Analysis, Interpretation, and

Prescription for the

Ametropias

Indra P Sharma

Optometrist

MRRH, Ministry of Health

Bhutan

Hypermetropia50%

Emmetropia25%

Myopia< 1D13%

Myopia> 1D12%

Ref: Borish IM, Clinical Refraction , ed 3 pp 861-937

Refractive Error distribution in

normal population

Sharmaindra, Bhutan

But why do we see more myopes

in the OPD?

• Because the problem is so readily apparent,

myopes account for disproportionate share of OPD.

• “Myopia causes blurring of distance vision”

• Easily observed either by self observation , noted

by comparison, or brought to notice by occupational

or school needs, or by screening.

• Patient may also develop a habitual squint to get

pinhole effect, and a “furrowed brow”

Sharmaindra, Bhutan

Normal refractive age norms

• Generally, manifest refraction conform to

the age norms

• Important for clinicians to know

• ? Outside normal norms- Alert to the

clinician to search for secondary

causes.

• Example:

Sharmaindra, Bhutan

0

1

2

3

4

5

6

0 5 10 20 30 40 50 60 70 80 90

Refr

action (

+ D

)

Age(in years)

Refraction based on age

Sharmaindra, Bhutan

Emmetropia

• Parallel light form infinity focus on the

retina, with accommodation relaxed

AccommodationSharmaindra, Bhutan

Emmetropes also complain

• The main reasons an emmetrope would

have refractive complaints are near-point

asthenopia as a result of accommodative

dysfunction or convergence problems.

• Possibly manifested in form of headache,

eyestrain or diplopia

• The management of emmetrope are

usually directed towards problems of

accommodation and convergence.

Sharmaindra, Bhutan

Synkinetic traid of near reflex

Accommodation

Pupillary

Constriction

Convergence

Sharmaindra, Bhutan

Prescription for Myopia

Sharmaindra, Bhutan

Myopia

Sharmaindra, Bhutan

Uncorrected Myopia

• Requires a medium or large magnitude of

minus lens power

• In addition to distance blur, patients may

also complain of problems at habitual

reading distance.

• Near problem-1. blurred vision

2. asthenopia secondary to difficulties at the far point

3. Photophobia }

Sharmaindra, Bhutan

Prescribing guidelines for Myopia

• Caution should be exercised during

subjective refraction because myopes

report that more minus increases clarity.

• “minification of image by minus lens is

seen as increased clarity”

• So its best to always compare between

unaided visual acuity, objective refraction

and subjective refraction(it helps to ascertain the appropriate amount of minus to

be prescribed)

• AVOID overcorrecting myopia

Sharmaindra, Bhutan

Sharmaindra, Bhutan

• Blur during reading- when reading material

is held further than patients far point

• Pt must move the reading material closer

to secure clarity

• “More myopia, closer the far point”

• Eg: - 4 D Myope made hold book at 25cm,allowing

near vision clarity with minimal/no accommodation.

This under accommodation increase in pupil

diameter beyond normal size PHOTOPHOBIA

Reading, while uncorrected, at this point may lead

to asthenopia

Sharmaindra, Bhutan

• When myopia is fairly large or problems

related to age related amplitude of

accommodation or near esophoria exist,

adaptation to reading with full-time

correction may be difficult.

• Solution: Adaptation effort can be minimized by

under correcting myopia.

• Initially undercorrect increase minus power in

subsequent visits

• “Minimum correction with maximum vision”

Sharmaindra, Bhutan

Prescription for Hypermetropia

Sharmaindra, Bhutan

Sharmaindra, Bhutan

• Unlike myopia, hyperope can usually secure

resultant clear distant vision by use of the

ability to accommodate.

• Low-mod hyperopia can sometimes function

asymptomatically until:

1.age reduces the accommodative amplitude

2.the accommodation is exhausted from

prolonged use

• The term “farsighted” is misnomer in older

patients because both distance and near

vision are blurred.

Sharmaindra, Bhutan

Uncorrected Hypermetropia

• Because of added accommodation required

– blur or asthenopia at near point

• Near point difficulty amplitude of

accommodation i.e older the uncorrected

hyperope, more likely the complaints

• Complaints:1. Headache(usually frontal)

2.Asthenopia(due to strain)

3.Tearing due to excessive

4.Excessive rubbing during accommodation

near work

5.Conjunctival irritation

}Sharmaindra, Bhutan

Challenge Prescribing for

Hyperopic Compensation

• In uncorrected hypermetropia, overaccommodation causes a perceived enhancement of contrast.

• Enhanced contrast removed by correction

• Patients perception may be that of “blur” even if visual acuity remains same.

• Initially, to minimize adaptive problems “cut-some-plus”

• Increase correction in subsequent visit till full hypermetropic compensation is reached

Sharmaindra, Bhutan

Prescribing guidelines for

Hyperopic CompensationConsideration Management

Birth to 6 years No compensation, except for strabismus, suppression or poor school performance

6 to 20 years No compensation, except for strabismus, suppression or poor school performance, nearasthenopia or acuity loss; prescribe cautiously with liberal cut in + power

20 to 40 years Compensate for complaints , with moderate cut in plus power for distance, yet full compensation for near activity

40 + years Usually compensate with full plus power with near add for presbyopia

Esotropes Fully correct , with possible near correction

Exotropes Partially correct to minimize secondary exoproblems

Sharmaindra, Bhutan

• As a general thumb rule,

‘prescribe for the hyperope to answer the

patients complaints’

Sharmaindra, Bhutan

Cycloplegic Refraction

* Used when control of accommodation by fogging or other method is not ensured

* Used in difficult hyperopes, mentally retarded patients, children with short attention span, younger hyperopes where latent hypermetropia is common,andmalingerers.

