Investigative Optometry

Visual acuity
Types of VA:
a) Detection acuity (Catford Dot Test, Dot Acuity Test, Cake decoration Test)
b) Discrimination Acuity( Snellen’s E-Chart)
c) Recognition Acuity ( Snellen’s letter Chart)
d) Vernier Acuity

Theory behind Visual Acuity
Receptor Theory
- At the fovea, the cones are separated by 2µm,
corresponding to a visual angle of 25 seconds of arc.
(50” of arc)
Resolution theory; Rayeligh’s criterion
• 47” of arc = MAR

Visual Acuity Measurement
VA measurement in infants: (Birth- 14 months)
Infants:
i) Light response
ii) OKN (Optokinetic Nystagmus Drum)
iii) VEP (Flash VEP, Pattern VEP, Sweep VEP (precise response across
various Spatial Acuity)
iv) Preferential Looking test ( Forced choice preferential, Cardiff Acuity
Test, Lea Paddle Test)
v) Catford Drum test
vi) Hundred or thousands test/Cake
Decoration test (6/24)
vii) Prism base out test (10 PD)
viii) Fixation behaviour Test (CSM-Central
Steady Maintained)

VA measurement in Toddlers:
(14 months-21/2yrs)
a) Dot VA Test
b) Coin Test
c) Boek’s Candy Test (6/60)
d) Sheridan Ball Test
e) Miniature Toy Test
f) Worth’s Ivory ball Test
g) Marble Game Test

VA measurement in Pre-School:
(21/2 to 5 yrs)
a) Sheridan Gardiner HOTV Test)
b) Isolated Hand Figure Test
c) Allen Preschool Test
d) Broken Wheel Test
e) Lea symbols
f) Lighthouse picture Cards
g) Ffooks Tests
h) Patti Pics
I) Illiterate E-cut out/Tumbliong E-Test
j) Kay Picture Test

VA measurement > 5Yrs:
a) HOTV Test
b) Lanolt’s C ring
c) Bailey Lovie Chart
a) Snellen’s Letter Chart
b) Snellen’s E chart

Bailey Lovie Log MAR Vision Test
• Log of the MAR used to notate the acuity
• Used in research and low vision
• Every letter read counts as 0.02 of each line (every line has 5 letters)
• Letter to Letter scoring system
• Each line = 0.1 log unit &
- 25% larger, than the preceding line
- Every third line is double/ half sized = 0.3 Log unit diff.
• Usual testing distances : 4 or 3 meters (depending on charts)
• Can be done at 2 or 1.5 m,
VA value = Log MAR score + 0.3
• The log MAR chart Optotypes
Landolt C, Tumbling E ,Numeric and Alphabet optotypes

Near Vision Test:
In Children:
a) Reduced Snellen’s Test
b) Log MAR Near Test Chart
c) MCclure Reading book
d) Reduced Kay Picture
In Adult:
a) Reduced Snellen’s Test
b) Log MAR Near Test Chart
c) N-Chart
d) M- Chart
e) Jaeger continuous Reading
Chart

Distance Unit Conversion
Suppose Snellen VA is 6/60.
Decimal : Just divide i.e. 6/60 = 0.1
MAR value : Just invert Snellen fraction i.e. 60/6= 10
Log MAR: Log of MAR i.e. Log 10 = 1
20/20 format: Multiply Snellen fraction by 20/20 i.e. don’t touch 20 in
numerator
6/60x20/20 = 20/200

Near Unit Conversion
If Letter size is 2.90mm what is M and N size of letter?
(Note: 1M=1.45 mm)
Hence, Letter size is: 2.90/1.45 mm = 2M
N size: Just multiply M size by 8 i.e. 2 x 8 = 16 i.e. N16
For N to M conversion just divide by 8:
N24 is how much M?
24/8 = 3M

Contrast Sensitivity
• Ability to distinguish difference in Luminance Profile.
• Thus ability to separate target from Background.
MICHELSON FORMULA:
Contrast: (L max- Lmin) / (Lmax+Lmin)
Lmax = Luminance on the lighter surface
Lmin = Luminance on the darker surface
WEBERS FORMULA:
Contrast: : (Lt- Lb) / Lb
Where, Lt & Lb are the luminance of the Target and Background
respectively

Pelli-Robson CS Chart:
- Test done at 1 meter
- Each letter read value adds by 0.05
- First three letters carry 0 score
- Letter size is set at 4 Cycles/degree frequency
Score of 2= Normal young adults
Score <1.5 = Visual impairment
Score <1.0 = Visual disability, have difficulties in reading or walking

• Bailey Lovie (10%) Low contrast charts: Vistech contrast Test charts:

• MELBOURNE EDGE TESTFUNCTIONAL ACUITY CONTRAST TESTING
(FACT)

THE ARDEN PLATE TEST REGAN LOW CONTRAST LETTER CHARTS

CAMBRIDGE LOW CONTRAST GRATING
1000 series
• The test distant 2.5 meters
• Test the spatial frequencies of
3,7,12,and 18 cycles/degree

MARS letter contrast sensitivity
test

Hiding Heidi for children
Normal Contrast
Range around 2.5%

Color Vision
Color Vision Deficiency: (Trichromats)
Protanomaly: Red deficiency
Deutanomaly: Green deficiency
Tritanomaly: Blue Deficiency
Absence of single color
Cone(Dichromat):
Protanopia: Red cone absent
Deutanopia: Green cone absent
Tritanopia: Blue Cone absent
Monochromat: Absence of 2 cone
Achromat or Rod monochromat: All Cone Absent

Color Vision Tests
a) Pseudo-isochromatic tests
b) Color arrangement Tests
c) Color naming Test
d) Color mixing/matching Test

Pseudo-isochromatic tests
• Ishihara Color Vision Tests
• Dvorine Color Vision Test
• American Optical Handy Rand Ritter

