Ultrasound Biomicroscopy in Silicone Oil-filled Eyes

Silicone oil can leave the eye difficult to visualize. Ultrasound biomicroscopy might help.

Pradeep Venkatesh, MD

Conventional ultrasonography has little ability to determine the status of the retina in silicone oil-filled eyes, in which direct visualization is not possible due to media opacity. Alternative methods with debatable efficacy have been reported earlier.

We herein report our observations using ultrasound biomicroscopy (UBM) in patients with retinal detachment before any surgical intervention and in patients with silicone oil-filled eyes following vitreoretinal surgery.

MATERIAL AND METHODS

Ultrasound biomicroscopy was performed using the Paradigm Model P40. The ultrasound frequency used was 50 MHz, with a gain of 80 dB. The examination was performed using a 2% methylcellulose bath in a polymethylmethacrylate cup of appropriate size after using paracaine eyedrops for topical anesthesia. UBM scanning was performed on all accessible quadrants, and the findings were recorded.

Initially, patients with retinal detachments alone (n=3) and patients with silicone oil with retinal detachments and clear media (n=2) were chosen for UBM evaluation. The UBM image patterns were studied in these cases, and consistently evident landmarks were identified with regard to the presumed ora serrata region. In addition, limitations of the imaging were also assessed.

Experience from studying and analyzing the above patients were then applied to UBM images obtained from patients with silicone oil and opaque media due to cataracts and who were awaiting cataract surgery (n=2). These findings were then compared with the intraoperative findings at the time of cataract surgery, with or without silicone oil removal.

RESULTS

The region of the ora serrata was characterized on UBM by the disappearance of a narrow hypoechoeic slit seen in all cases. This possibly represented the supraciliary space. Detachment of the retina could be deciphered by detection of a membrane attached to the presumed ora serrata and passing posteriorly with a downward slope (Figures 1 and 2). In the two patients with silicone oil-filled eyes and opaque media due to cataracts, intraoperative confirmation of a peripheral retinal detachment was obtained in one patient and its absence in the other. Disinsertion of the retina from the ora serrata (retinal dialysis) in a patient with posttraumatic retinal detachment with dialysis was also documented (Figure 3).

Figure 1. UBM image (top) shows a narrow hypoechoeic slit (black arrow) at the termination of which arises a membrane coursing posteriorly (white arrow). The latter is the commencement of the retinal detachment from the ora serrata. A UBM montage (bottom) shows a membrane attached at the presumed ora and coursing posteriorly in a patient with retinal detachment before surgical intervention.

Figure 2. UBM image (top) of a silicone-filled eye showing the origin of a membrane (white arrow) at the termination of a hypoechoeic slit (black arrow) in a patient with opaque media due to cataracts. Note the point-like hyper-reflective echoes indicative of emulsified silicone oil globules. A UBM (bottom) montage shows the origin of a retinal detachment in an aphakic patient with anterior migration of silicone oil.

Figure 3. UBM image showing disinsertion of the retina from the ora in a patient with post-traumatic retinal detachment and superonasal dialysis.

Based on visualization of the silicone oil meniscus close to the ocular coats, we can also comment on the completeness or otherwise of silicone oil fill. Of the four silicone oil-filled eyes scanned, three eyes with retinal detachment were found to be underfilled, while the fourth eye had a complete fill (Figure 4).

Figure 4. Silicone oil meniscus is visible up to the lens equator and beneath the ocular coats, indicating an almost complete silicone oil fill.

DISCUSSION

The principal reason that conventional ultrasonography has limited usefulness in predicting the status of the peripheral retina in silicone-filled eyes is that the frequency of the conventional ultrasound probe permits deeper penetration, so only the tissues diametrically away and at a distance from the probe can be studied.

Second, changes in velocity due to silicone oil tamponade alter the scanning ability of the probe, so other than pseudoenlargement of the globe, no details pertaining to the status of the retina can be obtained. In contrast, the UBM probe has a high frequency of 50 MHz and is capable of scanning structures immediately in its vicinity (usually to a depth of 5 mm). This enables it to identify structures adjacent to and lying underneath the ora serrata.

The only limiting factor is the inability of the probe to access the ora serrata in a particular region based on the size of the palpebral aperture, the presence of a scleral buckle, etc. The temporal, superior, and sometimes inferior regions are usually accessible in eyes with adequate palpebral aperture.

This scanning ability of UBM, with the known knowledge that rhegmatogenous retinal detachment almost always reaches up to the ora serrata, makes it a potential tool for predicting the status of the retina in silicone-filled eyes with media opacity. Earlier reports using UBM have described characteristic changes in the silicone oil and patterns of peripheral vitreoretinal proliferation.¹ However, they have not commented on its ability to decipher the presence of a retinal detachment.

Other reports have studied the relationship between emulsified oil in the anterior chamber on UBM and intraocular pressure.² Another study with UBM reported on the persistence of silicone oil in the anterior chamber despite surgery for oil removal.³ As ultrasonic evaluation of the posterior segment in silicone-filled eyes is subject to significant image artifacts, magnetic resonance imaging and other mathematical models have been described to overcome this limitation.^4,5

The method of using UBM to predict the status of the peripheral retina has a few limitations other than the accessibility of the ora serrata. In patients with very bullous detachments, the membrane coursing posteriorly from the ora may be difficult to detect, as it goes beyond the depth of imaging rapidly. Imaging the detached peripheral retina is also difficult in patients with concurrent significant choroidal detachments.

In patients with silicone-filled eyes and retinal detachments, the detachments are usually not very bullous, and concurrent choroidal detachments almost never occur. Hence, these limitations would not hinder the imaging of the peripheral retina using UBM in silicone oil -filled eyes.

CONCLUSION

To the best of our knowledge, this is the first report documenting the ultrasound biomicroscopic features in patients with rhegmatogenous retinal detachment and its usefulness in predicting status of the peripheral retina in silicone-filled eyes. RP

REFERENCES

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