Electrical Bio-Impedance Proximity Sensing for Vitreo-Retinal Micro-Surgery

Retinal vein occlusion is a widespread eye vascular disease leading to vision loss because of clots obstructing a retinal vessel. Recent research investigated retinal vein cannulation, a promising treatment in which a microneedle injects a thrombolytic agent inside the clotted vessel. During this surgical procedure, the surgeon only relies on a microscope, looking through the patient eye's lens. Such visual feedback gives poor depth perception, with a risk to inject the agent underneath the retina. Such situation endangers the patient's eyesight and is to be avoided. This letter explores bio-impedance as a means to estimate the proximity between an insulated electrode and a retinal vessel. Experiments on five
ex-vivo
pig eyes showed that the impedance phase decreases as the electrode tip approaches the retinal vessel. Specific patterns in the impedance magnitude were detected. A dedicated algorithm showed 98
$\%$
sensitivity and 100
$\%$
specificity at a distance of 775 ± 275 μm away from the retinal vessels. Knowledge of the proximity can help the surgeon prevent damaging fragile retinal structures during retinal surgeries such as cannulation and scar tissue peeling. A minimally-invasive measurement principle of this technique is demonstrated on an
ex-vivo
pig head to show the feasibility of such technique in more realistic conditions.