Anterior segment imaging in glaucoma: An updated review.

Anterior segment imaging allows for an objective method of visualizing the anterior segment angle. Two of the most commonly used devices for anterior segment imaging include the anterior segment optical coherence tomography (AS-OCT) and the ultrasound biomicroscopy (UBM). AS-OCT technology has several types, including time-domain, swept-source, and spectral-domain-based configurations. We performed a literature search on PubMed for articles containing the text "anterior segment OCT," "ultrasound biomicroscopy," and "anterior segment imaging" since 2004, with some pertinent references before 2004 included for completeness. This review compares the advantages and disadvantages of AS-OCT and UBM, and summarizes the most recent literature regarding the importance of these devices in glaucoma diagnosis and management. These devices not only aid in visualization of the angle, but also have important postsurgical applications in bleb and tube imaging.

Unresolved Issues and Take Home Message

Anterior segment imaging, including AS-OCT and UBM, are objective methods of visualizing the anterior segment angle

AS-OCT is most commonly used for appositional angle-closure and should be used when the cornea is clear and the patient can sit upright

UBM should be used when the cornea is cloudy, for an examination in the operating room, or if plateau iris, ciliary effusion syndrome, lens subluxation, ciliary body cyst, or tumor is suspected

Both AS-OCT and UBM are excellent for visualizing intrableb structure and glaucoma drainage device placement, and thus have important postsurgical implications

Recent advances in anterior segment imaging include irido-trabecular contact (ITC) index, a software that can estimate the percentage of angle-closure in a given eye once the scleral spur has been manually identified

Despite these advances in imaging, clinical examination cannot be replaced.

Visualization of the anterior chamber (AC) angle is a critical step in the diagnosis of glaucoma, especially angle-closure variants. Gonioscopy remains the clinical gold standard for the diagnosis of narrow angles; however, this method is fraught with several limitations. Gonioscopy is subjective and highly dependent on the examiner's skill and interpretation and the patient's cooperation. For example, a closed or narrow angle may erroneously appear open if too much pressure is exerted by the examiner during the dynamic gonioscopy examination or if too much light is shone into the eye.[1] Practically, many ophthalmologists do not include gonioscopy in their routine examination or even in the examination of glaucoma patients.[23]

Recently, the advent of anterior segment imaging devices has allowed for an objective quantitative method of analyzing the AC angle. This review will discuss the two most common types of anterior segment imaging devices, anterior segment optical coherence tomography (AS-OCT), and ultrasound biomicroscopy (UBM), and their clinical applications in the field of glaucoma.

Anterior Segment Optical Coherence Tomography

Background

The AS-OCT is a noncontact, rapid imaging device that uses low-coherence interferometry to obtain cross-sectional images of the anterior segment.[4] The measurements are semiautomated and have good reproducibility,[56] and, unlike gonioscopy, it is not operator dependent. The AS-OCT can be classified into time-domain (TD), swept-source (SS), and spectral-domain (SD) based configurations.[7]

Table 1 summarizes the features of each of these types. SS and SD-based imaging are considered a type of Fourier-domain (FD) OCT. Due to its inherent signal-to-noise ratio advantage, it has a higher imaging speed (up to 20–40 kHz line-scan rate) than those that are based in a TD configuration. Of the TD configuration of AS-OCT, there are two common commercially available types, the Visante OCT (Carl Zeiss Meditec, Dublin, CA, USA) and the slit-lamp OCT ([SL-OCT] Heidelberg Engineering, GmbH, Dossenheim, Germany). The Visante OCT is most commonly used in the United States and has a scan speed of 2000 A-scan/s and axial resolution of 18–25 µm. It uses a 1310 nm super luminescent light-emitting diode as a light source. The scan resolution is dependent on the wavelength of the light source with shorter wavelengths allowing for higher resolution images. The downside of a shorter wavelength is that it decreases the depth penetration of the photos. The SL-OCT is only available in Europe and scans at a speed of 200 A-scan/s. Spectralis OCT (Heidelberg Engineering, Heidelberg, Germany) is an SD AS-OCT and has an axial resolution of 3.9 µm. The resolution is better on a SD-OCT. The Spectralis OCT has enhanced depth imaging (EDI), which increases the imaging sensitivity of the structures at greater depth. The SS-OCT, utilizing an SS laser wavelength of 1310 nm based on FD technology and employing a scan speed of 30,000 A-scans/s and an axial resolution of 10 µm, has recently become commercially available and is able to capture extremely high-resolution images. One that is commonly used is the Casia OCT (Tomey, Nagoya, Japan). Less than 3 s are needed to image the angle morphology in high-resolution and circumferentially 360° [Fig. 1a and b].