Ishihara Color Vision Tests
- For Red-green defect only
Plates ( 38 Plates edition)
- Demonstration: (1) Even total Color blind can see)
- Transformation: (2-9) Color vision and normal person see different
digits/path
- Vanishing : (10-17) Color vision defect can’t see anything
- Hidden digit: (18-21) Normal can’t see number but Color vision defect
sees)
- Diagnostic: (22-25) To distinguish between Protan and Deutan Defect
24 plated edition
Plates missed 2 or less – normal CV
38 plated edition
Plates missed4 or less – normal CV
Testing /distance: 75-100 cm
Best VA: better than 6/60
Illumination-: 500-600 lux
Plates form 26-38 is Winding Path plates

Dvorine Color Vision Test:
- Contains total 23 plates like ishihara
- For Red-Green Defect only
15 plates: Arabic Numerals
8 plates: Winding path
• 3 or more plates misses: Color Vision defect
• Severity of defect can be measure as Mild, Moderate and Severe

American Optical Handy Rand Ritter (AOHRR)
- For Red, green and Blue color vision Defect
- Total 24 plates
- 10 for screening (4 demo, 6 Vanishing) and
remaining for diagnosis and severity
- It consists of geometric figures rather than
numbers.
Now discontinued and not
available

Color arrangement Tests
• Fransworth D-15 test
• Munsell 100 Hue test

Fransworth D-15 test:
• Total 16 caps ( 1 reference cap other 15 test caps)
• Red, green and Blue color vision defect
• Mild cases go undetected
• Sequence is plotted in report chart.
• 2 or more lines parallel to given defect line means that the
person has that particular color vision defect.
• D-15 can also detect achromatopsia where lines become parallel
with Scotopic line.

Munsell 100 Hue test:
-Tests Red, Green and Blue CV defect (Gold
Standard)
- Consist up of 85 caps, divided approximately
equally in four boxes.
- Need to arrange the color caps in order of hue
discrimination b/w fixed reference caps
Scoring :
Minimum:2 Maximum: 14
Calculate error score by positional difference b/w caps of either sides.
5,6,7 ( correct sequence) - score of 2
5,6,11 ( incorrect sequence) - score of 6

Horizontally extended Plot: Protan
Obliquely oriented Plot: Deutan
Vertical: Tritan
Deutan Protan Tritan

Color naming Test
• Lantern test: Subjects asked to identify the name of a projected color
• Yarn test: Pts choose the told color from pile of colored yarns

Color mixing/matching Test
Anomaloscope:
- Only instrument that can tell whether its deficiency or absence of cones.
- Separates opia from omaly.
- Patient is presented with Bi-partite field.
- Works best for R-G defect
- One field is test color.
- Another field is R-G matching field.
- You can add Red and Green at needed proportion to match the field.
- Add more Red than needed: You are Red deficient
- Add more Green than needed: You are Green deficient
- Add no Red or Green to match: there is absence of Red or Green Cone

• Nagel Anomaloscope
• Pickford Nicholson Anomalsocope
• Neitz OT Anomaloscope

Ophthalmoscopy
1. Monocular direct ophthalmoscopy
2. Monocular Indirect ophthalmoscopy
3. Binocular Indirect ophthalmoscopy

Monocular direct ophthalmoscopy
Lens selection
wheel

Principle of Monocular direct ophthalmoscopy
Uses May Prism.

Filters:
a) Blue filter: For Fluorescein staining
b) Green Filter(Red free Filter): For nerve fiber layer and Blood vessel Viewing
c) Visuoscopy filter: For Eccentric fixation up to 5 prism diopters only
d) Large Full circle: Generalized View in dilated Pupil
e) Small Full circle : Generalized View in Undilated Pupil
f) Half Circle: In case of Media opacities
g) Slit view: Anterior chamber Depth, Disc Edema.
Subjective Watzke Allen Test for Macular Hole test.

Distance Direct ophthalmoscopy:
Media Opacities: 50cm to 1 meter
Anterior to iris plane: against the movement
Posterior to iris plane: With the movement
Small deviations: done at 25 cm
Bruckner Test: deviated eye reflex is Brighter
Near Direct ophthalmoscopy:
• High to Low Power lens to view Cornea to
Fundus
• FOV (Field of View): 6.50 – 100
• Magnification: 15 x (M=60/4) i.e. F/4
Where, F= total eye power
For Myopia: Magnification increases
For Hyperopia: Magnification Decreases

Monocular Indirect ophthalmoscopy
• Combined hand-held condensing system with
a direct ophthalmoscope.
• Provides a real, erect image of the fundus.
Field of view: 120
Magnification: 5X magnification
Provides a monocular view of 70% of the retinal
surface but lacks stereopsis

Binocular Indirect ophthalmoscopy
a) Slit lamp bio-microscope
b) Head band indirect ophthalmoscope

Head band indirect ophthalmoscope:
• Forms a real, inverted, reversed and stereoscopic
image
• Aerial Image formed between lens and observer.

Slit lamp Bio-microscopy
Principle:
Haag-Street Model

Filters
Kodak Wratten No.12 (Yellow)
• Barrier filter placed in front of viewing system
• Enhancing green staining
Green(red free)-
-Blood vessels (neovascularization)
ND filters-
- Reduce beam brightness and increase comfort for the pt
Polarizing filters
- Reduce unwanted specular reflection and enhance
visibility of
subtle defects
Cobalt blue-
• Fluorescein staining
• Keratoconus- fleischer’s ring Graticule: contact lens fit, HVID

Magnification:
• Low 7x-10x (general eye)
• Medium 20x-25x (structure layer)
• High 30x-40x (details)
Galilean Telescope, angled at 10-15 degrees
Objective(2 planoconvex lens)
Porro Prism is use to invert image

Illumination
Diffuse (broad/Wide beam illumination):
- Direct illumination
- 45 degree observation to illumination system
Direct
- Optic section, parallelepiped, conical
- Specular
- Tangential
Indirect
- Proximal
- Retro
- Sclerotic scatter

Direct illumination
- Observation and illumination system focus at the
same point.
• Optic section:
- <0.25 mm slit width
- Corneal nerves, blood vessels, infiltrates, Cataract
grading, Anterior Chamber angle.
• Parallelepiped :
- Slit 1-2 mm, illumination 45-60 degrees,
- Layered view of the cornea and the lens, corneal
abrasion or FB depth
• Conical :
- Reduced height of Parallelepiped
- AC-cells, Flare
Optic section Parallelepiped

