Normal Optics of Eye & Refractory Errors

Pincushion Distortion

• Seen with convex lenses when
• Lens is thick at center.
• Lens is thin at periphery.

• Effects
• Causes gradual worsening of vision.
• Affects both meridians.
• Impaired binocular vision.

• Cause Aniseikonia:
• Difference in perceived size or shape of images between two eyes.

Normal Optics of Eye

Axes of the eye

• Anatomical axis (ANR)
• Cornea → lens → retina
• Not through fovea

• Visual axis (OENF)
• Object → nodal point → fovea
• Functional axis

• Pupillary axis (BE)
• Perpendicular to cornea
• Through center of pupil

• Optical axis
• Centers of curvature of cornea + lens
• Theoretical
• Not through fovea

• Visual axis ≠ Optical axis

Angles

• Kappa (k): Between pupillary and visual axes
• kaPPa → B/w visual and Pupillary

• Alpha (a): Between anatomical and visual axes
• Alpha → B/w visual and Anatomical

Gullstrand’s Schematic Eye

• Describes 4 refractory surfaces (Purkinje images).

• Based on light reflections.

• Purkinje Surfaces:
• I (Anterior Cornea →+48.8D)
• II (Posterior Cornea → -5.88D)
• III (Anterior Lens)
• IV (Posterior Lens)
• Anterior + Posterior Together = +19D

Listing’s Reduced Eye

• All elements are measured from the anterior corneal surface.

• Power of eye: 58.6 D.

• Factors determining power:
• Curvature of anterior corneal surface
• Length of eyeball

• Principal point: 1.35mm

• Nodal point: 7.08mm

• Focal Point: 24.13mm

Refractory Error

Types

• Correctable with:
• Monocular lens / Binocular lens
• Spherical lens / Cylindrical lens

Myopia vs. Hypermetropia

Hypermetropia

• Axial hypermetropia
• 1 mm shortening of axial length → +3 D

• Curvature hypermetropia
• 1 mm increase in radius of curvature → +6 D

25 in general

• A newborn is hypermetropic by + 2.5 D

• BW → 2.5 kg

Myopia

• Myopia female likes
• Temple and seeing cresent moon (Temproal myopic cresent)
• Nail lacquer (Lacquer cracks)
• Fuching () → Posteriorly (Posterior staphyolma) → RHegmatogenous RD
• Tiger like appearance

Fuchs Terms	Notes
Fuchs heterochromia iridocyclitis	Chronic AU ⇒ • U/L Diffuse Iris atrophy + Heterochromia + Posterior SCC • Painless, No redness, No posterior synechiae Positive Amsler's Sign: ◦ Bleeding into Anterior chamber on paracentesis ◦ Without trauma to Iris/Angle ◦ D/t abnormal fragile Iris Stellate Keratin Precipitates ↳ Herpetic uveitis ↳ Toxoplasmosis ↳ Fuchs Heterochromia Iridocyclitis • Young stella → Fucked () by Toxic () Herpes () Guy
Dalens Fuchs	• Seen in Sympathetic ophthalmitis ↳ (granulomatous panuveitis) • Dalen Fucked Granny () sympathetically ()
Foster Fuchs	• In Pathological Myopia • Bleeding at macula • Fucking in Foster () home ↳ Blind child (Pathological myopia) ↳ Bled (Bleeding at macula)
Fuchs Endothelial dystrophy	• Cornea guttata: • Wart-like excrescences on posterior cornea • Fuck her Guts→ endothelial Stages • Stage 1: Central corneal guttata that spreads peripherally • Stage 2: Corneal oedema - beaten metal-like appearance • Stage 3: Bullous keratopathy • Stage 4: Subepithelial scarring and superficial vascularization

 

Key Note

• 1 mm increase in axial length = 3D myopia.

Fundus Signs

• Only seen in pathological myopia (Power >6D).

• Temporal myopic crescent
• Choroidal & retinal atrophy

• Posterior staphyloma

• Lacquer cracks 
• Break in Bruch’s membrane
• Mnemonic: Apply nail lacquer with a brush

• Foster-Fuch’s spots 
• Bleeding at macula

• Vitreous hemorrhage

• Retinal detachment 
• M/c cause of rhegmatogenous retinal detachment

Surgical Correction

• Photorefractive Keratectomy (PRK):
• Multiple peripheral corneal incisions.
• Flattens cornea.
• Decreases converging power.

