Keratometry and Dynamic Retinoscopy

KERATOMETRY & DYNAMIC RETINOSCOPY
Resource Person:
Dr. Sanjeev Bhattarai
Presenters:
Kamal Luitel
B.Optom,MMC

Keratometry (Ophthalmometry)
 Kerato= Cornea
 Metry= Measurement

Zones of the Cornea
Central Zone (apical zone/corneal
cap/central spherical zone) – 4 mm,
radius of curvature does not vary by
more than 1 D or 0.05 mm
• Area where refraction differs by
<0.25 D
• Paracentral zone – 4 - 8 mm.
• Peripheral zone – 8 - 11 mm.
• Limbal zone – rim of cornea, 0.55
mm wide.

What is Keratometry?
 Measurement of the anterior surface of the
cornea, across the fixed chord length, usually
2-3mm, which lies within the optical spherical
zone of the cornea.

Principle of Keratometry
 Anterior surface of the cornea- CONVEX MIRROR
 higher the Curvature = smaller Image size
 Image (ie.1st Purkinje image) formed in cornea
 With this image the radius of the curvature of the
cornea can be calculated.

Principle…
 Due to presence of involuntary miniature eye
movements during fixation of a eye
 The image formed by anterior surface of the cornea
also moves –
 Use of Doubling principle
CHALLENGE

DOUBLING PRINCIPLE
 Measurement of image height
 Doubling device - Plano prism
 Lateral disp. of doubled image = IMAGE HEIGHT
 Prism is moved along the optical axis until two images
are just touching
 At this point, the prismatic displacement is exactly equal
to the size of the image

DOUBLING SYSTEMS
 Fixed doubling
 Variable doubling
 Divided doubling
 Full doubling
• TYPES

DOUBLING SYSTEMS
 FIXED DOUBLING –
 VARIABLE
Image size and mire separation
 FIXED
Object height and doubling device distance
Ex. B & L , Topcon & Magnon
► VARIABLE DOUBLING –
 Fixed
 image size & mire separation
 Variable
 object size & doubling device
distance
Ex. Haag streit & Javal Schiotz

Conversion to corneal Power
 Radius of curvature can be converted into corneal
power using equation:
K = (n – 1) / r
K = corneal power (D)
n = refractive index of cornea = 1.3375
r = radius of curvature of anterior corneal surface (m)
Refractive index of the cornea is actually 1.376 but we use n = 1.3375 to
compensate for the -ve power of the posterior corneal
surface
∴K = 0.3375/r
Or for r in mm K= 337.5/r

Preparation of Keratometry
 Focus the eyepiece of the keratometer
for the examiner’s eye
 Set the adjustable eyepiece as far
counter-clockwise as possible
 Place a white sheet of paper in front of
the instrument’s objective lens to
retroilluminate the reticle (i.e., cross hairs)
 Turn the eyepiece clockwise until the
reticle is first seen in sharp focus

 Adjust height of patient’s chair & instrument to a
comfortable position for both patient & examiner.
 Instruct patient to place chin on chin rest & forehead
against forehead rest & adjust for the patient.
 Raise or lower chin rest until patient’s outer canthus is
aligned with hash mark on upright support of instrument.
 From outside instrument, roughly align barrel with
patient’s eye by raising or lowering instrument and by
moving it to left or right until a reflection of mire is seen
on patient’s cornea.
Preparation--- Adjust instrument for patient

Procedure:- Instruct patient
• Keep eyes open wide and blink
normally.
• Try not to move the head nor
speak.
• Look at the reflection of own eye in the
keratometer barrel.

Procedure cont..
 Look into the keratometer
and refine the alignment of
the image of the mires
(three circles) on the
patient’s cornea.

Procedure cont..
 Focus the mires and adjust
the instrument so that the
reticle is centered in the
lower right hand circle.

Procedure cont..
 Adjust the horizontal
power wheels until the
horizontal mires are in
close apposition.
 Adjust the vertical power
wheels until the vertical
mires are in close
apposition.

