Sunday 23 March 2014

Intraocular silicon gel CT

Axial CT study of a 50 yr old male with history of minor trauma.
CT Brain within normal limits.
Orbital sections show hyper dense right side vitreous.

Retrograde inquiry revealed history of previous silicone injection into the right globe to treat retinal detachment. The increased opacification in the right globe which could easily be mistaken for blood in the acute traumatic setting.
References: Radiology, Bart's and The Royal London Hospitals - London/UK

Tamponade for retinal detachment 

The mechanism of action of tamponade agents is twofold. They close the retinal breaks, and the buoyancy force helps appose the retina to the eye wall while the retinopexy matures or heals. The former is considered by far the more important mechanism of action. The existing subretinal fluid is absorbed, leading to retinal reattachment.

Intraocular gas tamponade agents include air and mixtures of air and long-acting gases (sulfur hexafluoride [SF6] and perfluoropropane [C3F8]). The main difference in these agents is their duration of action. An air bubble will resorb in 3–5 days after injection, a mixture of SF6 in 10–14 days, and a mixture of C3F8 in 6–8 weeks. Gases may be injected in their pure form, after which they will expand into larger bubbles, driven by the difference in the partial pressure of nitrogen in the gas bubble versus in the body. Intraocular gas tamponade results in air attenuation and fluid levels within the nondependent portion of the vitreous cavity. There are no attenuation differences between air and the longer-acting gases at CT. The presence of air within the globe following retinal reattachment should not be misinterpreted as evidence of a postoperative complication such as intraocular infection.

Silicone oil (polydimethylsiloxane) is different chemically from silicone rubber and has a lower specific gravity than vitreous fluid. This natural buoyancy, together with surface tension differences, makes it a useful agent for intraocular tamponade. Silicone oil provides several distinct advantages over air tamponade. Head positioning is less critical with silicone oil, making it a preferred agent for treatment of children with retinal detachment. Unlike the gas-filled eye, which temporarily leaves the patient with no useful vision, silicone oil is transparent (it does not mix with intraocular fluids or blood) and permits the patient to see after proper refraction. It is left in for at least 8 weeks, after which it is usually removed, although it can be left permanently at the discretion of the surgeon. The retina surgeon may choose to leave the silicone oil indefinitely if there is evidence of residual traction on the retina or recurrent detachment is noted in a patient with acceptable vision. Oil may also be left in cases of ocular hypotony to prevent development of phthisis bulbi.

On CT Silicone oil is hyper attenuating to normal vitreous fluid and should not be mistaken for intra ocular blood. Density of silicone gel is more than 100 HU vs blood less than 90 HU.

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