refraction 1 k n jha, 24.08.16
TRANSCRIPT
Learning Aims
• To understand the Optics of the human eye
• Introduction to Refractive Errors
• Myopia: definition, etiology, clinical features,
diagnosis, complications, and treatment
Optics of the human eye
• Human eye is a complex optical system.
• It consists of cornea , aqueous humor,
crystalline lens, and the vitreous humor.
• This arrangements permits to make the eye
compact and small.
Main refracting surfaces
• Air-cornea interface
• Aqueous-lens interface
• Lens-vitreous humor interface
Reduced schematic eye
• Schema= a diagrammatic representation.
• In the reduced schematic eye the whole eye is
regarded as single ideal refracting element , an
ideal spherical refracting surface separating two
media of different refractive indices with one
optical centre( the nodal point , N).
Reduced schematic eye
Dimensions of the reduced eye
• Distance from anterior corneal surface to the
nodal point: 5.6 mm
• Distance from nodal point to the fovea: 17
mm
• Length of the eye 5.6 + 17 mm= 22.6 mm.
Refractive status of the eye
• Emmetropia
• Ametropia
• Refractive status ( static refraction) of the eye:
emmetropia , myopia, hypermetropia, astigmatism.
• Anisometropia: difference between the spherical
equivalents between the two eye.
Ametropia : Classification
• Axial
• Refractive: curvature, index
Emmetropic eye
Myopic eye
Hypermetropic eye
Far Point
Ametropia and Axial Length of the Eye
Pupil size and Visual acuity
Refraction and subjective correction of refractive errors
• Objective refraction
Retinoscopy without cycloplegia
Cycloplegic refraction
Use of autorefractometer
Keratometry
• Subjective verification of refraction
Retinoscopy Subjective verification
MYOPIA
Myopic eye
Prevalence
• Prevalence increases steadily with age.
• Some ethnic groups have higher prevalence at
all ages.
Myopia
Congenital myopia
Developmental myopia
- Juvenile onset myopia
- Adult onset myopia
• Simple myopia
• Pathological axial
myopia
Juvenile-onset myopia
• It is myopia with onset between 7-16 years of age, that occurs
due primarily to growth in axial length.
• Risk factors: Esophoria, against-the-rule astigmatism,
premature birth , family history, and intense near work.
• Earlier onset of myopia shows greater progression.
• In ¾ of teen myopia stabilize at about 15-16 years.
• In the rest , progression continues into the 20s and 30s.
Adult-onset myopia
• Begins at about 20 years of age
• Extensive near work is a risk factor for
development of myopia.
Etiologic factors
• Genetic: Some severe forms of myopia suggest
dominant , recessive, and even sex-linked
inheritance pattern.
• Environmental: near work, higher educational
achievements, nutritional factors.
Simple myopia
• Occurs as a variant in the frequency curve in the axial
length and the curvature.
• Abnormal axial length is the most important factor.
• Fundi : No degenerative changes.
• Myopia do not usually exceed -5 to -6 D.
• Does not progress beyond adolescence.
Pathological axial myopia
• It is degenerative and progressive.
• Refractive changes appear in childhood, usually
between 5-10 years of age.
• Increases steadily up to 25 years of age and beyond.
• Myopia may amount to 15-25 D or more.
• Degenerative changes in the fundus appear late in life.
Pathological axial myopia
• Strongly hereditary.
• Commoner in male than female.
• Racial predilection in Jews and Japanese.
• Other etiological factors : endocrine and
nutritional factors, debility or illness.
Pathological axial myopia
Changes in the eye ball
- Elongation of the eye ball, particularly
involving the posterior pole.
- Posterior staphyloma may form.
Symptoms of myopia
• Indistinct distance vision.
• Eye strain due to convergence accommodation
dissociation and exophoria.
• Floaters, and occasionally flashes of light.
Signs
• Eyes may appear prominent.
• Anterior chamber deeper than normal.
• Pupils are larger.
• Apparent divergent squint.
Fundus changes in Myopia
• Optic disc and the posterior pole: myopic crescent , Central
choroidal atrophy, Foster Fuchs spot, lacquer cracks.
• Peripheral retinal degeneration
- Cystoid degeneration
- Lattice degeneration
- Formation of retinal holes and detachment
• Vitreous degeneration
Prognosis
• Moderate myopia ( -5 to - 6D): prognosis is
good.
• High myopia: Prognosis depends on the
-corrected visual acuity
-fundus changes
Prevention of complications
• Examination of the fundus periphery with
indirect ophthalmoscope.
• Prophylactic treatment of retinal tear/ and
holes likely to lead to retinal detachment.
• Avoidance of contact sport in patient with
degenerative myopia.
Management of Myopia
• In children:
- Cycloplegic refraction
- Full correction ( spherical and cylindrical)
- Frequent refraction ( every 6-12 months)
and periodic changes of glasses.
- contact lens for older children with high
refractive errors .
Correction of Myopia
Surgery for Myopia
• Corneal surgery: RK, LASIK
• Clear Lens extraction
• Phakic intraocular lens implantation
Points to Remember
• Schematic eye
• Refractive status of the eye
• Myopia: Definition, clinical features, complications, diagnosis, and treatment