Ординатура / Офтальмология / Английские материалы / Glaucoma Surgery_Trope_2005

image the relationship of subsurface structures in real time can clarify mechanisms and aid in understanding complications in glaucoma surgery.

2.THEORETICAL CONSIDERATIONS

Mechanical waves and vibrations occur over a wide range of frequencies called the acoustic spectrum. This spectrum extends from the audible range (10 20,000 Hz), with which we are all familiar, to the range of phonons (.1012 Hz) that comprise the vibrational states of matter.

Higher frequency ultrasound provides higher resolution on the order of 20 40 mm, but the penalty to be paid is loss of penetration. All human tissues exhibit ultrasound attenuation coefficients that increase with frequency. The maximum penetration that could be achieved for a 10 MHz system is 50 mm. For a 60 MHz system, penetration is only 5 mm. The penetration limits prevent imaging of the posterior pole, but are sufficient to gain valuable information on events in the anterior segment of glaucoma entities (4 8).

3.CLINICAL USE OF ULTRASOUND BIOMICROSCOPY

High resolution ultrasound scans used in this chapter have been performed with the original instrument constructed in our laboratories and the commercial instrument based on this design. In the laboratory, we use instruments with frequencies between 40 and 100 MHz. The commercial instrument uses a 50 MHz transducer, which is a good compromise between resolution and penetration. Several other instruments are currently available with frequencies varying from 20 to 50 MHz.

3.1.Technique

The technique of eye examination using ultrasound biomicroscopy is similar to conventional B-scan examination of the anterior segment. A fluid immersion technique is required to provide an adequate standoff from the structures being examined. This is necessary to avoid distortion of the image close to the transducer and to prevent contact of the transducer with the eye. An eyecup is used to hold the eyelids open and allow more rapid patient preparation. These eyecups resemble those used in conventional ultrasound biometry, with a lip that slides under the eyelids and holds the cup in place. They differ from biometry eyecups in being shallower and having a distinct flair that allows a view of scanning head position. Figure 13.1 shows an examination being performed with one of these eyecups. A solution of 1% methyl cellulose is an excellent coupling medium with sufficient viscosity to prevent fluid loss during examination. Air bubbles have to be carefully avoided, both in the fluid and on the concave surface of the transducer.

Unlike conventional 10 MHz B-scan, high frequency transducers are generally not covered by a membrane. A membrane would provide excessive sound attenuation and defeat the purpose of doing examinations at this frequency. Since the transducer is moving, contact with the eye and resulting corneal abrasion must be carefully avoided. The presence of an articulated arm is valuable in improving control of the scanning head. Careful attention must be paid to the screen image to prevent the scanning head from getting too close to the eye. In practice, we have found that contact with the eye has been an extremely rare occurrence.

Any part of the eye that can be approached directly over the surface can be examined. The cornea and anterior segment structures are easily examined in any meridian. The most easily interpreted images are radial and orientated, so that the sclera is on the

Figure 13.1 Ultrasound biomicroscopic examination being performed in an eye cup filled with 1% methylcellulose.

left side of the screen. The conjunctiva, underlying sclera, and peripheral retina can be examined by rotating the eye as far as possible away from the region being examined.

3.2.Measuring Ocular Structures

Ultrasound biomicroscopy expands our ability to accurately measure ocular structures (4,9). Measurement accuracy is improved by ultrasound biomicroscopy, which has an axial resolution 5 10 times that of conventional 10 MHz ultrasound. We perform measurements on the screen during examination using electronic calipers. Stored images can be transferred to a computer and measured using imaging software. Measuring a structure accurately with ultrasound requires a knowledge of the speed of sound in the structure being examined. We have used a speed of sound of 1540 m/s to make the majority of measurements. This speed is used in conventional ultrasound scanning to measure distances in most tissue.

Conventional ultrasound is capable of measuring relatively large distances such as anterior chamber depth. However, ultrasound biomicroscopy increases measurement accuracy of such structures because the shorter wavelength allows a finer positioning of end points and the exact measurement position can be defined more precisely. A number of ocular structures such as the ciliary body, sclera, and iris cannot be measured by other techniques because of inadequate resolution and the inability to differentiate these structures from adjacent tissue.

4.ULTRASOUND BIOMICROSCOPY IN GLAUCOMA SURGERY

4.1.Filtering Surgery

Ultrasound biomicroscopy can be used to image at depth the surgical site of filtering surgery. Features that can be defined include the internal scleral ostium, the intrascleral

Figure 13.2 A medium reflective filtering bleb (B). The scleral opening (arrow) is imaged communicating with a intrascleral pathway.

pathway, and the filtering bleb itself (Fig. 13.2). The internal ostium usually appears as a wedge-shaped opening with clear fluid in the gap. The intrascleral pathway varies in size. At times, there is a distinct fluid filled pathway through the sclera that can be measured. At other times, this pathway can be discerned as a more subtle lower reflective line

Figure 13.3 (See color insert) A filtering bleb (B) contains clear spaces and low reflective episcleral tissue.

