This document discusses refractive errors and their management through prescription of corrective lenses. It begins by outlining the typical distribution of refractive errors in a normal population. It then discusses why myopia is more commonly seen in optometry clinics compared to other refractive errors. The document provides guidelines for prescribing lenses to correct myopia, hyperopia, astigmatism, and other refractive errors. It emphasizes undercorrecting initially to aid adaptation and only prescribing lenses when vision or symptoms can be improved.

1. Analysis, Interpretation, and
Prescription for the
Ametropias
Indra P Sharma
Optometrist
MRRH, Ministry of Health
Bhutan

2. Refractive Error distribution in
normal population
Hypermetropia
50%
Myopia> 1D
Myopia< 1D
13%
Emmetropia
25%
12%
Ref: Borish IM, Clinical Refraction , ed 3 pp 861-937
Sharmaindra, Bhutan

3. 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

4. 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

5. Refraction based on age
6
5
4
3
2
1
0
0 5 10 20 30 40 50 60 70 80 90
Refraction (+ D)
Age(in years)
Sharmaindra, Bhutan

6. Emmetropia
• Parallel light form infinity focus on the
retina, with accommodation relaxed
Sharmaindra, Bhutan Accommodation

7. 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

8. Synkinetic traid of near reflex
Accommodation
Convergence
Pupillary
Constriction
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9. Prescription for Myopia
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10. Myopia
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11. 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

12. 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

13. Sharmaindra, Bhutan

14. • 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

15. • 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

16. Prescription for Hypermetropia
Sharmaindra, Bhutan

17. Sharmaindra, Bhutan

18. • 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

19. 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

20. 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

21. Prescribing guidelines for
Hyperopic Compensation
Consideration 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, near
asthenopia 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 exo
problems
Sharmaindra, Bhutan

22. • As a general thumb rule,
‘prescribe for the hyperope to answer the
patients complaints’
Sharmaindra, Bhutan

23. 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,and
malingerers.
* Cycloplegic refraction values not necessary
prescribed, but gives starting point for
subjective refraction
Sharmaindra, Bhutan

24. Guidelines in Cycloplegic
Refraction Prescribing
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.
Prescription
History
For first prescription, plus power should
be cut from wet refraction for adaptive
purpose
Residual
accommodation
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
Sharmaindra, Bhutan

25. Prescription for Astigmatism
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26. Image formation in astigmatism
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27. • 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

28. Strum’s conoid
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29. With-the-rule: -cyl@180 Against-the-rule: -cyl@90
Sharmaindra, Bhutan

30. 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

31. 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

32. High-Degree Astigmatism
• High degree astigmatism(>0.75D) causes
asthenopia as well as decreased vision
• They are usually with-the-rule or oblique.
• Ascribed to genetic disposition
• Pressure of the upper eyelid on the
cornea
With-the-rule
• Considered congenital
Oblique • Precursor to conical corneal distortion
• Pt exhibit ‘fixed squint’ or ‘squeezing of lids’
Sharmaindra, Bhutan

33. 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 children
Sharmaindra, Bhutan

34. 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

35. 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

36. 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

37. 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

38. Sharmaindra, Bhutan

39. 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