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Optics of human eye Subjective refraction Eye as a camera

Components

Dr. Ali Abusharha Schematic eye and reduced eyes

Axes and visual angles

Optical aberrations

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Eye as a camera

Components

Eyelids- shutter The cornea

Cornea- focusing system Lens- focusing system The anterior chamber

Iris- diaphragm The iris and pupil Choroid- dark chamber

Retina-light sensitive film The crystalline lens

The retina

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Cornea  Vertical diameter slightly less than horizontal  Reasons of refraction:  Front apical radius 7.5 - 7.7 mm  Curvature.  Back apical radius 6.4 - 6.8 mm  Significant difference in refractive indices of air  and cornea. Actual refractive index cornea= 1.376  Power of cornea +43D (2/3 of total eye power)

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Iris and Pupil The anterior chamber •Regulate amount of light entering the eye •At 2.4mm pupil size, best retinal image obtained,  Cavity between cornea and iris as aberration and diffraction are balanced.  Filled with aqueous humor. Average  Depth of AC – about 2.5-4.0 mm • 2-4mm  Change in AC depth change the total power. 1mm size: forward shift of lens- increase about 1.4D in power  Refractive index of aqueous humor= 1.336 • depth of focus increases Small pupil • Concept used as pin hole test in refraction

Large pupil • Retinal image quality improves

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The crystalline lens  Lens accounts for about one third of the refraction • Birth 3.5 – 4 mm of the eye. Thickness • Adult life 4.75 – 5 mm  ACCOMODATION  Provides a mechanism of focusing at different • Ant surface 10 mm Radius of curvature • Post surface 6 mm distances.

• Nucleus 1.41 Refractive index of lens • Pole 1.385  OPTICAL CHANGES IN CATARACTOUS LENS • Equator 1.375  reduction. Total power • 15 -18 d.  Myopic shift.  Monocular diplopia. • At birth- 14-16 D Accommodative power • At 25yrs- 7-8D  Glare. • At 50yrs- 1-2D

9  Color shift. 10

Retina Axes and visual angles  Maximum resolving power at fovea.  A concave spherical surface with r =-12 mm.  Advantages of curvature of retina over plane image forming surfaces of cameras and optical instruments:  The curved images formed by the optical system is brought in the right order.  A much wider field of view is covered by the steeply curved retina

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Optical aberrations  OPTICAL AXIS: line passing through centre of  Diffraction of light cornea, lens and meets retina on nasal side of  Spherical aberrations fovea  Chromatic aberrations  VISUAL AXIS: line joining fixation point, nodal  Decentering point and fovea  Oblique aberrations  FIXATION AXIS: line joining fixation point and  Coma centre of rotation

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REFRACTIVE ERRORS Emmetropia • Ametropia: a is present

• : Near sightedness 43 diopters • Hyperopia (Hypermetropia): Far sightedness 24-25mm

• Presbyopia: Loss of accommodative ability of the lens resulting in difficulties with near tasks 18 diopters • : the curvature of the cornea and/or lens is not spherical and therefore causes image blur on the retina

Accommodation at rest

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REFRACTIVE ERRORS Myopia

• Anisometropia: a refractive power difference between the 2 eyes (> 2D)  A form of refractive error in which parallel rays of

light entering the eye are focused in front of retina • Aniseikonia: a difference of image size between the 2 eyes as perceived by the patient with accommodation being at rest. • Aphakia: (Phakos=lens), aphakia is no lens

• Pseudophakia: artificial lens in the eye

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Etiological types  Axial(MC)-increased AP length of eyeball  Curvatural-increased curvature of cornea, lens or both  Index-increased refractive index of lens with nuclear sclerosis  Positional-anterior placement of lens  Myopia due to excessive accommodation

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Clinical types of myopia Clinical features - Symptoms  Congenital  Distant blurred vision

 Simple or developmental  Half shutting of eyes  Asthenopic symptoms  Degenerative or pathological  Night blindness  Acquired  Divergent squint Assignment:  write an essay about clinical types of myopia discussing the difference between these types and mechanism of each type

