Day 6 (1): Pachymetry, Specular Microscopy, Keratometry and Fundamentals of Corneal Topography

Question 1

What is pachymetry?

Answer 1

Measurement of the thickness of the cornea
Provides an insight into the overall health of the cornea
Correlate findings with refraction, VA and IOP

Normal cornea: THIN centrally (WARM), THICKER peripherally (COOL)

Question 2

What are the different kinds of pachymeters?

Answer 2

1. Optical Pachymeter: oldest
   - quantifies corneal slit thickness
2. Ultrasound Pachymeter
   - sound waves emitted by transducer bounces off the Descemet’s membrane and is processed by the probe
   - disadvantages:
   + invasive: requires contact with the cornea
   + less precise (rough localization)
   + poor reproducibility (even with same examiner)
   + operator-dependent
3. Scanning Slit/Scheimpflug
   - gold standard for refractive surgery use
   - able to measure thickness of ENTIRE cornea
   - automatically measures and localizes thinnest point

Others:

4. Ocular Coherence Tomography (OCT)
   - uses light instead of sound waves
   - able to visualize and measure different layers of cornea
5. Ocular Low Coherence Reflectometer (OCLR)
   - measure corneal thickness at the point of fixation
6. Specular Microscopy
   - endothelial cell density and morphology are measured together with the corneal thickness in the same area for correlation

Question 3

What are corneal biomechanics analyzers?

Answer 3

1. Oculyzer (Oculus)
2. Ocular Response Analyzer (Reichert)

Parameters measured:
1. Corneal Hysteresis: corneal pliability or rigidity as a function of corneal thickness
2. Pachymetry
3. IOP

Question 4

Why is corneal thickness measured?

Answer 4

1. Screening for refractive surgery eligibility
   - most common reason
   - to see how much surgery can be done to the cornea
2. Diagnosis of corneal pathologies
   - thick: edema (Fuch’s Dystrophy)
   - thin: ectasia (Keratoconus)
3. Corneal graft health monitoring
   - N: thin and clear
   - A: thick and opaque
4. Assessing true IOP
   - Cornea may thin out with chronically elevated IOP
   - Tonometric measurements of IOP depend on corneal biomechanics

Question 5

What is specular microscopy?

Answer 5

• Qualitative and quantitative assessment of the corneal endothelium
• Settings: high magnification + extreme angle (80-90 degrees)

Parameters:
1. Density
2. Size Variation
3. Shape Variation
4. Corneal Thickness

Question 6

What are common indications for specular microscopy?

Answer 6

1. Pre-op evaluation for cataract, intraocular surgery or keratoplasty and post-op keratoplasty follow-up
   - stress from surgery –> expected decline in ECD
   - ensures that pre-op ECD will tolerate the decrease and cornea will remain clear
2. Diagnosis of corneal pathologies

Question 7

What is the normal endothelial cell density?

Answer 7

Birth: 350,000 cells/cornea
Normal attrition rate: 0.5-0.6%/year
Children: 3000 - 4000 cells/sq. mm
Adult: 2000 - 3000 cells/sq. mm (Ave: 2500)
Elderly: < 2000 cells/sq. mm
Minimum ECD for normal function: 1500 cells/sq. mm
Possible corneal edema: < 800 cells/sq. mm

Normal ECD declines with age
N: hexagonal, compact, uniform in size and shape
A: decreased count with larger irregularly shaped cells
- remaining cells enlarge (POLYMEGATHISM), taking on different shapes (PLEOMORPHISM) depending on the size of the uncovered areas; however, cellular enlargement is limited thus some areas remain uncovered with endothelial cells (GUTTAE)

Question 8

What is polymegathism?

Answer 8

• Variability in cell SIZE
• Some cells become bigger than others
• Normal Coefficient of Variation: 22 - 31 (Ave: 27 - 28)

Question 9

What is pleomorphism?

Answer 9

• Variability in cell SHAPE
• Cells are irregularly shaped (4-, 5- or 7-sided)
• N: hexagonal with 6 sides
• A: < 50% are hexagonal

Question 10

What is keratoscopy?

Answer 10

• QUALItative, subjective assessment of the corneal curvature using rings of light projected onto and reflected back by the cornea
• Curvature is NOT quantified but the appearance of rings give a clue whether a pathology is present or not
• May diagnose astigmatism but severity will not be quantified
• Findings:
  + Rings closer together: steep
  + Rings farther apart: flat

Question 11

Things to remember prior to doing keratometry.

Answer 11

1. Examiner presumed EMMETROPIC.
2. Visual axis of both examiner and patient are ALIGNED to the keratometer axis.
3. Cornea is LUBRICATED with no artificial changes in curvature (from contact lens use)
4. Mires should be CENTERED.
5. Image is properly FOCUSED before values are recorded.
6. Optimal: refractive index of keratometer is 1.3375 (similar to cornea)
7. Measurement:
   - Within 5 SECONDS after the last blink or BEFORE the tear break up time
   - Wetting agent: Balanced Salt Solution; avoid hypromellose

Question 12

What are the two types of keratometers?