Specular Reflection
- Microscope and slit beam at equal angles
from the normal to the cornea. Best at 50
degree.
- Highest possible magnification
- Endothelial cell layer, tear film debris, tear film
lipid layer thickness
Tangential Illumination
- Angle between the slit and microscope 70 – 80
deg
- Iris freckles and tumors, general integrity of
cornea and iris

INDIRECT ILLUMINATION:
- Observation and illumination system are not focused
at the same point with Variable angle.
Proximal:
- Structure just close to illuminated area
- Iris pathology, iris sphincters, epithelial vesicles and
erosions
Retro Illumination:
-Structure visible by reflected light Lens retro or Iris
retro
- Vascularization, epithelial edema, microcysts,
vacuoles, dystrophies, lens opacities and CL deposits
SCLEROTIC SCATTER
- Illuminate limbus of one side
- Another gets illuminated ( by total internal reflection)
• Central corneal epithelial edema, corneal abrasions,
corneal nebula and macula, FB in cornea

Visual Field
Visual field normal extent:
Temporal: 90 degrees
Superior/Nasal:50 degrees
Inferior: 60 degrees
Peak : fovea- greatest Sensitivity
Blind Spot: 15⁰ temporal to Peak
Traquair’s Field of Vision
*Sensitivity drops sharply for Temporal Retina.

Threshold:
If a particular intensity of light is shown 100 times and if it is appreciated 50
times, then that particular intensity of light is termed as threshold.
Higher db = lower intensity = high retinal sensitivity
Lower db= higher intensity = lower retinal sensitivity

10 db
20 db
30 db
More sensitivity
Less sensitivity

Factors affecting stimulus visibility are:
- Size of stimulus
- Background illumination
- Intensity of light

Stimulus presentation:
- Kinetic Method
- Static method
Isopter:
- Locus of the retinal points having equal sensitivity
Stimulus threshold:
Subthreshold, Threshold, Suprathreshold
Scotoma:
Relative scotoma: Decreased Retinal Sensitivity
Absolute Scotoma: No retinal Sensitivity
Qualitative Perimetry: Color Targets
Quantitative Perimetry:

Location of VF defect:
Central
• 50 or less from fixation
• Foveal defect– 0
• Off-centre defect – 10 or less
• Parafoveal defect – 30 or less but > 10
• Paramacular defect – 50 or less but >30
Paracentral (>50 up to 300)
• Ceacal, paracecal, pericecal (represents defects at, by, or around optic disc respectively)
• Cecocentral (between fovea & optic disc)
Peripheral : 300 away from central fixation point

Terms for visual field defects
• Central : involves fixation only
• Cecocentral: extends from fixation
temporally to blind spot
• Paracentral: involves region next to,
but not including fixation
• Pericentral: involves a region
symmetrically surrounding but not
involving fixation.

Terms for visual field defects
• Arcuate:
- Corresponds to and represents
nerve fiber bundle loss
• Altitudinal:
- Involves two quadrants in
either superior or inferior field
• Quadrantanopia:
- One quadrant of visual field
involved.

• Homonymous: Same side of visual space
affected in each eye
• Bitemporal: Opposite temporal sides of visual
field space affected in each eye
• Complete: Entire field affected
• Incomplete: A portion of field spared
• Congruity: Tendency for homonymous field
defect to be symmetrical (i.e to have a similar
size, location, and shape in each eye’s field)
Terms for bilateral VF defects

Perimetry Tools
Central
• Amsler Grid: 200
• Tangent (Bjerrum screen): 300
• Goldmann
• Automated (Actopus / Humphery) :300
Peripheral
• Confrontation
• Disc perimeter
• Goldmann
• Automated 900 program

• The patient sits facing us, about one meter away, without glasses.
• The patient is instructed to look at examiner’s nose and to cover
his/her left eye (the field of the right eye is being tested).
• We cover our right eye so that our visual fields will correspond.
• We hold up fingers in each of the visual field quadrants for the patient
to count.
Confrontation
Good for Hemianopia, quadranopia and Large Defects

• Each square subtends 10 (0.5cm)
• Allows assessment of the central 200 of the VF.
• Total 20 x 20 = 400 squares
Amsler Grid

Amsler Chart 1 Amsler Chart 2 Amsler Chart 3
Optic Nerve and Chiasmal
disorder or Toxic amblyopia
Where Central fixation is not
possible
Normal chart

Amsler Chart 4 Amsler Chart 5 Amsler Chart 6
• 20 evenly spaced white horizontal lines
• Central or paracentral metamorphopsia
resulting from various retinal and
choroidal disorders
• areas 10 above and below the
fixation dot
• Metamorphopsia along the
reading level

Amsler Chart 7
• The chart breaks the horizontally oriented 60X80
central area
• Sensitive to macular Compromise

Bjerrum’s screen (Tangent Screen)
– test a field of radius up to 300
– usual testing distance : 1m
– tangent screen would be 1.15mX1.15m
– test targets of 1mm , 2mm ,3mm,5mm ,10mm
– as 20 to 30 black headed pins for recordings
the findings.
– The patient should be wearing their Rx.

Goldman Visual Field
• Both Kinetic and Static Perimetry
• Manual method
• Can investigate extreme periphery as
well
Background illumination: 31.5 apostilbs (
approx. 10cd/m2)

Size of Target denoted by Roman Numerals:
V: largest target (64 mm2)
IV: (64 mm2) III: (4 mm2) II: (1mm2)
I: (1/4 mm2) 0: (1/16 mm2)
Luminance settings:
Larger Steps: (Numbers)
1,2,3,4 settings represent 0.5 log units changes =5db
Smaller Steps: (Letters)
a, b, c, d, e settings represent 0.1 log unit changes
=1db
Targets used are V-4e, I-3e, I-2e
Blind Spot plotting: I-4e

• While plotting the peripheral fields ,no Rx is worn (Except if patient is highly myopic ,hyperopic (-/+8.00D))
• Aphakic: Contact lens
• plotting the central 30 degrees Rx may be required
Cylinder:
• For Rx with -0.25DC , disregard
• For Rx with ≤ -1.00 DC , use spherical equivalent
For myopes with distance Rx between -0.25 and -3.25;
▫ Use distance Rx plus add for age
For myopes with distance Rx > -3.25;
▫ Use distance Rx plus +3.25 for all ages
For aphakic patients;
▫ Use contact lenses corrected for vertex distance Rx plus 3.25 add

