Get Familiar With SD-OCT
Spectral domain allows you to detect and monitor several retinal pathologies that you might not see ophthalmoscopically. Here’s how to effectively interpret SD-OCT imagery.
Release Date: June 2009
Expiration Date: June 30, 2012
Consistent advancements in imaging technology have allowed us to better examine various ocular pathologies. The most recent advance in imaging, spectral-domain optical coherence tomography (SD-OCT), provides a level of detail that parallels—and even enhances—histological observation of retinal integrity. This article provides an extensive tutorial on how to interpret SD-OCT images of outer, middle and inner retinal layer pathologies.
Samantha Slotnick, O.D., and Jerome Sherman, O.D.
This course is COPE approved for 2 hours of CE credit. COPE ID: 25588-PS. Please check your state licensing board to see if this approval counts towards your CE requirement for relicensure.
This continuing education course is joint-sponsored by the Pennsylvania College of Optometry.
Dr. Slotnick has no relationships to disclose. Dr. Sherman lectures on behalf of Carl Zeiss Meditec.
During the last 10 years, consistent advancements in
imaging technology have
allowed us to better examine various ocular pathologies. Several technologies, including optical
coherence tomography (OCT),
scanning laser ophthalmoscopy
(SLO) and scanning laser polarimetry (SLP), have expanded our ability to detect, follow and diagnose
retinal complications and glaucomatous presentations. Today, once
our suspicions have been raised, we
can employ such high-tech equipment to explore any presentation in
detail and monitor subtle changes
in both structure and function.
The most recent advance in
imaging, spectral-domain optical
coherence tomography (SD-OCT),
provides a level of detail that parallels—and even enhances—histological observation of retinal integrity.
Currently, several SD-OCT units
are available from different manufacturers, including 3D OCT-1000
(Topcon), Cirrus HD-OCT (Carl
Zeiss Meditec), Spectralis OCT
(Heidelberg Engineering), RTVue-100 (OptoVue) and 3D SD-OCT
Even in the absence of clinical
signs and/or symptoms, SD-OCT
allows us to detect retinal pathologies that might not be seen
SD-OCT enables us to gain more
detailed insight into the etiology of
clinical symptoms and signs than
we can detect ophthalmoscopically,
but cannot accurately diagnose
based upon clinical presentation
alone. With this deeper understanding of the problem at hand, SDOCT might even present an
opportunity for intervention before
an emerging pathology becomes
However, to take full advantage
of SD-OCT’s enhanced capabilities,
we must be able to interpret the
data it produces effectively. So,
drawing upon five case reports, this article provides an extensive tutorial on how to interpret SD-OCT
images of outer, middle and inner
retinal layer pathologies.
SD-OCT of a Healthy Eye
Like conventional time-domain
optical coherence tomography (TDOCT), SD-OCT shows a cross-section of the retina, which appears as
a histological slice perpendicular to
the fundus. OCT technology identifies changes in optical density and
illustrates them in a color-coded
format. When two adjacent structures demonstrate large differences
in refractive index, a greater volume
of light is reflected at their interface.
Large reflections are depicted by
warm colors (red through yellow)
and mild reflections are depicted by
cool colors (green through blue).
An absence of reflection appears
black. Images in grayscale utilize
brighter shading in lieu of warmer
OCT images do not, in fact,
depict histological layers, but rather
reflective interfaces between structures. So, the layers that appear in a
normal SD-OCT image show differences in optical density––some of
which occur within a single histo-logical cell layer. Cell bodies, for
example, are far denser than the
dendritic aspects of the cell, and
photoreceptor outer segments are
optically denser than photoreceptor
• Figure 1 depicts an SD-OCT
slice through a healthy fovea. The
well-organized structures of the
retina form distinct layers. There is
a marked change in reflectivity
between the vitreous and the internal limiting membrane/retinal nerve
fiber layer, depicting the vitreoretinal interface. Proceeding from inner
to outer retina, the ganglion cell
bodies, the inner plexiform layer
and the outer plexiform layer show
heightened zones of reflectivity.
Continuing outward, a relatively
dark area depicts a
lack of change in
optical density as
light passes through
the tightly packed
outer nuclear layer.
A fine, but distinct, boundary
appears next. This
boundary demarcates the adjacent
inner and outer segments of the photoreceptors. The
layer is extremely
demonstrating both horizontal
alignment between adjacent photoreceptor cells and vertical
alignment with an orientation perpendicular to the fundus. This nonhistological boundary is referred to
as the photoreceptor integrity line
(PIL), and it serves as the junction
between the inner and outer segments of the photoreceptors. On
SD-OCT, the PIL is present in virtually all healthy eyes.
