The
Topography of the Optic Nerve and Retina: What’s Hot
Joel
S. Schuman, MD
1)
Optic Nerve Imaging: Confocal
Scanning Laser Ophthalmoscopy
a)
An illuminating laser is scanned across the retina along
with the detector system. Only
a small spot is illuminated, and light can only enter the detector
through a pinhole aperture. This
permits only light from the region of interest to reach the detector;
all other light is blocked. Reflections
from out of focus planes are blocked by the pinhole, allowing
optical sectioning of the tissue in coronal planes.
b)
The commercially available confocal
laser scanning ophthalmoscope, the Heidelberg Retinal Tomograph
(HRT II, Heidelberg Engineering, GmbH, Heidelberg, Germany) uses a diode laser for retinal and ONH illumination.
c)
The HRT II each produces a series of 16-64 coronal planes
through the ONH. The number
of planes varies dependent on the scan depth.
d)
Minimum pupil size is 1 mm; image acquisition time is
0.9-1.5 seconds.
e)
The HRT II produces an excellent map of the optic nerve,
delineating the cup and disc, as well as the sloping area from
neuroretinal rim to cup. The
printout provides numerous parameters which can be followed. The most valuable of these include Cup Shape
Measure (the more negative the better), Rim Area and Rim Volume
(the higher the better), as well as several NFL height measures. Valuable discriminant
parameters are also calculated by the software.
The most clinically useful analysis uses the Moorfield’s
Algorithm, which evaluates the ONH by sector and classifies each
sector and the overall ONH as within normal limits, borderline
or outside normal limits.
f)
Patients can be followed sequentially
using software to detect change over time.
g)
Limitations of CSLO
i)
Variability of measurements is
<100 µm in vivo.
ii)
Because the axial resolution is only 300 µm cross‑sectional
imaging of the fundus and retinal nerve
fiber layer (RNFL) with confocal scanning
laser ophthalmoscopes is not feasible.
iii)
Due to limited depth resolution, the substructure of
the retina cannot be resolved, and only mean depth measurements
can be made accurately. Nerve
fiber layer thickness cannot be determined with precision,
as the posterior margin of the NFL is not easily detected using
this instrumentation; however, NFL surface contour is mapped.
2)
Scanning Laser
Polarimetry
a)
Scanning laser polarimetry
(SLP) is performed with the GDx (Nerve
Fiber Analyzer, NFA, Laser Diagnostic Technologies, Inc., San Diego, CA), and more recently the GDx
with a variable corneal compensator (GDx
VCC, see (v) below).
i)
Uses birefringence of NFL to change
the polarization rotation of light illuminating eye.
NFL thickness is measured from the change in polarization
called retaradance.
ii)
Reproducibility of measurements made with the NFA is
about 13 µm or better.
iii)
Limitations
(1)
Several assumptions are made in order to calculate NFL
thickness using this technique:
(a)
The NFL is the main source of birefringence in the retina.
(b)
Most of the light is reflected from the outer margin
of the retina, and therefore, double passes the full thickness
of the NFL.
(c)
The birefringence of the NFL is homogeneous.
(2)
There are birefringent structures
in the eye other than the NFL, such as the lens and cornea, and
their effect on polarized light may change over time, independent
of the NFL.
(3)
LASIK and PRK can significantly affect GDx measurements, most likely due to changes in corneal birefringence.
iv)
The GDx device has a large
normative database, and several studies have shown concordance
between GDx measurements and glaucoma status; however, discrepancies
are also found in increasing numbers as use of this unit becomes
more prevalent.
v)
The new version of the SLP, the GDx
VCC, promises to neutralize anterior segment birefringence actively,
by minimizing birefringence detected on macular scanning. Early reports support the improved discriminating
power of this modified device.
3)
Optical Coherence
Tomography (OCT)
a)
Optical coherence tomography (OCT Carl Zeiss Meditec, Dublin, CA) is a technique for high-resolution cross-sectional
optical imaging of ocular structures using light.
b)
OCT is a noncontact, noninvasive
tomographic imaging technique utilizing
short coherence length light to achieve a high resolution of about
8-10 µm with high sensitivity (OCT 3). OCT 1 and OCT 2 have axial
resolutions of ~10-15 µm.
c)
A superluminescent diode serves
as the light source for an interferometer-based fiber-optic system.
The light beam is scanned transversely across the eye, analogous
to B-mode ultrasound, to produce a cross-sectional image of the
tissue of interest, in this case the retina and optic nerve head
(ONH).
d)
Images
i)
The OCT and video image are displayed
in real time, the OCT signal being transmitted to a computer for
analysis.
ii)
Image acquisition time is < 1 second.
iii)
512 A-scans are used to create a single standard OCT
image.
(1)
There are 256 A-scans in each peripapillary
“fast” scan.