Color coding for isopter
1. I-2e blue
2. I-3e orange
3. I-4e red
4. II-4e green
5. III-4e purple
6. IV-4e brown
7. V-4e black

Patients under 50 years of age
1. Peripheral I-4e (size=same, brighter luminance)
2. Intermediate I-3e
3. Central I-2e (size=same ,dimmer luminance )
Patients 50 years or older
1. Peripheral II-4e(size=larger, brighter luminance )
2. Intermediate I-4e
3. Central I-2e or I-3e (size=smaller, dimmer luminance )

Automated Visual Field
• In Humphrey system the 40 db stimulus is equal to 1 asb ( the least intensity of
light projected by Humphrey visual field)
• 0 db equals to 10,000 asb (maximum intensity of light projected by HVF).

30-2 point pattern
- VF extent = 30⁰ radius
- There are no points on both the horizontal and vertical axis.
- Distance between two points: 6⁰
- 76 points measured
- 3⁰ bare area
24-2 point pattern
- Similar to 30-2 except VF extent is 24⁰ and points measures is 54
(**note: in 24 -2 point pattern extra 2 points above and below
horizontal on nasal side is taken making it 27⁰ at nasal field, thus 24-
2 pattern is not circular and extra points indicates area where there
is high possibility of VF loss)
Suspicious glaucoma: 30-2 or 24-2 Pattern use
Established case of glaucoma: 24-2 point pattern

10-2 point pattern
- VF tested= 10⁰
- No points in horizontal and vertical axis
- Distance between points=2⁰
- 68 points tested
- Bare area= 1⁰( extra 12 points are checked inside 3⁰ )
Macular point pattern
- 3 ⁰ VF tested in 10-2 pattern
- No points in vertical and horizontal axis
- Distance between points= 2⁰
- 16 points are checked in 3⁰ area
Custom test 6-2 point pattern
- Subset of 10-2 program
- VF extent= 6⁰
- Distance between points= 2⁰
- 24 points measured
- Bare area= 1⁰

Threshold testing strategies
1) Old standard threshold strategies
- Full threshold strategy (↑4db and ↓2 db bracketing
technique)
2) Newer threshold strategy
- Fast PAC
- SITA Standard
- SITA Fast
(SITA= Swedish Interactive Threshold Algorithm)
• Staircase method (bracketing method) is used for determining
threshold. This method is used in full threshold strategy.
• 2 times crossing of threshold occurs.
• Newer threshold strategy however use alternative strategy to
reduced testing time by 40% i.e uses 3 db steps and crosses the
threshold only once.

SITA strategies are not available for macular program, nasal step and custom tests point pattern) i.e only
Full threshold and Fast PAC available.

Visual field printout
1. Patient data/ Test data/Demographic data
2. Reliability indices/foveal threshold
3. Raw data
4. Grey scale
5. Total Deviation Numerical Plot (TDNP)
6. Total Deviation Probability Plot (TDPP)
7. Pattern Deviation Numerical Plot (PDNP)
8. Pattern Deviation Probability Plot (PDPP)
9. Global indices (MD, PSD, CPSD, SP)
10. Glaucoma Hemi-field test (GHT)

1. Patient data and Test data (Demographic data)
-
• Patient name, DOB, Age, VA, Refractive error.
• Fixation target, fixation monitor, color of stimulus,
background illumination, stimulus size

• Central:
• Small diamond:
• Large diamond:
• Bottom LED: For some test requiring evaluation of superior visual Field

2. Foveal threshold and reliability indices
• Fixation losses : shift in fixation. Pt responds when
stimulus presented in Blind Spot.(fixation loss > 20%
is considered unreliable)
• False positive response: ( > 33% unreliable and
printout will indicate printing ‘xx’ next to rate) 14/14
xx)
• False negative response : ( > 20% is considered
unreliable, though machine defaults to > 33%,
because it may be high in advanced glaucoma)

3. Raw data
• It is exact retinal sensitivity expressed in db
units as calculated by VF machine.
• In raw data “0” indicates absolute scotoma
( i.e 0 is brightest stimuli that is presented)
• “40” indicates highest retinal sensitivity
recorded by humphrey.

4. Grey scale
• Retinal sensitivity values from the best retinal
sensitivity value (50 db) to absolute scotoma
(0 db) are divided into 10 groups.
• Each step of pattern corresponds to change of
5 db intensity except the first column
represented by 50db to 41db.

5. Total Deviation Numerical Plot (TDNP)
• Measured retinal sensitivity of each point (raw) is
now compared with the normative data of same age
group.
• Then difference between normative data and raw
data at each point is calculated and plotted.
• TDNP gives depth of defect also.
• TDNP becomes the platform for the calculation of
global indices ( mean deviation index and pattern
standard deviation)

6. Total Deviation Probability Plot (TDPP)
• The loss of retinal sensitivity at each point is now
expressed in terms of its P-value and each P-value is given
as symbol.
• loss of sensitivity (numerical value) is converted into
symbolic form, the extent and pattern of field defect is
well appreciated in the scotomatous form of probability
plots.
• So, probability plot gives extent and pattern of field
defect.
• Darker the symbol greater the probability of abnormality

7. Pattern deviation Numerical plot (PDNP)
• Pattern deviation plot is created to now the
pattern and extent of the deep scotomas,
masked by generalized depression in the Total
Deviation Probability Plots.
• Decreased by media opacity.

8. Pattern Deviation Probability Plots (PDPP)
• Pattern Deviation Probability Plots never shows
generalized depression. ( as generalized depression
is removed)
• Probability Plots for new PDNP is made from PDNP
data.
• Generalized field defect can be due to cataract,
media opacities, uncorrected refractive error, optic
atrophy.

