ASTIGMATISM
ASTIGMATISM: PROGRAM
Astigmatism: program
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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
135°
160°
180°
110° 90° 70°
45°
20°
0°
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)
135°
160°
180°
110° 90° 70°
45°
20°
0°
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°
135°
160°
180°
110° 90° 70°
45°
20°
0°
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
Example 1: +3,50-2,00x180º
a. Optical cross:
c.
+3,50-2,00=+1,50
+3,50
b. The weaker meridian (flattest)
is the one at 180º
Transposition formula:
+1,50+2,00x90º
d. Classification according to main
meridians:
Direct astigmatism
c.
Classification according to
refraction:
Compound hypermetropic
astigmatism
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/Atoz/e
ncy/astigmatism.jsp
• http://www.eyemdlink.com/Condition.asp?Co
nditionID=250