Table 1

Available types of anterior segment OCT systems

Figure 1

(a) Casia anterior segment optical coherence tomography imaging of closed angles. C: Cornea, S: Sclera, I: Iris. (b) Casia anterior segment optical coherence tomography panoramic view of peripheral anterior synechiae, 360

Compared to UBM, AS-OCT achieves better resolution and does not require contact with the ocular surface.[8] However, the use of AS-OCT is not without disadvantages. Poor agreement between gonioscopic and AS-OCT findings has been reported in the literature. Reproducibility of the AS-OCT findings in the inferior quadrant is poor, due to the variable placement of the scleral spur, especially compared with the reproducibility in the nasal and temporal angles.[91011] The main limitation of AS-OCT is that the light energy cannot penetrate tissues behind the iris pigment epithelium, so AS-OCT cannot visualize any structures posterior to the iris pigment epithelium. Thus, AS-OCT is not useful in diagnoses such as plateau iris syndrome or phacomorphic angle-closure.

The biometric analysis of the AC angle requires a reference landmark from which the angle measurements are derived. Typically, the scleral spur is used as a reference point for parameters such as the iris area and volume,[1213] angle opening distance (AOD),[14] angle recess area,[15] scleral thickness,[16] trabecular meshwork-ciliary process distance,[16] trabecular iris angle,[14] and trabecular iris space area.[17] Other biometric parameters that can be measured by the AS-OCT include: Iris thickness, iris curvature, AC depth, AC width, and lens vault.[18] These parameters are further described in Table 2. Difficulty in identifying the scleral spur as a reference point has been cited in numerous prior studies, with a rate of 15–28% of AS-OCT images unable to identify the scleral spur.[1019] In the current literature, there is no consensus regarding the relationships of AS-OCT obtained measurements of the aqueous humor outflow structures to each other. One study demonstrated that the Spectralis OCT with EDI was able to identify the Schwalbe's line and scleral spur in all nasal and temporal scans.[20] In a recent study by Cheung et al., using a modified Cirrus SD OCT, the Schwalbe's line was identifiable in 95% of the scans and the scleral spur was identifiable in 85%.[21] In the Casia OCT, the scleral spur was identifiable in all subjects; however, Schlemm's canal was only identifiable in 32% of the scans. Its identification has also been previously reported to be subject to measurement error and variability.[891622]

Table 2

Biometric parameters which can be measured with the AS-OCT

Variables such as eye quadrant,[10] a smaller AC depth, or a diagnosis of narrow angle, shorter axial length, and older age[23] can all increase the difficulty of an accurate identification of the scleral spur. As the accurate identification of the position of the scleral spur using AS-OCT is very important, several studies have investigated the techniques to best identify the scleral spur. The three most common techniques are (1) location of Schwalbe's line relative to the scleral spur, (2) the intersection of the ciliary muscle (CM) and the inner corneal margin, and (3) a bump-like structure in the inner corneal-meshwork margin. A study by Seager et al. demonstrated that of these three different methods, the CM approach demonstrated the highest rate of scleral spur identification with the lowest intra-observer and inter-observer variability.[24]

Anatomically narrow angles or angle-closure glaucoma

For patients with primary angle-closure glaucoma, gonioscopy has historically been the gold standard to diagnose narrow angles. AS-OCT is known to have higher sensitivity when detecting angle-closure as compared to gonioscopy.[25] An excellent AS-OCT imaging of appositional angle-closure beginning at Schwalbe's line is shown in Fig. 2.

