Post-operative visual loss (POVL) is a rare but serious complication after surgery.
One of the known associations is prone spinal surgery with a prolonged duration.
The cause is not completely understood, but some of the risk factors for developing POVL include anaemia, hypotension, the duration of surgery and patient factors such as age, hypertension and atherosclerosis. There are also some unidentifiablerisk factors that may predispose young healthy patients to POVL. Once the patient develops POVL, it is usually irreversible, and the patient is left with a permanent disability.
The majority of POVL cases that occur with prone spinal surgery are due to posterior ischaemic optic neuropathy (PION). PION is directly related to the ocular perfusion pressure (OPP), which is estimated as the difference between mean arterial blood pressure (MAP) and intraocular pressure (IOP). Autoregulation is believed to maintain a constant perfusion to the optic nerve despite fluctuations in the perfusion pressure.
However, there is a knowledge gap in the physiological changes of IOP during prone surgery. IOP is a highly dynamic parameter, and the most widely used method to measure it is using a handheld tonometer. The main limitation of this technique is the isolated nature of its measurements, as it can only be taken at regular intervals, and may not reflect the full range of IOP changes. Studies on IOP during prone position in the current literature have all used this technique. 1-5 Hence subtle changes of IOP can be undetected, as interval measurements cannot reflect short-term variations that occur within seconds or minutes. Furthermore, most of the studies are performed on animals or healthy volunteers 5,6 , which further limit the interpretation of their results. For the few studies performed on anaesthetised patients undergoing spinal surgeries, the duration of surgery is short or mixed, so the critical time point may not have been studied. The American Society of Anesthesiologists Postoperative Visual Loss Registry suggests that an anaesthetic duration of greater than 6 hours is more correlated with POVL.
Using state of the art technology Over the past five years, advances in technology have led to the development of devices that allow 24 hour continuous IOP monitoring. The new system makes use of microelectromechanical systems, nanotechnology, and telemetry to allow continuous monitoring. A disposable contact lens sensor, first proposed by Leonardi et al. 8 , is now commercially available (SENSIMED Triggerfish; Sensimed AG, Lausanne, Switzerland). This technology can acquire data points over a 24 hour period corresponding to 30 seconds of continuous measurements. It measures changes in the ocular dimensions at the corneoscleral junction, which corresponds to changes to IOP and volume. The microprocessor within the lens transmits data to an external antenna, and the profiles can be stored in a portable recorder. 9 Although this sensor does not measure IOP directly, it gives measurements with a composite of IOP and volume changes, and it is most valuable in documenting relative changes of IOP-related events and their timing. It is also well tolerated in real life patients, and have been used to document IOP changes in patients with glaucoma and thyroid eye disease. 9-11 This is the only non-invasive temporary continuous monitoring in the system that can be used in human beings. This new technology is therefore ideal in detecting subtle changes in IOP pattern in prone spinal surgery. It will provide a continuous monitoring which can be correlated with the patient's level of anaesthesia, arterial pressure fluctuations and volume status, and can contribute to improved understanding of IOP changes during prone spinal surgery, which may in turn bridge the knowledge gap of the causes of POVL.
