Deck #3: AS imaging

What are the 3 main anterior segment imaging procedures?

1. AS-OCT

2. Specular Microscopy

3. Wavefront Aberrometry

Do auxiliary tests replace provider judgment?

No, they are used in conjunction with the clinical picture and provider expertise

What is Optical Coherence Tomography (OCT)?

→ non-contact technique that creates high-resolution, cross-sectional images of ocular structures using reflected light

• (+) allows for quantitative analysis of imaged tissues without biopsy

• (+) can monitor disease progression

What 4 areas of the eye is OCT used for?

1. Cornea (e.g., CT)

2. Anterior chamber angle

3. Retina

4. Optic nerve

Why is OCT more repeatable than ultrasound biomicroscopy?

lower coefficient of variation (CV)

List 7 specific uses for Anterior Segment OCT (AS-OCT).

1. Corneal assessment

   • Thickness

   • Opacifications

   • Disorders/dystrophies

2. Anterior chamber angle

3. Anterior segment tumors

4. Planning for anterior segment surgeries

5. CL fittings/eval

6. Tear meniscus measurements

What 5 corneal layers can be seen on AS-OCT?

1. Epithelium

2. Bowman’s

3. Stroma (thickest, where nerves are)

4. Descemet’s

5. Endothelium

What 3 tear film metrics can AS-OCT quantify?

Tear meniscus height, depth, & area

Other than tear meniscus, what else can the AS-OCT measure?

• Corneal thickness

  • epithelium - can detect thickness (helpful for kerataconus)

• corneal edema

Note: Measurements don’t always correlate to ultrasound pachymetry

What signs of Fuch’s dystrophy appear on AS-OCT?

• Thickened Descemet’s membrane

• guttata in endothelium

What corneal dystrophy is this an image of?

Epithelial Basement Membrane Dystrophy

• whitening of tissue is peripheral, so VA isn’t affected

What corneal dystrophy is this an image of?

Granular Corneal Dystrophy Type 2

• red spots = corneal opacities

• useful to determine stroma layer that’s affected

What is this an image of?

Descemet’s Membrane Detachment (from stroma)

What is this an image of?

Bullous Keratopathy

What is this an image of?

Infective Keratitis

What is this an image of?

Corneal Hydrops

• break in decements → massive influx of aq fluid into cornea → blister on surface that can rupture

Is AS-OCT angle assessment functional or structural?

structural (can’t see how aq is flowing, BUT can see if angle is open/closed)

What angle configurations can AS-OCT visualize?

• Narrow angles

• Plateau iris

• PAS

• Pupillary block

Note: Can be observed in dark vs light

What is the gold standard for measuring angle configuration?

Gonioscopy (AS-OCT can be used in conjunction)

How would you describe this angle?

Narrow angle, but not touching (might occlude in dim light)

How would you describe this angle?

Iridocorneal touch (closed angle)

• iris to cornea touch (closed angle)

How would you describe this angle?

PAS (Peripheral Anterior Synechiae)

• iris to cornea touch (closed angle)

What type of iris is this?

Plateau iris

How does AS-OCT help with iris lesions?

→ distinguishes between tumors and cysts and images the anterior architecture

• (+) avoid intraocular biopsy

• (+) good in small or hypopigmented lesions

• (-) poor analysis of posterior aspect/extension of tumor

What is this an image of?

AS-OCT of Iris cysts (clear interior = fluid)

What is this an image of?

AS-OCT of ant seg tumours

• 3rd row is a B-scan (which doesn’t give that posterior aspect, you can only see the anterior bowing)

What are the applications of AS-OCT in specialty contact lens fittings?

• Sagittal depth of the anterior segment

• Lens clearance

  • Posterior lens tear film thickness

  • Lens bearing in any location

• Scleral and limbal interaction

• Lens thickness

• Lens defects

What do each of these arrows represent?

How does AS-OCT assist with soft contact lenses?

Evaluate pachymetry & edema pre-/post-lens wear

What power do you think this soft CL has?

Minus

What is specular microscopy? What 3 aspects of the endothelium does it analyze?

→ Noninvasive, photographic technique that visualizes the endothelial morphology:

1. Size

2. Shape

3. Cell density

List 2 ways to visualize the Corneal endothelium.

1. Specular reflection

2. Specular microscopy

What is Specular reflection?

→ Light source & microscope at equal angles from cornea

• Focused on the endothelium

• Elevations = dark

What is Specular microscopy?

Microscopic visualization of endothelial cells in vivo

What is the Endothelium & its functions?

→ 1 layer thick

• Maintain corneal health & transparency

• Semipermeable membrane - facilitate diffusion of nutrients into the cornea from aq

• Secrete the Descemet’s membrane

How does the endothelium restrict fluid from entering the cornea?

via tight junctions that b/w endothelial cells

What is the metabolic pump function of the endothelium?

→ Controlled by junctional complex between endothelial cells

Endo → Ant chamber:

• Active transport fluid out of the stroma

• Translocation of HCO₃ across endothelial membrane

• Passive transport of H₂O into anterior chamber

  

How many endothelial cells in young patients?