The next distinct boundary
occurs at the interface between the
outer segments of the photoreceptors and the retinal pigment epithelium (RPE).
Finally, the choroid is the most
external layer, which also demonstrates areas of dense reflectivity.
Figure 1 depicts the foveal pit
while simultaneously demonstrating
intact and continuous seeing structures of the fovea. Here, the PIL
provides evidence of an intact photoreceptor layer. The dark zone
beneath the foveal pit (between the
PIL and RPE boundaries) is
normal––not a cyst. This area
represents the extent of the photoreceptors’ outer segments, which
are elongated at the fovea.
• Figure 2 shows another healthy
patient with intact cell layers. In this grayscale scan, a
band of highly reflective intensity is
observed at the continuous PIL,
above the RPE. The fundus image
shows the location of the scan
(superior to the macula), which
transects a large blood vessel. The
dim area observed on the line scan
at the cursor’s location is not a retinal defect, but a result of the blood
vessel attenuating the amount of
light reaching the underlying anatomy. It is important to recognize
that this attenuation occurs evenly
to all layers below the level of the
blood vessel. The PIL is faint but
present, as is the RPE.
A closer look reveals several
small areas of apparent signal dropout along the length of the line
scan. The accompanying fundus
photo reveals that this slice was
made through several arterioles
much smaller than the branch retinal artery, and their presence is also
depicted as vertical linear shadows
with equal signal attenuation
As we proceed through the following cases, keep in mind the
ways that the SD-OCT images have
been formed in addition to retinal
anatomy. It is critical to understand
how the technology works to effectively interpret the imagery.
Case 1: Outer Retinal
• History and diagnostic data. A
78-year-old male presented with no
ocular complaints; he came in only
because he had broken his glasses.
His best-corrected visual acuity
measured 20/30 O.U. On fundus
examination, we identified some
pigment mottling O.D. and two
subtle areas of geographic atrophy
of the RPE, both superior and inferior to the fovea O.S.
How does a patient with geographic atrophy maintain 20/30
visual acuity? What is the effect of
geographic atrophy on retinal
integrity? Where is the disruption
to the visual pathway? Is this disruption reversible, now or in the
future? Close inspection of figure 3
provides answers to these particular
• Discussion. The area indicated
centrally (at the red arrow),
beneath the foveal pit, shows an
intact PIL. This patient was fortunate because the central fovea was
not impacted, preserving visual acuity in the left eye.
The PIL and RPE are interrupted
both superior and inferior to the
fovea, coincident with the zones of
geographic atrophy seen in the fundus photo. Both the PIL and RPE
re-emerge as a unit in the inferior
aspect of the scan.
SD-OCT clarifies the physiological effect of dry macular degeneration, which often progresses to
geographic atrophy.1 This process
causes deterioration of the RPE,
which results in apoptosis of the
Take a moment to recall the
effect of blood vessels in the normal
eye (figure 2)–– additional light is
absorbed by the blood vessel,
resulting in the attenuation of light
transmitted to (and reflected from)
the areas below the vessel. In figure
3, however, note that the areas
below the geographic atrophy show
a much more detailed appreciation
of the choroid, including visualization of blood vessel cross-sections
in this underlying space. Why are the blood vessels apparent here, but
not underneath the areas where the
PIL and RPE are still intact? Where
these two highly reflective boundaries (the PIL and RPE) are missing,
more light is transmitted to the
structures below. Accordingly, both
Bruch’s membrane and the choroid
are more clearly visualized.
What can we appreciate about
the integrity of the retina’s inner
layers? SD-OCT shows that the
overlying portions maintain their
structure and organization. If the
unaffected components of the visual
pathway are intact (without ischemia, there is no present cause for
cell death), then the potential to
transfer visual information to the
The potential for sight exists if
technological advancements provide a method for light detection
and signal transfer to the visual
pathway with retinal implantation
above the areas of geographic
atrophy. Currently, this sort of
cutting-edge technology is under
development, and it is important to
understand in which cases the
potential for sight remains.
Case 2: Outer Retinal
• History and diagnostic data. A
16-year-old white female presented
after she failed a high school eye
screening.2 She reported blurred
vision, but claimed that her acuity
had been stable during the last several years. Clinical evaluation
revealed a small refractive error
with unreliable responses on subjective testing. Her best-corrected visual acuity was 20/60 O.D. and 20/70
O.S. The patient correctly identified
2/8 Ishihara color plates. No nystagmus was observed. Fundus
examination revealed a very subtle
macular lesion O.U. Carefully evaluate figure 4 before reading on.