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Signs Optical treatment  Prominent eyeballs  Concave lenses  Large cornea  (Minus lens)

 Anterior chamber is deep  Large & sluggishly reacting pupil  Fundus examination-changes seen only in pathological myopia  Contact lenses

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Optical treatment  Adults: Surgical treatment  <30years-full correction   >30years-less than full correction with which patient is Radial keratotomy comfortable for near vision.  Lamellar corneal refractive procedures  Laser based procedures HIGH MYOPIA  PRK  under correction is done to avoid  LASIK near vision problem  LASEK magnification of images  C-LASIK contact lenses are better(to avoid image magnification)  E-LASIK

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Hypermetropia Etiological types  It is the refractive state of eye where in parallel rays of  Axial(m.c)-decreased AP diameter of eyeball light coming from infinity are focused behind the sensitive layer of retina with accommodation being at  Curvatural-flattening of cornea, lens or both rest  Index –old age, diabetics under treatment  Positional-posteriorly placed lens  Absence of lens-aphakia

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CLINICAL TYPES TOTAL HYPERMETROPIA  It is the total amount of refractive error,estimated after  SIMPLE HYPERMETROPIA complete cycloplegia with atropine  PATHOLOGICAL  FUNCTIONAL HYPEROPIA  Divided into latent & manifest

Assignment:  write an essay about clinical types of hyperopia  discussing the difference between these types and mechanism of each type

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MANIFEST HYPERMETROPIA LATENT HYPERMETROPIA  Remaining part of total hypermetropia  Correct by accommodation and convex lens  Consists of facultative & absolute  Corrected by inherent tone of ciliary muscle  High in children FACULTATIVE HYPERMETROPIA  Decreases with age  Corrected by patients accommodative effort  Revealed after abolishing tone of ciliary muscle with atropine ABSOLUTE HYPERMETROPIA  Residual part not corrected by patients accommodative effort

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NORMAL AGE VARIATION  At birth +2+3D HM  Slightly increase in one year of life,  Gradually diminished by the age 5-10 years Facultative hyper metropia  In old age after 50 year again tendency to Manifest hyper HM metropia Total hyper Absolute hyper . Tone of ciliary muscle decreases metropia metropia . Accommodative power decreases Latent hyper metropia . Some amount of latent HM become manifest . More amount of facultative HM become absolute 33 34

SYMPTOMS  Principal symptom is blurring of vision for TREATMENT close work BASIS FOR TREATMENT  Symptoms vary depending upon age of  No Treatment patient & degree of refractive error Error is small   Asymptomatic Asymptomatic Visual acuity normal  Asthenopic symptoms No muscular imbalance  Defective vision only (particularly near vision)

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Young children(<6 or 7yrs) Adults Some degree of hypermetropia is physiological so no correction If symptoms of eye-strain are marked, we correct as much of the total hypermetropia as possible, Treatment required if error is high or strabismus is trying as far as we can to relieve the present accommodation  working in school small error may require When there is spasm of accommodation we correction correct the whole of the error In children error tends normally to diminish with Some patients with hypermetropia do not initially growth so refraction should be carried out every six tolerate the full correction indicated by manifest month and if necessary the correction should be refraction so we under correct them reduced, ortherwise a lens which is overcorrecting their error may induce an artificial myopia No deduction of tonus allowance in strabismus 37 38

PRESBYOPIA The physiologic loss of MODE OF TREATMENT accommodation in the  SPECTACLES eyes in advancing age OPTICAL TREATMENT  CONTACT LENS

 SURGICAL

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 Physiologic loss of accommodation in SYMPTOMS advancing age  deposit of insoluble proteins in lens in  The need to hold reading material at advancing age-->elasticity of lens progressively arm's length. decrease-->decrease accommodation  around 40 years of age , accommodation  Blurred near vision become less than 4.00 D, causing difficultly with reading fine print , headache , visual  Headache fatigue  Fatigue

 Symptoms worse in dim light. 41 42

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SPECTACLES Surgery  Monovision LASIK