(2)
There are 128 A-scans in each optic nerve head or macular
“fast” scan.
iv)
The image displayed is a cross-section in false color,
with color corresponding to the strength of the reflected signal.
e)
Patterns
i)
Scans can be taken in any geometric
pattern. For glaucoma, circular scans around the ONH and linear
scans across the ONH have proven most valuable.
ii)
The circular scans around the ONH produce a cylinder
of information, which is unfolded and viewed in cross section.
A circle diameter of 3.37 mm has proven optimal for NFL assessment
in normal and glaucomatous eyes.
iii)
NFL thickness is measured directly from the scan using
an automated computer algorithm. NFL and total retinal thickness
are summarized by clock hour and by quadrant, as well as by overall
mean thickness.
iv)
Unlike ONH analyzers and confocal
scanning laser ophthalmoscopes, no reference plane is required
to determine NFL thickness, since OCT provides an absolute cross-sectional
measurement of retinal substructure from which the NFL thickness
is calculated.
f)
Fixation
i)
Fixation is maintained by means
of an internal fixation light. A beam is placed in the fovea.
The difference in position between that beam and the circle around
the ONH is recorded, and the same position is used on all subsequent
scans.
g)
Reproducibility of Measurements
i)
Standard deviation of measurements
is about 8–10 µm.
h)
Advances
i)
Advances, such as ultrahigh resolution
OCT (UHR OCT, not commercially available), provide 2–3 µm
resolution.
i)
Limitations
i)
There is no normative database
as yet for OCT.
ii)
The technology is young, still in evolution.
iii)
One cannot obtain an OCT image with media opacities
such as dense central corneal scarring, moderate to severe posterior
subcapsular cataract, or dense vitreous hemorrhage.
j)
Structure and Function
i)
There is good structural and functional
correlation in normal and glaucomatous eyes evaluated with OCT
and other technologies; OCT may perform better than other structural
measures (CSLO, SLP) or functional measures (SAP, SITA, SWAP,
FDT).
k)
Normal versus Glaucoma
i)
There is a significant difference
in NFL thickness between normal and glaucomatous eyes.
ii)
NFL thickness correlates with visual field defects:
thinner inferiorly with superior defects, and thinner superiorly
with inferior defects.
iii)
Focal NFL defects are readily seen and quantitated, and they can be followed over time.
iv)
Although there is no relationship between cup-to-disc
ratio or neuroretinal rim area and aging,
NFL thickness does thin with increasing age.
l)
Macular Thickness and Glaucoma
i)
Ganglion cells are specifically
lost in glaucoma.
ii)
The macula is defined anatomically as that region of
the retina where the ganglion cell layer is more than one cell
thick.
iii)
A ganglion cell body (soma) is 15 microns or more in
size.
iv)
A ganglion cell axon is 1–2 microns in size.
v)
Most ganglion cells are in the macula.
vi)
Macular OCT does not sample regions outside macula.
vii)
360 degree circumpapillary
scan includes entire retina.
viii)
Each axon is thinner than its cell body.
m)
Glaucoma Diagnosis
i)
Retinal nerve fiber layer thickness
evaluation
ii)
Macular ganglion cell layer thickness evaluation
n)
Ultrahigh Resolution OCT
i)
New technology for improving the
resolution of optical coherence tomography (OCT)
ii)
Allows enhanced imaging of retinal microstructures
iii)
Retinal anatomy and pathology evaluation
iv)
Longitudinal resolution of retinal imaging with OCT
is mainly determined by the bandwidth of the low coherence light
source used for imaging.
v)
Conventional ophthalmic OCT
(1)
Superluminescent diodes emit low coherence light with a 20–25
nm bandwidth at 830 nm.
(2)
Superluminescent diodes yield a 10–15 μm
longitudinal image resolution (OCT 1 or OCT 2), or 8-10 μm
with dispersion compensation (OCT 3).
vi)
The resolution determines the performance of OCT retinal
imaging and the performance for quantifying the retinal and RNFL
thickness.
vii)
This is a novel high-resolution ophthalmic OCT system
for improving the longitudinal resolution of OCT by a factor of
~3x
viii)
Short pulse Ti:Al2O3 laser
(1)
Pulses less than 7 fs duration
(2)
Bandwidths up to 200 nm at 800 nm center wavelength
ix)
OCT imaging in the eyes of normal volunteers and subjects
with eye pathology
(1)
Longitudinal resolutions of less than 3 μm
(2)
Highest OCT resolutions achieved for retinal imaging
x)
Higher longitudinal resolution can contribute to a better
visualization of intraretinal structures
and pathologies.
xi)
Higher longitudinal resolution can enable increased
sensitivity and specificity of retinal and RNFL thickness changes
for early diagnosis.
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