Oblique Astigmatism
2 + signs will not be aligned
Entire optical instrument is rotated till
the two plus signs are aligned
Procedure (for oblique astigmatism)

Extended Keratometry
 Range from 36 Ds to 52 Ds
 If K reading is very high
 For very high
 Place +1.25 Ds trial lens over eye piece –
increase range by 9 D or
 Multiply k reading by 1.185
 Ex: if with +1.25 D, dial reading = 49 D, Actual
K = +58 D
 Precise 49 D x 1.185 = + 58.07 Ds

To expand the range of measurement
 For very low
 Place – 1.00 Ds trail lens over the eye piece –
shift 6 D
 Multiply k reading by 0.840
 Ex: With –1.0Ds, dial reading = +38 Ds , Actual
reading = +32 Ds
 Accurate, +38 x 0.840 = + 31.92 Ds

Types of Keratometers
 One-position keratometers:
 that don’t require rotation through 90 ̊ in
order to measure the second principal
meridian
 the principal meridians are assumed to be at right
angles to each other.
 Doubling device variable & object height
constant.
 Ex: B & L or Magnon

Types of Keratometers
 Two-position keratometers:
 that require rotation through 90 ̊ in order to
measure the second principal meridian
 Fixed amount of image doubling & object
height adjusted.
 Ex: Javal-Schiotz Keratometer manufactured by
Haag -Streit

Types of Keratometer
(according to Operation)
1. Manual Keratometer
 Bausch & Lomb keratometer
 Javal-Schiotz keratometer
 Zeiss Opthalmometer
 Haag streit Opthalmometer
 Topcon OM-4 keratometer
2. Automated Keratometer
 IOL master
 Pentacam
 Orbscan
 Corneal Topography
 The Humphrey auto-keratometer

Caliberation
 Should be done regularly to ensure the
accuracy of “K” readings
 Mount a 5/8 inch steel ball bearing at the
position close to that normally of the patient’s
eye.
 The steel ball has a known radius of
curvature, which upon proper calibration of
the keratometer, can be correctly read.

Keratometry
 Calibration Index:
 The keratometer uses a specific
refractive index to account for both the
front and back surface corneal
curvatures.
 The calibration index adopted is normally
1.332 or 1.3375.

Uses of keratometer
 Measurement of corneal astigmatism. ie. Diff in
power btn two Principle meridians= the amount
of corneal astigmatism
 In contact lens fitting
 Assess integrity of tear film
 Monitors the shape of cornea- Keratoconus,
Keratoglobus.
 Assess refractive error in cases of hazy media.
 IOL power calculation.
 To monitor pre-& post –surgical astigmatism.
 Used for differential diagnosis of axial versus
curvatural anismetropia.

Limitations of Keratometry
 Measures refractive status of a very small central area of
cornea (3 mm), ignoring the peripheral corneal zones.
 Accuracy lost when measuring very flat or very steep cornea.
 Small corneal irregularities would preclude the use of
keratometer due to irregular
 High astigmatism.
 One position instruments assume regular astigmatism.
 Distance to focal point is approximated by distance to the
image.
 Autokeratometers do not evaluate the quality of cornea

Some of the troubles shooting tips
PROBLEMS SOLUTIONS
Keratometric mires not visible Align the instrument with the patients eye follow
cantal marking
Clarity of the mires are not stable. Allow the patient to blink and quickly take the
movement .
Not getting the Knob after full rotation. Adjust headrest by rotating its knob.
Patients gaze is changing. Occlude the other eye.
+ + & - - signs are not overlapping Patient is having irregular astigmatism
Only one – sign is visible Patients eye is drooping; widen the eye
Only one + sign is visible. Occluder is coming on the way; take it away

What is retinoscope ?
 Is an instrument used to determine the refractive error
 Is an objective method
What is retinoscopy ?
 The purpose of retinoscopy is to obtain an objective
measurement of patient’s refractive state
 it is based on the fact that when the light is reflected from a
mirror into the eye, the direction in which the light will
travel across the pupil will depend upon the refractive state
of the eye