Plus lens  Monovision & CK

(or)  IntraCor Convex lens  Refractive lens exchange

 Corneal Inlays & Onlays

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Types ASTIGMATISM  Regular astigmatism – change in refractive power is uniform from one meridian to

another A defect of an optical system causing light rays  With-the-rule astigmatism from a point source to fail  Against-the-rule astigmatism to meet in a focal point  resulting in a blurred and Oblique astigmatism imperfect image.  Bi-oblique astigmatism  Irregular astigmatism –Irregular change of refractive power in different meridians

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 Types of regular astigmatism  Simple astigmatism  Simple hyperopic astigmatism  Simple myopic astigmatism  Compound astigmatism  Compound hyperopic astigmatism  Compound myopic astigmatism  Mixed astigmatism

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• Symptoms : Regular Astigmatism : Blurring of vision  Correctable by Spherocylindrical lenses Asthenopic symptoms Etiology : Tilting of head 1. Corneal - abnormalities of curvature [common] Squinting [Half closure of eyelid]

2. Lenticular is rare. It may be: i. Curvatural - abnormalities of curvature of lens as seen in lenticonus. ii. Positional - tilting or oblique placement of lens , subluxation.

3. Retinal - oblique placement of macula [rare]

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Investigations: Treatment

 Retinoscopy  Optical treatment  Keratometry  Spectacles  Computerized corneal Tomography  RGP contact lenses  Astigmatic fan test  Toric contact lenses  Jackson cross cylinder  Surgical correction

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Irregular Astigmatism Guidelines for optical treatment • Etiology :  Small astigmatism- treatment is required Corneal -[ Scars , Keratoconus , flap complications,  In presence of asthenopic symptoms marginal degenration ]  Decreased vision

• High astigmatism- full correction Lenticular -[Cataract maturation] • Better to avoid new astigmatic correction in adults because of intolerable Retinal-[scarring of distraction macula,tumours of retina,choroid]

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Treatment :  Optical treatment : - RGP contact lenses -Hybrid contact lenses -Scleral lenses

 Surgical treatment: - penetrating keratoplasty

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Anisometropia Anisometropia  Difference in refractive power between eyes  Glasses : magnified or minified 2% per 1 D  Contact lens : change less than glasses  refractive correction often leads to different image sizes on the retinas( aniseikonia)  Tolerate aniseikonia ~ 5-8%  Symptoms : usually congenital and often  aniseikonia depend on degree of refractive asymptomatic anomaly and type of correction  Treatment  anisometropia > 3-4 D-->contact lens  unilateral aphakia-->contact lens or intraocular lens

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What is Refraction? Subjective Refraction

It is Determination of the refractive status (prescription) of the eye. To determine by subjective means the combination of spherical and cylindrical lenses necessary to provide best

visual acuity. (with accommodation relaxed)  Refraction could be performed Objectively (using

Retinoscopye or Autorefractometer) or subjectively.

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Principles of Refraction How to ensure accommodation is relaxed?

 1. Accommodation-relaxed state . Use PLUS lens to FOG  2. Maximum PLUS, minimum minus . Ensure image is located infront of retina  3. Always trial frame before prescribing . This causes image / VA to become worse if eye attempts to accommodate (Image point becomes further away from the  4. Take into account vertex distance fovea) especially for high prescription individuals

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How to achieve BVS? FOGGING TO ACHIEVE BVS

• Highest positive spherical lens to give •Place enough PLUS lenses to FOG vision best VA to ~6/12 line •ROT: every line= ~0.25 DS •Slowly reduce the plus power until best VA FOGGING DUOC ROME is obtained

•Remember: “Maximum plus power for best visual acuity”

USING DUOCHROME TEST TO JACKSON CROSS CYLNDER (JCC) ACHIEVE BVS

JCC used to find used to determine the cylindrical axis and PT the cylindrical power for the patient. TP

• Based on chromatic aberration • PaLETTtientE aRskS edin :t h“ eA rger eeLETTEn darRkeSr ?in” the red darker or

• Green letters clearer = Add ‘+ 0.25DS’ • Red letters clearer = Add ‘- 0.25DS’ • GrEndee-pon cinhat irst appobtaineaersd equwheanll yt hdae rlektt oerr swhe onn t hae reversal Red- occurs.

