical tips for managing myopia Michael Morton This article presents a summary of Online Education Coordinator: practical approaches to diagnosing Brien Holden Vision myopia, myopia management Institute, Sydney, Australia. (with particular attention to low resource settings), reviewing myopia progression, and collecting data for myopia management programmes. Ling Lee Research Officer/ Optometrist: Part 1 Diagnosing and prescribing Brien Holden Vision Institute, Sydney, for myopia Australia. While myopia might be initially detected by a patient EDGARDO CONTRERAS, COURTESY OF IAPB (e.g. reporting distance blur), or an adult observing Refraction is the first step. MEXICO behaviour changes in a child (e.g. squinting or • Monocular estimate method (MEM) retinoscopy. viewing things closer than expected), myopia is generally An objective method to determine a child’s diagnosed by an eye care professional. accommodative (near focussing) status at near. Priya Morjaria Equipment Retinoscopy should be conducted with a near target. Research Fellow: Accommodative facility. A subjective method to Department of The minimum required equipment to diagnose myopia • Clinical Research, and assess progression includes: assess accommodation function (ability of eye to London School focus at near). A high-contrast distance visual acuity (VA) chart (e.g., of Hygiene and • • Subjective phorias. A subjective method to Tropical Medicine, Snellen, logMAR, E, or LEA) determine whether the eyes prefer to converge in or International Centre • A room or space where the viewing distance for VA diverge out, at distance and near. for Eye Health, is at least 3m/10ft. The chart should be well lit and • Vergence reserves. A subjective method that London, UK. calibrated for the working distance measures the eyes’ ability to converge in and • Occluder (ideally with pinhole occluder) diverge out.
    [Show full text]
  • Presbyopia Presbyopia Is a Common Type of Vision Disorder That Occurs As You Age
    National Eye Institute Eye Institute National Institutes Institutes of Health of Health Presbyopia Presbyopia is a common type of vision disorder that occurs as you age. It is often referred to as the aging eye condition. Presbyopia results in the inability to focus up close, a problem associated with refraction in the eye. What is presbyopia? Presbyopia is a common type of vision disorder that occurs as you age. It is often referred to as the aging eye condition. Presbyopia results in the inability to focus up close, a problem associated with refraction in the eye. Can I have presbyopia and another type of refractive error at the same time? Yes. It is common to have presbyopia and another type of refractive error at the same time. There are several other types of refractive errors: myopia (nearsightedness), hyperopia (farsightedness), and astigmatism. An individual may have one type of refractive error in one eye and a different type of refractive error in the other. Presbyopia 1 What is refraction? Refraction is the bending of light as it passes through one object to another. Vision occurs when light rays are bent (refracted) by the cornea and lens. The light is then focused directly on the retina, which is a light-sensitive tissue at the back of the eye. The retina converts the light-rays into messages that are sent through the optic nerve to the brain. The brain interprets these messages into the images we see. How does presbyopia occur? Presbyopia happens naturally in people as they age. The eye is not able to focus light directly on to the retina due to the hardening of the natural lens.
    [Show full text]
  • 6. Refractive Errors .Pdf
    Refractive Errors Objectives: ● Not given. [ Color index : Important | Notes | Extra ] ​ ​ ​ ​ ​ ​ ​ Resources: Slides+Notes+Lecture notes of ophthalmology+434team. ​ Done by : Abdulrahman Al-Shammari & Munira alhussaini ​ Edited by: Munerah alOmari ​ Revised by : Adel Al Shihri, Lina Alshehri. ​ FACTS: ● 75% of avoidable blindness is due to: ● Uncorrected refractive error ● Cataract ● Trachoma Physiology: ● To have a clear picture in the retina & to be seen in the brain, there should be a clear cornea, clear anterior chamber, and clear lens, clear vitreous cavity then the picture should be focused on the retina with normal refractive index. ● The retina is responsible for the perception of light. It converts light rays into impulses; sent through the optic nerve to your brain, where they are recognized as images. ● Normal refractive power of the eye is 60 diopters. (The cornea accounts for approximately two-thirds ​ ​ ​ ​ .(and the crystalline lens contributes the remaining (ﺛﺎﺑﺖ of this refractive power (about 40 diopters 60 is the power when we’re looking at something far ( the lens is relaxed). but when we look at near objects the lens’s power increases according to the distance of the object we’re looking at. ● The normal axial length is 22.5 mml (it’s measured from the tip of the cornea to the surface of the ​ ​ ​ ​ retina). ● If the axial length is longer = the picture will be in front of the retina “Myopia”. ​ ​ ​ ​ ​ ​ ● If the axial length is shorter = the picture will be behind the retina “Hyperopia”. ​ ​ ​ ​ ​ ​ ​ ❖Refraction: ● In optics, refraction occurs when light waves travel from a medium with a given refractive index to a medium with another.