• LASIK:
• Uses Excimer laser (Argon & Fluoride).
• C/I
• < 18 yrs
• unstable power
• CT < 450 Micrometer

• SMILE (Small Incision Lenticule Extraction):
• Uses Nd: Glass laser

• ICL (Implantable Collamer Lens) / PRL (Phakic refractive lens):
• If power >8D
• Site of implantation: Sulcus

• INTACS (Intracorneal stromal ring segment)

Lasers:

Principle	Laser	Uses
Photocoagulation	ㅤ	• POAG, ROP, PDR = Retina Problems
ㅤ	• Nd: YAG (532 nm) ↳ Double frequency	• Proliferative DR → Panretinal photocoagulation • Laser trabeculoplasty
ㅤ	• Argon green (514 nm)	• Rx of POAG • AG → Argon Green
ㅤ	• SLT (532 nm)	• Prophylaxis in other eye in POAG
ㅤ	• Diode red laser	• Retinopathy Of Prematurity (ROP)
Photodisruption	Disrupt Yagam	ㅤ
ㅤ	• Nd-YAG/Nd-glass	• SMILE (Myopia surgery) • Flash (FLACS) adichapo • Cap (PCO) vach Smile (SMILE) • Glass (NdGlass) vachond photodirupt () ayi.
ㅤ	• Nd-glass 1054 nm	• FLACS (Cataract Sx) → 1054 nm ↳ Femtosecond Laser ↳ Used for 2 steps 1. CCC 2. Nuclear fragmentation.
ㅤ	• Nd-YAG 1064 nm	• Posterior Capsular Opacification (PCO) / ↳ After-cataract ↳ Occurs 6-12 months postop Cataract Sx ↳ M/c complication. ↳ Elschnig’s pearls (90%) ↳ Nd-YAG posterior capsulotomy / (Peripheral Iridotomy)
Photoablation	ㅤ	ㅤ
ㅤ	• Excimer	In Psoriasis • Targeted UVB (Phototherapy) • Xenon Chloride (308 nm) In LASIK, PRK for myopia • Argon & Fluoride (193nm) • Most commonly used in refractory surgeries

 

Astigmatism

• Refractive error due to irregular cornea or lens.

• D/t Unequal curvature in different meridians

• Result in Blurred or distorted vision.

Treatment

1. Cylindrical glasses



• Have power 90° to their axis
• Types:
• Only 1 axis has power
1. Plano convex
• Used for hypermetropic astigmatism
2. Plano concave
• Used for myopic astigmatism

2. Toric contact lens
• A lens with surfaces with different powers

3. Surgery
1. Astigmatism LASIK
• Selective burning of the axis with higher curvature in the cornea
2. Accurate Keratotomy
3. LRI (Limbal Relaxing Incisions)
• Principle: Partial thickness cuts are made on the axis with higher curvature

Types

• Axis determination:
• Mnemonic: With CV, against VC

• With the rule:
• Vertical meridian more curved.
• Correction at 180°.

• Against the rule:
• Horizontal meridian more curved.
• Correction at 90°.

• Oblique:
• Two perpendicular meridians.
• Not vertical or horizontal.

• Irregular:
• Two principal meridians are not perpendicular.
• Caused by corneal/retinal scarring.
• No Rx (not correctable with conventional spectacles).

Sturm’s Conoid

• Configuration of light rays when refracted from a toric surface.

• With the rule
• Vc > Hc
• Vp > Hp
• Vertical rays focus first

Correction of Refractory Errors

Prescription of Spectacles

Determination of Refractory Errors

Objective Methods

• Manual Refractometer

Retinoscopy:

• Streak Retinoscope (M/c)

• Light produced in vertical/horizontal beams.

• Examination distance: 1 mt.

• Compares movement of red streak to movement of retinoscope.

• Retinoscopy values (at 1 meter):
• No movement → 1D myopia.
• With movement → <1D myopia or emmetropia/hypermetropia.
• Against movement → power >1D myopia

• Mnemonic:
• Retinoscopy → Infants
• Less than 1 year (<1D Myopia or Hypermetropia) → With our movement
• More than 1 year (>1D → Myopia) → Against movement

Correction factors (C):

• C1 = 1/distance (d) of examination.

