Refractive errors

REFRACTIVE ERRORS

REFRACTION

Refractive Errors

• Emmetropia

Ametropia

Emmetropia

Emmetropia means no Refractive error It is the ideal condition in which the incident parallel rays come to a perfect focus upon the light sensitive layer of the retina, When accommodation is at rest

Ametropia

Ametropia means Refractive error Eye It is the opposite condition , where in the parallel rays of light are not focused exactly upon the retina , When the accommodation is at rest

Ametropia

• Myopia
• Hypermetropia

Astigmatism

Myopia

Principal focus is formed in front of the retina

Causes

Axial Myopia

Curvature Myopia

Index Myopia

Abnormal position of the lens

Axial Myopia

• Axial myopia results from increase in anteroposterior length of the eye ball.
• Normal Axial length- 23mm to 24mm

1mm increase in AL – 3Ds of Myopia

Curvature Myopia

• Curvatural myopia occurs due to increased curvature of the cornea and Lens or both.
• Anterior surface of the cornea +7.8mm
• Posterior surface of the cornea -6.5mm

1mm increases in radius of curvature results in – 6 Ds of Myopia

Index myopia

• Index myopia results from increase in the refractive index of crystalline lens.

Refractive index of normal Lens 1.42

Abnormal position of the lens

Positional myopia is produced by anterior displacement of crystalline lens in the eye.

Types

• Congenital myopia
• Simple Myopia (or) Developmental myopia
• Pathological Myopia (or) Degenerative myopia

Acquired myopia

Congenital myopia

Congenital myopia is present since birth however, it is usually diagnosed by the age of 2 – 3 years.

Simple myopia

Simple or developmental myopia is the commonest variety. It is considered as a physiological error not associated with any disease of the eye.

Power limit less than 6D

Aetiology

• Axial type of simple myopia

Curvatural type of simple myopia

Pathological myopia

• Myopia associated with degenerative changes in the eye.

Myopia more than 6D to25D or More than 25D

Aetiology

• Axial growth
• (i) Heredity

(ii) General growth process

Acquired myopia

• Some of the causes of acquired myopia
• * Index myopia
• * Curvatural myopia
• * Positional myopia
• * Consecutive myopia
• * Pseudo myopia
• * Space myopia
• * Night myopia (or) Twilight myopia

* Drug induced myopia

Symptoms

• Poor vision for distance
• Asthenopic symptoms
• Exophoria
• Muscae volitantes (pathological)

Night blindness (pathological)

Signs

• Large eye ball
• deep Anterior chamber
• sluggish Pupil

Large Disc

Complications

• Retinal tear – Vitreous haemorrhage
• Retinal detachment
• Degeneration of the vitreous
• Primary open angle Glaucoma
• Posterior cortical cataract

Posterior staphyloma

Treatment

• Optical

Spectacle Correction (Concave Lens)

• Contact lens
• Surgical
• PRK
• Keratomileusis
• Epikeratophakia

Redial Keratotomy

Optical Treatment

• Concave lens

• Myopic with Esophoria give under

correction.

HypermetropiaPrincipal focus is formed behind the retina

Causes

Axial Hypermetropia

Curvature Hypermetropia

Index Hypermetropia

Abnormal position of the lens

Axial Hypermetropia

Axial hypermetropia is by far the commonest In fact, all the new- borns are almost invariably hypermetropic (approx,+2.50D) This is due to shortness of the globe, and is physiological.

• Normal axial length – 23mm to 24mm

1mm decrease in AL – 3Ds of hypermetropia

Curvature Hypermetropia

In which the curvature of cornea, Lens or both is flatter than the normal resulting in a decrease in the refractive power of the eye.

• Anterior surface of the cornea- 7.8mm
• Posterior surface of the cornea- 6.5mm

1mm increase in radius of curvature results in – 6Ds of hypermetropia

Index Hypermetropia

Index hypermetropia occurs due to change in refractive index of the lens in old age. It may also occur in diabetics under treatment.

Classification

• Total Hypermetropia may be divided into
• (a) Latent Hypermetropia
• (b) Manifest Hypermetropia
• (i) Facultive Hypermetropia

(ii)Absolute Hypermetropia

Latent Hypermetropia

LH which is corrected physiologically by the tone of ciliary muscle. As a rule latent hypermetropia amounts to only one dioptre. It can be revealed only after atropine cycloplegia.

Manifest HypermetropiaMH is made up of two components

Facultative hypermetropia is that part of hypermetropia which can be corrected by the effort of accommodation.

Absolute hypermetropia which can not be overcome by the effort of accommodation.

Clinical Types

• Simple hypermetropia
• Pathological hypermetropia

Functional hypermetropia

Simple hypermetropia

It results from normal biological variation in the development of the eye ball. It includes Axial and Curvatural HM. It may be hereditary.

