Continuous Intraocular Pressure Patterns in Spine Surgery
Condition: Intraocular Pressure
Study type: Interventional

Summary: 
      Post-operative visual loss is a rare but serious complication after surgery. It is most
      significantly correlated with prone spinal surgery with a prolonged duration. The cause and
      risk factors are not elucidated, but the majority of the cases are due posterior ischaemic
      optic neuropathy (PION). This entity 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.

      IOP is known to increase in the prone position thus putting susceptible patients at risk for
      inadequate ocular nerve perfusion. Most of the evidence comes from animal and healthy
      volunteer studies and cannot give an accurate insight into the subtle changes of
      intra-operative IOP. All the published studies employed the use of a tonometer, which may
      have rooms for measurement errors due to inadvertent pressure on the globe while retracting
      the eyelids, particularly when there is significant periorbital/conjunctival swelling in the
      prone position. The majority of the studies recruited healthy volunteers in a simulated
      surgical setting so other fluctuating parameters, which can affect intraoperative IOP, cannot
      be measured. In the few reports where patients undergoing surgery in the prone position were
      studies, IOP measurements were taken at non-continuous monitoring at time intervals, thus the
      effects of changes in blood volume, MAP, central venous pressure (CVP) and PaCO 2 could not
      be studied. Lastly, all the literature consists of case series only with no control group so
      the effect of position cannot be evaluated independently of the other factors.

      The investigators therefore propose to conduct a prospective study with continuous
      intraoperative IOP monitoring to give us further insight into the physiological changes of
      IOP in patients undergoing spine surgery, and identify the risk factors related to
      fluctuations of IOP during prone spinal surgery.
    

Detailed description: 
      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.
    

Intervention name(s): Contact lens sensor
Intervention type(s): Device
Intervention description(s): Patient will continue to wear the contact lens postoperatively for a total of 24 hours or until the patient cannot tolerate the contact lens. After removal of the contact lens sensor, the recorded profiles will be collected and visualized graphically on a computer interface.
Intervention model: Single Group Assignment
Intervention model description: All eligible patients who have given informed consent will attend the ophthalmology clinic for screening.
Patient will undergo general anaesthesia for prone spinal surgery. The degree of inclination was set prior to the start of the case and independently confirmed using a goniometer. Patients will be placed in the prone position on a Jackson Spine table.
Once positioned, the head is carefully placed on the GentleTouch prone positioning pillow of the appropriate size.
Patient will continue to wear the contact lens postoperatively for a total of 24 hours or until the patient cannot tolerate the contact lens. After removal of the contact lens sensor, the recorded profiles will be collected and visualized graphically on a computer interface. The output of the sensor expressed in voltage, and can be analysed in real time points corresponding to the intraoperative events.
Masking: None (Open Label)
Primary purpose: Diagnostic
Keywords: IOP