Types of retinoscopy
 Static retinoscopy: the patient is looking at a
distance object, with accommodation relaxed
 Dynamic retinoscopy: the patient is looking at a
near object ,with accommodation active
 Near retinoscopy: the patients is looking at a near
object, with accommodation relaxed

Dynamic retinoscopy
 Objectively determines the point that is conjugate
to the retina when the pt. is viewing a particular
target
 NO WORKING DISTANCE POWER IS ADDED OR
SUBSTRACTED FROM THE FINDING

Movements
same as that of static retinoscopy
 With movement : eye conjugate to a point either
behind the eye or behind the retinoscope.
 Against movement : eye conjugate to a point
between the eye (patient’s) and retinoscope.
 Neutrality : eye conjugate with retinoscope

History
 Early 1900s, various investigators began utilizing the
retinoscope to determine the amplitude or status of
accommodation in non-verbal patients - term
dynamic retinoscope emerged
 A.J. Cross is credited with introducing the basic theory
and method for dynamic retinoscopy
 Sheard, Nott, and Skeffington - elaborated on the
theory and procedure

Goals
 to determine accommodative Response
 also helped to determine the most appropriate near
prescription with testing conditions
 Reveals the degree to which accommodation is
fluctuating when attending to a near target & if the
eyes are balanced equally at near
 provide the information and insights regarding the
patient’s abilities and level of visual processing at the
chosen distance

Accomodation
 Accomodative stimulus is defined by the near target
stimulus
 Because of depth of focus and depth of field the
accommodative response is generally less than the
stimulus
 Near point is usually located around 10-17cm
beyond near target at 40cm

Accommodation
 Accomodative demand is provided by the target
distance as well as the refractive error
 Over minus or under plussed: has extra
accommodative demand required to see target clearly
 Under minused :does not have to accommodate as
much

Accommodation
 Accommodative response is a measure of the actual
accommodation that is present
 If your accommodative system likes to “hang
out”
 Right on the target accommodative
response = stimulus
 In front of the target accommodative
response >stimulus (i.e. accommodative lead)
 Behind the target accommodative
response< stimulus ( i.e.accommodative lag)

Lag of accommodation
 Time lapse between the presentation of an
accommodative stimulus and occurrence of the
accommodative response
 Average time
 - Far to near accommodation is 0.64 seconds
 - Near to far accommodation is 0.56 seconds

Lag of accommodation
 Accommodative lag = accommodative demand (
+2.50D at 40 cm) – accommodative response
 Lags are greater when closer test distances are used
 Lag of accommodation exhibits a slow but
progressive increase to adult levels
 Binocular accommodative system normally respond
with only +1.75D to +2.00D of increased plus power

 Normal Lag: +0.50 or +0.75 diopters
 High Lag: +1.00 diopters or higher
 Lead : +0.25 diopters or less

Lag > +0.75D/ High Lag
 Inadequate accommodative response:-
 as a result of :- near esophoria
poor negative vergences
accommodative insufficiency
uncorrected hyperopia
Patient is Overminused

Low Lag /lead of accommodation <
+0.50
 Overaccommodating
 As a result of :- near exophoria
spasm of accommodation
Over Plus Correction
inadequate positive vergences

Types of dynamic
retinoscopy
Monocular Estimation Method
(MEM)
Nott retinoscopy
Bell retinoscopy

MEM (monocular estimated
method)
 Founder Dr. Harold Haynes
 Clinician neutralize the reflex of the eye while
patient accommodates to fixate a target placed at
the patient’s customary reading distance (usually
at 40cm)

Materials
 series of cards with a central aperture mounted on
a retinoscope
 cards can have printed letters, or words, or pictures
that range in size from 20/160 (6/120) to 20/30 (6/9)
 Arranged around the aperture