Pathological hypermetropia

PH results due to either congenital or acquired conditions of the eye ball which are out side the normal biological variations of the development.

The Normal Age Variation

• At birth:- 2D to 3 D Commonly Present
• At the age of 5 Yrs- 90% of Children’s are Hypermetropic

At Puberty:- Emmetropic

Symptoms

• Head ache
• Blurred vision particular near work
• Convergent squint
• Early onset of presbyopia

Eye Strain

Complications

• Eye appears to be small including cornea and anterior chamber becomes shallow
• Extreme cases – Microphthalmos

Retinal reflex – Shot silk-Retina

Treatment

• Optical

Spectacle ( Convex Lens )

• Contact lens

• Surgical

Thermokeratoplasty

Astigmatism Astigmatism is that condition of Refraction where the point focus of light cannot be formed upon the Retina

Causes

• Curvature
• Ex: Keratoconus, Lenticonus etc..
• Centering error
• Ex: Sub location of the lens
• Refractive index

Ex: Cataract

• Retinal

Oblique placement of macula

Types

• Regular

Irregular

Regular astigmatism

• Refractive types

Physiological types

Refractive types

• Simple astigmatism
• Compound astigmatism

Mixed astigmatism

Physiological types

• With rule astigmatism
• Against rule astigmatism
• Oblique astigmatism

Bioblique astigmatism

Symptoms

• Head ache
• Blurring of vision
• Eye tired
• Eye ache

Head Tilt

• Half-closure of the lids (High astigmatism)

Blurring & Itching (Low astigmatism)

Treatment

Optical Treatment

• * Cylindrical lens
• * Under correction
• * Contact lens (RGP, Toric)
• Refractive surgery

* Astigmatic Keratotomy

* PRK, LASIK

Study Reports

• Percentage of astigmatism

• Percentage of Types

Duo chrome test To test if the eye has been under corrected or over corrected or is properly corrected

Astigmatic Fan To know the axis and power in Astigmatism

Jackson cross cylinder To refine the axis and power of cylinder

Presbyopia

This is a physiological aging process, In which the near point gradually recedes beyond the normal reading or working distance

Causes

• Lens matrix is harder and less easily moulded
• Lens capsule is less elastic
• Progressive increase in size of the lens

Weakening of the ciliary muscle

Symptoms

• Patient holds the book at arms length
• Patient prefers to read in bright light
• Eye strain
• Head ache

Eyes feels tired and ache

TreatmentConvex lens

• Methods of prescription
• * Occupation
• * Working distance
• * Age
• Surgical
• * Anterior ciliary sclerotomy
• * Laser thermal keratoplasty

* Small diameter corneal inlays

Aphakia

Aphakia means absence of the Crystalline lens from the Eye ball

Causes

• Congenital
• Surgery

Traumatic

Optics of Aphakia

• Anterior focal distance – 23mm (N-15mm)
• Posterior focal distance- 31mm (N-24mm)
• The Nodel point of the eye is thus moved forward

Strong converging (convex) lens- +10D

Signs

• Anterior chamber – Deep
• Iris
• (i) Iridodonesis (or) Tremulousness
• (ii) Peripheral button-hole iridectomy mark
• Pupil – Jet black reflex
• Absence of the 3rd and 4th Purkinje images
• Retinoscopy – reveals high hypermetropia and astigmatism

Ophthalmoscopy – As in hypermetropic fundus with a small optic disc

Disadvantages

• Image magnification of about 25-30%
• Spherical aberration, Peripheral and Pincushion
• Roving ring scotoma (The scotoma extents
• from 50°- 65° from central fixation)
• Jack in the box
• Restriction of the visual field
• Coloured vision

Inaccurate spectacle correction because of errorneous vertex distance

Treatment

• Spectacle ( Convex lens )
• Contact lens
• Secondary IOL
• Epikeratophakia

Keratophakia

Aphakic formula

P = X / 2 +10.00D

• P = IOL power

X = Refractive power

Pseudophakia

Pseudophakia means False lens

Image magnification

Calculation of IOL power

Refractive stages of a Pseudophakic eye

• Emmetropia
• Consecutive myopia

Consecutive hypermetropia

Advantages

• Image magnification is only 0- 2%
• No spherical and prismatic aberrations
• Minimum (or) No Anisokonia with rapid return of binocularity
• Normal Peripheral field of vision and eccentric vision
• Freedom from handling of the optical devices

Cosmetically it is well accepted

Disadvantages

• Risks and complications may be more
• Initially, the cost is more
• PCO(posterial capsular edema)
• CME (cystoid macula edema)

IOL related complications

Thank “U”