Ultrasound Biomicroscopy – Enduring Clinical Value After a Quarter Century
By CARLA LUTCHMAN, MD, FRCSC, and E. RAND SIMPSON, MD, FRCSC
The technique of ultrasound biomicroscopy (UBM) was described by Pavlin in Ophthalmology Rounds 10 years after the introduction of this important imaging technology. Despite the development of newer imaging technology, UBM remains the mainstay of anterior-segment imaging in that sound energy is able to penetrate structures that limit light transmission. In this issue of Ophthalmology Rounds, UBM technology is described to outline the enduring benefits of this form of imaging.
Imaging the human eye continues to play an indispensable role in the diagnosis and management of ocular abnormality and disease. Both light- and sound-based technologies have achieved impressive levels of structural definition and are used extensively throughout all aspects of ophthalmic practice. Ultrasound has provided unique advantages in this regard by employing B-scan, 2-dimensional, cross-sectional views of the intraocular space and orbit, particularly when anterior-segment pathology prevents adequate light penetration or when views of more peripheral posterior abnormalities are required. The capacity to view the anterior segment has been challenging both for light- and ultrasound-based devices due to the loss of light transmission with pigment absorption for light-based technology and the limitation of image resolution at high frequencies for ultrasound techniques. Ultrasound biomicroscopy (UBM), developed by Drs. Stuart Foster and Charles Pavlin 25 years ago in Toronto,1 has provided a unique tool to evaluate the anterior segment of the eye which, at microscopic resolution, allows imaging of living structure in a noninvasive and relatively uncomplicated procedure. Dr. Pavlin described the technique and its clinical benefits in a 2004 issue of Ophthalmology Rounds.2
Since then, another technique of imaging the anterior segment of the eye, anterior segment optical coherence tomography (AS-OCT), has been developed. OCT employs the time delays of light waves reflected from the various depths of the target sample to reconstruct a 3-dimensional image. Despite the introduction and refinement of AS-OCT, UBM continues to be the mainstay of anterior-segment imaging.3
Basics for Imaging
Ultrasound employs the region of sound waves in the acoustic spectrum above the limits of human audibility. The frequency of sound waves determines its tone or pitch; low frequencies produce low tones and high frequencies produce high tones. Ultrasound produces vibrations with a pitch so high that it is not audible to the human ear, and frequencies above 18 kHz are usually considered to be ultrasonic. Higher image resolution suffers the penalty of limited penetration into tissue. The maximum penetration achievable for a 10-MHz system is about 50 mm, but for a 60-MHz system (a range employed in some UBM systems) the image resolution is reduced to about 5 mm. Posterior pole imaging is therefore impossible with high-frequency ultrasound; however, anterior-segment definition can be resolved in a manner to be clinically valuable. Although laboratory systems have achieved frequencies of 100 MHz, currently available instrumentation employs between 20 MHz and 50 MHz to provide more than satisfactory resolution for analysis of the anterior segment.
A Noninvasive Procedure
Examination of the eye by UBM uses a fluid immersion technique, similar to that of conventional B-scan imaging. This allows sufficient standoff from the structures being examined to avoid image distortion close to the transducer as well as prevention of the transducer from coming into contact with the ocular surface. Figure 1 shows the patient positioned supine, with the probe held perpendicularly over the eye through the immersion fluid. The eye cup is designed with a small lip to allow the cup to be held in place under the eyelids. Some UBM designs allow a fluid-filled membrane over the transducer to be applied directly to the surface of the eye, eliminating the need for the patient to be positioned horizontally. Although the anterior segment structures are easily examined in any meridian using UBM, the conjunctiva, sclera, and peripheral retina can also be examined by rotating the eye as far as possible away from the region being examined.
Figure 1: Performance of ultrasound biomicroscopy
UBM in Ocular Disease
A wide range of anterior-segment disease and structural abnormalities can be evaluated by UBM within the resolution and penetration limitations of this high-frequency technology. It is particularly valuable when adequate evaluation by standard examination techniques cannot determine iris, sub-iris, angle, and ciliary body abnormalities as well as intraocular lens (IOL) positioning and other lenticular anomalies. The technique is particularly useful in assessing management outcomes.