Procedures
 instructed to keep the targets clear
 sweeps the retinoscope beam
 observes the motion of the retinoscopic reflex
 quickly interposes a trial lens at the spectacle plane

Interpretation
 “lag of accommodation” is the amount of plus
lens that neutralizes the reflex
 has been found to accurately measure the lag
of accommodation in an objective manner
Example
If the retinoscopic reflex is neutralized by
+1.75D then lag is
ADD = +1.75 – (+0.75)
= +1.00

Limitation
 Plus lenses – relaxation of
accommodation – accommodative
response measured by this value found to
be 10% less
 No longer than one fifth of a second

Bell retinoscopy
 Developed by Drs. W.R. Henry and R.J. Appel
 Evaluate the performance of the
accommodative system under moving & real life
conditions in free space
 cognitive demand is low
 term “Bell” is used because the procedure was
done originally using a cat-bell suspended on a
string.

Materials
 Three dimensional viewing target
 a small, highly reflective bell dangling from
String – replaced with a Wolff Wand(½ inch
diameter, metal ball mounted on the end of a
rod)

Procedures
 wand is held by the examiner
 moved closer to and farther from the patient -
slower than 2 inches/sec
 retinoscope is positioned at a fixed distance of 50
cm (20 inches)
 patient fixates the target and the examiner notes
the direction of the reflex

Contd…
 target is moved closer to the patient there will
be a point where the motion changes from
“with” to“against’’
 Target is again moved away from patient until
with motion is observed

Interpretation
 The two measurements are recorded as a fraction
e.g. 30/40 (meaning that the inward change from
“with” to “against” occurred at 30cm and the
outward change from “against” to “with” occurred
at 40cm.
 The expected values for Bell retinoscopy are:
Inward shift at 42.5 to 35cm and outward shift at
37.5 to 45cm.
 If the lag of accommodation does not fall within
these ranges, the procedure is repeated with plus
lenses. Lenses which normalize these ranges are
considered an acceptable nearpoint prescription.

Contd..
 eye movement control can be assessed by
judging the extent to which the ball can be
fixated
 eye-hand coordination can be evaluated by
asking the patient to touch the Wolff Ball during
the procedure
 NPC can be determined by the normal means
Limitation
patient converges - scoping more off axis

Nott’s retinoscopy
 developed by I. S. Nott in the 1920s
 main purpose is identical to the MEM method
 cognitive demand is moderate

Materials..
 reduced block of 20/20 (6/6) letters is
placed at 16 inches (40 cm) from the patient

Procedures
 Patient wearing their best correction is
instructed to view a detailed and high contrast
target placed on the retinoscope
 Retinoscopic reflex is examined from the plane
of target and retinoscope is moved closer or
farther away from the target until neutrality is
achieved

Interpretation
 Dioptric difference between these two distances
equals the lag of accommodation
Example
Distance from the target to spectacle plane = 40cm
Distance from retinoscope to spectacle plane = 50cm
Lag of accommodation = +2.50D – 2.00D
= +0.50D

Book retinoscopy
 Also known as Getman
retinoscopy.
 Developed at Gesell institute of
child development at Yale
university.
 Develop to obtain information
about the visual processing of
nonverbal infants .
 Cognitive demand is high.

 Getman and Kephart described the following response levels
with this technique.
A. free reading level : Desirable , reflex varies from neutral to
with
B. Instructional level : more demanding than the free reading
level , reflex is a varying fast against motion. •
C. Frustration level : Even though the subject is “focused” on
the page he is not interpreting the information properly slow
against motion
 Reflex color is bright and white when the words are
understood.

Contd..
 Reflex color is more pink and dims slightly if
the patient is struggling to comprehend a
word or passage.
 Reflex color is dull and brick colored when
the patient has given up on comprehending
a word or reading passage.