• Discussion. SD-OCT reveals a
distinct gap beneath the foveal pit.
The PIL is absent in an area that
corresponds to the central 5° of the
visual field. A lack of a PIL explains
the reduced visual acuity; but, what
etiology explains the lack of a PIL?
Recall the patient’s clinical profile––she is a 16-year-old female
who performed poorly on color
vision testing. Her best visual acuity
measures about 20/60. The gap in
the SD-OCT is limited to the central 5°. Recall that the cones are
concentrated at the fovea and that
the rods predominate outside of the
central 5°. The most likely diagnosis is incomplete rod monochromatism. There is some evidence of
cone preservation, which explains
why her vision is relatively good.
Additional evidence on SD-OCT
confirms a lack of cones. Note that
there is an area of increased reflectance above the gap in the PIL,
and that this interface is not
observed in the adjacent areas
where the PIL is present. Where the
cones are absent, there is a distinct
lack of cells beyond the outer limiting membrane (likely composed of
Mueller cells). The increase in
reflectivity represents the transition
between the presence and absence
of cells beyond the outer limiting
membrane. The next area of
reflectivity occurs at the interface
between this gap in the photoreceptors and the RPE.
Careful inspection of the SDOCT image reveals the presence of
some cones in this gap area, which
is consistent with limited color
detection and a moderately reduced
visual acuity level.
Case 3: Middle Retinal
• History and diagnostic data. A
75-year-old Hispanic male presented with blurred vision O.D. that
had persisted for several months.
His general health was unremarkable. Clinical evaluation revealed
4.00D of hyperopia O.U., with a
best-corrected visual acuity of
20/40 O.D. and 20/25- O.S.
On fundus examination, we
noted a slight elevation of the macular area O.D. We performed SDOCT imaging to investigate the
nature of the probable macular elevation O.D. Figure 5 shows both the fundus and the SD-OCT cross-section taken through the macula
and the optic nerve head. Evaluate
this image before reading on.
• Discussion. There are several
common differential diagnoses for
retinal elevation near the macula,
including intraretinal cystoid macular edema, central serous chorioretinopathy, retinoschisis and
retinal detachment.3,4 This SD-OCT
image identifies several locations of
intraretinal insult. We can see multiple retinal splits that extend from
the optic disc to the macula O.D.
with cystoid spaces occurring
between different retinal layers.
Now, assess the area below the
foveal depression. What is the
potential for vision here? Note that
the PIL remains intact, although
less organized in its present location. The thin layer visualized
above the PIL is the outer limiting
membrane, and this too remains
continuous temporal to the macula
and at the fovea. At the macula, the
retinal separation occurs between
the RPE and the PIL. The retinal
elevation may induce some distortion in addition to a hyperopic
refractive shift, but the acuity is
only mildly reduced to 20/40 O.D.,
which is consistent with the observation of a relatively intact PIL. A
separation that occurs between the
RPE and the photoreceptors is a
neurosensory retinal detachment.
Next, observe the cystoid areas
between the macula and the optic
nerve head within the layers of the
middle retina. The presentation
seen here is a retinoschisis.5 Retinoschisis may occur when multiple
cystoid spaces within the retinal
layers coalesce and divide the retina
into an inner layer and an outer
layer. Alternatively, the cystoid
spaces may be secondary to longstanding retinal splits.
A well-known etiology of
retinoschisis that occurs adjacent to
the optic nerve head is the presence
of an optic pit. Optic pits (a form of
coloboma) create a communication
for fluid between the optic cup and
retinal stroma. However, a review
of all 128 sections scanned around
this patient’s optic disc failed to
reveal an optic pit. Subsequently,
we performed a fluorescein
angiogram, which did not reveal
any leakage between the optic disc
and the area of the retinoschisis,
ruling out the presence of an optic
pit. Therefore, we considered this
presentation to be an idiopathic
retinoschisis. Whether intravitreal
anti-VEGF (vascular endothelial
growth factor) treatment or intravitreal gas injections to flatten the
macula would be of any value in
this case is unclear.
Case 4: Inner Retinal
• History and diagnostic data. A
73-year-old white male presented
for a routine eye exam with no
complaints. He was pseudophakic,
and his best-corrected visual acuity
was 20/25 O.U. Fundus examination revealed a probable epiretinal
membrane at the macula (EMM)
O.D. We performed SD-OCT imaging to better visualize the EMM.