* Cycloplegic refraction values not necessary prescribed, but gives starting point for subjective refraction

Sharmaindra, Bhutan

Consideration Management options

Ciliary tonicity Cut about +1.0 D from ‘wet’ refraction

Patient age The younger the patient the more liberal cuts from plus power.

PrescriptionHistory

For first prescription, plus power should be cut from wet refraction for adaptive purpose

Residualaccommodation

If less than 1.oD,good cycloplegic effect. So liberal plus cut from wet refraction

Dry Refraction The closer the dry refraction is to the wet, the less likely to cut plus power in the final prescription

Guidelines in Cycloplegic

Refraction Prescribing

Sharmaindra, Bhutan

Prescription for Astigmatism

Sharmaindra, Bhutan

Image formation in astigmatism

Sharmaindra, Bhutan

• Astigmatism presents a greater challenge

• Low amount – usually varying anatomical

etiological origins

• Large astigmatic errors- mainly result of

corneal curvature

• Focal line formed on the retina and not a

point focus

Sharmaindra, Bhutan

Strum’s conoid

Sharmaindra, Bhutan

With-the-rule: -cyl@180 Against-the-rule: -cyl@90

Sharmaindra, Bhutan

Uncorrected Astigmatism

• Symptoms frequently similar to

uncorrected hyperope- asthenopia and

headache

• In some- decreased visual acuity and

squinting to increase clarity

• Tilting head or habitual spectacle to induce

cylindrical component

Sharmaindra, Bhutan

Adaptation problem may occur-when marked changes in cylindrical power or axis or initial introduction of cylindrical power.

Younger the patient, easier the adaptation to the cylindrical. Converse true for older patients.

In low degrees of astigmatism, uncorrected against-the-rule affects visual acuity more than with -the-rule astigmatism

Even Low degree against-the-rule: Visual acuity may decrease ,so compensatation is advisable

Sharmaindra, Bhutan

High-Degree Astigmatism

• High degree astigmatism(>0.75D) causes

asthenopia as well as decreased vision

• They are usually with-the-rule or oblique.

• Pt exhibit ‘fixed squint’ or ‘squeezing of lids’

• Ascribed to genetic disposition

• Pressure of the upper eyelid on the cornea

With-the-rule

• Considered congenital

• Precursor to conical corneal distortionOblique

Sharmaindra, Bhutan

Cont….

• Patient may exhibit a ‘fixed squint’ and

‘squeezing of lids’ to obtain stenopaic slit

• Uncorrected for long time- may develop

meridional amblyopia

• Subjective refraction often difficult- because

patients are grown firmly adjusted to image blur or

strong habitual tendency to squint

• Correct high-degree astigmatism at the

earliest in childrenSharmaindra, Bhutan

Astigmatism Management

Type Visual acuity Symptoms Management Adaptation

Low Little reduction Near asthenopia,

distance driving

fatique

Prescribe if

symptomatic

Minimal

Small amount

with-the-rule

Little reduction Near asthenopia Prescribe if

symptomatic

Minimal

Large amount

with-the-rule

Reduction at far

and near

Blur vision at

distance and

near

Prescribe to

increaser visual

acuity

Pronounced

Against the rule Slight reduction

at far and near

Near asthenopia,

slight near blur

Prescribe if

symptomatic

Moderate

Oblique Little reduction Near asthenopia Prescribe if

symptomatic

Moderate

Sharmaindra, Bhutan

High spherical with low

astigmatism

• Necessary to estimate if cylinder is

causing patients symptoms

• Correct cylindrical or not?- initially matter

of diagnostic judgement

• Often large spherical correction provides

satisfactory acuity

• Patient symptoms on subsequent

evaluation will possibly indicate weather

the initially omitted should be prescribed

Sharmaindra, Bhutan

Relationship between Visual Acuity

and Refractive Error

Relationship between Visual acuity and refractive error

Snellen Visual Acuity Uncorrected Spherical Error(DS)

Uncorrected Cylindrical Error (DC)

6/6 (20/20) <= 0.25 <= 0.25

6/9 (20/30) 0.50 1.00

6/12 (20/40) 0.75 1.50

6/18 (20/60) 1.00 2.00

6/24 (20/80) 1.50 3.00

6/36 (20/120) 2.00 4.00

6/60 (20/200) 2.00- 3.00 >= 5.00

Sharmaindra, Bhutan

General guidelines to glass

prescription

• Aim for 6/9 or better.

• If less than one line improvement in vision there is

no real benefit in prescribing new glasses.

• Convergence insufficiency/ exophoria

Low myopic correction is helpful

Low hypermetropia-Do not prescribe

• Low hyperopes, especially the young-Do not

prescribe until symptomatic.

• Patient must always be counseled about the

intention of lens correction

Sharmaindra, Bhutan

Sharmaindra, Bhutan

Eg.

Case: 50 years old patient suddenly reveals a

pronounced shift towards less plus power or

more minus power that exceeds expected

change at this age.

• Directly prescribing new glasses, without determining the

cause for the change is NOT WISE

• Underlying causes may be recent trauma, blood glucose

fluctuation,cataract development and the like.

Sharmaindra, Bhutan