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Visual Optics 2007/2008. Chapter 4 Emmetropia and Ametropia. For 5% bonus points in Visual Optics II, I would prefer:. Clicker question format as in Visual Optics I Pop quizzes ~ once per week Clicker questions and Pop quizzes

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Visual optics 2007 2008

Visual Optics 2007/2008

Chapter 4

Emmetropia and Ametropia


For 5 bonus points in visual optics ii i would prefer
For 5% bonus points in Visual Optics II, I would prefer:

  • Clicker question format as in Visual Optics I

  • Pop quizzes ~ once per week

  • Clicker questions and Pop quizzes

  • not to be tested at all and I’m happy to forego the bonus points


For the clicker questions i would prefer the option
For the clicker questions, I would prefer the option:

  • of discussing the question with the person sitting next to me and being able to refer to my notes

  • of being able to refer to my notes, but NO discussion allowed

  • of “test” conditions with NO reference to notes and NO discussion


Number of visual optics ii tests i want
Number of Visual Optics II tests. I want:

  • Two. (1) midterm and (2) final

  • Three. (1) ~end of week 4; (2) ~end of week 8; (3) finals week


Visual optics 2007 20081

Visual Optics 2007/2008

Chapter 4

Emmetropia and Ametropia


Most newborn babies are
Most newborn babies are:

  • Emmetropic

  • Hyperopic

  • Myopic

  • Presbyopic



Emmetropization two processes
Emmetropization: Two Processes

Page 4.9

  • Passive

    • ocular scaling: shorter eyes have steeper (higher power) cornea and crystalline lens

  • Active

    • Form Deprivation (no “signal”) → axial myopia

    • Optical Defocus – eye “grows toward” focused image


Visual optics 20072008

How does the Retina Interpret Optical Defocus?

Page 4.10

  • Animal Models: optical defocus-sensing mechanism must differentiate hyperopic from myopic defocus. Possible candidates:

    • Chromatic aberration? – asymmetric wavelength spread could be interpreted by R, G, B retinal cones


Visual optics 20072008

Paraxial

Focus

LCA

Chromatic Aberration

n = 1

n' > 1

400

nm

700

nm


Visual optics 20072008

How does the Retina Interpret Optical Defocus?

Page 4.10

  • Optical Defocus sensing mechanism must differentiate hyperopic from myopic defocus. Possible candidates:

    • Chromatic aberration? – asymmetric wavelength spread could be interpreted by R, G, B retinal cones

    • Off-axis astigmatism? – focal line and COLC distribution


Visual optics 20072008

Focal Lines in Off-axis Astigmatism

Figure 3.52 – Oblique (radial) astigmatism. A point object (B) below the optic axis (A) creates an image spread along the optic axis direction from the tangential to sagittal focus.


Visual optics 20072008

How does the Retina Interpret Optical Defocus?

Page 4.10

  • Optical Defocus sensing mechanism must differentiate hyperopic from myopic defocus. Possible candidates:

    • Chromatic aberration? – asymmetric wavelength spread could be interpreted by R, G, B retinal cones

    • Off-axis astigmatism? – focal line and COLC distribution

    • Other monochromatic aberrations? – e.g. SA: image asymmetric either side of the WOLA


Spherical aberration

Paraxial

Focus

Marginal

Focus

WOLA

Spherical Aberration


Visual optics 20072008

How does the Retina Interpret Optical Defocus?

Page 4.10

  • Optical Defocus sensing mechanism must differentiate hyperopic from myopic defocus. Possible candidates:

    • Chromatic aberration? – asymmetric wavelength spread could be interpreted by R, G, B retinal cones

    • Off-axis astigmatism? – focal line and COLC distribution

    • Other monochromatic aberrations? – e.g. SA, image asymmetric either side of the WOLA

Nature of the defocus-sensing mechanism is not known.

May be some combination of the above factors

No proof in humans that the visual system can differentiate hyperopic from myopic defocus


Visual optics 20072008

Emmetropization and Development of Ametropia

Page 4.11

  • Emmetropization: close to complete by age 5, but does not produce the ideal outcome in ALL cases

  • Successful emmetropization does not guarantee that the eye will remain emmetropic throughout life

  • Low Ametropia: most cases result from imperfect emmetropization. Ametropia is not clearly axial or refractive; simply a mismatch between Fe and ax. “Correlation” ametropia

  • Higher Ametropia: usually axial in origin. Most evident cause-and-effect relationship in higher myopia (axial). “Component” ametropia


Visual optics 20072008

Ametropia: Age-related Patterns

Page 4.11

  • Hyperopia

    • emerges post-“emmetropization;” relatively stable through life

    • higher hyperopia often associated with higher astigmatism.Suggestsnormal emmetropization was blocked (newborns are usually astigmatic).