Anatomic variation of the anterior segment, particularly involving the iris and outflow mechanism, is the basis for a variety of glaucoma-related conditions. UBM has provided an important imaging technique to better define certain structural anomalies affecting aqueous circulation in the glaucomatous eye.
Pressure differences between the posterior and anterior chambers result in a convex iris configuration, which can be sufficient to compromise the angle. Iris-lens contact may be minimal in pupillary block but with dilatation, the iris thickens, becomes anteriorly bowed, and moves to obstruct the angle producing a potential rise in intraocular pressure (Figure 2). A provocative darkroom test during the UBM procedure can amplify the degree of angle closure as the iris thickens.4 Peripheral iridotomy will usually provide relief for this condition with straightening of the iris and deepening of the anterior chamber. The patency and location of the iridotomy, as assessed with UBM, is helpful in determining the degree of success for this procedure, which may be limited by the presence of peripheral anterior synechiae, imperforate iridotomy, location of the iridotomy, or plateau iris configuration.
Figure 2: Anterior bowing of the iris and significant narrowing of the angle
Peripheral anterior synechiae
The anterior chamber angle is variably blunted echographically at the site of synechial closure, compromising the angle in this location. Angle definition can nevertheless be determined behind the synechia and the extent of angle closure evaluated.
Plateau iris configuration
An important reason for insufficient improvement of the iris/angle anatomy following iridotomy can be related to anterior rotation of the ciliary body (Figure 3), which acts to support the iris base, closing the angle, despite an adequate and well placed peripheral iridotomy.5 This rotated position of the ciliary processes prevents the peripheral iris from falling away from the trabecular meshwork following the iridectomy. Management of this condition can be a challenge since there is some evidence to suggest that the possibility of a higher than normal lens rise may contribute to the malposition of the ciliary body/zonule platform. The lens position and morphology can be assessed by UBM in this and other conditions, providing insight into a possible mechanism to account for the angle narrowing with a view to the potential need for lens removal. The degree of angle relief following iridoplasty or induced miosis can also be monitored using this technique to assess whether pupil dilatation may be accomplished without compromising the angle. The approach to managing the plateau iris configuration remains controversial.6
Figure 3: Anterior rotation of the ciliary body supporting the iris base following iridotomy, narrowing the angle
The supraciliary space can be imaged with UBM and can play a role in narrowing or closing the angle in certain conditions such as venous occlusive disease and inflammatory conditions involving the anterior segment and following retinal detachment surgery. As this space widens with the effusion,7 the angle can close and produce a rise in intraocular pressure, particularly if there is initial narrowing. It should be noted that the presence of intraocular oil placed at the time of retinal surgery will preclude successful imaging of the anterior segment with UBM.
Pigmentary dispersion syndrome
Although rare, temporary reversal of the posterior- and anterior-chamber pressures may bring about an inward or posterior bowing of the iris (Figure 4). This occurrence can result in pigment loss from the pigment epithelial layer of the iris from iris-zonule contact, causing deposition of pigment in the angle and subsequent intraocular pressure rise. As to whether this situation can be relieved with an iridotomy is open to question.
Figure 4: Posterior bowing of the iris in pigment dispersion syndrome
Tumours of the anterior segment
Given that the resolution is excellent for anterior-segment imaging due to the short penetration of ultrasound waves, UBM is an important adjunct in the management of anterior-segment tumours. The ability to measure these lesions accurately adds to a critical analysis for diagnosis and demonstrating growth. This allows for greater precision in following lesions that are being observed, and assists in the ultimate management of these cases. The most common tumours followed in this way are those originating from the iris and ciliary body. Although resolution is not at the level of histological differentiation, UBM can help determine the extent of the lesion (Figure 5). For instance, iris nevi can be frequently distinguished as an echogenic layer on the surface of the iris. In contrast, iris melanomas may not show a distinction between the iris stroma and underlying iris pigment epithelium. UBM can be useful in defining a peripheral lesion localized to the iris and whether the ciliary body is involved. Mortality for iris melanoma is extremely low if the tumour is confined to the iris, but can increase if the ciliary body is involved.8,9
Figure 5: Imaging of a large ciliary body melanoma
If a melanoma is suspected and progressive with growth and extent, it is important to determine whether the tumour is in contact with the corneal endothelium. Resection of these lesions will occasionally be compromised if there is contact with the cornea since malignant cells can remain adherent to the endothelium despite apparent complete resection. The position and nature of the melanoma is also important to define if brachytherapy (plaque radiotherapy) is to be entertained. Following the management of these lesions by resection or brachytherapy, UBM is useful in determining therapeutic efficacy and long-term follow-up.