• C2 = Tonus allowance of cycloplegic drug:
• Atropine 1
• Cyclopentolate 0.75
• Homatropine 0.50

• Corrected retinoscopic value (RF) = RF - C1 - C2.

• Automated Refractometer

Subjective Methods

1. Adjustment of refraction
• Trial & error method for refraction refinement.

2. Refinement for Cylindrical Lenses:

Cylinder

• Mnemonic: Cylinder vach jackson (Jackson cross) fan (astigmatism fan) undakki
• a. Jackson Cross Cylinder (JCC) test:
• Uses -0.5 D sph/+1.0 cyl combinations.
• Or -0.25 D sph/+0.5 cyl combinations.



• b. Astigmatic fan test:
• Power refined until patient can see all axes clearly.



Sphere

• Spherical hole (Pinhole) of 2 color (duochrome)
• 1. Duochrome test:



• Residual hypermetropia → better vision in red.
• Residual myopia → better vision in green.
• 2. Pinhole test:



• Eliminates peripheral light rays.
• Allows central light rays.
• Visual acuity ↑↑
• residual refractory error.
• Visual acuity ↓↓
• with macular disease.
• Size of hole: 1 mm.

3. Binocular balancing

• Duchrome test with fogging

• Prism dissociation method

Stenopaic Slit (SS):

• 4 Clinical Uses
• Diagnosis & Refinement of Astigmatism
• 1. Initial Detection of Astigmatism
• 2. Confirmation & Refinement of Astigmatic Correction
• Fincham’s Test
• 3. Differentiating Coloured Halos
• Unbroken halos → Suggests Acute Congestive Glaucoma (ACG).
• Broken halos → Suggests Cataract
• Broken people - old age
• Optical Iridectomy
• 4. Determining Best Visual Meridian for Iridectomy

OPTICS AND REFRACTION

Tests for Vision

Snellen’s Chart:

• Distance of examination: 6 metres.

• 7 rows of letters

• Normal vision in decreasing order
• 6/5 > 6/6 > 6/9 > 6/12 > 6/18 > 6/24 > 6/36 > 6/60 > 5/60 > 4/60 > 3/60 > 2/60 > 1/60
• Finger Counting → HM: Hand Movement → PL: Perception of Light

• Least vision that can be measured with Snellens: 1/60

• Principle:
• Each letter subtends an angle of 5 minutes of arc at nodal point when viewed from its respective distance.

Bailey-Lovie/Log chart:

• Not used in India.

• 5 letters in each row

Landolt C-Ring.

Snellen’s E-Chart:

• For illiterate patients.

Tests Used in Infants

Optokinetic nystagmus (OKN):

• Child normally focuses on OKN/Catford drum.

Preferential looking test:

• Child prefers to look at Teller acuity cards/Keeler gratings.

Blink reflex:

• Blinks on exposing eye to light (present since birth).

Menace reflex:

• Object brought close to child’s eye makes child move away (develops at 4 to 6 months).

Cardiff Acuity Test.

Tests for Near Vision

• Distance of examination: 35 cms

• Snellen’s Near Chart:



• Best near vision: N6.
• Worst near vision: N36.
• Most commonly used.

• Jaeger’s Chart:
• J1 to J7 vision.

Test for Contrast Sensitivity

• Pelli Robson Chart:
• Used in cases of cataract
• where contrast vision is lost early
• before the actual vision loss
• Mnemonic: Palli in wall → cannot be seen in cataract old age → due to contrasting colour

• Cambridge test

• Regan chart

• Ardens gratings

• Bailey Love contrast

• FACT chart

Accommodation

Mechanism

• Helps near vision by increasing power.

• Far vision:
• Ciliary muscles relaxed → Ciliary zonules tense → Lens thin (↓Power).

• Near vision:
• Ciliary muscles contracted → Ciliary zonules slack → Lens thick (↑Power).

Presbyopia

• Physiological insufficiency of accommodation.

• Difficulty in near vision.

• Onset age: >40 yrs.

• Treatment: Convex spectacle (+ lens).

Spasm of Accommodation

• AKA Pseudomyopia.

• Issue with distant vision.

• Due to spasm of ciliary muscles.

• Treatment: Cycloplegics (e.g., Atropine) to relax ciliary muscle.

Newborn

• At birth:
• Axial Length (AL) ~17 mm (<24mm).
• Refractory error: 2-3 D hypermetropia.

• Becomes emmetropic by 5-6 yrs.