    • emmetropization appears to arrive at the wrong “balance point” between axial length and refractive power

    • High hyperopia tends to be axial, but rarely exceeds 8 D (high myopia can exceed 25 D)


Visual optics 20072008

Ametropia: Age-related Patterns

Page 4.11

  • Myopia

    • Much more complex and variable than hyperopia; often progressive; suggests multiple etiological factors

    • Higher myopia: typically develops and progresses after the emmetropization phase (but, can begin during emmetropization)

    • Age of Onset: Caucasians 10-16; Asians 10 years of age

    • Asian children typically less hyperopic at birth than Caucasians


Visual optics 20072008

Ametropia: Nature (Heredity) vs. Nurture (Environment)

Page 4.11-12

E

Higher prevalence of myopia in Taiwanese cities vs. villages __

Children with one or two myopic parents 4 more likely to be myopic __

Higher rate of myopia in mono- than di-zygotic twins __

Number of daily “book hours” definitely a factor in myopia __

Myopia genes recently identified __

Recent studies indicate that hereditary factors are likely to drive both susceptibility and resistance to environmentally-induced myopia (only susceptible patients will develop myopia if exposed to environmental “risk factors”) __

Increased prevalence of myopia in Asian communities has occurred more rapidly than can be accounted for by Asian gene pool turnover __

H

H

E

H

&E

H

E


Visual optics 20072008

Classification of Myopia (U.S. Figures)

Page 4.12

  • Early Onset/School/Juvenile Myopia (9-11 years)

    • Majority of myopes in U.S. (60%)

    • Progresses through early teenage years (“lag” of accommodation?)

    • Stabilizes at around 3 - 4 D in early adulthood

  • High Myopia (> 6 D)

    • 1% of Caucasian adolescents; 15% of Asian adolescents

  • Late Onset Myopia (15 – 18 years; sometimes later)

    • 8-15% of myopes; probably a delayed version of school myopia

    • Slower progression than school myopia; rarely > 2 D

    • Sustained and/or “high cognitive demand” near work appear causal

  • Other types: congenital, disease-related and lenticular


Visual optics 20072008

Myopia Statistical Facts (Zadnik)

Myopes (ages 8-13) have a deficient accommodative response to a near target relative to emmetropes, on the order of 0.20 D difference for a 4 D stimulus

Best single predictor of myopia is spherical refraction at age 8-9 years (threshold > or < +0.75 D hyperopia)

Sporting activities in youth appear to protect against myopic progression (equal “book” hours to non-participants)


Visual optics 20072008

Myopia “Treatments” (Zadnik)

  • Progressive Addition Lenses (PALs): limited success (COMET Study)

  • Rigid Gas Permeable (RGP) Contact Lenses (Singapore Study): limited success. Is the “cure” worse than the “disease”?

  • Pharmacological Treatment of Myopia (pirenzepine)

    • progression in 1 year with pirenzepine (US) in 174 children 8-12 years old vs. placebo:

      • 0.26 D compared to 0.53 D

      • >1.00 D of progression: 2% compared to 20%


Visual optics 20072008

Hyperopia (Zadnik)

  • 1. Why doesn’t anybody care about hyperopia?

    • Less prevalent than myopia

    • Often asymptomatic in childhood

    • Onset and progression poorly understood

    • Vision scientists are myopes


Visual optics 20072008

Ametropia: Summary

  • The human eye “tries” to develop emmetropia

  • Most cases of low ametropia didn’t quite get it right (Correlation Ametropia)

  • Component Ametropia

    • Hyperopia: breakdown in emmetropization (wrong endpoint)?

    • Myopia: multifactorial

      • Prevalence increasing worldwide, especially in Asian countries

      • Both hereditary and environmental factors involved

    • Etiology of both hyperopia and myopia poorly understood

  • Key question: what drives susceptibility?


Visual optics 20072008

Examples of Axial & Refractive Ametropia

Page 4.13

  • High myopia is nearly always axial in origin

  • Aphakia (e.g. cataract extraction without IOL implant) is refractive. The previously emmetropic patient becomes ~20 D hyperopic in the crystalline lens plane and ~17 D hyperopic at the cornea


Visual optics 20072008

Simplified Schematic Eye - Removing Cr. Lens

+43.08 D

~ +20 D

Feff (CrLens)~ +17 D



Visual optics 20072008

Examples of Axial & Refractive Ametropia

Page 4.13

  • High myopia is nearly always axial in origin

  • Aphakia (e.g. cataract extraction without IOL implant) is refractive. The previously emmetropic patient becomes ~20 D hyperopic in the crystalline lens plane and ~17 D hyperopic at the cornea

  • Cataract often leads to increased crystalline lens index and therefore a myopic shift. This would also be refractive in origin

  • Astigmatism is always refractive in origin. Because the two ocular principal meridians have different powers, at least one meridian must have refractive ametropia. Astigmatism can be combined with axial ametropia.


