Download How Valuable Is Anterior Segment Imaging?

COVER STORY
ONLINE SURVEY
How Valuable Is
Anterior Segment
Imaging?
Imaging devices are useful for further screening after first-line diagnostic assessment.
BY SHAN LIN, MD
T
he primary value of anterior segment imaging in glaucoma care is for the detection and
quantification of angle closure. Angle-closure
glaucoma (ACG) is a major cause of blindness
worldwide.1 It should be distinguished from open-angle
glaucoma, because the treatment algorithms for each
disease differ significantly, with potentially devastating
results for patients in whom angle closure is not identified. The prevalence of ACG is especially high among
Asians1: in China, ACG is the leading cause of bilateral
blindness.2 That said, although ACG and the anatomic
risk factors for this disease are most often identified in
Asians,3-7 the prevalence of this disease is not insignificant in whites.8
Even as the role of anterior segment imaging in glaucoma care grows, many glaucoma specialists are questioning if it is imperative to have such a device in the
office, especially if gonioscopy is the gold standard for
assessing closed angles.
GONIOSCOPY
Gonioscopy has many advantages, including rapidity of use, low to no expense, the ability to distinguish
synechial from appositional closure, a 360º view, and a
direct view of the angle recess. Unfortunately, gonioscopy is also subjective and thus has a lot of variability.9
Moreover, the technique is uncomfortable for both
the patient and the surgeon. It is generally accepted
that inadvertent compression during gonioscopy with
Figure 1. An ultrasound biomicroscopy (UBM) image showing a narrow angle secondary to plateau iris anatomy. Note
the large, anteriorly positioned ciliary process causing angle
narrowing.
a Zeiss-style lens may lead to artifactual opening of the
angle and erroneous grading by the examiner.
Perhaps the greatest limitation of gonioscopy is its
lack of use. The examination is not performed in most
glaucoma cases,10 possibly because many physicians
feel that the assessment of the Van Herick grading is
sufficient to determine occludability of the angle. The
approach’s correlation with gonioscopy, although good,
SEPTEMBER/OCTOBER 2014 GLAUCOMA TODAY 53
COVER STORY
resolution can be up to 5 µm with
Fourier-domain OCT (FD-OCT). The
Visante OCT (Carl Zeiss Meditec)
is a time-domain OCT device that
scans the anterior segment, generating images that extend from limbus
to limbus. FD-OCT units intended
for posterior segment imaging are
increasingly evolving to include anterior segment imaging. Compared to
traditional time-domain AS-OCT
devices, FD-OCT units have shorter
wavelengths (typically 800-840 nm)
and are less able to penetrate the
angle recess. Although limbus-tolimbus scans are possible, the top of
Figure 2. An anterior segment OCT (AS-OCT) image showing mild to moderate
the cornea is usually removed from
pupillary block causing narrowing of the angles. Note the anterior bowing of
the image, because there is a limited
the iris.
depth of imaging.
A new swept source FD-OCT device
is not perfect, however, and there is a high probability
(Casia SS-1000; Tomey Corporation)
of missing clinically significant cases when only the Van can rapidly obtain high-resolution scans with threeHerick method is used.11 Also, peripheral anterior syndimensional reconstruction of the anterior segment.
echiae or vertical quadrants cannot be viewed with this
Using a 1,310-nm light source, similar to the Visante
technique.
OCT, the Casia SS-1000 can penetrate the angle recess.
Approval of the device by the FDA is pending.
ULTRASOUND BIOMICROSCOPY
Downsides of OCT include a relatively high cost and
Ultrasound biomicroscopy (UBM) is a relatively lowan inability to image deep to uveal tissues, including
cost imaging technology that typically ranges from 25 to
the iris.
75 MHz and has a resolution of about 50 µm (Figure 1).
Traditional UBM involves the placement of an eyecup
OTHER MODALITIES
and a coupling solution for transmission of the echogenic
Other modalities for imaging the anterior segment
signal while the patient is supine. More recent advances
include the Scanning Peripheral Anterior Chamber
include containment of the coupling solution within a
Depth Analyzer (model SM-70; Takagi Seiko) and the
condom tip or plastic tip that is placed directly on the
EyeCam (Clarity Medical Systems). Although the former
ocular surface. UBM captures cross-sectional images
is a noncontact device and is relatively inexpensive, it has
of the anterior segment, typically to a depth of 5 mm.
poor resolution and often provides poor visualization of
Compared to optically based devices, UBM has the
the angle recess. The EyeCam uses the RetCam (Clarity
advantage of penetrating the iris and imaging the ciliary
Medical Systems) to allow gonioscopy-like imaging of
processes, important for distinguishing the anatomy of
the angle. It requires direct contact with the eye using a
the plateau iris from other causes of narrow or closed
coupling agent.
angles (Figure 2).