Figure 2

Anterior segment optical coherence tomography photo demonstrates S-type angle-closure with apposition beginning at Schwalbe's line. SS: Scleral spur

AS-OCT allows for better diagnosis of angle-closure glaucoma given its ease of use, nonoperator dependence, and objective measurements of important quantitative data. AS-OCT can detect clinically important changes in the AC angle structure in patients with angle-closure glaucoma under light versus dark conditions.[26]

While shallow AC depth and short axial length are known risk factors for the development of primary angle-closure glaucoma, AS-OCT has revealed that it is much more than just these two factors. Several new AS-OCT parameters have been associated with angle-closure, including smaller AC width, area, and volume;[2728] larger lens vault;[2930] and a greater iris thickness, curvature, and area.[31] AC area and volume and lens vault have been shown to be the most important determinants of angle width.[32] A study by Cheung et al. used AS-OCT to demonstrate that iris bowing is associated with angle width, independent of the AC depth.[33] AS-OCT has been used to understand the anatomic factors causing acute primary angle-closure glaucoma attacks. A recent study by Sng investigated AS-OCT measurements in 31 patients with unilateral acute primary angle-closure glaucoma prior to implementation of therapeutic interventions.[34] AS-OCT revealed that patients with acute primary angle-closure glaucoma tended to have smaller AC depth and iris curvature.[34] In pupillary block, the iris adopts a convex, forward-bowing appearance to its contour due to the pressure gradient between the AC and posterior chambers. The findings of another study in which AS-OCT was used on eyes with acute primary angle-closure glaucoma prior to therapy agreed with the above findings.[35] This study also revealed that these eyes had significantly greater lens vault compared to unaffected fellow eyes.[35]

AS-OCT is also excellent at measuring iris parameters, which gonioscopy cannot do. Increase in iris curvature, area, and thickness has been shown to be independently associated with narrow angles in prior studies.[1931]

AS-OCT has been used in eyes with narrow angles to demonstrate widening of the angles after laser peripheral iridotomy (LPI).[36] In addition, AS-OCT can be used to demonstrate whether an LPI is truly patent, which is sometimes difficult to evaluate on clinical examination [Fig. 3]. A recent longitudinal study using AS-OCT to measure angle structure 2 weeks and 6 months after the procedure found that while significant angle widening was measured after LPI early after the procedure, this widening was significantly reduced to 6 months, suggesting that nonpupillary block mechanisms may contribute to primary angle-closure glaucoma.[37] A stepwise regression model with variables from these AS-OCT parameters has been shown to have a high diagnostic capability in detecting patients with angle-closure glaucoma.[38]

Figure 3

(a) Visante anterior segment optical coherence tomography demonstrating residual membrane occluding laser peripheral iridotomy (arrow), though iris transillumination defect is apparent on slit-lamp examination. I: Iris. (b) Following an additional neodymium: yttrium-aluminum-garnet (Nd: YAG) laser shot, the laser peripheral iridotomy is patent with a residual “burst” of pigment (arrow) demonstrated on the anterior segment optical coherence tomography. I: Iris

The SS-OCT, given its ability to provide extremely high-resolution images, has been shown to have accurate and reproducible measurements of peripheral anterior synechia (PAS), which ordinary SD-OCT cannot measure well and gonioscopy cannot measure to the same degree of precision.[39] This may allow for an excellent method of monitoring risk assessment and PAS progression in the development of angle-closure glaucoma.[39] In addition, the presence of PAS can be confirmed on SL-OCT with an indentation technique applied to the cornea.[40] SS-OCT is also excellent at calculating iris volume, which has been shown to be an important determinant of the AC angle.[41]

In the Casia, manual identification of the scleral spur can allow the user to use the irido-trabecular contact (ITC) index, software that can estimate the percentage of angle-closure in a given eye once the scleral spur has been manually identified. A recent study by Baskaran et al. demonstrated that the ITC index has good diagnostic performance compared to gonioscopy for estimating the degree of angle-closure.[42] Indeed, automated angle grading by software programs such as the ITC index are likely in the future for AS-OCT. AS-OCT has proven to be an important imaging device in the detection and monitoring of eyes with angle-closure glaucoma and will continue to be so in the future.

