3 Physiological optics. Physical refraction, clinical refraction (emmetropia, myopia, astigmatism). Research methods. Correction methods. 4 Oral survey. TPCBL. Situational tasks.

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Physiological Optics — Clinical Refraction: Study Guide

1. Physical Optics of the Eye

The Eye as a Refracting System

The eye contains four refracting surfaces: the anterior and posterior surfaces of the cornea, and the anterior and posterior surfaces of the crystalline lens. The total refractive power of the normal eye is approximately +59 diopters (D) when the lens is relaxed.

Cornea: Provides ~+43 D (the single most powerful refracting surface, due to the large air-to-tissue refractive index difference)
Crystalline lens: Provides ~+15–20 D (variable via accommodation)
Aqueous and vitreous humors: Refractive index ≈ 1.336

Diopter (D): Unit of lens power = reciprocal of focal length in meters. A lens with a focal length of 1 m = 1 D; 0.5 m = 2 D.

Snell's Law (Physical Refraction)

When light passes from one medium to another of different density, it bends. The degree of bending follows:

n₁ · sin θ₁ = n₂ · sin θ₂

where n₁, n₂ are refractive indices and θ₁, θ₂ are angles of incidence and refraction.

Accommodation

The lens can change its shape to alter refractive power, allowing focus at varying distances:

Ciliary muscle contracts → zonular fibers relax → lens becomes thicker and more convex → increased refractive power → near focus
Controlled by parasympathetic nerves via CN III
Power of accommodation: ~14 D in children, declining to ~2 D at age 45–50, and ~0 D at 70 (presbyopia)

2. Clinical Refraction: Ametropia

Emmetropia (Normal)

Parallel light rays from a distant object focus exactly on the retina when the ciliary muscle is completely relaxed. The emmetropic eye sees all distant objects clearly without accommodation.

Parallel light rays focus on the retina in emmetropia, behind the retina in hyperopia, and in front of the retina in myopia.

Myopia (Nearsightedness)

Feature	Detail
Cause	Eyeball too long, or lens system too powerful
Focus	Parallel light focuses in front of the retina
Vision	Distant objects blurred; near objects clear
Far point	A definite limiting distance for clear vision
Accommodation	Cannot compensate (can't reduce lens power below minimum)

Hyperopia / Hypermetropia (Farsightedness)

Feature	Detail
Cause	Eyeball too short, or lens system too weak
Focus	Parallel light would focus behind the retina
Vision	Distant objects may still be seen by using accommodation; near objects require even more effort
Accommodation	Partially compensates for distance; fails in presbyopia

Astigmatism

Definition: Refractive error in which the curvature of the cornea (or less often the lens) differs in two perpendicular meridians, so light rays from a single point focus at two different planes rather than one.

Example: An egg-shaped cornea — the curvature in the vertical plane is greater than in the horizontal plane. Light in the steeper plane focuses closer than light in the flatter plane.

Key property: Accommodation cannot correct astigmatism, because the ciliary muscle increases lens curvature equally in all meridians.

Types by axis:

Regular astigmatism — principal meridians are perpendicular (with-the-rule, against-the-rule, oblique)
Irregular astigmatism — no consistent perpendicular relationship (e.g., keratoconus)

3. Research (Examination) Methods

Subjective Refraction

Method	Principle
Trial frame & lens box	"Best vision" found by systematically adding spherical then cylindrical lenses
Snellen chart	Standard distance visual acuity (6/6 or 20/20 = normal)
Cross-cylinder test	Refines cylinder axis and power
Duochrome (bichrome) test	Uses chromatic aberration of the eye to balance under/over-correction
Near vision testing (Jaeger chart)	Tests reading vision

Objective Refraction

Method	Principle
Retinoscopy (skiascopy)	Observe reflex of light from the fundus; neutralize movement with lenses; works under cycloplegia
Autorefractometer	Automated infrared optometer; gives objective sphere/cylinder/axis
Keratometry (ophthalmometry)	Measures anterior corneal curvature in two meridians; quantifies astigmatism (Javal–Schiötz keratometer)
Corneal topography	Placido disc-based mapping of the entire anterior corneal surface; detects keratoconus, irregular astigmatism
Corneal pachymetry	Measures corneal thickness; essential pre-refractive surgery
Wavefront aberrometry	Maps higher-order aberrations of the entire optical system

Cycloplegia (cyclopentolate, atropine) paralyzes accommodation — essential for accurate refraction in children and young adults.