    [Show full text]
  • The Definition and Classification of Dry Eye Disease
    DEWS Definition and Classification The Definition and Classification of Dry Eye Disease: Report of the Definition and Classification Subcommittee of the International Dry E y e W ork Shop (2 0 0 7 ) ABSTRACT The aim of the DEWS Definition and Classifica- I. INTRODUCTION tion Subcommittee was to provide a contemporary definition he Definition and Classification Subcommittee of dry eye disease, supported within a comprehensive clas- reviewed previous definitions and classification sification framework. A new definition of dry eye was devel- T schemes for dry eye, as well as the current clinical oped to reflect current understanding of the disease, and the and basic science literature that has increased and clarified committee recommended a three-part classification system. knowledge of the factors that characteriz e and contribute to The first part is etiopathogenic and illustrates the multiple dry eye. Based on its findings, the Subcommittee presents causes of dry eye. The second is mechanistic and shows how herein an updated definition of dry eye and classifications each cause of dry eye may act through a common pathway. based on etiology, mechanisms, and severity of disease. It is stressed that any form of dry eye can interact with and exacerbate other forms of dry eye, as part of a vicious circle. II. GOALS OF THE DEFINITION AND Finally, a scheme is presented, based on the severity of the CLASSIFICATION SUBCOMMITTEE dry eye disease, which is expected to provide a rational basis The goals of the DEWS Definition and Classification for therapy. These guidelines are not intended to override the Subcommittee were to develop a contemporary definition of clinical assessment and judgment of an expert clinician in dry eye disease and to develop a three-part classification of individual cases, but they should prove helpful in the conduct dry eye, based on etiology, mechanisms, and disease stage.
    [Show full text]
  • The Relation Between Myopia and Cataract Morphology
    Br J Ophthalmol: first published as 10.1136/bjo.71.6.405 on 1 June 1987. Downloaded from British Journal ofOphthalmology, 1987, 71, 405-414 Cataract: the relation between myopia and cataract morphology NICHOLAS A P BROWN, AND ADRIAN R HILL From the Nuffield Laboratory of Ophthalmology, University of Oxford, Walton Street, Oxford SUMMARY The association between high myopia and cataract is already well established and an association between simple myopia and cataract has been suggested, but it has not been clear to what extent the myopia precedes the cataract or is the result of it. The present study compares the refraction of a group of 100 British patients at the time of first presentation with cataract in whom the refraction was also known four years previously, with a group of matched controls in whom the refraction was also known four years previous to presentation. The study shows that simple myopia does not appear to predispose to cataract. It is the development of the cataract itself, in particular nuclear sclerosis, which causes the refractive change towards myopia. The myopic change precedes the development of cataract, and patients over the age of 55 showing a myopic change in refraction have a very high probability of developing nuclear sclerotic cataract. The healthy aging eye and eyes with cortical cataract or subcapsular cataract, but without nuclear sclerosis, continue to show a gradual hypermetropic change with time. The relationship of cataract to high (or degenerative) was known four years prior to the development of myopia is a well accepted concept.' It has also been cataract, and to compare them with the same number http://bjo.bmj.com/ considered that this relationship may extend to of controls, matched for age, in whom the refraction involve simple myopia and attention has been focused was also known four years prior to the time of on this recently by Weale,2 Perkins,' and Von presentation.
    [Show full text]
  • REFRACTIVE ERRORS (Nearsightedness, Farsightedness, Etc.)
