VENNILA .C.RREFRACTION
Refractive ErrorsEmmetropiaAmetropia
EmmetropiaEmmetropia means no Refractive errorIt is the ideal condition in which the incident parallel rays come to a perfect focus upon the light sensitive layer of the retina, When accommodation is at rest
AmetropiaAmetropia means Refractive error EyeIt is the opposite condition , wherein the parallel rays of light are not focused exactly upon the retina , When the accommodation is at rest
AmetropiaMyopiaHypermetropiaAstigmatism
MyopiaPrincipal focus is formed in front of the retina
Causes Axial MyopiaCurvature MyopiaIndex MyopiaAbnormal position of the lens
Axial MyopiaAxial myopia results from increase in anteroposterior length of the eye ball.Normal Axial length- 23mm to 24mm1mm increase in AL 3Ds of Myopia
Curvature MyopiaCurvatural myopia occurs due to increased curvature of the cornea and Lens or both.Anterior surface of the cornea- 7.8mmPosterior surface of the cornea- 6.5mm1mm increases in radius of curvature results in 6 Ds of Myopia
Index myopiaIndex myopia results from increase in the refractive index of crystalline lens. Refractive index of normal Lens - 1.42
Abnormal position of the lensPositional myopia is produced by anterior displacement of crystalline lens in the eye.
Accommodative Myopia:. Myopia due to excessive accommodation.
Types Congenital myopiaSimple Myopia (or) Developmental myopia Pathological Myopia (or) Degenerative myopiaAcquired myopia
Congenital myopiaCongenital myopia is present since birth however, it is usually diagnosed by the age of 2 3 years.
Simple myopiaSimple or developmental myopia is the commonest variety. It is considered as a physiological error not associated with any disease of the eye.Power limit less than 6D
AetiologyAxial type of simple myopiaCurvatural type of simple myopia
Pathological myopiaMyopia associated with degenerative changes in the eye.Myopia more than 6D to25D or More than 25D
AetiologyAxial growth(i) Heredity(ii) General growth process
Acquired myopiaSome of the causes of acquired myopia* Index myopia* Curvatural myopia* Positional myopia* Consecutive myopia * Pseudo myopia* Space myopia* Night myopia (or) Twilight myopia* Drug induced myopia
SymptomsPoor vision for distanceAsthenopic symptomsExophoriaMuscae volitantes (pathological)Night blindness (pathological)
SignsLarge eye balldeep Anterior chamber sluggish PupilLarge Disc
ComplicationsRetinal tear Vitreous haemorrhageRetinal detachmentDegeneration of the vitreousPrimary open angle GlaucomaPosterior cortical cataractPosterior staphyloma
Treatment Optical Spectacle Correction (Concave Lens)Contact lensSurgicalPRKKeratomileusisEpikeratophakiaRedial Keratotomy
Optical TreatmentConcave lensMyopic with Exophoria give full correction.Myopic with Esophoria give undercorrection.
Hypermetropia
Principal focus is formed behind the retina
Causes Axial HypermetropiaCurvature HypermetropiaIndex HypermetropiaAbnormal position of the lens
Axial HypermetropiaAxial hypermetropia is by far the commonest In fact, all the new- borns are almost invariably hypermetropic (approx,+2.50D) This is due to shortness of the globe, and is physiological.Normal axial length 23mm to 24mm1mm decrease in AL 3Ds of hypermetropia
Curvature HypermetropiaIn which the curvature of cornea, Lens or both is flatter than the normal resulting in a decrease in the refractive power of the eye.Anterior surface of the cornea- 7.8mmPosterior surface of the cornea- 6.5mm1mm increase in radius of curvature results in 6Ds of hypermetropia
Index HypermetropiaIndex hypermetropia occurs due to change in refractive index of the lens in old age. It may also occur in diabetics under treatment.Refractive index of Normal Lens- 1.42
ClassificationTotal Hypermetropia may be divided into(a) Latent Hypermetropia(b) Manifest Hypermetropia (i) Facultive Hypermetropia (ii)Absolute Hypermetropia
Latent HypermetropiaLH which is corrected physiologically by the tone of ciliary muscle. As a rule latent hypermetropia amounts to only one dioptre. It can be revealed only after atropine cycloplegia.
Manifest HypermetropiaMH is made up of two componentsFacultative hypermetropia is that part of hypermetropia which can be corrected by the effort of accommodation.Absolute hypermetropia which can not be overcome by the effort of accommodation.