Global indices
i) Mean Deviation (MD)
ii) Pattern Standard Deviation (PSD)
iii) Short Term Fluctuation (SF)
iv) Corrected Pattern Standard Deviation (CSPD)

i) Mean deviation Index (MD)
• Mean Deviation index signifies the average of overall severity of field loss
i.e average of all numbers of TDNP except two points in area of blind
spot.
• If MD is lower than found in 10% of normal subjects in parameter
database, a significant level is printed (p<10%)
• Difference of MD index between both eyes should be taken as a serious
clue in confirming the diagnosis of glaucoma.
• 1 db MD difference means , 52 db difference in 2 eyes in 24-2 point
pattern.( 54 points in 24-2)
• Expected change in MD per year is 0.08 db per year should be considered
normal.
• MD can be increased due to reasons that cause generalized field defect.
Eg: cataract, refractive error.

ii) Pattern Standard Deviation
• Expresses dissimilar deviation values in the Total Deviation
Numerical Plot whether it is smooth
• PSD will be a simple number without P-value.
• If the deviation of slope is significant it will be represented
by P-value.
• So, PSD with significant P-value indicates the numbers in
TDNP are not similar to each other.
• PSD is higher in localized or irregular generalized field
defect but lower in uniform generalized field defect.

iii) Short term fluctuation (SF)
• Short term fluctuation and corrected Pattern Standard Deviation will be
calculated by full threshold or FAST PAC strategies. (SITA strategies do not
calculate SF and CSPD).
• Full threshold and FASTPAC are not in use these days.
• SF is an index of intra test variation.
• The sensitivity will be calculated twice at pre-selected points.
• SF is usually < 3db i.e between 1 to 2.5 db, value higher than this shows an
index of unreliability or pathology.
iv) CSPD
- Calculated as adjustment to PSD by removing intra-testing variability.

10. Glaucoma Hemi-field test (GHT)
• GHT evaluates five zones in upper field
and compares these zones to mirror
zones in lower field.
• Zones are constructed in approximate
patterns of retinal nerve fibers.

There can be five results for GHT test:
i) Outside Normal Limit: least one sector pairs’ sore difference must exceed that found in 99 % of normal population.
ii) Borderline : least one zone pair difference exceeds that found in 97% of normal individuals.
iii) General Reduction in sensitivity: Not “outside normal limit” but general height calculation shows the best part
of field to be depressed to a degree that occurs in fewer than 0.5%
iv) Abnormally high sensitivity :
- The general height calculation shows the overall sensitivity in the best part of the field to be higher than that found in
others.
- No comparision between zones.
v) Within normal limits :
- This message appears if none of above four conditions are met.

Anderson Criteria for Glaucoma

Corneal Topography
• Reflection Base techniques
- Photokeratoscope (Placido ring )
- Keratometer
- Videokeratoscopes
• Projection Based Techniques
- Scheimpflug principle
- Slit photography
- Rasterstereography
- Moiré interference
- Laser interferometry

Keratoscopy
• Topographic abnormalities of the corneal surface by direct observation of the images of mires reflected from the
surface of the cornea.
• When a photographic film camera is attached to a keratoscope, it is a photokeratoscope.
• corneal cylinders of up to 3 D can escape detection
• optically important central 2-3 mm as well as the peripheral cornea
• Even best photokeratoscope covers upto 75 % corneal Surface.

Videokeratoscopes
• Television camera is attached to a keratoscope, it is a videokeratoscope .
• covers approximately 95% of the corneal surface
Computer-Assisted Videokeratoscopes

Placido ring
- Qualitative assessment of corneal curvature
- Instrument with mire attached with battery
handle.
- Mire ( 10-12 rings) glows and is reflected back
from cornea.
- Observer sees through peephole.
- Done at 20 cm.
- Peephole has +2 Diopters lens.
Closer mire Separation: Steep cornea
Larger mire separation: Flat Cornea
Abrupt change in mire separation in irregular
cornea.

Keratometer (Ophthalmometer)
• Corneal anterior surface acts as a convex mirror – utilizes 1st Purkinje image
• Measures the Central cap – 3 mm
• Sagittal radius
r= (-2d h’)/h
Where,
r = radius of curvature
d = distance between object
and 1st Purkinje image
h’= image height
h = mire separation
NIBUT: qualitative analysis of tear film

Fixed doubling Variable doubling

Two Types of doubling
Fixed doubling
• Fixed object height & doubling device system
• Variable image size & mire separation
• Bausch and Lomb Keratometer (One position Keratometer)
Variable doubling
• Fixed mire separation & image size
• Variable distance of doubling device & variable object size
• Javal-Schiotz Keratometer (Two position Keratometer)

Extended Keratometry
• Range from 36 Ds to 52 Ds
• If K reading is very low or high
• Trial lens in front of object to increase range
When curvature is Beyond the normal measurement limit:
 +1.25 Ds increase reading by 9 D (i.e. up to 61 Diopters)
 -1.00 Ds increase reading range by 6 D (i.e. up to 30 Diopters)

Javal’s Rule
• It’s the way to determine the total astigmatic error of the eye based on Corneal
Astigmatism.
• Internal astigmatism is due to corneal back surface toricity and lens tilting.
At=P (Ac)+ k, where At = refractive astigmatism
Ac = Corneal astigmatism
P= 1.25
k= 0.50 D (against-the-rule induced by lens tilt)

K value of a Right eye of a person in diopters is given below. What is his Total
refractive Astigmatism?
42.00 @ 180 43.50 @ 90
Using Javal’s rule:
At = P (Ac)+ k
= 1.25 (-1.50 x 180) + (-0.50 x 90)
= 1.25 (-1.50 x 180) +( +0.50x 180)
= -1.875 x180 + 0.50 x 180
= -1.375 x 180
+ 42.00
+ 43.50
Transposition:
-0.50 x 90
-0.50 / +0.50 x 180

Simplified Javal’s Rule
(Grosvenor’s Modified Rule)
Total Astigmatism (TA) = Corneal Astigmatism (CA) + Lenticular Astigmatism (LA)
i.e. TA = CA + LA
e.g. if a person has -0.75 x 90 cornea astigmatism. What is his total astigmatism?
TA = -0.75 x 90 +(-0.50 x 90)
= -1.25 x 90

When to use Standard Javal’s and Simplified Javal’s?
Corneal Astigmatism : ≤ 1.50 ( Use Simplified Javal’s)
Corneal Astigmatism : 1.50 to 2.50 ( Can use either of them)
Corneal Astigmatism : 2.50 ( use standard Javal’s)

Projection-Based Systems
• The Principle of Projection
• In projection-based methods, an image is formed on the surface of the tear film in the same way as
a slide is projected onto a screen.
Two Problems with this technique:
• Cornea is normally transparent and therefore transmits light, resulting in a low signal.
• Light is reflected by the surface of the tear film, resulting in high noise.