Cross retinoscopy
 Andrew J. Cross (1911) •
 Start with static retinoscopy finding .
 Patient made to view target at 40cm .
 Examiner performs retinoscopy adding plus lens
till neutrality.
 A alternative to cycloplegic refraction
 Method of adding plus lens power to obtain a
reversal

 Determining the correction in cases of
Astigmatism
Presbyopia
Subnormal accommodation in young
patients

Limitation
 A measurement of negative relative
accommodation
 Plus power recommended – patient
would not persist

Sheard’s method
 Charles Sheard (1920)
 Introduced the concept of “ Lag of accommodation”
 add plus lens power until neutrality occurred

Tait’s method
 Tait(1953)
 Working distance = 33cm
 Fogging with a considerable amount of plus
lens power and then approaches neutral by
reducing the plus lens power
 Found an average of approximately +1.50 D
more than sheard system , thus total lag of
accommodation = +2.25 D
 Close to +2.50D i.e Negative relative
accommodation.

Low neutral and high neutral
methods
Sheard ( low neutral method)
 The end point is the least plus power required for
a neutral reflex to be observed.
Cross ( high neutral method)
 Addition of plus power beyond neutrality until a
reversal occurs.

Stress point retinoscopy
 developed by Harmon and Kraskin
 evaluate the response of the entire
organism to stress
 in stress-point retnoscopy - looking at
the change in reflex quality
 Cognitive demand is moderate to high

 reasoning behind stress-point retinoscopy is that
vision is intimately related to the whole body and that
a physiological change in stress occurring in the body
can be perceived through a change in the retinal
reflex
 Three things occur when near-point stress is
experienced
 Firstly - there is a change in the individual's pulse
 Secondly - there is an inner canthal twitch and
 lastly - change in the colour of the retinal reflex is
observed

Procedures
 Wolff ball is moved closer to the patient - looks
at which distance the reflex "pops"
 initially brightened and then became dull and
finally brightened again - termed "popping" of
the reflex - about 4 inches in front of the patient
 distance is noted and then different lenses are
placed binocularly and the procedure is repeated

 ideal lens is the one which makes the stress point
as close to the subject as possible
 more desirable to have the stress-point closer to
the patient - they are not working under
physiological stress
 For example; if the stress-point of a subject is
40cm and they habitually read at 30cm they
would be under constant near-point stress

 plus lenses move the stress-
point closer to the subject and
minus lenses move it away
 in children the stress-point
should be 10cm closer to the
subject than the Harmon
distance.
 In adults, the stress point is 20
to 22.5cms from face.

Near retinoscopy by
Mohindra
 Near retinoscopy by Mohindra in 1977.
 For use in determining the refractive state of
infants and children
 The stimulus or fixation is the dimmed light
source of the retinoscope in a darkened
room.
 The retinoscope is held at a distance of 50
cm with hand-held trial lenses.

 Near retinoscopy differs from other forms of
dynamic retinoscopy in the following ways:
1. it is performed in complete darkness , the only
illumination in the room is supplied by retinoscope
with child fixating at retinoscope light .
2. It is monocular procedure that is eye not being
examined is occluded.
3. The adjustment factor of -1.25 D is algebrically
combined with the spherical component of the
gross sphero - cylindrical lens powers.

Source of error
 Same as those with static: scissors, small
pupils, dim media (cataracts, etc.), angle
 More sensitive to physical arrangement for
the measurement (distance, lens adaptation),
instructions given and patient’s cooperation
 Changes in patient’s fixation or accommodative
level (often related to failure to understand task
or to cooperate)

 Patient looking at a target at a different
distance than requested
A +0.50 to +0.75 lag is not normal if not
testing at 40cm
Lag increases as fixation distance is
reduced
 Adaptation to lenses with MEM: relaxes with
plus lenses, stimulates with minus lenses

Refrences..
o Clinical Procedures in Optometry by J.D. Bartlett, J.B.
Eskridge, J.F. Amos
o Theory and Practice of Squint and Orthoptics by
A.K.Khurana
o Borish’s Clinical Refraction by W.J. Benjamin
o Internet

Keratometry and Dynamic Retinoscopy

Keratometry and Dynamic Retinoscopy

In this document