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CMDT 2013
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Disorders of the Eyes & Lids Paul Riordan-Eva, FRCOphth
Refractive Errors Refractive errors are the most common cause of reduced clarity of vision (visual acuity) and may be a readily treatable component of poor vision in patients with other diagnoses. In the normal state (emmetropia), objects at infinity are seen clearly. Focusing on objects nearer than infinity requires an increase in the refractive power of the crystalline lens by the process of accommodation. In farsightedness (hyperopia), objects at infinity are not seen clearly unless accommodation is used, and near objects may not be seen because accommodative capacity is finite. Farsightedness is corrected with plus (convex) lenses. In nearsightedness (myopia), the unaccommodated eye focuses on objects closer than infinity. The markedly nearsighted eye is able to focus on very near objects without glasses. Objects beyond this distance cannot be seen without the aid of corrective (minus, concave) lenses. Astigmatism, in which the refractive errors in the horizontal and vertical axes differ, is corrected with cylindrical lenses. Presbyopia is the natural loss of accommodative capacity with age. Persons with emmetropia usually notice inability to focus on objects at a normal reading distance at about age 45. Farsighted individuals experience symptoms at an earlier age. Presbyopia is corrected with plus lenses for near work. Use of a pinhole will overcome most refractive errors and thus allows their identification as a cause of reduced visual acuity.
rotein deposits as required. Sterilization is usually by p chemical methods. For individuals developing reactions to preservatives in contact lens solutions, preservative-free systems are available. The major risk from contact lens wear is bacterial, amebic, or fungal corneal infection, potentially a blinding condition. Such infections occur more commonly with soft lenses, particularly extended wear, for which there is at least a fivefold increase in risk of corneal ulceration compared with daily wear, and with certain contact lens solutions. Contact lens wearers should be made aware of the risks they face and ways to minimize them, such as avoiding overnight wear and maintaining meticulous lens hygiene, including not using tap water or saliva for lens cleaning. Whenever there is ocular discomfort or redness, contact lenses should be removed. Ophthalmologic care should be sought if symptoms persist.
``When to Refer Any contact lens wearer with an acute painful red eye must be referred emergently to an ophthalmologist. Cavanagh HD et al. Castroviejo Lecture 2009: 40 years in search of the perfect contact lens. Cornea. 2010 Oct;29(10):1075–85. [PMID: 20622672] Jeng BH et al. Epidemiology of ulcerative keratitis in Northern California. Arch Ophthalmol. 2010 Aug:128(8):1022–8. [PMID: 20697003]
1. Contact Lenses
2. Surgical Correction
Contact lenses are used mostly for correction of refractive errors, for which they provide better optical correction than glasses, as well as for management of diseases of the cornea, conjunctiva, or lids. The major types of lenses are rigid (gas-permeable) and soft. Rigid lenses, which need to be removed each day, are more durable and easier to care for than soft lenses but are more difficult to tolerate. Usually removed each day, soft lenses are available for extended wear. Disposable soft lenses to be renewed daily, or less frequently (up to monthly), are available. Contact lens care includes cleaning and sterilization whenever the lenses are to be reused and removal of
Various surgical techniques are available for the correction of refractive errors, particularly nearsightedness. Laser corneal refractive surgery reshapes the mid-portion (stroma) of the cornea with an excimer laser. Laser assisted in situ keratomileusis (LASIK), including femtosecond laser assisted LASIK (IntraLASIK), and the surface ablation techniques epithelial LASIK (Epi-LASIK), laser epithelial keratomileusis (also known as laser-assisted subepithelial keratectomy) (LASEK), and photorefractive keratectomy (PRK) differ according to how access to the stroma is achieved. LASIK is most commonly performed because postoperative visual recovery is rapid and there is little