انت هنا الان : شبكة جامعة بابل > موقع الكلية > نظام التعليم الالكتروني > مشاهدة المحاضرة
الكلية كلية الطب
القسم الجراحة
المرحلة 5
أستاذ المادة اسعد جعفر عبد السادة الطائي
4/19/2011 8:25:59 PM
Refractive errors:
Accommodation:
It is the ability of the human lens to reflexly increase it’s refractive power to bring images of objects situated nearer than infinity on the retina.
Refractive state:
Human eye is designed so that parallel rays of light in the resting state ( i.e; without exertion of accommodation) are refracted by the normal refractive elements of the eye ( mainly the cornea 43 D.and the lens 19 D.) to be focused on the retina, this normal state, which is most commonly encountered, is called emmetropia and the person having it is called emmetrope.If such rays are not focused on the retina the refractive state is called ametropia and the patient ametrope.So if the refractive power of the eye is excessive the image will fall in front of the retina and we call this myopia or it is inadequate and the image will fall behind the retina and we call it hypermetropia or hyperopia.
Hypermetropia:
Aetiology:
(1) Axial hypermetropia: the eye is short.
(2) Curvature hypermetropia: flat cornea.
(3) Index hypermetropia: lens refractive power is inadequate e.g: hypoglycaemia.
Types:
(a) Manifest: It is the maximum hypermetropia that can be crrected with convex lenses with the patient fully accommodated i.e; without the use of cycloplegia.
(b) Latent: It is the additional amount of hypermetropia revealed when we use cycloplegia.
(c) Total: sum of latent + manifest hypermetropia.
Natural history:
Hpermetropia is the commonest error of refraction.At birth practically 90-95% of the newborn have physiological hypermetropia. This usually disappears with time so that at 5 years of age 50% and at 16 years 15% only are hypermetropic.
Clinical picture:
The main symptom is eye-strain ( tired eye ) and if the patient complains from blurred vision, it will be for near.Other symptoms are: heaviness, grittiness, soreness, and blepharitis.
Myopia:
Aetiology:
(1) Axial myopia: Eyes are big.
(2) Curvature myopia: The cornea has a steep curvature.
(3) Index myopia: increase in refractive index of the lens e.g: nuclear sclerosis and in hyperglycaemia.
Clinical picture:
The main symptom is blurring of distant vision making the patient squeeze his lids.
Types:
(a) Simple: up to 5-6 D.
(b) Pathological: more than 6 and may reach up to 30 D.Here there is thinning of the retina making it susceptible to hole or tear formation and hence the danger of retinal detachment. Posterior staphyloma is bulging of the sclera posteriorly at the optic disc area due to the weakness induced by the thinning aggravating the condition.
Astigmatism:
In this condition there is no point focus of light on the retina.The focus is in the form of an interval conoid called the conoid of Sturm instead of being a point.
Aetiology:
(1) Curvature astigmatism: meridia of the cornea perpendicular to each other have different curvatures.
(2) Lens subluxation.
Types:
(a) Regular: the 2 meridia are perpendicular to each other.
(b) Irregular:meridia are not perpendicular and maybe more than 2.
Presbyopia:
The loss of accommodation that comes with aging to all people is called presbyopia.A person with emmetropic eyes will begin to notice inability to read small print or discriminate fine close objects at about the age 44-46. This is worse in dim light and usually worse early in the morning or when the subject is fatigued. These symptoms increase until about age 55, when they stabilize but persist.
Anisometropia:
It is a difference in refractive error of 2 diopters or more between the 2 eyes. The importance of the condition lies in that the more ametropic eye will develop amblyopia.
Aphakia:
It is the condition of an eye with no lens behind the pupil regardless of the cause.As expected the effective refractive power of the eye will be reduced by an amount equal to the refractive power of the lens,so it results in high hypermetropia.
Methods of refraction:
Determination of a patient’s refractive state can be achieved by objective or subjective means and is best accomplished by a combination of the 2 methods where possible.
Objective refraction:
1- Retinoscopy.
2- Automated refractometry.
3- OPD scanning.
Subjective refraction:
In cooperative patients, subjective refraction produces more accurate results than objective refraction. It relies on the patient’s response to alterations in lens power and orientation, using objective refraction or the patient’s current refractive correction as the starting point.
Cycloplegic refraction:
It is usually impossible to force young adults and especially children to relax their accommodation during examination in order to reveal their latent hypermetropia, so it becomes mandatory to use a cycloplegic to negate accommodation. The most powerful cycloplegic is atropine 1% eye drops bid for 3 days.
Correction of refractive errors:
(a) Spectacle lenses: Spectacles continue to be the safest method.A minus sphere is used for myopia, a plus sphere for hypermetropia, presbyopia and aphakia, and a cylinder for astigmatism.
(b) Contact lenses: Those are optically more efficient and yeild better visual quality than spectacles with much improved visual field and practically absent aberrations or prismatic effect.They can reduce image size disparity in anisometropia.
(c) Keratorefractive surgery: This represents a range of methods for changing the curvature of the anterior surface of the eye e.g: radial keratotomy and LASIK.
(d) Intraocular lenses:
(1) After cataract extraction: In correction for the resultant aphakia.
(2) Phakic I.O.L implantation:To correct high dgrees of refractive errors.
(3) Clear lens extraction: Here a clear ( non-cataractous ) lens is removed to correct moderate to high myopia.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
الرجوع الى لوحة التحكم
|