ASTIGMATISM

ASTIGMATISM: PROGRAM

Astigmatism: program

• Definition• Epidemiology• Classification• Symptoms and signs• Methods of measurement• Prescription criteria• Resolution of clinical cases

ASTIGMATISM: DEFINITION

Astigmatism: definition

• Refractive condition in which the image of an object is not formed on a solo plane, since the different ocular meridians are of distinct potency (distinct focal distances).

• Habitually, there are 2 main meridians, of maximum and minimum potency, and perpindicular to one another.

Astigmatism: epidemiology I

• The majority of eyes show weak astigmatism.

• Astigmatism can present itself in an isolated form or, with greater frequency, associated with myopia or hypermetropia.

• Between 2-6% of the population has an astigmatism > 2,00 dioptre

Astigmatism: epidemiology II

• Changes with age– An significant percentage of newborns show inverse

astigmatism.– During the first few months of life the astigmatism

dimishishes gradually.– At school age, direct astigmatism of low magnitude

tends to exist.– Throughout youth and adulthood, astigmatisms do not

tend to pass through any important changes.– From the 50-60 and on, increases in inverse

astigmatism or decreases in direct astigmatism exist.

Astigmatism: epidemiology III

• Genetics – In corneal astigmatisms >1,50/2,00D there is a

strong genetic component

• Environmental factores– The use of rigid contact lenses can induce variations

in the corneal astigmatism of 2 or more dioptres.

• Various authors suggest that astigmatism and its variations are the consequence of the relationship between the palpebral tarsus and the cornea.

Astigmatism: classification I

• According to the regularity of the corneal surface

• According to the direction of the main meridians

• According to the refraction of the eye

Astigmatism: classification II

• According to regularity of the corneal surface– Regular (habitual):

• Main meridians are perpindicular to one another

– Irregular (infrequent): • Main meridians are not perpindicular• Curvature of one of the meridians is not

constant

Spherical cornea

Regular corneal astigmatism

Irregular corneal astigmatism

Astigmatism: classification III

• According to the direction of the main medians of the astigmatism of the eye– Direct astigmastism or “in favor of the rule”

• The horizontal meridian is the flattest• The horizontal meridian is less powerful• The axis of the refractive astigmatism,

expressed in negative potency, is around 0º-180º (±20º)

• It is the most frequent

180°

135°

0°

90°

45°

160°

110°

20°

70°

Astigmatism: classification IV

• According to the direction of the main meridians of the astigmatism of the eye– Inverse astigmatism or “against the rule”

• The vertical meridian is the flattest• The vertical meridian is less powerful• The axis of the refractive astigmatism,

expresed in negative potency, is around 90º (±20º)

180°

135°

0°

90°

45°

160°

110°

20°

70°

Astigmatism: classification V

• According to the direction of the main meridians of the astigmatism of the eye– Oblique astigmatism

• The main meridians are between 20° and 70° and between 110° and 160°

180°

135°

0°

90°

45°

160°

110°

20°

70°

Astigmatism: classification VI

• According to the refraction of the eye:– Simple: only one meridian is ametrope (only astigmatism

exists).• Example 1: -0,50x90º (simple myopic astigmatism)• Example 2: +1,25x5º (simple hypermetropic

astigmatism)– Compound: the two meridians show the same type of

ametropia.• Example 1: +2,50+1,75x15º (Compound hypermetropic

astigmatism)• Example 2: -1,00-0,75x30º (Compound myopic

astigmatism)– Mixed: the two meridians are ametropic and of a different

type.• Example 1: +0,50-1,50x10º (the potency of one

meridian is +0,50 and the other -1,00)

Astigmatism: classificaction VII

• In the following schemes the formation of images in the retina according to the eye’s refraction are shown:

Simple H. Astig.

Compound H. Astig.

Mixed Isodioptric Astig.

Simple M. Astig.

Compund M. Astig.

Mixed Isodioptric Astig.

Astigmatism: classification VIII

a. Optical cross:

b. The weaker meridian (flattest) is the one at 180º

c. Transposition formula:+1,50+2,00x90º

d. Classification according to main meridians:Direct astigmatism

c. Classification according to refraction:Compound hypermetropic astigmatism

Example 1: +3,50-2,00x180º

+3,50-2,00=+1,50

+3,50

Astigmatisms: symptoms and signs I

• The symptoms tend to depend on the magnitude of the astigmatism.– Moderate and evelated astigmatism

•Blury vision in DV and NV•Symptoms of visual fatigue,

headache, ocular irritation, etc.•Symptoms of image distortion and

absence of comfort upon initial use of lenses that compensate for astigmatism

Astigmatisms: symptoms and signs

– Low astigmatism (<1,50D)•The VA does not tend to be very

affected, but it is difficult to determine it precisely

•Visual fatigue associated with prolonged use of vision

• Inverse astigmatisms tend to produce greater symptomology than direct ones

•Significant difficulties to adapt to the new prescription do not tend to appear

Astigmatisms: symptoms and signs III

• Imprecision in the determination of VA– Low astigmatisms < 1.50 D

• If it is hypermetropic the VA can easily reach 20/25 or even 20/20

• If it is myopic the VA is affected more and is near 20/30– Moderate to high astigmatisms (≥1,50-2,00 D)

