Day 10 (4): Evaluation of the ONH and RNFL in Glaucoma Flashcards
Cardinal features of glaucomatous ON
- Widening of cup (advancing inner border)
- Thinning of neuroretinal rim (NRR) due to tissue loss
What are the five parameters assessed in the optic disc in cases of glaucoma?
- Size and limits of the optic disc
- Size of the NRR
- Retinal nerve fiber layer
- Peripapillary atrophy
- Retinal and optic disc hemorrhages
What is the normal optic disc size?
Optic Disc
- optic cup + neuroretinal rim
- approximates the scleral ring/posterior scleral foramen
- individual variability in size
- affected by:
1. Genetics
2. Race: Asians with larger disc (> 4.0 mm) and cup (> 2.0 mm)
3. Refractive error: Myopes have larger discs
- GLAUCOMA: LARGER cup + THINNED rim
- size of the cup varies with size of the disc
+ LARGER optic disc (vertical diameter) –> LARGER optic cup –> LARGER cup-disc ratio
+ similar trend in both Caucasians and Asians
+ Asian optic cup and disc size > Caucasians BUT with similar CDR
Vertical Optic Disc size
Small: < 1.4 mm
Average: 1.4 - 2.5 mm
Large: > 2.5 mm (OCT: > 3.0 mm)
Direct Ophthalmoscopy
- use a small aperture light spot (5 degrees)
- size of the light spot ~ size of an average-sized OD
Slit Lamp Biomicroscopy
- measure using slit beam sizes
- uses Volk lenses
+ 60D: x 1.0 (NO correction needed)
+ 90D: x 1.3 (PREFERRED because disc is visualized through a small pupil without dilation)
NOTE: Myopes
- larger discs and cups
- harder to delineate extent of disc
- cup border/rim: where vessels BEND
What is the normal neuroretinal rim size?
Neuroretinal Rim (NRR)
- more important parameter than cup size
- doughnut-shaped area:
+ INNER border: optic CUP
+ OUTER border: optic DISC
- WIDTH: distance between optic disc border and area of blood vessel bending (optic cup)
ISNT Rule: largest NRR in the I = S > N > T
- not followed in 50% of discs
- Temporal: narrowest INDEPENDENT of ethnicity
STRONGLY SUGGESTIVE:
- NOT applicable if disc is very large or tilted
1. CD ratio > 0.70
2. Thinner ISN rim compared to T side
SUSPICIOUS:
1. Diffuse loss or localized rim thinning (notching) (especially if reaching the disc margins)
2. Very thin IS rim width: rim-to-disc ratio < 0.1
3. Asymmetry of rim width between I and S sectors: should be roughly similar in size
4. CD ratio asymmetry between eyes > 0.20 OR asymmetry of rim width between eyes with symmetric disc sizes
5. Hemorrhage crossing the rim
6. Undercutting of rim: NORMAL in large cups
7. Nasal cupping
Remember:
NON-glaucomatous optic neuropathy
- pallor > cup size (based on vessel kinking)
How is RNFL examination carried out?
- Use red-free light/photographs or green light
Parameters:
1. Striations
2. Brightness/reflectivity
3. Visibility of peripapillary retinal vessels
NORMAL RNFL
1. Retina appears FUZZY or hazy due to an overlying veil-like RNFL layer
2. Striations at the S-T and I-T regions appear brighter or more reflective due to denser concentration of fibers
3. (+) Alternating bright and dark striations especially surrounding the optic disc
4. Retinal vessels have BLURRED outlines
5. Parapapillary retina vessels visualized due to contrast of white fibers with red vessels
ABNORMAL RNFL
1. Retina appears bright red with NO haziness
2. (+) Drop-outs: hyporeflective or dark areas due to absence of fibers
3. NO alternating bright and dark striations
4. Retinal vessels have SHARPER outlines
5. Parapapillary retina vessels NOT visualized because retina appears uniformly red with NO white striations from the fibers
Patterns:
1. Diffuse loss
- generalized loss of striations
- sharper outline of vessels in involved areas
- Localized loss
- wedge-shaped hyporeflective area = drop-out
- follows the distribution of nerve fiber layer
What is peripapillary atrophy?
Alpha Zone
- patchy area with hypo and hyperpigmentation interspersed with each other
- immediately adjacent to the optic disc
- NSR, RPE and Bruch’s membrane INTACT
- (+) in normal and glaucomatous eyes
Beta Zone
- NOT normal: (+) in glaucomatous eyes
- formed due to the ATROPHY of the RPE, Bruch’s membrane and choriocapillaris
- large choroidal vessels and sclera visible
- denotes deeper and larger atrophy
- width INVERSELY proportional to rim width in the same area: larger beta zone –> thinner rim
NORMAL: alpha zone ADJACENT to disc
GLAUCOMA:
- beta zone appears adjacent to disc
- alpha zone moves further peripherally
- PROGRESSION:
+ WIDENING beta zone
+ MORE PERIPHERAL alpha zone
How does optic disc hemorrhage correlate with glaucoma?