350-550 K

What is the normal ECD for young patients?

~3500 cells/mm²

How many endothelial cell loss is normal? Do endothelial cells regenerate?

100-500 cells/year (slough into anterior chamber)

• these DON’T regenerate

What ECD difference between eyes is considered abnormal?

Loss is symmetrical, however >280 cells/mm² difference in cell loss b/w eyes is abnormal

Once endothelial cells are shed, what happens?

→ Cellular repair

1. Stretching & sliding of existing cells into new position

2. Fusion of adjacent cells

What is the critical density range for the endothelium? What occurs if it falls below this range?

300-500 cells/mm²

• lose the ability to regulate fluid → corneal edema

How does age contribute to the average cell density?

10-59 = stable cell loss

60-80+ = cell density declines

What is the corneal endothelial cell density in newborns, adults and old adults? How does cell diameter vary?

Newborn = 3000-4000 cell/mm² (18-20 μm)

Adult = 2500 cell/mm²

Old age = 2000 cell/mm² (40 μm)

What occurs during Endothelial cell damage? List factors that cause it.

→ Physiologic stress alters normal endothelium, causing the breakdown of fluid barrier function → edema

• damage alters central and peripheral cell density + shape

.

Causes

• Normal aging

• CLs wear

• Trauma

• Ocular surgeries

• Ocular diseases/disorder

• Systemic disease

Define pleomorphism. List its causes.

→ Disruption in regular hexagonal pattern of cells (shape change)

• ↓ cellular stability

.

Causes

• Normal aging

• CL wear

• Physiologic stress

• Ocular disease

What is Hexagonality (Hex)?

→ Variability in hexagonal shape of cells

• measure of pleomorphism (% of cells with 6, >6, <6 sides)

• deviation from hexagonal shape

What % of pleomorphism is considered “clinically significant”?

Normal = >50% hexagonal cells (6 sides)

Abnormal / pathology = >50% pleomorphic cells (anything with less/more than 6 sides)

• greater than 50% (if more than 50% of cells are not hexagon it's bad → disrupts fluid barrier function)

Define polymegethism. List its causes.

→ change in uniform size of endothelial cells (size change)

• 1st sign of endothelial disease ⭐

.

Causes

• Physiologic stress of cornea

• Overactive wound repair

How can you track polymegathism? What are normal, borderline and abnormal values?

→ via Coefficient of Variation (CV)

• Normal CV = 0.22-0.31 (22-31%)

• Borderline CV = 0.32-0.40 (32-40%)

• Abnormal CV = >0.40 (greater than 40%)

Low CV = similar in size

What is the formula for CV?

What are guttata?

→ Secretions of abnormal BM that accumulates under cells

• made of collagen

• nodules covered by endothelial cells

• cause thickening of Decemet

• can coalesce over time

• common in periphery

• found in 70% of patients over 40

  • part of normal aging, but pathologic when there’s loss of fluid barrier function

Compare contact vs non-contact specular microscopy.

What does the accuracy & precision of measurements taken during Specular microscopy depends highly on?

→  area of corneal image selected for analysis

• cell density varies in different endo locations (periphery has more endo cells, than central)

→ presence/absence of defects

List the factors that affect image quality of Specular microscopy.

• Dry eye

• CL use

• Keratoconus

• Patient compliance

• Pt’s age

• Training or experience of technician/doctor

What does Specular microscopy measure?

1. Cell density (CD)

   • Cells/mm²

2. Cell morphology

   • Size (cell area ± SD)

   • Shape

   • Variability

3. Optical pachymetry

   • CT

List the indications for performing Specular Microscopy.

• General corneal health

• Diagnose & manage corneal dystrophies/degeneration

  • Fuch’s dystrophy

  • Posterior polymorphous dystrophy

  • Iridocorneal dysgenesis conditions

  • ICE syndromes

• Assessing idiopathic decreased vision in pt

• Pre-operative risk assessment & post-operative management (cataracts, LASIK)

• CL evaluations & management (e.g., overnight / extended wear lenses, Scleral lenses)

What is the most sensitive measure of corneal health?

1. Pleomorphism

2. Polymegethism

🌟 Both can indicate loss of barrier & pump function

What is the least sensitive measure of corneal health?

Cell density

🌟 cornea can still function well at low densities

Diagnose this.

Early Fuch’s dystrophy

Diagnose this.

Advanced Fuch’s dystrophy

Diagnose this.

ICE syndrome

Diagnose this.

CL changes

Define Wavefront aberrations.

Deviation of a wavefront exiting the pupil after progressing through the optics of an eye compared to a reference unaberrated wavefront free of aberration

What is a Wavefront aberrometer?

→ Instrument used to measure aberrations within an optical system

• allows for deeper understanding of aberrations & effect on vision

• provides means for improved optics via refractive surgery, custom design CLs

How are ocular aberrations measured?