Iris pigment epithelial cysts are easily imaged by UBM, and this technique is advantageous over AS-OCT where there is incomplete imaging due to the attenuation of light by the iris pigment epithelium.10 Iris pigment epithelial cysts, or iridociliary cysts, are frequently diagnosed on routine slit-lamp evaluation with or without dilatation. While localized elevation of the iris can be clinically apparent, the typical UBM appearance is that of a thin-walled lesion that is optically empty beneath and often elevating the overlying iris (Figure 6). Iris pigment epithelial cysts are frequently small, and are often multiloculated and extensive in their involvement. These cysts generally do not progress but do require review periodically in the event of enlargement. They rarely affect visual acuity adversely but can narrow the angle throughout the extent of the cyst.11
Figure 6: Elevation of the iris from an underlying iridociliary cyst
Corneal and scleral disease
In the setting of an opaque cornea or partial corneal opacity,12 UBM is a crucial tool for imaging the anterior segment beneath the opacity. UBM can reveal details such as the anterior-chamber depth and iris configuration, presence or absence of peripheral anterior synechiae, and the presence and/or the position of a natural or implanted intraocular lens Unexpected foreign material following a traumatic event can be frequently visualized with this technique. UBM is also helpful in assessing and localizing scleritis, and in the differentiation between extra- and intrascleral disease, including the degree of scleral integrity. This becomes important in determining the extent of intraocular involvement of anterior segment tumor progression prior to management.13
IOL position and complications
UBM has been proven a valuable tool in the identification of malpositioned IOLs.14 The haptics can often be seen on UBM to determine whether they are placed in the sulcus or the capsular bag. In addition, malpositioned IOLs (Figure 7) can be seen and irido/IOL contact assessed, especially in some cases of uveitis-glaucoma-hyphema syndromes. This information assists in determining whether an IOL exchange or repositioning may be beneficial. Retained cortical material, as well as retained nuclear fragments in relation to iris involvement can be observed with this imaging technique.
Figure 7: A tilted intraocular lens (IOL) with IOL-iris contact
Hypotony and trauma
UBM can assess the status of ciliary body effusions or dialysis in patients post-trauma or post-surgery.15 The presence of 360º supraciliary fluid may be evident, as is the separation of the iris root from the scleral spur indicating the presence of a cleft. Other causes of hypotony, such as ciliary body membranes and occult wound leaks, can often be detected on UBM. The presence of intraocular oil can obscure accurate visualization of the anterior segment. The presence of retained foreign bodies embedded in the anterior chamber post-trauma or intentional foreign bodies such as the iStent inject® can be imaged with UBM.16
In many cases, UBM is a useful adjunct in determining the extent of conjunctival lesions. The presence or absence of scleral involvement may be an important consideration if surgical management is contemplated.17 Sentinel vasculature on the sclera may be investigated with UBM for the possibility of underlying pathology.
For 20 years, UBM has been an important tool to evaluate the anterior segment of the eye at high resolution, noninvasively and with relative patient comfort. Many aspects of ocular anatomy and disease can be displayed with this technology to produce images cross-sectionally and at microscopic resolution that cannot otherwise be evaluated with any other clinical method. The images are generally not affected by pigment content, as occurs with light imaging technology, and therefore are particularly useful to define angle and ciliary body abnormalities that cannot be otherwise determined by other clinical methods. Newer methods employing ultrasound technology continue to emerge in clinical practice and ophthalmic research, which will complement our approach to ophthalmic management and benefit patient care.
Dr. Lutchman is a lecturer, Department of Ophthalmology and Vision Sciences, University of Toronto, and staff member, Mount Sinai Hospital, Toronto, Ontario. Dr. Simpson is a consultant, Mount Sinai Hospital, Toronto, Ontario. The authors would like to acknowledge the skilled techniques by Zain-ul Abidin at Mount Sinai Hospital’s Ocular Function unit.
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