Disadvantages of UBM include the need for a skilled
CLINICAL RECOMMENDATIONS
technician, the potential for corneal abrasion or discomIn my opinion, slit-lamp examination and gonioscopy
fort due to its contact nature, the possibility of ocular
are still the first line of diagnostic assessment for angle
compression if the contact tip is used, and the difference closure. If found to have narrow or closed angles, my
in anatomy related to supine positioning for the eyecup
patients undergo AS-OCT for further screening. I also
approach.
order AS-OCT imaging to determine if the angle has
opened adequately after a laser peripheral iridotomy. I
OPTICAL COHERENCE TOMOGRAPHY
may request UBM when I suspect plateau iris, iris cysts, a
AS-OCT obtains a high-resolution cross-section of
tumor, or other secondary causes of angle closure.
the anterior segment without touching the eye. The
(Continued on page 58)
54 GLAUCOMA TODAY SEPTEMBER/OCTOBER 2014
COVER STORY
(Continued from page 54)
Weigh in on
this topic now!
https://www.surveymonkey.com/s/GToday25
1. Do you currently use anterior segment imaging in
your practice?
Yes
No
2. Are you considering purchasing a device to image
the anterior segment?
Yes
No
CONCLUSION
Anterior segment imaging is an important facet of
glaucoma care. The various devices have their own
pros and cons, and the decision to use these modalities
depends on their cost, technicians’ expertise, and medical professionals’ clinical preferences. n
Shan Lin, MD, is a professor of clinical ophthalmology and the director of the Glaucoma Service,
Department of Ophthalmology, University of
California, San Francisco. He has a research relationship with Carl Zeiss Meditec. Dr. Lin may be
reached at (415) 514-0952; [email protected].
1. Tham YC, Li X, Wong TY, et al. Global prevalence of glaucoma and projections of glaucoma burden through 2040: a
systematic review and meta-analysis. Ophthalmology. In press.
2. Foster PJ, Johnson GJ. Glaucoma in China: how big is the problem? Br J Ophthalmol. 2001;85(11):1277-1282.
3. Wang D, Huang G, He M, et al. Comparison of anterior ocular segment biometry features and related factors among
American Caucasians, American Chinese, and Mainland Chinese. Clin Experiment Ophthalmol. 2012;40(6):542-549.
4. Wang D, Qi M, He M, et al. Ethnic difference of the anterior chamber area and volume and its association with angle
width. Invest Ophthalmol Vis Sci. 2012;53(6):3139-3144.
5. Wang D, He M, Wu L, et al. Differences in iris structural measurements among American Caucasians, American
Chinese, and Mainland Chinese. Clin Experiment Ophthalmol. 2012;40(2):162-169.
6. Wang D, He Mingguang, Wu Lingling, et al. Dark-light change of iris parameters and related factors among
American Caucasians, American Chinese, and Mainland Chinese. Curr Eye Res. 2012;37(7):599-605.
7. Wang D, Chiu C, He M, et al. Differences in baseline dark and the dark-to-light changes in anterior chamber angle
parameters in whites and ethnic Chinese. Invest Ophthalmol Vis Sci. 2011;52(13):9404-9410.
8. Day AC, Baio G, Gazzard G, et al. The prevalence of primary angle closure glaucoma in European derived populations:
a systematic review. Br J Ophthalmol. 2012;96(9):1162-1167.
9. Congdon NG, Spaeth GL, Augsburger J, et al. A proposed simple method for measurement in the anterior chamber
angle: biometric gonioscopy. Ophthalmology. 1999;106(11):2161-2167.
10. Hertzog LH, Albrecht KG, LaBree L, Lee PP. Glaucoma care and conformance with preferred practice patterns.
Examination of the private, community-based ophthalmologist. Ophthalmology. 1996;103(7):1009-1013.
11. Park SB, Sung KR, Kang SY, et al. Assessment of narrow angles by gonioscopy, Van Herick method and anterior
segment optical coherence tomography. Jpn J Ophthalmol. 2011;55(4):343-350.
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