Refractive Errors of eye

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Refractive  Errors
Refractive error is the most common visual problem
This defect prevents light rays from converging into a single focus on the retina

REFRACTION:


Refraction of light occurs when light passes from one medium to another of different refractive index (ie. density)
Refractive Components of the Eye: Cornea, Aqueous humor, Lens, Vitreous humor

REFRACTIVE ERRORS

MYOPIA (Near Sightedness)



·         Causes light rays to be focused in front of the retina
·         Myopia may occur because of excessive light refraction by the cornea or lens or because of an abnormally long eye.
·         There is inability to accommodate for objects at a distance
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HYPEROPIA (farsighted)    




·         Causes the light rays to focus behind the retina & requires the patient to use accommodation to focus the light rays on the retina for near & far objects.
·         This type of refractive error occurs when the cornea or lens does not have adequate focusing power or when the eyeball is too short.
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ASTIGMATISM

 


·         Is caused by an irregular corneal curvature
·         This irregularity causes the incoming light rays to be bent unequally.
·         Consequently, the light rays do not come to a single point of focus on the retina.        Occur in conjunction with any of the other refractive errors.






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PRESBYOPIA




·         Is the loss of accommodation associated with age.
·         This condition generally appears at about age 45yrs.
·         As the eye ages, lens becomes larger, firmer & less elastic
·         These changes, which progress with aging, decrease the eye's accommodating ability
·         There is an inability to accommodate for near objects
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APHAKIA



·         Is defined as the absence of the lens.
·         The lens may be absent congenitally or it may be removed during cataract surgery or a perforating wound or ulcer.
·         The absence of the lens results in a significant refractive error
·         Without the focusing ability of the lens, images are projected behind the retina.

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MANAGEMENT
a. Nonsurgical Corrections
l. Corrective glasses
·         Myopia- minus corrective glasses (concave) 
·         Hyperopia-plus corrective lens (convex)
·         Presbyopia -plus corrective lens (convex)/ Reading glasses
·         Presbyopia+myopia or astigmatism- combined glasses with bifocal or trifocal lower glasses OR 'no-line' bifocal (multifocal)
·         Aphakia- plus corrective lens (convex) which are very thick, heavy and unattractive.
2.Contact lenses
·         Provide better vision than glasses
·         Made from various plastic and silicone substances
·         The person must remove contact lenses immediately, if any of the following problems occur..
o   l. Redness
o   2. Sensitivity
o   3. Vision problems
o   4. Pain

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Types of contact glasses
 Rigid Lenses:
1.       Standard Rigid Lenses
·         Rigid plastic; smaller than cornea.
·         Long lasting, least expensive to purchase; corrects all types of pass through the cornea refractive errors.
·         Daily wear.    
·         Requires separate care solution

2.       Gas Permeable rigid lenses
·         Similar, but plastic allows oxygen to pass through the cornea
·         Long lasting, more comfortable;  flexible wearing schedule.
·         More expensive and requires separate care solution
 Soft Lenses:
·         Standard
·         High water content
·         Toric
·         Disposable
·         Daily disposable
Corneal molding, also called as orthokeratology, is the sue of specially designed, rigid, gas permeable contact lenses to alter the shape of the cornea.

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SURGICAL THERAPY
These are designed to eliminate or reduce the need for eyeglasses or contact lenses and correct refractive errors by changing the focus of the eye.
Surgical management include
·         Laser surgery
·         Intraocular lens (IOL) implantation
·         Thermal procedures

l. Laser assisted in situ keratomileusis (lasik)
·         For patients with low to moderately high amounts of myopia, hyperopia, and astigmatism.                
·         Using a laser or surgical blade to create a thin flap in the cornea.
·         Using new technology called 'wave-front' the laser is then programmed to use a map of the patient's cornea to sculpt the cornea and correct the refractive error.
·         The flap is then repositioned and adheres on its own without sutures in a few minutes.
·         placed in front of the natural lens.

2.         Thermal procedures
·         Laser thermal keratoplasty (LTK)  
·         Conductive keratoplasty (CK)
·         For patients with hyperopia or presbyopia
·         Using laser or high radio frequency, heat is applied to the peripheral area of the cornea to tighten it like a belt and make the central cornea steeper.
·         Only the less dominant eye is treated

3.         Photorefractive keratectomy (PRK)
·         Indicated for low to moderate amounts of errors.
·         Good option for a patient with insufficient corneal thickness for LASIK flap.
·         Only the epithelium is removed and the laser sculpts the cornea to correct the refractive error

Implants
·         Intra corneal ring segments (ICRs) are 2 semi-circular pieces of plastic that are implanted between the layers of the cornea to treat mild myopia
·         Refractive intraocular lens (refractive IOL) for patients with high degree of myopia or hyperopia. Involves removal patients natural lens and implantation of IOL
·         Phakic intraocular lenses (phakic IOLs) for patients with high degree of myopia or hyperopia. IOL is placed in front of the natural lens.

4.         Thermal procedures
·         Laser thermal keratoplasty (LTK)
·         Conductive keratoplasty (CK)
For patients with hyperopia or presbyopia
Using laser or high radio frequency, heat is applied to the peripheral area of the cornea to tighten it like a belt and make the central cornea steeper.
Only the less dominant eye is treated


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