4. Correction Methods

4A. Spectacle (Ophthalmic) Lenses

Correction of myopia with a concave lens (top) and correction of hyperopia with a convex lens (bottom).

Error	Lens Type	Mechanism
Myopia	Concave (diverging, –)	Diverges rays so the image moves back to the retina
Hyperopia	Convex (converging, +)	Converges rays so the image moves forward to the retina
Astigmatism	Cylindrical (combined with spherical)	Corrects one meridian more than the other
Presbyopia	Bifocal / trifocal / progressive	Upper segment for distance, lower for reading

4B. Contact Lenses

Soft lenses: Flexible hydrogel; drape the cornea; correct sphere and cylinder
Rigid gas-permeable (RGP): Replace corneal surface; correct irregular astigmatism; tear film fills the gap
Orthokeratology: Worn overnight; temporarily flatten cornea to reduce myopia

4C. Surgical Correction (Refractive Surgery)

Laser corneal procedures

Procedure	Full Name	Mechanism	Range
PRK	Photorefractive keratectomy	Surface ablation — removes epithelium, ablates stroma	Myopia ≤ –6 D, astigmatism ≤ 3 D
LASEK	Laser epithelial keratomileusis	Preserves epithelial flap	Similar to PRK
LASIK	Laser in situ keratomileusis	Corneal flap raised, stroma ablated, flap replaced	Myopia –6 to –8 D, hyperopia ≤ 3–4 D, astigmatism ≤ 5 D
SMILE	Small incision lenticule extraction	Femtosecond laser cuts a lenticule; removed through a small incision; no flap	Myopia and myopic astigmatism

LASIK: Central ablation flattens cornea for myopia; mid-peripheral ablation steepens center for hyperopia. Residual stromal bed must be ≥ 250 μm to prevent ectasia.

Intraocular procedures

Procedure	Indication	Notes
Phakic IOL (iris claw / ICL)	High myopia or hyperopia not suitable for laser	Implanted in anterior or posterior chamber; natural lens preserved
Clear lens extraction (CLE/RLE)	Very high refractive errors; presbyopia	Replaces natural lens with monofocal, multifocal, toric, or EDOF IOL
Toric IOL	Astigmatism correction at time of cataract surgery	Correct axis alignment critical
Limbal relaxing incisions (LRI) / arcuate keratotomy	Mild–moderate astigmatism	Paired arcuate incisions flatten the steep corneal meridian

5. Situational Task Summary (Exam-Ready Points)

Emmetropia: No refractive error; parallel rays → sharp retinal focus; no lens needed at rest.
Myopia: Concave (–) lens; LASIK central ablation; "far point" is the limit of clear distant vision.
Hyperopia: Convex (+) lens; may be latent (masked by accommodation) — reveal with cycloplegia.
Astigmatism: Cylindrical lens; LASIK/PRK/toric IOL; accommodation cannot correct it.
Presbyopia: Loss of accommodation (lens elasticity); corrected with bifocals or multifocal IOL.
Retinoscopy: Gold standard objective method; performed under cycloplegia in children.
Keratometry: Measures corneal curvature → corneal astigmatism; unit: diopters or mm radius.
LASIK contraindications: Dry eye, thin cornea (<480 µm), keratoconus, unstable refraction, autoimmune disease, pregnancy.
SMILE advantage: No flap → no flap complications; better corneal biomechanical stability.
Phakic IOL: For myopia > –8 D when LASIK is not feasible due to insufficient corneal thickness.

Sources: Guyton & Hall Textbook of Medical Physiology (Ch. 50); Kanski's Clinical Ophthalmology: A Systematic Approach, 10th ed. (Ch. 8)

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