INTRAOPERATIVE OCT

Four weeks after a shaken baby with epiretinal membrane was treated with a 25-gauge vitrectomy and retinal peel, optical coherence tomography (OCT) revealed a second membrane layer missed at the first surgery. Following its removal in a second procedure, the infant’s foveal contour returned and vision improved.

Many repeat surgeries such as this might be prevented with intraoperative OCT for vitreoretinal surgery, Albert J Augustin MD, professor and chairman of the Department of Ophthalmology at Klinikum Karlsruhe, Karlsruhe, Germany, told the 12th EURETINA Congress. The emerging technology “allows you to judge the different [tissue] layers and the results of your surgery immediately, and you still have access to the retina if you missed something.â€

Microscope-mounted OCT systems now in development also could improve the accuracy of diagnosis and prognosis, prevent second peels and other procedures that may turn out to be unnecessary as surgery progresses, and avoid surgical mishaps such as inducing full-thickness macular holes, Prof Augustin said. Detailed 3-D imaging of how tissues respond during surgery also would advance understanding of the pathophysiology of vitreoretinal conditions, he added. “I think we can clearly state that vitreoretinal surgery overall would benefit from real-time feedback on depth and tissue interaction,†Prof Augustin said.

Handheld OCT

The use of OCT during vitreoretinal surgery began with handheld devices and dates back at least to 2009, with much of the pioneering clinical work done by Cynthia Toth MD and colleagues at Duke Eye Centre, Durham, North Carolina, US, Prof Augustin said. Such devices provide rapid, non-contact, high-resolution, crosssectional retinal imaging that can be very useful for assessing progress intraoperatively.

For example, it can help identify surgical changes difficult to see under the surgical microscope, such as relief of macular traction around a full-thickness macular hole following internal limiting membrane peel (Dayani et al. Retina. 2009;29(10):1457-1468). It can also show development of a macular hole during a peeling procedure. This may lead to a modification of your surgical technique and tamponade, Prof Augustin said.

Handheld intraoperative OCT also has provided insight into disease pathogenesis, such as showing a possible connection between the vitreous cavity and intraretinal fluid in optic pit-related maculopathy (Ehlers et al. Arch Ophthalmol. 2011;129(11):1483-1486).

However, handheld OCT also presents several drawbacks, Prof Augustin noted. The system must be covered by a sterile plastic bag, making it somewhat clumsier to use than in non-surgical settings, and potentially compromising the sterility of the operating field. More significantly, it requires halting surgery and does not allow imaging of live surgical manoeuvres with instruments in the eye.

The next logical step is to mount the OCT on the surgical microscope so it can be used during surgery, Prof Augustin said. Work with spectral domain OCT in porcine eyes has shown that it can image instruments and retinal tissues in surgery (Ehlers et al. IOVS, May 2011).

“The diamond-dusted scraper can clearly be seen, which means you can judge your manoeuvres by visualising the instrument you are using in surgery.†Integrating the OCT image with the surgical image, so that both can be seen simultaneously, will allow surgeons even greater control, Prof Augustin said. Prototypes have been demonstrated on animal eyes and are in development at Duke and by Susanne Binder MD and colleagues at Rudolph Foundation Hospital, Vienna, Austria.

Dr Binder is collaborating with Carl Zeiss Meditec, who have mounted a Cirrus HD-OCT system running 4.5.1.11 Cirrus HD-OCT acquisition software adapted to the optical pathway of a Zeiss OPMI VISU 200 surgical microscope. The shared pathway enables simultaneous direct and OCT visualisation of the same surgical area.

Prof Augustin presented several cases illustrating the capability of microscope-mounted OCT. In one, of a preretinal membrane, the flat membrane without anteriorposterior traction and an intact foveal contour are clearly visible at the start of surgery, as are the clean retina and foveal depression at the end of vitrectomy.

In another case, the lifting of the internal limiting membrane is clearly visible mid-procedure. A third shows how the technology can be used to identify residual membrane after the first peeling. Intraoperative OCT is also useful for evaluating macular hole closure directly after membranectomy and can help determine how to proceed, Prof Augustin said. It can help in choosing which gas to use or if just air is an option.

Prof Augustin noted that ultrasound imaging is already used to guide placement of posterior radioactive plaque for treating choroidal melanoma. OCT can provide very high resolution images of the choroid, and so is a logical next step. Recording OCT images during surgery also opens the door to further research in pathophysiology, Prof Augustin said.