Ultrasound biomicroscopy in uveitis

ABSTRACT

Ultrasound Biomicroscopy (UBM) is an imaging technique using ultrasound waves to produce a two-dimensional real time view of the anterior segment of the eye up to a depth of five millimeters. The cross section images are obtained using a high frequency ultrasound (50 to 100 MHz) transducer, incorporated into a B-mode clinical scanner. Axial and lateral resolutions of the anterior segment structures are within 20 to 60 mm. This is ten times the resolution achieved by the conventional ophthalmic ultrasound imaging frequencies. As a relation exists between image resolution and waves, tissue penetration ultrasound biomicroscopy has a penetration depth of only five millimeters, but can be increased to 15 mm using an orbital cup filled with methylcellulose that allows a shorter work transducer distance. Commerically available scanners produce a 5 X 5-mm transducer field with 256 image lines at a scan rate of 8 frames per second. This system generates images of the anterior segment of the eye, the pars plana and peripheral retina. Clincial applications of UBM have been previously described, including imaging in cases of glaucoma, anterior segment and peripheral choroidal tumours, ocular trauma, corneoscleral phakic and pseudophakic pathologies. With UBM, the anterior segment of the eye can be better observed in cases of corneal opacities, cataract, hyphaema and intraocular anterior membranes. UBM also provides information that cannot be obtained with the clinical examination alone. This talk highlights some of the findings of ultrasound biomicroscopy in anterior uveitis. UBM provides a view of the inferior angle in cases of hypopyon. Sometimes what looks like a hypopyon can be a cyst filled with inflammatory cells. A sign of granulomatous inflammation that can be observed with UBM is a localized thickening of the interior peripheral cornea at the six o'clock position (prismatic effect). Many cases that present this prismatic effect will be associated with sarcoidosis. Inflammation of the ciliary body and the vitreous base can be observed in ways that are not possible with the direct ophthalmoscope and the tree mirror contact lens. Circumscribed areas of reflectivity and thickness changes in UBM images resembing nodules are often seen in the ciliary bodies in cases of sarcoidosis or in severe chronic granulomatous uveitis.(AU)