Visual optics 20072008

Astigmatism - Refractive Ametropia

Page 4.13, 14

Fig. 4.4 - Reduced eye representation of an astigmatic eye (relaxed) showing focal lines formed by parallel incident light (distant point object).


Visual optics 20072008

Determining the Nature of Ametropia

Page 4.14

  • When are we interested in determining the nature of ametropia?

  • Certainly not for the routine patient with low to moderate refractive errors and isometropia (unless history suggests risk factors  set baseline)

  • We become interested when:

    • The patient has high ametropia

    • The patient is anisometropic; especially higher levels

    • The patient has only moderate ametropia, or slight anisometropia, but is symptomatic


Visual optics 20072008

Determining the Nature of Ametropia

Page 4.10

  • Important signs during routine examination:

    • Clinical refraction will identify candidates – e.g. high ametropia or anisometropia. Routine refraction gives no information about origin

    • Ophthalmoscopy – myopic conus (crescent) seen in patients with axial myopia. Caused by stretching of the globe


Visual optics 20072008

Myopic crescent

Temporal

Nasal



Visual optics 20072008

Determining the Nature of Ametropia

p 4.15

  • Important signs during routine examination:

    • Clinical refraction will identify candidates – e.g. high ametropia or anisometropia. Routine refraction gives no information about origin

    • Ophthalmoscopy – myopic conus (crescent) seen in patients with axial myopia. Caused by stretching of the globe

  • Retinoscopy or biomicroscopy (slit-lamp) – cataract often produces a myopic shift. In particular, anisometropia in a previously emmetropic, older patient may be due to different stages of cataract development between eyes


Visual optics 20072008

More advanced nuclear cataract in left eye

O.D.

O.S.

http://www.cvr.org.au/pages/common-eye-diseases/cataract-risk-factors.htm


Visual optics 20072008

Cortical Cataract (“spoke” opacities)

http://www.cvr.org.au/pages/common-eye-diseases/cataract-risk-factors.htm


Visual optics 20072008

Posterior Subcapsular Cataract

Why would this type of cataract be associated with significant loss of visual function?

http://www.cvr.org.au/pages/common-eye-diseases/cataract-risk-factors.htm


Visual optics 20072008

Clinical Methods to Determine the Nature of Ametropia

Page 4.15

  • A-Scan Ultrasonography – measures axial length

    • Same principle as sonar; echoes occur when sound waves strike interface between “refractive” media

    • A –scan detector measures return time for each echo

    • Transducer aligned with patient’s visual axis, and 10-20 MHz sound waves directed at fovea

    • Sound wave velocity varies between “refractive” media. Usually set device to a weighted average velocity for given patient, e.g. normal, cataract, aphake, etc.

    • Theoretical accuracy ±0.02 mm; practical accuracy ±0.1 – 0.2 mm

    • Practical accuracy consistent with dioptric tolerance ± 0.25 – 0.50 D


Visual optics 20072008

Single corneal peak

Measuring Axial Length – A-Scan Biometry

(Applanation Method)

Page 4.15

Fig. 4.6 - A-Scan (time-amplitude) ultrasonograph. Time intervals between peaks (in sec) are multiplied by sound velocity in each medium to compute axial length).


Visual optics 20072008

Applanation A-Scan Biometry

Page 4.15

  • “Probe” applied directly to patient’s cornea (with gel)

  • Pressing on cornea “compresses” axial length

  • Result: variable underestimation


Visual optics 20072008

Immersion A-Scan Biometry

Page 4.15

  • Fluid-filled immersion cup applied to suppine patient’s cornea

  • Probe does not contact cornea

  • More accurate axial length measurement

  • Down-side: more time-consuming


Visual optics 20072008

Measuring Axial Length – Immersion A-Scan Biometry

Page 4.11

Immersion A-Scan biometry. Note two corneal peaks (C1 and C2)


Visual optics 20072008

Optical Coherence Tomography

  • Broad bandwidth polychromatic source

  • Light split into two beams: one enters eye and reflects back from retina; other reflects from movable reference mirror

  • Beams recombine at detector. Only produce constructive interference when pathlengths almost identical

  • Reference mirror movement produces clear image at corneal surfaces, cr. lens surfaces, and retina  precise axl value

Page 4.17


Visual optics 20072008

Optical Coherence Tomography

Page 4.17

  • Precision of OCT to within lens “tolerance” (0.25 D)

  • Method of choice for post-cataract IOL design (main design parameters are axial length, “total” corneal power, and anterior chamber depth)


Visual optics 20072008

Clinical Methods to Determine the Nature of Ametropia

Page 4.16

  • A-Scan Ultrasonography – measures axial length

  • Keratometry – measures anterior corneal radius. Provides reasonable estimate of total corneal power