- N: disappears after 2 - 6 months
- indicates glaucoma PROGRESSION: ongoing damage to the ONH
- increases risk of conversion to POAG by 6x
- recurrence increases risk of ON damage by 3-4x
What diagnostic tests are used to examine the optic disc and the RNFL in glaucoma?
A. Clinical Evaluation:
- subjective
- qualitative
Optic Disc Drawing
1. Direct Ophthalmoscopy
- higher magnification than IO but limited field
2. Indirect Ophthalmoscopy
- smaller view compared to other diagnostics
- lower magnification: ONH not as detailed
3. Slit Lamp Biomicroscopy (Volk 60D, 90D)
Fundus Photographs
1. Stereoscopic Optic Disc Photography
2. Red-Free RNFL Photography
B. Imaging:
- objective
- quantitative
- ideal for monitoring of disease progression
- Optical Coherence Tomography
- Confocal Scanning Laser Ophthalmoscopy
What are the parts of the optic disc drawings?
Parts:
1. Optic disc: indicate size
2. Neuroretinal Rim:
- include defining vessels
- (+/-) notches, shelving, vessel bayonetting
- key areas: indicate rim-disc ratio
- narrowest part: indicate vertical cup-disc ratio
3. RNFL defects
4. Peripapillary atrophy: alpha and beta zones
5. Hemorrhages
Advantages
- inexpensive
Disadvantages
- subjective
- qualitative: NOT ideal for monitoring
- can be inaccurate due to lack of details
What are advantages and disadvantages of fundus photographs?
- Stereoscopic Optic Disc Photography
- compare 3D structure of ONH between visits - Red-free RNFL Photography
- difficult to assess in light-colored fundus and those with tessellated fundus (choroidal sclerosis)
Advantages:
- simple
- inexpensive
Disadvantages:
- slightly subjective: no standardized system for interpretation
- need to obtain good quality photos for correct interpretation
- affected by media or lens opacities
- pupils have to be dilated
What imaging modalities can be used to QUANTITATIVELY assess the optic disc and RNFL?
A. Optical Coherence Tomography (OCT)
- most commonly used
1. Time-domain (TD) OCT: Stratus OCT
2. Spectral-domain (SD) OCT: Cirrus OCT
3. Swept-source (SS) OCT: can assess as deep as the lamina cribrosa
B. Scanning Laser Ophthalmoscopy (SLO)
- Heidelberg Retina Tomography (HRT)
C. Spectralis: SD OCT + SLO
What is Optical Coherence Tomography?
- Imaging technique that uses low-coherence light to capture micron-resolution, 2D and 3D images from within an optical media
- Multiple A scans fired in rapid succession
- Tomography: generating a 2D image of a section through a 3D object using a penetrating wave
- Based on low-coherence interferometry using:
1. Michelson interferometer
2. Near-infrared light: wavelength ~ 800 nm - Long wavelength light: allows penetration into the scattering medium
- Analogue: B-Mode Ultrasound
+ But uses light instead of sound
+ Light travels faster permitting > 100X greater resolution
How does it work?
- relies on optical differences of tissues
- Light source is directed towards a beam splitter which splits beam into two.
- One beam directed into a reference mirror while the other is directed into the retina.
- Reflections of both light waves are received by an interferometer which superimposes the two to form an interference pattern.
- Interference pattern is analyzed by a detector and forms a 2D or 3D image of the retina.
What are the three basic kinds of OCTs?
- Time Domain OCT: lowest resolution
- Spectral Domain OCT
- Swept Source OCT: highest resolution similar to a histologic section even at the fovea
How does a Time Domain OCT work?
- Light source: broad band of light from a super luminescent diode
- Reference mirror: moves back and forth
- Image: 10 um (grainy)
- Disadvantage:
+ Mirror movement and speed determines image quality and resolution (TIME-domain)
+ Speed of movement is limited thus image is poorer in quality
How does a Spectral Domain OCT work?
- Light source: broad band of light from a super luminescent diode (similar to TD-OCT)
- Reference mirror: FIXED
- Utilizes the Fourier analysis: converts a signal from its original domain (often time) to a representation in the frequency domain
- Frequency domain: analysis of signals with respect to frequency, rather than time.
- Image: 5-7 um (better resolution than TD-OCT)
- Added components:
1. Diffraction Grating Detector: - receives interference pattern from the interferometer
- separates the light wave into its component wavelengths
2. Spectrophotometer or Spectrum Analyzer - receives light from the DGD
- analyzes spectrum of light and forms a spectral interferogram
- processed interferogram subjected to fast Fourier transform to form the final image