Ultrasound Biomicroscopy

UBM provides high-definition, reliable, and repeatable images of the anterior segment, as well as quantitative measurements. UBM uses high frequency ultrasound at 50–100 MHz for anterior segment imaging. A computer program then converts these sound waves into a high-resolution B scan image. The probe provides a scan rate of 8 Hz, with a lateral resolution of 50 µm and an axial resolution of 25 µm.[4344]

UBM has previously been shown to have good agreement with gonioscopy in its ability to evaluate angle-closure when performed in a darkened room.[1] In addition, there are several advantages to UBM. Unlike AS-OCT, UBM can achieve visualization of structures posterior to the iris pigment epithelium[144344454647] as sound penetrates the pigment epithelium but light does not. Thus, UBM is better for visualizing the posterior chamber structures, including the lens zonules, ciliary body, and even the anterior choroid. Unlike AS-OCT, UBM can also be performed with the subject lying down, and thus it is useful in the operating room when an examination needs to be performed under anesthesia. Table 3 highlights the main differences between AS-OCT and UBM.

Table 3

Comparison of AS-OCT and UBM

There are some disadvantages of UBM. Prior studies have reported excellent intra-observer reproducibility but poor inter-observer reproducibility in assessing the AC angle or iris dimensions.[1648495051525354] There are two recent studies that investigated the repeatability of UBM in measurement of the ciliary sulcus diameter that demonstrated that the inter-observer reproducibility had more variability than the intra-observer measurements.[5556] In addition, UBM may have a narrower field of view compared to the AS-OCT.[17575859]

Manual identification of the scleral spur prior to measurements is important to the accuracy of the measurements, but there are several disadvantages to this method.[1660] There is currently no technology available that can automatically identify the scleral spur, but some programs are semi-automatic.[61] Using the scleral spur as a reference point allows UBM to make measurements of several angle parameters, including the trabecular iris angle, the AOD, the trabecular-ciliary process distance, iris thickness, iris ciliary process distance, iris-lens contact distance, iris zonular distance, AC angle, iris-lens angle, and AC depth.[62] Many of these parameters can also be measured with AS-OCT and are described further in Table 3.

Ultrasound biomicroscopy and glaucoma

UBM has been used to help lead further insight into the pathogenesis and mechanisms of several types of glaucoma. Indeed, because of its ability to image structures posterior to the iris, UBM has been especially useful for elucidating mechanisms of angle-closure, such as plateau iris, ciliary effusion syndrome, lens subluxation, ciliary body cyst, or tumor. Fig. 4a shows UBM of a ciliary body tumor extending up to the pars plana. On clinical examination, this patient had a pseudo-plateau iris configuration with PAS on gonioscopy [Fig. 4b].

Figure 4

(a) Ultrasound biomicroscopy of a ciliary body tumor extending up to the pars plana. C: Cornea, S: Sclera, CB: Ciliary body. (b) Peripheral anterior synechiae on gonioscopy in the same patient

An important study by Pavlin et al. demonstrated, using UBM, that those eyes with plateau iris syndrome have anteriorly situated ciliary processes.[46] UBM has been pivotal in developing further insights into this disease. It is an excellent method to clarify and confirm the clinical examination. Plateau iris has been previously defined as appearing as a relatively deep AC but with peripheral angle narrowing on gonioscopy. Notably, the angle then does not open adequately after peripheral iridotomy. In a more recent study by Mandell et al., they evaluated 181 eyes with plateau iris syndrome using UBM and found that the AC depth is shallower than that of normal eyes and shallower than that of eyes with pupillary block.[63] This is in contrast to previously held beliefs,[6465] that the AC in plateau iris syndrome is deep. This previously held belief is thought to be secondary to a clinical observation of deeper AC after iridotomy, which is when plateau iris syndrome is typically diagnosed. UBM is important in plateau iris syndrome as it allows for an image of the position of the ciliary processes in relationship to the iris, and it may indeed be the most definitive method of establishing this diagnosis.[46] UBM can be used to demonstrate changes in plateau iris before and after laser iridoplasty [Fig. 5].

