CLINICAL APPROACH TO REFRACTIVE ERRORS

Ayesha Abdullah14.09.2012

Learning objectives

By the end of this lecture the students would be able to;

• Correlate optics with the various types of refractive errors & their correction with lenses

• Describe the clinical presentation of refractive errors

• Describe the clinical protocol for the assessment of refractive errors

To begin with

Refraction ?Refractive errors?Main refractive surface of the eye?How much is the refractive power of the cornea?40-44 diopters Why is it so?So its mainly corneal refractive error!How much is the role of lens in refraction?What is the role of accommodation in refraction?During accommodation in a youthful eye the lens can change its

refractive power from 19 D to 33 D

What determines the type and amount of refractive error?

• Refractive power of the cornea and the lens• Length of the eye (1m.m. changes represents

about 3 dioptres change in refraction)• Refractive errors of up to 5 D are considered to

be biological variation • Higher degrees of refractive errors are

associated with structural anomalies of the ocular structures e.g. the cornea, lens, choroid and the retina

What is emmetropia & ametropia?

• The state of having no refractive error is emmetropia ; a balanced state of refractive power of the cornea, lens and the length of the eyeball.

• Parallel rays of light are brought to a focus on the fovea

• The state of the eye when parallel rays of light are not focused on the fovea is called ametropia

Emmetropia

Axial length matches dioptric power of the eye & parallel rays of light are brought to a focus onto the retina

Ametropia

When parallel rays of light are NOT brought to a focus onto the retina

Ametropia could be : a. Axial (1m.m. changes represents about

3 dioptres change in refraction. )b. Curvature (1m.m. change in radius of

curvature of cornea represents about 6 dioptres of change in refraction. )

c. Index ; due to change in the refractive index of the refractive media

Types of ametropia

• Myopia, common in young age group• Hypermetropia/ hyperopia, common in very

young children & old age• Astigmatism; common in young age group but

less common than myopia

Myopia (near-sightedness)

• Parallel rays brought to a focus in front of the retina. The eye is stronger for the axial length of the eye

Simple myopia

Usual onset by adolescence but may begin as late as 25 years of age.

Gradually increases until the eye is fully grown.

Seldom exceeds -6 dioptres.

Pathological myopia

Commonly begins as physiological but rather than stabilizing when adult size of the eyeball is achieved, the eye continues to enlarge. It is associated with pathological changes in the posterior segment that can be seen on ophthalmoscopyIt can lead to complications like retinal breaks and detachment Can go upto over -20 diopters

Hyperopia/ Hypermetropia Parallel rays brought to a focus BEHIND the retina. The eye is weaker for the axial length of the eye.What would happen if the person accommodates?

Hyperopia

• Manifest• Latent – to measure this the accommodation

has to be knocked off through cycloplegic agents like cyclopentolate and atropine

• At which age do you think the latent would become manifest?

• Total

Hyperopia

At birth practically all eyes are hypermetropic to the extent of +2.5 to +3.0 diopters.

Emmetropisation ensues as the eye grows.

Emmetropia may not be reached and hypermetropia may persist.

May also occur pathologically due to orbital mass, intraocular tumour, retinal oedema and RD.

Astigmatism

• The eye has different refractive power in different meridians of the eye e.g.

• Vertical rays being focused in one position (in front, behind or on the fovea) and horizontal rays focused on another

• When the two meridians are at right angle to each other its called regular astigmatism otherwise its termed as irregular astigmatism

Astigmatism

May also be classified: a. Simple – One axis ametropic either

myopic/hyperopic b. Compound – both axes ametropic but either

myopic or hypermetropic. c. Mixed – each axis of opposite power.

Simple Myopic astigmatism

Mixed astigmatism

How do refractive errors present?

• Asthenopia ,eyestrain & visual fatigue• Blurring of vision• Ocular discomfort with itching, burning of the eyes

and at times increased sensitivity to light etc• Headache, rarely could be attributed to refractive

errors. Headache presenting after visual work especially in those above 40 years could be because of RE, however a headache presenting early morning is extremely unlikely to be because of RE

In children

• Can present in a variety of ways• In preverbal children it can present as delayed

milestones of visual development; inability to focus at visually stimulating objects, follow light or bright object

• Squint • Lazy eye or eyes

In school going children

• Lack of interest in visual tasks, class work• Generally apathic or withdrawn behaviour• Difficulty in reading or seeing the black/ white

board from distance• Squint• Lazy eye

Signs

• Decreased visual acuity that improves with pinhole

• The eyeball may be obviously small(hyperopia) or large (myopia)

• The cornea my be conical in shape (irregular astigmatism (keratoconus)

• Pupils are normal• Posterior segment may show abnormalities

Posterior segment signs

• In pathological myopia retinal degenerations ( myopic crescent, lattice) , breaks ( holes and tears) etc

• In hyperopia psedupapilloedema ( blurring of optic disc margins with hyperopia of greater than 5 D)

• In high degrees of astigmatism the optic disc may appear oval

Normal fundus

Treatment of RE

• Spectacles• Contact lens• Refractive surgery

Refractive assessment

1. Check VA with and without spectacles

2. Check with a pin hole

3. Pupillary examination

4. Ophthalmoscopy.

5. Cover test with and without correction

6. Objective refraction/retinoscopy/refraction/

autorefraction (cycloplegic in children)

7. Subjective refraction

Subjective verificationDuochrome test.

Muscle balance – Maddox rod for distance.

Near vision correction

Maddox wing for near

Presbyopia

• What is presbyopia?• How is it corrected?