Feature of Projection-Based Systems
a) Measurement of Corneal Height
• Measurements are made in terms of height or elevation above a reference plane (* In contrast to
systems using reflection, which measure surface slope)
• corneal map therefore follow lines of equal height, rather than lines of equal slope.
• The radius of curvature or corneal power data can then be calculated directly.
• Measurements of corneal height are also useful in planning refractive surgery
b) Irregular and Non-reflective Surfaces
- They can make measurements from irregular or non-reflective surfaces. (unlike reflection based)
c) Entire Corneal Coverage
• able to make measurements from the whole cornea, including the very center and the limbus
d) High Resolution and Accuracy
• some devices, the resolution is in the order of 2–5 μm.

Disadvantages of Projection-Based Systems
a) Influence of Tear Fluid
• Both reflection and projection based topography systems image the air-tear fluid interface rather than
the
surface of the corneal epithelium
• Hence tear thickness with unknown uniformity affects the result.
b) Lack of Standardized Presentation Formats
• Each presentation format may have its individual merits, but there could be considerable benefits in
developing a standard format.

Scanning Slit Photography
Have to ever seen corneal thickness with slit beam of Slit Lamp?
• The machine is capable of calculating thickness in similar way at different
areas of cornea.
• Each slit contains up to 240 data points, giving a total of over 9000 data
points on each surface, each with a resolution of 2 μm
• Orbscan

• Limitations of this technology include the relatively long time
(0.8 seconds) required to individually image 40 slits and the
resultant possibility of introducing.
• Artefacts due to eye movement
• potentially inaccurate at locating the posterior corneal surface
• tends to underestimate corneal thickness after refractive
surgery

Rasterstereography (Rasterphotogrammetry)
• A grid is projected onto the tear film surface and imaged from a known angle.
• The topographic elevation is calculated from the displacement of components within the grid image
when
projected onto the corneal surface compared to their known position when projected onto a flat surface.
• The number of data points used by this method was initially limited by the number of grid
intersections.

Moiré Interference
• Moiré interference occurs when two sets of parallel lines are superimposed at different orientations.
• When parallel gratings are projected obliquely onto a cornea, the image on the corneal surface is a
series of parallel lines, curved in a similar manner to that seen when using a slit lamp beam.
• Addition of these two images results in moiré interference which generates ring-shaped interference
fringes visible on the corneal surface
• Their orientation is dependent upon the relative orientations of the two grating images and therefore
the shape of the surface on which they are formed

Laser Interferometry
• Interferometry records the interference pattern generated on the corneal surface by the interference
of two coherent wave fronts.
• The two wave fronts may be generated by light from separate illuminating and reference lasers, or
the light from an illuminating laser may be directed through two distinct optical pathways by the use
of a beam splitter.
• The corneal elevation is calculated from analysis of the interference pattern.
• The density of data points generated is dependent upon the wavelength of the light.

Scheimpflug principle
• Permit detailed mapping of the anterior and posterior corneal
surface the measurement of corneal pachymetry and a number
of other anterior chamber parameters.
• Enables the camera to capture sharp, focused images of
objects that are not parallel to the camera and lens.
• The principle describes the orientation of the plane of focus of
an optical system when the object plane, lens plane and the
image plane are not parallel but intersect at a common point in
space.

4-Maps:
• Axial curvature
• Anterior float (elevation)
• Pachymetry map
• Posterior float (elevation)

Gonioscopy
Gonioscopy is the evaluation of anterior chamber for seeing whether the
aqueous drainage system is ok or not.
Two types of Gonioscopy:
a) Direct Gonioscopy
b) Indirect Gonioscopy
All these lenses have a diameter larger than that
of the cornea (15 mm in adults).
Lens curvature is steeper than corneal curvature needing
coupling agent.

a) Direct Gonioscopy
• Provide direct view of angle.
• Direct gonioscopy is performed with a binocular microscope.
• The lens is placed on the eye, and saline solution is used to fill the space between the cornea and the lens
• Has an erect view of the angle structures, which is essential when performing goniotomies
• Commonly used in the operating room for examination of the eyes of infants under anesthesia
• E.g.: Koeppe Lens

Indirect gonioscopy
• Eliminates the total internal reflection at the surface of the cornea.
• Light reflected from the chamber angle passes into the indirect gonioscopy lens and is
reflected by a mirror within the lens.
• Yields an inverted image of the opposite angle
• Slightly foreshortened image underestimates AC angle compared to direct gonio lens.
• Requires a viscous fluid such as methylcellulose for optical coupling with the cornea
• E.g. Goldmann-type goniolens

1,2, or 3- mirror gonio lens: single side AC depth hence needs rotation
4-mirror gonio lens : no rotation needed.
Posner, Sussman, and Zeiss 4-mirror gonio lenses allow all 4 quadrants of the
chamber angle to be visualized without rotation

Dynamic /Indentation Gonioscopy
• Gonioscopy have a smaller diameter than that of the cornea (9 mm).
• Their curvature is (almost) the same as that of a regular cornea so no contact gel is needed for
examination
• Very important in differential diagnoses in angle-closure pathologies
• Gentle pressure is placed on the cornea, and aqueous humor is forced into the chamber angle.
• Posterior pressure can be used to force open a narrowed angle.

Fundus Fluorescein Angiography (FFA)
• Fundal photography, performed in rapid sequence following intravenous injection of Fluorescein
dye.
• Sodium Fluorescein: C20 H10 O5 Na2
• Molecular weight: 376 daltons
• Peak Absorption at 490nm (blue)
• Emits light at 530nm (yellow-green)
• 80% bound to plasma albumin. The remaining 20% is seen during angiography.