Figure 13.4 In pupillary block, the iris shows anterior bowing narrowing the angle (arrow) and a small iris lens contact. S, sclera, C, cornea, I, iris, CP, ciliary processes, z, zonule, L, lens.

intrasclerally without obvious separation of the superior and inferior scleral walls. The intrascleral pathway can generally be traced back to the superficial entry point below the bleb. The bleb itself is quite variable in appearance. The height of the bleb can be measured from the conjunctival surface, to the highly reflective underlying sclera. The internal reflectivity can vary. The usual appearance is medium reflective tissue interspersed with some clear fluid spaces (Fig. 13.3). The medium reflective areas indicate the fluid filled, spongy episcleral tissue. Rarely is the entire bleb filled with clear fluid

Figure 13.5 In plateau iris the ciliary processes (CP) are forward supporting the peripheral iris producing peripheral angle narrowing (arrow).

4.2.Assessing Filtration

Ideally, ultrasound biomicroscopy could be used to predict the functional status of filtering blebs and to help ascertain the site of failure of filtration. This is often possible, but not always. Yamamoto et al. (10) classified blebs into four categories: low reflective, high reflective, encapsulated, and flattened. They found that good control was generally associated with low reflective blebs and poor control with encapsulated and flattened blebs. McWhae and Crichton (11) used a blinded study to predict filtering function in postsurgical eyes. They used the presence of a patent filtration pathway from the anterior chamber to the bleb and the presence of a bleb space to indicate good bleb function. Evidence of obstruction of the internal orifice, obstruction of the trapdoor flap, and absence of bleb cavity were evidence of poor function. Several cases did not fit clearly into these categories. They found a correlation of 86% between ultrasound biomicroscopy grade and clinical findings. Correlation was weakest in those in which medication was still required to control intraocular pressure. Other authors have found varied correlation of ultrasound biomicroscopy appearance and function (12 15).

4.3.Other Forms of Filtering Surgery

Various types of nonpenetrating filtering surgery have evolved over the past several years. These include deep sclerectomy with collagen implant and viscocanalostomy. Ultrasound biomicroscopy has been used to image these entities (16 18). In deep sclerectomy with implants, the presence of a filtering bleb, a supraciliary hypoechoic area, and hyporeflectivity of the scleral tissue around the decompression space have been associated with good control. In viscocanalostomy, the presence of a nonreflective scleral chamber has been associated with good glaucoma control.

4.4.Blood in the Filter

Autologous injection of blood has been used to prevent overfiltration. In eyes that have had this procedure, ultrasound biomicroscopic imaging shows the presence of red cells in the passageways of the filtering procedure (Fig. 13.8). Red cells can be imaged in the bleb, the intrascleral pathway, and the anterior chamber.

4.5.Valves

Various valves have been used to improve the results of filtering surgery in difficult cases. Ultrasound biomicroscopy can be a valuable means of detecting the position of these valves, and to determine the cause of nonfunctioning valves. The valve itself is easily imaged in its path into the anterior chamber because of the high reflectivity of the plastic used, and its distinctive tubular appearance. The position of the tip of the valve and its relationship to intraocular structures is easily determined (Fig. 13.9). Causes of nonfiltration that can be detected by imaging include failure of the valve to enter the anterior chamber, occlusion of the tip of the valve by iris, or obstruction of the tip of the valve by other materials.

4.6.Overfiltration

In patients with shallow chambers and hypotony following filtering surgery, several distinct features can be imaged with ultrasound biomicroscopy. The shallowing of the chamber can be imaged and quantitatively measured. A very common finding is the

Figure 13.8 Ultrasound biomicroscopy image of filtering bleb with injected blood (B) above the sclera (S).

presence of a supraciliary effusion. The appearance of an effusion consists of a separation of the ciliary body from the overlying sclera (Fig. 13.10). The effusion extends forward close to the scleral spur. Such an effusion can be part of a larger choroidal effusion or be confined to the ciliary body region. The space between the ciliary body and sclera is low reflective, and crossed by thin lines representing cross-sections of the thin connective tissue septae that join the ciliary body and sclera, which are now expanded by

Figure 13.9 (See color insert) Ultrasound biomicroscopy image of Ahmed valve. The iris is partially obstructing the opening of the tube (arrow).

Figure 13.10 A supraciliary effusion (E). The effusion appears as a low reflective space between the pars plana and sclera with cross sections of tissue septa.

fluid. The ciliary processes and iris are imaged as being rotated forward around the scleral spur.

Grigera et al. (19) presented 15 patients with flattening of the anterior chamber following filtering surgery. All patients were low or normotensive when examined, and

Figure 13.11 Ultrasound biomicroscopy image cyclodialysis cleft. The ciliary body is disinserted from the scleral spur (arrow).