→ Monochromatic light projected into the eye, off the fundus & back out to the photosensors of an instrument

• Ideal eye / optical system = Light from distance focuses sharply → ideal wavefront

• Aberrated eye = Light reflected out from the retina → abberated wavefront

List 5 factors affecting ocular aberrations.

1. Pupil size (dilated at dark, introducing more abberations)

2. Accommodation

3. Lid position

4. Eye movements

5. Tear film stability

What are lower order abberations? List them.

→ Account for ~80-90% of ocular aberrations

1. Defocus

   • Myopia, hyperopia

   •  related to spherical ametropia in RE

   • describe image postion

2. Astigmatism

   • eye can’t produce 1 point image

→ easily corrected with glasses, CL, refractive surgery

What are higher order abberations?

→ Account for ~10-20% of ocular aberrations (3er order + above)

1. Coma

2. Trefoil

3. Spherical aberration

4. Quatrefoil

5. 2er astigmatism

→ not easily corrected, worsened by refractive surgery

What is the amount of higher-order aberration proportional to?

pupil size

What is Coma?

→ Occurs due to misalignment of the center of the pupil, cornea & lens → creates an image that appears to have a “tail”

• Irregular astigmatism

• Frequent problem in pts with corneal ectatic or thinning disease (e.g., Keratoconus)

What is Trefoil?

• Irregular astigmatism

• makes the least amount of image degradation of higher order aberrations

What is Spherical aberration?

→ created by difference in refracting power of lenticular annular rings & pupil size → “haloes” around point images

• ↑ effect of uncorrected myopia in low light settings

• depends on pupil size

  • 2x pupil diameter = 16x SA

• affected by accommodation or age-related lens changes

(+) SA = peripheral refracted more than central

(-) SA = peripheral refracted less than central (accom, CL)

List the visual impact of Haloes created by SA?

• Poor night vision

• ↓ depth of focus for reading

• Glare / haloes around lights

• Diplopia

• Starbursts around lights

What is Root Mean Square (RMS)?

→ calculated using the difference between wavefront surfaces & average of the surface

• deviation of the wavefront from a plane wavefront

• abberations aren’t directly correlated to visual performance

• predictive value for likelihood of increasing HOAs after refractive surgery

What is Modulation Transfer Function (MTF)?

→ ratio of image contrast as a function of SF

• ↑ SF = ↓ MTF

• Quantitative measure of image quality

What is Point Spread function (PSF)?

→ Measure of how well an object point is imaged on the retina

• shows how aberrations affect a point of light

• Ideal optical system: PSF = 0

What are the indications for wavefront analysis (aberrometry)?

• Refractive surgery planning

• Cataract surgery evaluation

• CL fitting

• Custom spectacle design

• Patients with minimal VA decline

  • Patients with 20/20 VA but persistent visual complaints

List the clinical uses of wavefront aberrometry.

• Potential improvement in vision beyond correction with typical glasses or CLs

• Increasing use in refractive & cataract surgery

• Objective clinical refraction

  • Optimizing the sphero-cylindrical refraction

What makes up the majority of aberrations?

LOA (HOA is masked, but becomes noticeable when LOA is corrected)

What’s the difference between Conventional LASIK vs Custom LASIK?

Conventional LASIK (Laser In Situ Keratomileusis)

• ↓ Defocus (LOA)

• ↑ Coma & SA (HOA) → night glare, haloes, ghosting

.

Custom LASIK

• Aims to ↓ HOAs for better visual quality

What are ODs role in refractive surgery?

• Co-management of refractive surgery pt (both pre- & post-op)

• Wavefront aberrometry can help detect pts that may not be as good of candidates prior to surgical consult

We know that CL correct LOA. What have many soft lenses brands began to do?

→ incorporate aspheric optics to ↓SA

• (+) or (-) SA incorporated into lens to ↓ ocular aberrations

What are the limitations to Abberation control CL?

• Fluctuations in pupil size

• Expensive

• Lens movement/rotation

• Disease progression

What is Objective wavefront refraction?

→ Process of deriving sphero-cylindrical correction using wavefront aberration measurements

• takes into account amount & type of HOAs in an eye to optimize optical correction

• can create lenses to ↓ HOAs

What are limitations to wavefront refraction?

→ Wavefront refraction measures optical quality, but perception is not limited to optics alone, it must also consider:

• Retinal image quality

• How retinal images are processed

• Transfer of the image through the entire visual pathway

Why is measuring aberrations with monochromatic infrared light a limitation of wavefront refraction?

→ b/c it doesn’t account for chromatic aberration from white light

• White light has multiple wavelengths that focus at different points

What is the gold standard for comparing the success of an objective refraction (e.g., wavefront refraction)? List its limitations.

Subjective refraction

• vary between different practitioners

• produce different results from the same practitioner at different times/days

How do phoropters and trial lenses limit comparison with wavefront refraction?

quantified in 0.25 D steps → limits precision of subjective correction

Why might different aberrometers give different refractive results for the same patient?

• Use different measurement methods

• Analyze aberrations differently

• Calculate different estimated refractions