Fluorescein dye
- 5 ml of 10%
- 2-3ml of 25%
Injected intra-venous
- Preferably antecubital vein of the patient's arm.
Inject the dye in bolus very rapidly – 2 sec

Principle
• The eye is illuminated using blue light produced by a blue filter (excitation filter).
• The fundus is viewed through a yellow filter (barrier filter).
Kodak Wratten No. 57 – Green filter
• To take photos of macula (stereoscopic)

Phases of FFA
Early Phase
• Choroidal
• Arterial
• Arteriovenous
• Venous
• Mid Phase
• Late Phase

Choroidal phase:
• Choroidal filling via the short ciliary arteries (8 – 10 sec)
Arterial phase:
• the central retinal artery fills about 1 sec later than choroidal phase
Capillary phase (Arteriovenous Phase)
- Lamellar flow evident in veins
Venous phase:
• Early filling of the veins resulting in a tramline effect.
• Later the whole diameter of the veins is filled

• Venous phase divided into:
• Early:
• Mid Phase: 2 to 4 minutes after injection
• Late: After 10 to 15 minutes little dye remains

Foveal Region:
- Foveal Avascular Zone
- Blockage of choroidal because of more xanthophyll
pigments and melanin in RPE
Hyperfluorescence– excessive glow
Hypofluorescence– blockage of the glow

Hyperfluorescence
• Leak
• Pooling
• Staining or Transmission increase
• Abnormal vessels
Smoke Stack pattern is seen in CSR

Hypofluorescence
Blocked
Filling defect

Autofluorescence
• Innate property of fluorescein in certain ocular tissues (Fluoresce without Dye)
• Crystalline lens , basement membranes, Myelinated Nerve Fibers, Melanin
Granules, Certain Lipids

PSEUDOFLUORESCENCE :
False fluorescence
Overlapping between Transmittance curve of exciter and barrier filters:
- Results in Apparent Fluorescence
- To avoid Confusion Fundus Photos with both filters taken before Injection of Dye

Tonometry
• Tonometry is a procedure for IOP measurement.
• Normal IOP (10-20 mm of Hg)

Factors Influencing IOP
• Dilatation of pupil
• Heredity
• Age
• Sex
• Diurnal variation
• Postural variation
• Blood pressure

Tonometry Types:
1) Direct Tonometry
• Manometry
2) Indirect Tonometry
• Schiotz ( indentation tonometry)
• Applanation
Digital Tonometry/Palpation
Method

Applanation:
Non –contact applanation: air puff tonometer
Contact:
Goldman, Perkins, Tonopen (Static)
Ocular Response Analyser (ORA) or Dynamic Contour Tonometry (DCT) (Dynamic)

Indentation Tonometry
• Measure the IOP by relating a deformation of the globe to the force responsible
for the deformation.
• A known weight is placed on the cornea
e.g. Schiotz tonometer

• More the plunger indents the cornea, higher the Scale
reading and lower the IOP
• Each scale unit represents 0.05 mm protrusion of
the plunger.
• The 5.5 gm weight is initially used.
• If scale reading is 4 or less, additional weight is added to
plunger.
• IOP measurement is repeated until 3 consecutive
readings agree within 0.5 scale units.

Applanation Tonometry
• It is based on IMBERT FICKS LAW.
• It states that the pressure inside an
ideal sphere (P) is equal to force (F)
necessary to flatten its surface
divided by the area of the flattening
(A).
P=F/A
Uses biprism

Goldman Applanation tonometer
• Measures the force necessary to flatten an area of the cornea of 3.06 mm
diameter.
• The IOP (in mmHg) equals the flattening force (in grams) multiplied by 10.
• Also unaffected by ocular rigidity.

Effect Of Central Corneal Thickness
Thinner cornea : less force to applanate : Underestimation
Thicker cornea : more force to applanate : Overestimation
Goldman applanation tonometer was designed to give accurate readings when the CCT was
550 μm.
The deviation of CCT from 550 μm yields a change in applanation readings of 0.7 mm Hg per
10 μm.
Fluorescein thickness: overestimation with thicker dye band and underestimation with
thinner dye band.

Perkin’s Tonometer
Similar to Goldman:
- portable

Optical Coherence Tomography
• Noninvasive imaging technique for cross sectional images of the retina and anterior segment
• Longitudinal/Axial resolution up to 2 μm can be achieved
• Uses Long-wavelength light (near infrared 840 nm)
• The lateral resolution is usually about 20 μm due to diffraction caused by the pupil.
• measurement may be performed by an optical device known as an interferometer.

Types of OCT
a) Time Domain OCT
b) Spectral Domain OCT

spectral domain optical coherence tomography: Macular cube
(left), radial line scan (center), and raster scan (right)

1. nerve fiber layer ( hyper reflective )
2. ganglion cell layer ( hypo reflective )
3. internal limiting membrane ( hyper reflective )
4. inner plexiform layer ( hyper reflective )
5. inner nuclear layer ( hypo reflective )
6. outer plexiform layer ( hyper reflective )
7. outer nuclear layer ( hypo reflective )
8. external limiting membrane ( hyper reflective )
9. photoreceptors ( hyper reflective )
10.pigment epithelium ( hyper reflective )

Ophthalmic USG
• USG A-scan
• USG B-scan

• Ophthalmic ultrasonography uses frequency ranging from 6 to 20 MHz.
8 MHz in A scan
10 MHz in B scan
Unfocused beam: USG A
Focused beam : USG B

A-scan
• Time amplitude USG
• In A-scan USG echoes are represented as spikes arising from a baseline
• Tissue boundary
• Probe emits unfocused beam
• Contact technique and Immersion technique

0.4mm compression causes 1 D error in the calculated IOL power
1mm error in Axial length – 2.5 to 3.0 Ds error in IOL
Power

B-mode Display
• Stands for Brightness modulation
• Probe emits focused beam
Probe Orientations
Axial
Transverse
Longitudinal

Measured in decibels
Higher gain – Display weaker echoes like vitreous opacities, poor resolution, less frequency, more penetration
Lower gain: Stronger echoes (retina and sclera): Better resolution; more frequency, less penetration
Gain

T sign collection of fluid in subtenon space suggestive of Posterior Scleritis

Biometry
For calculation of IOL power:
Regression formulae
- Derived from f/u of real patients after cataract surgery
- SRK – I formula
- P = A- 2.5L-0.9K
Theoretical formulae-