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STEP 2: JACKSON CROSS (JCC) Determining Cylinder Axis CYLINDER • Patient directed to observe a round target • Align dots with trial lens axis ie: 180 • JCC used to find used to determine the cylindrical axis and the cylindrical power for the patient.

OR

Determining Cylinder Axis Refining the axis

• JCC is flipped such that two views are shown • If view one is clearer turn trial lens' axis Turn 10 degrees • Patient asked: “Is view one rounder, sharper, TOWARDS red lines (~5-10 degrees) clearer or view two?”

FL VIEW 1 I P VIEW 2 VIEW 1

Refining the axis Determining Cylinder Power

• If view two is clearer turn trial lens' axis TOWARDS • Patient directed to observe a round target red lines (~5 degrees) • Align red lines OR white lines to trial lens axis

O R VIEW 2 Turn 5 degre es

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Determining Cylinder Power Determining Cylinder Power

• If view one is clearer, ADD -0.25 DC • If view one is clearer, ADD +0.25 DC

To maintain the circle of least confusion To maintain the circle of least confusion on the retina, on the retina, a +0.25DS is added for every -0.50DC a -0.25DS is added for every +0.50DC

Another technique for measuring Astigmatism BINOCULAR BALANCING

• The technique is also known as "equalising". • During the monocular refraction, a different state of relaxation of accommodation may occur because one eye was under test while the other was not. • Thus, binocular balancing is performed to balance accommodation between eyes.

ALTERNATE HUMPHRISS OCCLUSION IMMEDIATE CONTRAST

The fan test Clock dial test

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BB: Alternate Occlusion BB: Humphriss immediate contrast

•Used only when VA is EQUAL in both eyes •Used when VA is EQUAL or UNEQUAL between both eyes

.1Fog both eyes with + 0.75DS .1Fog OS with +0.75DS .2Direct patient to view 3 lines above best VA .2Direct patient to view OD's best VA line .3Alternately occlude each eye for ~0.5 secs each ( .3Perform BVS in OD) while asking patient: “ Which eye sees .4Add +0.25DS in OD � VA same or better Add clearer/sharper”? +0.25DS, VA worse remove +0.25, until you achieve .4Add +0.25DS to the better eye max plus min minus .5Repeat step 3 and 4 until both eye's vision .5Repeat Step 1-3 to test OS is equalised .6Slowly reduce fog until best VA is reached

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BINOCULAR BEST VISION SPHERE What about for reading?

• After binocular balancing, spherical lenses are added in front of the 2 eyes at the same time to determine the Binocular Best Sphere. The most plus/ least lmeninus tsha t would not reduce VA would be the end- point. •Strongest positive spherical lens to give best VA

.1Direct patient to view best OU VA .2ADD +0.25DS � VA same= Add +0.25DS (Repeat with additional +0.25DS), VA worse= Remove “I can see clearly at distance, but I can’t read 0.25+DS my newspapers!”

Near Addition Finding the Near Addition

• Presbyopic • Step 1: Estimation from patient's age patients Age (in Years) Estimated Add (in D) would require 40 - 45 + 0.50 to + 1.00 a near 46 - 50 + 1.25 to + 1.75 addition for 51 - 55 + 2.00 to + 2.50 their reading. 56 - 60 + 2.50 to + 2.75 61 - 65 + 2.50 to + 2.75 66 - 70 + 2.50 to + 2.75 70 + + 2.50 and Above

Finding the Near Addition

• Step 2: Place the estimated near addition on top of the distance prescription Instruments for Refraction • Step 3: Patients holds the near vision chart at habitual distance. The amount of near Trial lens set and frame add is then adjusted to position the patient's Phoropter habitual reading distance in the middle of the range of clear vision.

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Two sets of commonly used Adjusting Sphere Powers instruments

Adjusting Cylinder Powers and Axis Cross Cylinder

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