Figure 5

Ultrasound biomicroscopy of plateau iris syndrome before and after laser irido-plasty. S: Sclera, CP: Ciliary sulcus, C: Cornea, AC: Anterior chamber, I: Iris, L: Lens

Pupillary block has previously been diagnosed when the iris appears bowed forward on SL biomicroscopy examination and the angle appears occludable on gonioscopy. Fig. 6 demonstrates pupillary block in lens-induced angle-closure. After peripheral iridotomy, the iris typically flattens out. This diagnosis can be confirmed with UBM as UBM allows imaging of the posterior iris epithelial surface and iris curvature.

Figure 6

Ultrasound biomicroscopy in a patient with lens-induced angle-closure. CB: Ciliary body, S: Sclera, C: Cornea, I: Iris

One of the first studies to demonstrate using UBM as an adjunct to the clinical examination in the diagnosis of pupillary block syndrome was published by Aslanides in 1995 and demonstrated that UBM is a valuable tool in helping make the diagnosis of pupillary block.[66] Using UBM, a recent study by Wang et al. demonstrated that Chinese patients had significantly higher proportion of nonbasal iris insertion in the nasal and temporal quadrants compared to the Caucasians, and that, this difference may be a reason for their increased risk for angle-closure glaucoma.[67] Postoperatively, UBM can help distinguish pupillary block glaucoma from malignant glaucoma in cases where the exam is equivocal.[68]

For pigment dispersion syndrome, which is associated with iris concavity, UBM has been used to demonstrate that the anterior surface of the lens moves forward with accommodation.[69] It has been theorized by Pavlin et al. that this decrease in the AC volume increases iris concavity in patients with pigmentary dispersion syndrome.[69] A study by Potash et al. demonstrated through UBM imaging of 16 eyes with pigment dispersion syndrome that mid-peripheral iris concavity and iridociliary contact were associated with the disease.[70] In addition, UBM has been used to demonstrate that eyes with pigment dispersion syndrome tend to be associated with a more posterior iris insertion compared to control eyes.[71]

UBM has even been used to help characterize pseudoexfoliation syndrome. Using UBM, Guo et al. found that a thicker anterior lens capsule and lens zonule nodules were associated with pseudoexfoliation.[72] A paper by Sbeity et al. found evidence that patients with clinically unilateral pseudoexfoliation had subclinical bilateral zonular involvement as detected by UBM.[73] They suggested that UBM may be helpful to assess the zonular integrity of the fellow eye prior to cataract surgery.[73]

UBM has been used to provide ciliary body measurements in eyes with malignant glaucoma after trabeculectomy. A recent study by Wang et al. compared UBM measurements from eyes with malignant glaucoma after trabeculectomy compared to their normal fellow eyes and discovered that the ciliary bodies were thinner and more anteriorly rotated in the eyes with malignant glaucoma.[74]

Ultrasound biomicroscopy in trauma and in the evaluation of cyclodialysis clefts

UBM is helpful in evaluating eyes after ocular trauma and is especially useful when visualization is limited by media opacities or there is distortion of anterior segment anatomy.[7576] UBM has been shown to be able to detect and localize very small ocular nonmetallic foreign bodies when computed tomography (CT) and ultrasound B scan failed to do so and can accurately determine their position relative to the sclera.[7778] A retrospective study by Deramo et al. demonstrated that UBM could detect small intraocular foreign bodies <1 mm in size missed by CT and B scan.[77] In addition, UBM has been shown to be an excellent method for identifying occult zonular damage from trauma not detected on clinical examination.[79] UBM is able to detect angle recession, iridodialysis, rupture of the anterior lens capsule, lens displacement, lens subluxation, ciliary body detachment, hyphema, and traumatic cataract.[80] UBM is also able to demonstrate cyclodialysis, which is described further below.