Answer 12

1. Manual: measures central 3.2 mm (more peripheral area overestimates corneal power)
   - Javal: align red image with green image
   - Bausch & Lomb: more common; align the (+) with the (-) signs and align the images into a reverse L shape
2. Automated: measures central 2.6 mm

Question 13

When should Keratometry findings be validated or repeated?

Answer 13

1. K in any meridian LESS THAN 40 D or GREATER THAN 47 D
   - Normal: 40-47 D
   - Check oldest available refractive error and axial length
2. Astigmatism in either eye GREATER THAN 4.0 D
   - Confirm with MANIFEST REFRACTION: manual way of determining corneal power by placing lenses of different powers in front of the eyes and asking which one results to clearer vision
   - Do CORNEAL TOPOGRAPHY: to assess entire anterior surface
3. Difference in K between eyes GREATER THAN 1.0 D
   - Is there an appropriate amount of corresponding ANISOMETROPIA (unequal refractive power between the two eyes)

NEVER accept INADEQUATE measurements!!!

Question 14

How to do keratometry in patients with poor fixation?

Answer 14

1. Determine cause: cataract, macular hole, etc.
2. If fixation NOT possible, align keratometer reflex at PUPIL CENTER. If pupil eccentric, align at CORNEAL CENTER.
3. If pt uncooperative or cannot understand, ask patient to come back another time.

Question 15

When to discontinue contact lens use when undergoing keratometry?

Answer 15

Hard (RGP) CL: 2 weeks prior
Soft CL: 3 - 7 days prior

Why?
Prevent masking of astigmatism and undetected central corneal flattening (corneal warpage)

Question 16

What is corneal warpage?

Answer 16

Contact lens induced irregular astigmatism with:
1. Loss of radial symmetry
2. Reversal of normal flattening of corneal contour
- N: center more curved than periphery
- A: center flattens
3. Reduced vision on post-lens-wear refraction

Question 17

What is the difference between Corneal Topography and Corneal Tomography?

Answer 17

Topography: imaging technique for mapping the surface curvature and shape of the ANTERIOR corneal surface.

Tomography: computes a 3-D image of the cornea and assesses the ANTERIOR and POSTERIOR surfaces of the ENTIRE cornea
- best captures OVERALL or TRUE corneal power

Question 18

What is Corneal Topography and when is it indicated?

Answer 18

• Technique that maps the ENTIRE surface of the cornea
  + vs Keratometry which measures only a SELECT area in the anterior surface
• Uncovers corneal pathologies in areas undetected by keratometry

2 General Principles/Kinds:

1. Placido Disc-Based Topography (Keratometric map)
   - Uses concentric rings or mires reflected off of the anterior cornea and converted to color scales
   - Displays REFRACTIVE POWER of cornea
   - Can only evaluate the ANTERIOR corneal curvature
   - E.g. Zeiss Atlas, NIDEK OPD-Scan
2. Purkinje Image-Based Topography (Elevation map)
   - Describes the corneal curvature with respect to a reference shape (best fit sphere)
   - Displays SHAPE/ELEVATION of cornea
   - Can evaluate both ANTERIOR and POSTERIOR curvature
   - Able to measure corneal thickness along the entire cornea
   - Gold standard for topographic measurements

A. Scanning Slit System (Orbscan)
- Uses rapidly scanning projected slit beams of light and a camera to capture the reflected beams to create a map of the anterior and posterior corneal surface

B. Scheimpflug Imaging (Pentacam)
- Uses a rotating camera to photograph corneal cross-sections illuminated by slit beams at different angles
- Corrects for the non-planar shape of the cornea and, thus, allows greater accuracy and resolution in creating a 3D map of cornea
- Can be considered a Tomogram
- In 3D renders:
+ Red: anterior cornea
+ Green: posterior cornea
+ Blue: iris
+ Yellow: crystalline lens

Question 19

What are the indications disadvantages of corneal topography?

Answer 19

Indications:
1. Investigation of keratometric findings (unusual keratometric readings, poor quality mires)
2. Post-keratoplasty and post-cataract surgery astigmatism evaluation and management
3. Refractive surgery screening and monitoring
4. Detection, monitoring and treatment of corneal ectasia (keratoconus, PMD, post-LASIK ectasia)
5. Ocular surface disorder evaluation (pterygia, Salzmann’s nodular degeneration)
6. Guiding suture removal and placement of corneal relaxing incisions

Disadvantage:
- Irregularities in tear film can significantly impact the quality and fidelity of a Placido disk topography.
- Decreased accuracy of posterior elevation values especially after refractive surgery

Question 20

How is Placido Disc-Based Topography done?

Answer 20

1. Keratoscope image (placido rings) projected onto the cornea
2. Reflected ring images are digitized
3. Power is calculated
   - for each ring, around 5,000 - 20,000 points are used to assess corneal curvature and power
4. Color-coded map is displayed according to computed power of each point in the cornea.

Question 21

What are the two types of placido-ring-based map readings?

