Intraoperative neuromonitoring (IONM) was introduced several decades ago and is an increasingly favourable option for delicate surgeries. A recent study1 indicates that IONM procedures have risen 296% from 2008–2014. So, what is it and how does it benefit spine surgery? Jay Jagannathan discusses IONM’s relevance to complex spinal surgeries.
IONM is the practice of using electrophysiological methods including electroencephalography (EEG), electromyography (EMG), and evoked potentials to scan and monitor the functionality—as well as integrity—of certain neural structures during a surgical procedure. While IONM has multiple purposes, it can be invaluable in the spinal theatre in reducing risk to a patient’s spinal cord or the central nervous system. IONM requires a team of specialists typically consisting of a surgeon, neuromonitoring technologist, neuromonitoring interpreting physician, anaesthesiologist and surgical team to collaborate during a procedure.
Intraoperative neuromonitoring has particular relevance for complex spine procedures. Its use in cervical, thoracic or lumbar procedures, especially those that pose any risk to nerves or the spinal cord, provide critical information to assist in achieving a positive surgical outcome. In extreme cases such as tumour removal, deformity correction or an injury to the spinal cord, intraoperative neuromonitoring should be considered a necessity.
In any neurosurgical procedure with a risk of injury to the nervous system or spinal cord, an extra set of eyes and clear guidance is a welcome addition. Collaboration and trust in working with neuromonitoring is key and begins well before any procedure is scheduled. To understand the importance of neuromonitoring in spine surgery, it is critical to recognise the specific importance of what is being monitored.
Somatosensory Evoked Potentials (SSEPs) monitor the transmission of sensory impulses to the brain. In spine surgery, this is often measured by providing a small electrical stimulus to a peripheral nerve ending. These impulses are then sent via a complex pathway involving the dorsal columns, the thalamus, and the somatosensory cortex, which is the portion of the brain which receives sensory impulses. In general, somatosensory impulses cross over from one side of the body to the other, so impulses received in the left side of the brain are from the right side of the body.
From a surgical perspective, continuous SSEP monitoring provides the surgeon with a general assurance of the integrity of the peripheral nerve/central nervous system circuit. Changes in SSEPs may be seen in a variety of situations—the initial movement of patients in unstable spinal cord injuries, during surgical manipulation around the spinal cord, when an extremity is not positioned properly and during circumstances such as excessive blood loss or drops in blood pressure. Each of these conditions require specific treatments. For example, if SSEP potentials alert when a spinal cord tumour is removed, it is often necessary to raise blood pressure, and in some instances steroid medications may be indicated. In other cases, changes in potentials may be addressed by doing something as simple as repositioning an extremity which may be under too much traction. Preservation of SSEP curves have been shown to predict long-term recovery from spinal cord injury in animals (Neurosurgery. 1987 Jan;20(1):138-42.). In another study, Langeloo et al found that an 80% decrease in SSEP amplitude in one of six recordings had 100% sensitivity and 91% specificity for spinal cord injury (Spine, Phila Pa 1976) 2003 May 15;28(10):1043-50).
Motor evoked potentials (MEPs) test the integrity of motor pathways that start from the brain. These pathways are relayed to the muscles and extremities via nerves in the cortico-spinal tract and subsequently by peripheral nerves. Motor neurons can be stimulated directly (D-waves), that reflect direct activation of the motor system, or indirectly (I-waves), that reflect indirect activation of motor neurons via other circuits in the brain. When operating in a region at risk of injury (for example in a spinal cord tumour), measuring D-waves above the location can provide information on whether the level of transmission from the brain is adequate.
More commonly, in degenerative spine surgery, the transmission of signals at the level of the muscle (the M-wave) is important as it demonstrates integrity of the cortico-spinal-peripheral nerve circuit. Unlike SSEP monitoring, which is continuous, MEP potentials are done at the surgeon’s request, when there is suspicion of injury. In patients with severe pathology, MEPs may be decreased prior to the surgery starting, and often a baseline MEP recording is needed before the operation. Profound drops in MEP may be a sign of neurologic injury.
Unlike SSEP and motor potentials, EMG monitoring measures the integrity of peripheral nerve function and the impulses it relays to the muscles–thus it measures nerve function outside the spinal cord. In EMG monitoring, electrodes are placed in the muscles innervated by a given nerve. Stimulation of intact motor nerves will create a Compound Motor Action Potential (CMAP) that indicates stimulation of the muscle innervated by the nerve. In addition to confirming the presence of a nerve (useful in lateral, transpsoas approaches where the nerve may be hidden in muscle), a train of EMG during decompression or placement of a cage may be a signal of nerve compression or injury and may require the surgeon to reconsider the extent of the decompression or the size of the cage being placed.
Typically, there are two types of EMG recordings, including free-running EMG, which is performed through the duration of an operation and provided second-by-second monitoring of the nerve function. Triggered EMG by contrast, measures muscle contraction after stimulation on or around a nerve. Triggered EMG has been described in placement of pedicle screws in spinal surgery, although it’s utility in these cases is controversial.
There is some industry debate among spine surgeons on when to utilise IONM on patients. While it is now widely accepted that there are benefits in achieving positive surgical outcomes, some dismiss the need for this advanced surgical approach among simpler procedures such as routine laminectomies or microdiscectomies.
There is no doubt that IONM adds expense to already expensive spinal procedures, but many insurers recognise the benefits. The reduced surgical injury risk coupled with quicker patient recovery are a bonus, not to mention the avoidance of a potential malpractice claim. IONM’s growing popularity can be noted in the steady increase of cases, which one study2 tracked at 1% of all spine procedures in 2007, to 12% of all cases by 2011.
If IONM is to continue growing in popularity and acceptance in the spine surgery theatre, there will need to be more training on its indications and limitations. The cohesiveness of the IONM team from surgeon, to anaesthesia to monitoring is also critical. In the future it is believed that applications of IONM will have benefits in the operating room, but also in on-site trauma and intensive care units. The potential benefits of applying remote monitoring techniques to the critically injured can greatly impact the level of knowledge and approach to spinal assessment and care prior to the surgical environment.
For further information, research and advocacy, please visit the American Society of Neurophysiological Monitoring (ASNM), a non-profit dedicated to advancing industry interest in the practice.
Jay Jagannathan, FAANS is a neurosurgeon specialising in cranial and spinal surgery at Jagannathan Neurosurgery (Michigan, USA).
References
1 Laratta JL, Shillingford, JN, Ha A, Lombardi JM, Reddy HP, Saifi C, Ludwig SC, Lehman RA, Lenke LG. Utilization of intraoperative neuromonitoring throughout the United States over a recent decade: an analysis of the nationwide inpatient sample. Journal of Spine Surgery. 2018; 4(2):211–219.
2 James WS, Rughani AI, Dumont TM. A socieconomic analysis of intraoperative neurophysiological monitoring during spine surgery: national use, regional variation, and patient outcomes. Neurosurgical Focus. 2014; 37(5):E10.