• If it is hypermetropic the VA is diminished, but not as much as it would be if it were myopic

• If it is hypermetropic the diminishment of the VA will be ≈ in DV and in NV

• If it is myopic the diminishment of the VA will be greater in DV than in NV

– Oblique astigmatisms demonstrate the worst VA• Comparing the same level, the VA in the oblique astigmatism

< VA in inverse astigmatism < VA in direct astigmatism

Astigmatism: methods of measurement I

• Keratometry: determination of the power of the main meridians of the cornea– Hemholtz– Javal– Automatics

• Corneal topography: determination of the morphology of the anterior corneal surface

Astigmatism: methods of measurement II

• Keratometry:– Clinical technique to measure the radius of

the curvature of the anterior face of the cornea.

– Based on the reflection of light in the cornea (convex mirror). It gives a small image, straight and virtual, of the object (“look”) which is of a known size

– The measurement is done in a diameter of 3 mm around the visual axis

Astigmatism:methods of measurement III

• Hemholtz’s keratometer

• Javal’s keratometer

• Automatic keratometers

Astigmatism: methods of measurement IV

• Corneal topography:– Can measure large areas– Is a quantitative evaluation– High resolution (approx. 5000 puntos)– Lots of presentation options

Astigmatism: methods of measurement V

• Clinical use– Informs on the quality/integrity of the corneal

surface• regular : clear and regular vision and the main

meridians are perpindicular• Irregular: irregular or distorted vision. The precise

determination of the main meridians is difficult

– Help in the determination of approximante astigmatic refraction • In cases of minimal collaboration• When ocular means are unclear

– Essential help in the selection of parameters for contact lenses

Astigmatism: methods of measurement VI

• Limitations of keratometry:– An astigmatism determined through keratometry

corresponds to the anterior face of the cornea.• Astigmatism also exists in the posterior face of

the cornea, being crystalline and even retinal.– The design of the keratometer is based on spherical

surfaces and this leads to errors in the measurement– The visual axis frequently remains displaced from

the geometric center of the cornea– The measurement is done is a 3 mm diameter

around the visual axis

Astigmatism: methods of measurement VII

• The total astigmatism (TA) is the sum of:– Astigmatism of the anterior face of the cornea

(FHC)– Internal physiological astigmatism (IPA)

• Javal’s rule:– In general, the IPA has an approximate value

of-0,50x90º

• Example 1:– FHC = -1,75x180º– Which TA is expected, if we follow Javal’s rule?

Astigmatism: prescription criteria I

• Age of the patient:– Small children (from 2 to 6): total

compensation if the VA is believed to be compromised. There tends to be good tolerance.

– Children (from 6 to 12): total compensation continues being recommended, but the tolerance tends to lessen.

– Adults: Variable tolerance to the changes:• If there are great improvements of the VA:

prescribe for the astigmatism• Oblique axes: partial compensation of the

astigmatism

Astigmatism: prescription criteria II

• Magnitude of the astigmatism:– The greater the astigmatism, the lesser the

tolerance to the total prescription– Elevated astigmatisms tend to be congenital

or of early appearance. If no prescription is made, they can provoke ambyopia.

– In cases of irregular elevated astigmatisms the best VA is obtained through the use of rigid contact lenses.

– Small astigmatisms (<1,00D) do not tend to require serious consideration.

Astigmatism: prescription criteria III

• Habitual astigmatic prescription:– When an adult patient does not show symptoms with

his/her habitual compensation, it seems wise not to realize important changes.

– Consider changes when symptoms, marked reduction of the VA or reduction of stereopsis exists.

– For adults that have never had astigmatism:• Reduce the cylindrical power, maintaining the

spherical equivalent.• With the passage of time try to align the level of

prescription to the refraction of the person.

Astigmatism: prescription criteria IV

• Method of the spherical equivalent (SE)– Method to reduce the power of the cylinder but

allowing that, without additional accomadative force, the circle of least confusion is situated over the retina.

– Half of the magnitude of the unprescribed cylinder (SE) sums up algebraically to the value of the sphere

– Example 1: +2,50-3,50x85º• 2,00 dioptres are prescribed• SE of the unprescribed astigmatism = -1,50/2 = -

0,75• The SE adds up to the value of the sphere: +2,50

+(-0,75) = +1,75• Final prescription: +1,75-2,00x85º

ASTIGMATISM: CASES

Astigmatism: case 1-I

• MJH, 12-year-old child. Student.• MC: Occasionally shows that he does

not see well in NV. Visual tiredness when studying. Occasional ocular hyperaemia.

• PH: Has never worn glasses. Previous pediatric check-ups. No illnesses or ingestion of medication.

• FH: Unimportant.