Answer 21

Axial/Sagittal Map
- measurements are based on a CENTRAL AXIS or central reference point for the ENTIRE corneal curvature
- Axial Power = (Refractive Index - 1)/Radius of Curvature
- best for: OVERALL curvature or power; screening
- advantages:
1. less prone to noise or minute variations in curvature
2. less accurate peripherally
- disadvantage: cannot pick-up LOCALIZED abnormality

Tangential Map
- each point of the cornea is interpreted as a part of a circle with different radii of curvature
- different points in cornea = different reference points
- best for:
1. detecting LOCALIZED or FOCAL abnormalities especially at the PERIPHERY
2. monitoring of corneal healing
- advantage: more sensitive to subtle changes
- disadvantage: more sensitive to noise

Question 22

How are placido-ring-based topography results interpreted?

Answer 22

Prior to interpretation:
1. Image quality?
- empty spaces or maps of dots = INADEQUATE data
- CRISP: distinct, well-formed mires = corneal surface is regular and smooth
- distorted, wavy mires = surface irregularities
2. How is the map oriented over the pupil?
- each square in the grid = 1 mm
- number of squares horizontally = corneal diameter
- steepest part of a normal cornea: CENTER of the PUPIL
3. Check and set the scale of K increments used
- choices: 0.25 D, 0.50 D (most common) or 1.0 D
- large increments: may make abnormal corneas appear normal (false negative)
- small increments: may make normal corneas appear abnormal (false positive)

Interpretation:
1. Are values within normal range?
2. If not: regular or irregular astigmatism?
- Regular: ovoid mires
- Irregular: irregularly distorted mires
3. Interpret the colored maps
- when reflected rings do not appear symmetric, they are interpreted as abnormal and displayed as different colors
- NORMAL: symmetric rings –> GREEN
- FLAT: widely spaced rings –> COOL colors (BLUE)
- STEEP: closely spaced rings –> WARM colors (YELLOW/ORANGE/RED)

Clue: STEEPing warm tea

Reporting:
- Steep K at ___ axis
- Flat K at ___ axis
- Astigmatism of ___ D (steep K minus flat K)

Question 23

What is the appearance of a post-myopic LASIK ablation on placido-disk-based topography?

Answer 23

• Center of the cornea is thinned out or flattened to decrease the converging power of the cornea
• Central cornea: COOL
• Periphery cornea: WARM

Question 24

What is the appearance of astigmatism on placido-ring-based corneal topography?

Answer 24

Regular Astigmatism (Figure-of-Eight)
- uniform steepening along a single corneal meridian that can be fully corrected with a cylindrical lens
- steepest meridian is 90 degrees away from the flattest meridian

1. With-the-rule astigmatism: hourglass (8)
   - steeper at vertical meridian
2. Against-the-rule astigmatism: bowtie (infinity sign)
   - steeper at horizontal meridian

Irregular Astigmatism
- non-uniform or localized steepening that cannot be corrected by cylindrical lens
- steepest and flattest meridians are NOT 90 degrees apart

Question 25

What is Pellucid Marginal Degeneration?

Answer 25

Bilateral non-inflammatory condition characterized by severe crescent-shaped thinning of the cornea

Appearance: Against-the-Rule astigmatism in a crab-claw pattern

Most affected area: Infero-Temporal Region (WARM)

Question 26

What is Purkinje Image Based Topography or Elevation Topography?

Answer 26

• Describes the corneal curvature with respect to a reference shape (usually a cut sphere)
• Evaluates both ANTERIOR and POSTERIOR curvature
• Able to measure corneal thickness along the entire cornea
• Gold standard for topographic measurements

Interpretation:
1. WARM colors (YELLOW/ORANGE/RED)
- cornea elevated ABOVE the best fit sphere
2. COOL colors (BLUE/PURPLE)
- cornea depressed BELOW the best fit sphere

E.g.
Post-myopic LASIK ablation: depressed center (COOL)
Keratoconus: elevated center (WARM)
WTR Astigmatism
- depressed along vertical meridian (COOL)
- elevated along horizontal meridian (WARM)
ATR Astigmatism
- depressed along horizontal meridian (COOL)
- elevated along vertical meridian (WARM)

Question 27

What is shape of the normal cornea?

Answer 27

Prolate: STEEPER CENTER, flat periphery
- meridional curvature DECREASES from center to periphery
- sphere that’s compressed along the horizontal meridian –> bulging poles (squeezed by praying hands)
- shape of a normal cornea

Oblate: FLATTER CENTER, steep periphery
- meridional curvature INCREASES from center to periphery
- sphere that’s compressed along the vertical meridian –> bulging equator (pounded by a fist on the table)

Question 28

What are the parts of an Orbscan/Pentacam reading?

Answer 28

Examples of elevation topography

1. Anterior Elevation Map
2. Posterior Elevation Map
3. Placido/Axial Map
4. Pachymetry/Corneal Thickness Map
   - WARM: THIN (thinner clothes in warm weather)
   - COOL: THICK (thicker clothes in cool weather)