One of the main causes of cyclodialysis, which is the disinsertion of the ciliary body from the scleral spur, is blunt trauma. Cyclodialysis may not be apparent on gonioscopy, especially in situations with hazy media, severe hypotony, or abnormal anterior segment morphology.[8182] UBM has been helpful in diagnosing cyclodialysis cleft [Fig. 7a and b] and can even be used to confirm and observe the re-attachment of the ciliary body.[83]

Figure 7

(a) Gonioscopy of cyclodialysis cleft. (b) Ultrasound biomicroscopy of cyclodialysis cleft demonstrating disinsertion of the ciliary body from the scleral spur (arrow). CB: Ciliary body, SS: Scleral spur

Anterior Segment Imaging and Bleb Morphology

Bleb morphology has known to be an important indicator of bleb function and possible future bleb success.[84] Bleb morphology can be assessed using SL biomicroscopy for its external appearance, but SL biomicroscopy cannot assess any internal structures of the bleb. AS-OCT is useful in this regard as it can show cross-sectional images of the internal structures of the bleb. AS-OCT has been used for the imaging of trabeculectomy blebs as well as aqueous humor drainage devices. AS-OCT may be especially useful for fragile post-trabeculectomy blebs given the noncontact nature of the test.[85]

Previous studies with AS-OCT showed that mature blebs with hyporeflective walls are more likely to function.[868788] Other additional studies demonstrated that internal fluid-filled cavities, low reflectivity of the bleb walls, microcysts, and internal ostia are associated with good filtration of the aqueous.[8990] SD-OCT may be superior to other types of AS-OCT to visualize the superficial features of post-trabeculectomy blebs given its improved resolution.[85] In addition, AS-OCT may be useful in guiding the management and decision for laser suture lysis in post-trabeculectomy blebs.[9192] UBM is also able to visualize these features of the bleb.[149394] UBM can demonstrate the location of the scleral flap, the presence of cystic spaces, and the patency of the internal ostium [Fig. 8].[94] UBM can also demonstrate blocked internal ostium in failed bleb [Fig. 9].

Figure 8

Ultrasound biomicroscopy of a functional filtering bleb (asterisk) with open internal ostium (arrow). S: Sclera, AC: Anterior chamber, I: Iris, C: Cornea

Figure 9

Ultrasound biomicroscopy of a failed bleb with a blocked internal ostium (arrow)

A recent cross-sectional, observational study by Jung et al. investigated the usage of AS-OCT in the visualization of blebs from Ahmed glaucoma valves in 76 patients in order to compare the differences between successful and unsuccessful surgeries.[95] AS-OCT measurements indicated that the maximum bleb wall was significantly thinner in successful surgeries when compared to unsuccessful surgeries.[95] A recent prospective study on 56 eyes that underwent trabeculectomy analyzed the postoperative blebs at 1 month and at 6 months.[96] AS-OCT imaging that demonstrated multiform bleb wall reflectivity with a pattern of multiple internal layers and microcysts was associated with increased success of the bleb.[96] Khamar et al. divided bleb wall reflectivity into two types, multiform or uniform reflectivity. Multiform bleb wall reflectivity describes a bleb with small, multiple fluid-filled spaces shown as hyporeflective areas in the conjunctiva or bleb wall. It is theorized that hyporeflectivity of the bleb wall and these microcysts are collections of aqueous humor humor within the bleb wall. In contrast, bleb walls that have uniform reflectivity at 1 month had poor bleb function at 6 months.[96] AS-OCT may be used in the early post-operative period to predict the functionality of blebs, and thus may help indicate earlier intervention for blebs that are destined to fail.