Clinical TypesSimple hypermetropiaPathological hypermetropiaFunctional hypermetropia
Simple hypermetropiaIt results from normal biological variation in the development of the eye ball. It includes Axial and Curvatural HM. It may be hereditary.
Pathological hypermetropiaPH results due to either congenital or acquired conditions of the eye ball which are out side the normal biological variations of the development.
The Normal Age VariationAt birth:- 2D to 3 D Commonly PresentAt the age of 5 Yrs- 90% of Childrens are HypermetropicAt Puberty:- Emmetropic
Symptoms Head acheBlurred vision particular near workConvergent squintEarly onset of presbyopiaEye Strain
ComplicationsEye appears to be small including cornea and anterior chamber becomes shallowExtreme cases MicrophthalmosRetinal reflex Shot silk-Retina
TreatmentOpticalSpectacle ( Convex Lens )Contact lensHypermetropic with Exophoria give under correctionHypermetropic with Esophoria give full correctionSurgicalThermokeratoplasty
Astigmatism Astigmatism is that condition of Refraction where the point focus of light cannot be formed upon the Retina
CausesCurvature Ex: Keratoconus, Lenticonus etc..Centering error Ex: Sub location of the lensRefractive index Ex: CataractRetinal Oblique placement of macula
TypesRegular Irregular
Regular astigmatismRefractive typesPhysiological types
Refractive typesSimple astigmatismCompound astigmatismMixed astigmatism
Physiological typesWith rule astigmatismAgainst rule astigmatismOblique astigmatismBioblique astigmatism
SymptomsHead acheBlurring of visionEye tiredEye acheHead TiltHalf-closure of the lids (High astigmatism)Blurring & Itching (Low astigmatism)
TreatmentOptical Treatment* Cylindrical lens* Under correction* Contact lens (RGP, Toric)Refractive surgery * Astigmatic Keratotomy* PRK, LASIK
Study ReportsPercentage of astigmatism* 0.25-0.50D50%* 0.75-1.00D25%* 1.00-4.00D24%*>4.00D1%Percentage of Types* with rule38%* Against rule30%* Oblique32%
Duo chrome test To test if the eye has been under corrected or over corrected or is properly corrected
Astigmatic Fan To know the axis and power in Astigmatism
Jackson cross cylinder To refine the axis and power of cylinder
PresbyopiaThis is a physiological aging process, In which the near point gradually recedes beyond the normal reading or working distance
CausesLens matrix is harder and less easily mouldedLens capsule is less elastic Progressive increase in size of the lensWeakening of the ciliary muscle
SymptomsPatient holds the book at arms lengthPatient prefers to read in bright lightEye strain Head acheEyes feels tired and ache
TreatmentConvex lens Methods of prescription * Occupation * Working distance * Age Surgical * Anterior ciliary sclerotomy * Laser thermal keratoplasty * Small diameter corneal inlays
Aphakia Aphakia means absence of the Crystalline lens from the Eye ball
Causes Congenital Surgery Traumatic
Optics of AphakiaAnterior focal distance 23mm (N-15mm)Posterior focal distance- 31mm (N-24mm)The Nodel point of the eye is thus moved forwardStrong converging (convex) lens- +10D
SignsAnterior chamber Deep Iris (i) Iridodonesis (or) Tremulousness(ii) Peripheral button-hole iridectomy markPupil - Jet black reflexAbsence of the 3rd and 4th Purkinje imagesRetinoscopy reveals high hypermetropia and astigmatismOphthalmoscopy As in hypermetropic fundus with a small optic disc
DisadvantagesImage magnification of about 25-30%Spherical aberration, Peripheral and PincushionRoving ring scotoma (The scotoma extents from 50- 65 from central fixation)Jack in the boxRestriction of the visual fieldColoured visionInaccurate spectacle correction because of errorneous vertex distance
TreatmentSpectacle ( Convex lens )Contact lensSecondary IOLEpikeratophakiaKeratophakia
Aphakic formulaP = X / 2 +10.00DP = IOL powerX = Refractive power
PseudophakiaPseudophakia means False lens
Image magnification
Calculation of IOL powerP = A-2.5*L -0.9KP = IOL PowerA= Constant value 2.5= AC depth L = Axial length in mm 0.9= Corneal curvatureK = Corneal diapters
Refractive stages of a Pseudophakic eyeEmmetropiaConsecutive myopiaConsecutive hypermetropia
AdvantagesImage ma