    REFRACTIVE ERRORS (Nearsightedness, Farsightedness, etc.) The eye focuses an image onto the retina by refracting (bending) light. If an eye requires glasses or contact lenses to sharply focus the image, than it is said to have a refractive error. Glasses, contact lenses, and laser eye surgery correct refractive errors. The procedure to determine a prescription for eye glasses or contact lenses is called a refraction. The four common refractive errors are: 1. Myopia, or nearsightedness - clear vision close up but blurry in the distance 2. Hyperopia, or farsightedness - clear vision in both up close and the distance when one is young, but blurry vision up close followed by blurry vision in the distance as one gets older. The age at which one first needs glasses or contacts depends on the amount of farsightedness. 3. Astigmatism - problems focusing caused by the cornea 4. Presbyopia - inability to focus close up as a result of aging Myopia (nearsightedness) If you have myopia you can see close objects clearly, but distant objects are blurry. Myopia is usually caused by the eyeball being too long. Myopia can range from minimal to extreme. The more myopic you are the blurrier your vision is at distance, and objects will have to be closer for you to see them clearly. Myopia often starts between the ages of 8 and12 years. Since the eyes grow with the rest of the body, nearsightedness commonly increases during the teen years. It tends to levels off in the 20s but can progress until about age 30. Frequent changes in prescription eyewear may be needed until the myopia levels off.
    [Show full text]
  • The Eye & Corrective Lenses
    Phys 102 – Lecture 20 The eye & corrective lenses 1 Today we will... • Apply concepts from ray optics & lenses Simple optical instruments – the camera & the eye • Learn about the human eye Accommodation Myopia, hyperopia, and corrective lenses • Learn about perception of size Angular size Magnifying glass & angular magnification Phys. 102, Lecture 19, Slide 2 The Camera Cameras are one of simplest optical instruments, produce real image onto sensor Pinhole camera (“camera obscura”) Modern camera Pinhole Imaging lens 11 1 += DEMO ddo i f lens Not a true imaging system. Each point from True imaging system. Each point from object creates a circle of light on screen. object has a corresponding point on screen. Phys. 102, Lecture 20, Slide 3 Evolution of the eye The eye is like a camera Pinhole eye Complex eye Nautilus Octopus Phys. 102, Lecture 20, Slide 4 Anatomy of the human eye As in a camera, eye lens creates image of object onto retina Ciliary muscles Vitreous Part of eye n Cornea fluid Cornea 1.351 Lens 1.437 Pupil Vitreous fluid 1.333 Retina Lens Iris Optic nerve DEMO Pupil controls amount of light – diameter typically 2-8 mm Retina has ~125 million photoreceptor cells (rods & cones) Phys. 102, Lecture 20, Slide 5 ACT: Anatomy of the Eye Which part of the eye is responsible for most of the bending of light? Vitreous Cornea fluid Part of eye n Retina Cornea 1.351 Lens 1.437 Laser eye surgery Lens Vitreous fluid 1.333 changes cornea! A. Lens B. Cornea C. Retina D. Vitreous fluid Shape and index of refraction mismatch determine how much light bends: Lens and cornea have similar shape and n n 1.44 n 1.35 nlens 1.44 lens = ≈1.06 cornea = ≈1.35 = ≈1.08 n 1.33 ncornea 1.35 nair 1 vitreous Most of bending occurs at air-cornea interface Phys.
    [Show full text]
  • Where the Current Data Takes Us on Myopia Control
    Eye (2014) 28, 142–146 & 2014 Macmillan Publishers Limited All rights reserved 0950-222X/14 www.nature.com/eye 1,2,3 1,2,3 2,4 CAMBRIDGE OPHTHALMOLOGICAL SYMPOSIUM Myopia, an B Holden , P Sankaridurg , E Smith , T Aller2,5, M Jong1 and M He6 underrated global challenge to vision: where the current data takes us on myopia control Abstract Eye (2014) 28, 142–146; doi:10.1038/eye.2013.256; published online 20 December 2013 Myopia is the most frequent cause of distance impairment in the world and is creating an Keywords: blindness; myopia; myopia control; alarming global epidemic with deleterious vision impairment ramifications for the quality of life and economic health of individuals and nations as a whole. In addition to being immediately disadvantageous, myopia increases the risk of Prevalence of myopia and high myopia serious disorders such as myopic macular degeneration, retinal detachment, glaucoma, On the basis of world-wide prevalence of and cataract and is a leading cause of visual myopia, it can be estimated that over 22% of the 1 Brien Holden Vision Institute, impairment and blindness across many current world population, that is, 1.5 billion Sydney, Australia countries. The reduction in age of onset of people are myopic, and although myopia has 2Vision Co-operative Research myopia is of great concern since the earlier often been referred to as a ‘simple’ refractive Centre, Sydney, Australia the onset, the more myopic the individual error, even in lower amounts it enhances the will become, with all the attendant increased risk of serious eye disorders such as retinal 3 School of Optometry and Vision risks of accompanying debilitating eye detachment, cataract, and glaucoma.
    [Show full text]