AS-OCT is also able to produce three-dimensional (3D) images. 3D imaging has been important in classifying blebs as diffuse, encapsulated, or nonfunctioning in post-trabeculectomy eyes.[97] In addition, in these eyes, 3D AS-OCT has been shown to be important in the identification and measurement of the filtration opening on the scleral flap margin.[98] Most recently, in a prospective study by Kojima et al., 3D AS-OCT was used to image post-trabeculectomy blebs in 23 eyes, and various bleb parameters were measured, including position and width of filtration opening at the scleral flap, the total bleb height, fluid-filled cavity height, bleb wall thickness, and bleb wall intensity.[99] The width of the filtration opening of the bleb two t t weeks was found to be significantly correlated with intraocular pressure (IOP) at 12 months, suggesting that this parameter may be a prognostic factor for long-term IOP control.[99]

A recent case series described utilizing 3D AS-OCT guidance to perform bleb revision in two patients after trabeculectomy, one had a leaking bleb and the other had an overhanging bleb.[100] In both cases, 3D AS-OCT was essential in pinpointing the anatomical causes and guiding surgical management. Indeed, 3D AS-OCT may play a pivotal role in guiding glaucoma management in the future, in not just bleb revision, but possibly bleb needling procedures as well. In other postglaucoma surgery patients, ASOCT has been shown to be useful in diagnosing Ahmed tube tip position and patency in three patients with opaque corneas after corneal transplantation.[101] UBM may also be used to further evaluate the position of the tube [Fig. 10] and may be an important adjunct to clinical exam. With various postsurgical applications, AS-OCT has an important role in visualization of intrableb structure.

Figure 10

Ultrasound biomicroscopy of glaucoma drainage device with tube (arrow) in ciliary sulcus and blocked with iris (I)

Anterior segment polarization-sensitive OCT (PS-OCT) has been used as a noninvasive method of evaluating phase retardation in blebs. PS-OCT is based on SS-OCT technology and can evaluate birefringence by imaging phase retardation of biological fibrous tissues.[102] Phase retardation is the phase difference induced by tissue birefringence. PS-OCT offers an excellent method of evaluating intrableb fibrosis not feasible with conventional AS-OCT, which may be useful in determining potential antifibrotic treatment for blebs.[102]

Conclusion

Two main imaging devices for the anterior segment, AS-OCT and UBM, offer rapid, objective, and reproducible methods to image the anterior segment. While each of these imaging devices has their advantages and disadvantages, both technologies allow the acquisition and comparison of objective data and parameters not possible with SL biomicroscopy or gonioscopy examinations. Both technologies have played important roles in furthering our understanding of the mechanisms of various types of glaucoma. While clinical examination can never be replaced by imaging devices, AS-OCT and UBM proved to be useful adjuncts to the clinical examination.

Future Directions

We anticipate that the field of anterior segment imaging will continue to grow. As more sophisticated technology is developed, anterior segment imaging will continue to have important roles in the management, diagnosis, and postsurgical management of glaucoma patients.

Salient Features

Anterior segment imaging, including AS-OCT and UBM, are objective methods of visualizing the anterior segment angle

AS-OCT should be used when the cornea is clear, the patient can sit upright and is most commonly used for appositional angle-closure

AS-OCT may have higher sensitivity for detecting angle-closure than gonioscopy

UBM should be used when the cornea is cloudy, for an examination in the operating room, or if plateau iris, ciliary effusion syndrome, lens subluxation, ciliary body cyst, or tumor is suspected

Both AS-OCT and UBM are excellent for visualizing intrableb structure and glaucoma drainage device placement, and thus have important postsurgical implications

AS-OCT may be used in the early postoperative period to predict the functionality of blebs, and may indicate earlier intervention for blebs that are destined to fail

Recent advances in anterior segment imaging include ITC index, a software that can estimate the percentage of angle-closure in a given eye once the scleral spur has been manually identified

Anterior segment imaging can acquire objective parameters and data that are not possible with clinical examination

Anterior segment imaging can help elucidate mechanisms of glaucoma not possible with standard clinical examination

Despite these advances in imaging, clinical examination cannot be replaced.

Literature Search

We performed a literature search on PubMed for articles containing the text “anterior segment OCT,” “ultrasound biomicroscopy,” and “anterior segment imaging.” Filters were used that included English only and date range from 2004 to present. These results were reviewed and only the articles deemed to be of the most clinical significance were cited in this review. Older articles cited are background information only.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.