Astigmatism: case 1-II

• Habitual VA in DV and NV:– RE: 20/20-2; NV: 20/25– LE: 20/25; NV: 20/25

• Binocularity in habitual conditions:– Cover test:

• DV: ortho• NV: ortho

– Proximal convergence: 5/8cm

Astigmatism: case 1-III

• Retinoscopy:– RE: +1,00-1,50x180º– LE: +0,50-1,50x5º

• Subjective DV and VA:– RE: +0,50-1,25x175º; VA: 20/20– LE: +0,25-1,25x5º; VA: 20/20– NV with the subjective: VA 20/20 in both eyes. Good

comfort

• Amplitude of accomodation with the subjective:– RE: 8cm≈12,5D– LE: 8cm≈12,5D

• Ocular health exams: within normal limits• Color vision: normal

Astigmatism: case 1-IV

• Complete diagnostic of the case• Proposed treatment and plan of check-

ups• Possible evolution of the condition

Astigmatism: case 1-V

• Complete diagnostic of the case– Low hypermetropia present in both eyes– Direct astigmatism in both eyes:

• According to the conoid: mixed in both eyes

– Binocularity and accomodation: within the normal limits

– Other tests within normal limits

Astigmatism: case 1-VI

• Proposed treatment:– Glasses with the value of the subjective:

• RE: +0,50-1,25x175º• LE: +0,25-1,25x5º

– Use mainly for school and work in NV. – They can be worn for all uses.– Revision in one year or before if new

symptomology appears.– Explain the condition to the patient and

his/her parents.

Astigmatism: case 1-VIII

• Possible evolution of the condition:– Stability of the stigmatism– Slight diminishment (or stability) of the

hypermetropia

Astigmatism: case 2-I

• JJB, 25-years-old. Waiter.• MC: Notices sporadic diminishment of

vision, as much in DV as in NV. Greater difficulty at the end of the day.

• PH: 15 years ago he was prescribed glasses but they were very uncomfortable and he never wore them. No illnesses or ingestion of medication.

• FH: Irrelevant.

Astigmatism: case 2-II

• Habitual VA in DV y NV:– RE: 20/40; NV: 20/40– LE: 20/30; NV: 20/25

• Binocularity in habitual conditions:– Cover test:

• DV: ortho• NV: ortho

– Proximal convergence: as far as the nose

Astigmatism: case 2-III

• Retinoscopy:– RE: +3,00-4,00x5º– LE: +1,50-2,50x20º

• Subjective DV and VA:– RE: +2,75-3,50x5º; VA: 20/25– LE: +1,00-2,00x15º; VA: 20/20+

– DV and NV with the subjective: notices better vision but is not comfortable. A reduction of the graduation is tried and tolerance is greater:

• RE:+2,00-2,00x5º; VA: 20/25-2

• LE: +0,75-1,50x15º; VA: 20/20• Amplitude of accomodation with the second refraction:

– RE: 14cm≈7D– LE: 11cm≈9D

• Exams of ocular health: within normal limits– Central fixation in both eyes

Astigmatism: case 2-IV

• Are other tests necessary for a correct diagnosis and treatment?

• Complete diagnosis of the case• Proposed treatment and plan of check-

ups• Possible evolution of the condition

Astigmatism: case 2-V

• Are other tests necessary for a correct diagnosis and treatment?– VA with a stenopaic disc?– Keratometry?

Astigmatism: case 2-VI

• Complete diagnosis of the case– Hypermetropic and astigmatic anisometropia– Hypermetropia becomes apparent in both

eyes (RE>LE)– Direct astigmatism in both eyes (RE>LE)

• According to the conoid: mixed astigmatism in both eyes

– Slight amblyopia in the RE– The rest of the tests within normal limits

Astigmatism: case 2-VII

• Proposed treatment:– Prescribe glasses with the determined

equivalent:• RE:+2,00-2,00x5º • LE: +0,75-1,50x15º

– Use as much in DV as in NV. – Explain the condition to the patient– New revision in 3-4 months

Astigmatism: case 2-VIII

• Possible evolution of the condition:– Significant changes to the value of the

refractive defect are not expected in the first few years

– Periodical check-ups are necessary since we want to get the prescription as near as possible to the value of the refractive defect

– Greater dependence on glasses with the passage of time

ASTIGMATISM: BIBLIOGRPHHY

Astigmatism: bibliography

• Amos JF. Diagnosis and management in vision care. Butterworth-Heinemann, 1987

• Grosvenor T, Flom MC. Refractive anomalies. Research and clinical applications. Butterworth-Heinemann, 1991

• Brookman KE. Refractive management of ametropia. Butterworth-Heinemann, 1996

• Werner DL, Press LJ. Clinical pearls in refractive care. Butterworth-Heinemann, 2002

Astigmatism: Bibliography

• http://en.wikipedia.org/wiki/Astigmatism• http://www.healthatoz.com/healthatoz/A

toz/ency/astigmatism.jsp• http://www.eyemdlink.com/Condition.as

p?ConditionID=250