Before reading on, examine figure
6, which demonstrates both the
fundus appearance O.D. and the
location of the SD-OCT section.
• Discussion. Upon evaluation of
the SD-OCT image, a few details
appear noteworthy. First, as this
section shows, the foveal pit was
not observed. (None of the sections
through the macula revealed a foveal pit.) The foveal pit appears
to be compromised because of traction created by the EMM.
The epiretinal membrane itself is
depicted on the SD-OCT scan as a
layer of increased reflectance that
overlies the center of the macula.
Centrally (beneath the EMM), the
macula appears distinctly thicker
than it does in the adjacent areas,
temporal and nasal to the membrane. In fact, it is possible to visualize an upsweep in the trajectory of
the middle retinal layers, which
point toward the membrane above.
Below the location of the EMM,
the PIL appears to be intact, explaining the nearly normal acuity.
The minor visual acuity reduction is
likely attributed to the epiretinal
membrane.6 This patient should be
monitored for changes in the
EMM. An Amsler grid may be used
to track progression at home.
Case 5: Inner Retinal
• History and diagnostic data. An 80-year-old black male presented for a routine eye exam. He has
had a history of type II diabetes for
about seven years, but has not
demonstrated diabetic retinopathy
to date. The patient’s best-corrected
visual acuity measured 20/30 O.U.
Fundus examination revealed a
subtle, round lesion at the macula
O.D. We performed an SD-OCT to
better assess the nature of the lesion
detected ophthalmoscopically. Before continuing, examine figure 7.
• Discussion. Evaluation of the
SD-OCT images reveals a surface of
reflectance above the level of the
retina. This additional surface
adheres to the retina at the fovea.
The retinal nerve fiber layer is clearly visualized below this additional
layer, confirming that the additional layer is internal to the retina.
What creates this additional
reflection? This is the posterior surface of the vitreous body, which has
begun to separate from the retina,
yet remains connected to the retina
via its macular adhesion. The distortion of the foveal pit at the location of this adhesion suggests that
this presentation is the result of vitreomacular traction. The subtle,
round lesion at the macula was created by a concentration of the vitreous at its central macular adhesion.
This is better seen on the 3-D SDOCT image. It is rather challenging
to appreciate vitreomacular traction
The 2-D image reveals that the
PIL is mostly intact through the
scan. A small interruption in the
PIL can be seen nasal to the fovea.
The 3-D image reveals the extent
of posterior hyaloid and the remaining traction of the macula. In
most cases, vitreomacular traction
subsides spontaneously as the posterior vitreal detachment becomes
complete. More infrequently, the traction results in a macular hole.
As these five cases show, SD-OCT is able to illustrate retinal
anatomy in better detail than clinical observation alone. Whether
exploring unsubstantiated clinical
symptoms or investigating a subtle
lesion detected ophthalmoscopically, SD-OCT provides a new dimension to our current clinical capacity.
And ultimately, the more familiar
we become with SD-OCT, the better we can care for our patients.
Dr. Slotnick is a behavioral
optometrist who practices in Dobbs
Ferry and Mahopac, N.Y. Dr. Sherman is a distinguished teaching professor at State University of New
York College of Optometry and the
Schnurmacher Institute of Vision
Research. He also practices at The
Eye Institute and Laser Center,
New York City, and is the current
president of the Optometric Retina
Society. They wish to thank Yuliya
Bababekova for her help in organizing this article.
- Lujan BJ, Rosenfeld PJ, Gregori G, et al. Spectral domain optical
coherence tomographic imaging of geographic atrophy. Ophthalmic
Surg Lasers Imaging 2009 Mar-Apr;40(2):96-101.
- Clinical case and imagery courtesy of Kerry Head, O.D. Photographs reprinted with permission of the author.
- Ozdemir H, Karacorlu M, Karacorlu SA. Serous detachment of
macula in cystoid macular edema associated with latanoprost. Eur J
Ophthalmol 2008 Nov-Dec;18(6):1014-6.
- Pollack AL. Peripheral retinoschisis and exudative retinal detachment in pars planitis. Retina 2002 Dec;22(6):719-24.
- Hassenstein A, Richard G. Optical coherence tomography in optic pit
and associated maculopathy. Ophthalmologe 2004 Feb;101(2): 170-6.
- Mitamura Y, Hirano K, Baba T, Yamamoto S. Correlation of visual
recovery with presence of photoreceptor inner/outer segment junction in optical coherence images after epiretinal membrane surgery.
Br J Ophthalmol 2009 Feb;93(2):171-5.