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Neuroradiology Case of the Week

Case 217

Kristina A. Siddall, MD and P-L Westesson, MD, PhD, DDS

Clinical Presentation: A 67-year-old female presented to the emergency department with a syncopal episode. Initial CT of the head revealed a large right cerebellar mass, later diagnosed as large B-cell lymphoma. An incidental finding was identified in the left orbit.

Radiological Findings: Axial unenhanced CT slice (A) at the level of the orbits shows a round high-attenuation mass measuring 60 HU seen in the anterior aspect of the posterior chamber of the left globe. The left bony orbit, extraocular muscles and optic nerve are unremarkable. Images from MRI of the head to evaluate the cerebellar mass are also provided. At MR, the left ocular mass is isointense to muscle and hyperintense to fluid on T1-weighted imaging (B). On T2-weighted imaging (C), the mass demonstrates hypointense signal and no enhancement. Chemical shift artifact is seen at the interface between the mass and the vitreous. There was no abnormal signal in the optic nerve and extraocular muscles and the contour of the globe was preserved. Contrast-enhanced CT (not shown) revealed no abnormal enhancement in the orbits.

Diagnosis: Intraocular silicone tamponade for retinal detachment

Discussion: Retinal detachment occurs in 1 to 5% of the population, with increasing frequency in the elderly and diabetics, and in patients with ocular tumors, head trauma and cytomegalovirus retinitis.  If left untreated, retinal detachment causes progressive vision loss and eventual blindness, both direct sequelae of membranous scar formation. Untreated retinal detachment also leads to phthisis bulbi, a shrunken ossified globe.
     To avoid these complications, prompt surgical treatment involves approximating the sensory retina and the retinal pigment epithelium. In the presence of an associated retinal tear, thermal or laser induction of chorioretinal scarring is also performed, a process called retinopexy. After retinopexy and surgery, the ophthalmologist will commonly replace the vitreous with silicone oil to tamponade the repaired retina against the wall of the globe.
     CT and MR are the imaging modalities of choice to evaluate ocular anatomy following silicone tamponade, as retinal localization by sonography in the presence of silicone is difficult. At CT, silicone is hyperdense to simple fluid and measures up to 120 HU, depending on the concentration of silicone. The hyperdense fluid can sometimes be confused with blood, but typical retinal hemorrhage layers dependently. Silicone oil’s most common appearance at MR is hyperintensity to water on T1 and hypointensity to water on T2, again sometimes mimicking hemorrhage. Chemical shift artifact at the interface between the silicone and vitreous can be used to best distinguish the two entities. Fat saturation pulses will also cause some degree of silicone suppression, distinguishing it from blood.
     The silicone oil used for tamponade is usually removed after 8 weeks, but may be left in permanently, depending on the risk of recurrent detachment.  In the patient shown, retinal detachment surgery and silicone tamponade had been performed 3 years prior to imaging and was unsuccessful in preventing the patient’s eventual left-sided blindness. Complications of silicone instillation include choroidal detachment, infection, scarring, cataracts, and optic atrophy. Case reports in the literature have also described intracranial migration of silicone, in patients with high intraocular pressure, along the optic nerve and then into the lateral ventricles.
     Scleral buckling, another treatment for detached retina, also has a characteristic appearance at CT. A circumferential silicone or hydrogel band is placed around the equator of the globe adjacent to the insertion of the rectus muscles. These bands are dense on CT and cause indentation of the eye wall.

References:

1. Som PM, Curtin HD. Head and Neck Imaging, Vol. 1, 4th ed. Mosby, 2002. 520-1.
2. Lane JI, Watson RE, Witte RJ, McCannel CA. Retinal detachment: imaging of surgical treatments and complications. Radiographics 23, 2003: 983-94. [Medline]
3. Eller AW, Friberg TR, Mah F. Migration of silicone oil into the brain: A complication of intraocular silicone oil for retinal tamponade. Am J Ophthalm 129, 2000: 685-8. [Medline]
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