Modified SRK II formula:
P = A- 2.5L-0.9K
Based on axial length, A constant is modified as:
• If L is < 20mm : A+ 1.5
• If L is 20-21 :A + 1.0
• If L is 21-22 : A+0.5
• If L is 22-24.5 : A
• If L is 24.5 – 26 :A-1.0
• If L is >26mm :A-1.5

TBUT/NI-BUT: Tear Stability
Cut-off: TBUT value less than 10s
NIBUT by keratometer

Schirmer Test – I & II
Schirmer I:
Basal and reflex secretion
Normal lower limit is 10mm of wetting after 5min
Performed without anesthesia.
Schirmer II:
Schirmer test with anaesthesia
Normal lower limit is 6mm after 5min
Measures Baal Secretion only

Phenol red thread test- Phenolsulphophthalein
• Basal secretion
• Standard clinical data suggests
• A 15-s test, wetting lengths should normally be between 9 and
20 mm.
• Patients with dry eyes have wetting values of less than 9 mm.

Tear Meniscus Height
• A tear meniscus height less than 0.25 mm is suggestive
of dry eye

ROPLAS (Regurgitation On Pressure Over the Lacrimal Sac)
• Mucopurulent material on compression
indicates patent canalicular system with
obstruction at lacrimal sac or NLD
• Steady pressure with index finger over
lacrimal sac area is applied.

Fluorescein dye disappearance test
Observations made after 2 min.
• No dye is seen in conjuctival sac-patent passage
• Retention of dye –inadequate drainage due to atonia of sac or mechanical
obstruction.

Jones dye testing
John test I:
- Cause of watering
- Partial obstruction or Hypersecretion of tear.
- (Negative: No dye is recovered) (Positive: Dye is recovered)
John test II:
- If Partial obstruction
- Identifies probable site of partial obstruction.

• Negative:
• Unstained saline is recovered from the nose.
• It indicates no entry of dye in lacrimal sac and
implies partial obstruction of puncta, canaliculi
or common canaliculus.
John Test II:

ERG (Electroretinogram)
• Mass response evoked from entire retina by a brief flash of light in
the form of action potential.

Components of ERG
• a wave –
• negative waveform
• generated by
photoreceptors.
• b wave –
• positive waveform
• generated by Müller and bipolar
cells.
• c wave –
• positive waveform
• generated by RPE.

A-Wave Amplitude
B-Wave Amplitude

Types of ERG
• Full Field ERG
• Focal ERG
• Multifocal ERG

ERG responses
Five different responses are standardized internationally
1. “Rod response”
2. Maximal combined response
3. Oscillatory potentials
4. Single flash “cone response”
5. 30 Hz flicker response

Abnormal ERG response
• Supernormal
• Early Siderosis bulbi
• Subnormal response
• Early cases of RP
• Chloroquine and quinine toxicities
• Vitamin A deficiency, mucopolysaccharidosis
• RD
• Extinguished response
• Negative response
• CRAO, CRVO, CSNB, giant cell arteritis

VEP( Visual Evoked potential)
(VER-Visually Evoked Response)
Visual Evoked Potential
Stimuli
Light
Pattern
Stimulate
Nerve impulse
Depolarization
Voltage
Principle

Techniques for VEP
Three Major techniques used for VEP
i) Pattern Reversal VEP
ii) Pattern onset/offset VEP
iii) Flash VEP

Pattern Onset/Offset VEP
• Elicited by Reversing Checkboard stimulus(200 msec) separated by
regular period of diffuse blank screen(400 msec)
• Less affected by poor fixation so in preverbal, Nystagmus patients.

Flash VEP
• Flash stimulus is delivered in full field Dome.
• Light adapting photopic Background
• Stimulus Rate: 2-3 Hz
• In infants ( can even be performed in close eye)

Clinical Uses of VEP in Children
• Estimation of VA
• Childhood Amblyopia and Binocular Function
• Oculomotor Disorders
• Delayed Visual maturation
• Optic nerve Hypoplasia

Clinical Uses of VEP in adults
Media Opacities
- Flash VEP used
- 15 ms delay, 50% reduced amplitude suggestive of dysfunction on Central Visual field

• Central Serious Retinopathy
- Latency is prolonged and returns to normal with recovery.
• Macular Disease
- In Macular hole, Macular Cyst prolonged latency.
• Optic Neuritis:
- Prolonged VEP latency doesn’t return normal even if VA returns to normal.

• Multiple Sclerosis:
- Large Delay in conduction ( prolonged latency)
• Dysthyroid Optic Neuropathy
- VEP latency decreased with compression of nerve ; should return normal after
treatment.
• Anterior Ischemic Optic Neuropathy
- Latency usually normal ( Amplitude amplitude significantly reduced)

EOG( Electro-Oculogram)
Measures activity of Retinal pigment
Epithelium

Techniques
• Instillation of appropriate mydriatic agent
• Electrodes placed at inner and outer canthus.
• Forehead electrode as ground electrode.

Techniques
• Stimulus – pair of fixation lights separated by 30
degrees of visual angle in ganzfeld bowl.

• The patient looks from right to left at an approximate rate of 16 to 20 rotations per minute.

• cornea is positive with respect to posterior
aspect of eye, eyeball acts as a dipole.
• eye movements can be recorded by
electrodes , so that changes in polarity can be
recorded and amplified with shifts in gaze .
• The amplitudes of the voltages generated by
constant eye movements in light & dark are
basic measures obtained in the EOG.

Light sensitive potential
After exposure to light , the potential per 300 of eye
excursion gradually increases.
Arden ratio= max height of light peak X 100
min height of dark trough
Normal 1.80-2
Subnormal 1.85- 1.65
Abnormal <1.6

Clinical application
• Any condition where ERG is abnormal, EOG is also abnormal.
• Helpful in diagnosis of conditions where fundus not visible.
• Certain clinical condition where ERG is normal but EOG is
abnormal.
-Juvenile Best disease (Vitelliform dystrophy)
-Fundus flavimaculatus
-Butterfly- shaped pigment dystrophy of fovea
-Advanced drusens.

Investigative Optometry

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