Intraoperative neurophysiological monitoring (INPM) is a subspecialty of the Clinical Neurophysiology specialty dedicated to continuously evaluate the integrity of the neurological structures of the anesthetized patient during surgery, to alert in case there are indications of possible damage in order to try to correct the causal factor and, in case the damage cannot be avoided, to document when it happened. In general, it has made it possible to reduce post-surgical morbidity in terms of neurological deficits, as well as allowing more aggressive surgeries to be performed that would otherwise not be done due to the risk involved. On the other hand, it allows the localization of neurological structures by means of mapping techniques.
Advances in monitoring techniques have gone hand in hand with the development of other techniques to evaluate the nervous system and have been applied to the peculiarities that the operating room demands. Thus, spinal surgery monitoring was performed years ago only with somatosensory potential techniques that only evaluate sensory pathways and, as transcranial magnetic stimulation studies were developed initially and, later, transcranial electrical stimulation, it was also possible to monitor motor pathways, whose affectation can occur independently and not associated with the previous ones. New technologies such as the application of trains of stimuli versus single stimuli or the development of multimodal monitoring with multiple neurological variables monitored simultaneously allow continuous control of the integrity of the nervous system.
Despite the progress they have made in surgical safety, these techniques are not free of complications, although they are not frequent. Biting of the tongue or, more seriously, of the orotracheal intubation tube itself, damage to the surgical field due to stimulation-induced movement, seizures, burns at the electrode sites or cardiovascular alterations can occur.
However, the most important complication is not detecting damage due to not recognizing all the circumstances that may have an influence or not having conveniently monitored the appropriate variables. Therefore, as in everything else, these techniques depend on the expertise of the person performing them and on the good interrelation of the professionals involved in the whole surgical procedure.
Techniques used in intraoperative neurophysiological monitoring and in which surgeries are they used?
Although the mainstay of IOM is the monitoring of motor and sensory potentials, there are many other neurophysiological techniques that are usually applied simultaneously and that will depend on the surgical procedure to be performed. We are going to mention some techniques and their application in different surgeries:
– Transcranial cortical PESS. They represent the reproducible electrical activity of cortical and subcortical structures after a peripheral nerve stimulus (commonly, median or posterior tibial nerve). Final cortical potentials named for their latency and polarity (N20 and P39 respectively) are obtained. They evaluate long sensory pathways, especially the post-sensory cords. They are indicated in surgeries with risk of direct mechanical damage on the sensitive pathways at any of their levels, as well as in vascular surgical processes that put the irrigation of the pathway at risk. They are very useful in the monitoring of ischemia. However, it is possible that, due to the different irrigation of the anterior and posterior medulla, motor deficits may occur without alteration of the PESS. On the other hand, because they present a very small amplitude, they need to be summed by averaging techniques, so it is not possible to detect damage at the exact moment it occurs.
– Transcranial motor evoked potentials. EMPs are produced by initiating a depolarizing action potential in the axons of pyramidal cells in response to a transcranially applied stimulus. Transcranial electrical stimulation is achieved with short, high-voltage electrical stimuli through subdermal electrodes. The recorded responses can be obtained at the spinal level, by means of epidural electrodes and consist of D waves (direct) and a series of I waves (indirect) or also at the muscular level. The muscle components of the PEM are biphasic responses recorded on the muscle belly. They assess long motor pathways (corticospinal pathway) at any central level. They would be indicated in surgeries that put at risk the motor cortex, the motor cords in the medulla or any part of the pathway between both sites. They are very sensitive in the assessment of motor function and are, together with the D wave, essential to predict the prognosis in intramedullary surgery. However, they are very susceptible to small changes in anesthesia, and especially to halogenated gases and muscle relaxants, so that even small changes in anesthesia can disrupt monitoring. On the other hand, a single stimulus is incapable of producing them, so we need a train of stimuli, which produces a certain movement in the patient that may disturb the surgeon.
Other techniques depending on the type of surgery:
– Supratentorial surgeries: PEM and PESS by direct cortical stimulation. In these cases, monitoring would be performed exclusively ipsilateral to the lesion.
– Brainstem surgeries: cortico-bulbar EMP, Blink Reflex, Brainstem Auditory Evoked Potentials (BAEP), Electromyography (EMG).
– Spine surgeries with instrumentation. Free-running electromyography (free-running EMG) or by screw stimulation (triguered-EMG). More recently, pedicular path stimulation techniques by means of stimulus trains and EMG recording in distal muscles. Bulbo-cavernous reflex (BCR).
– Surgery of intramedullary expansive processes. D wave.
– Epilepsy surgery, vascular surgeries and evaluation of anesthetic depth. Electroencephalography (EEG).
– Supratentorial surgeries requiring localization of eloquent areas of the cortex, either motor or language (cortical mapping), precentral sulcus (PESS phase inversion technique), localization in depth of the descending motor pathways (subcortical mapping)…
– Peripheral nerve surgeries. Peripheral nerve mapping.
Which pathologies need to be monitored?
In general, any surgery involving possible secondary neurological injury, either of central or peripheral origin. These lesions are not always due to the direct action of the surgical technique itself, but may also be secondary to the positioning of the patient during the intervention, vascular and/or spinal blood flow alterations, as well as body temperature.
Differences between an awake patient and an anesthetized patient
It is important to keep in mind that the problems in an operating room are totally different from those in the office.
Most anesthetics affect monitoring by altering evoked responses, both sensory and motor, so the choice of drugs may be limited depending on whether they affect evoked potentials to a greater or lesser extent. The anesthetic regimen that has been shown to be most effective is the combination of propofol (100-150 mg/kg/min) and remifentanil (1mg/kg/h), with the use of short-acting muscle relaxants only during intubation. In addition, it should be taken into account that the administration of halogenated gases (due to their action at the central synapses) and muscle relaxants should be avoided beyond the time of intubation (due to their characteristic inactivation of peripheral synapses).
Intraoperative management includes maintaining stable physiological conditions, which involves adjusting hemodynamic parameters, blood rheological constants to promote proper oxygen exchange, ensuring ventilation and avoiding temperature variations.
Close collaboration between the anesthesiologist, the surgeon and the neurophysiologist will ensure the success of NIM, and will make it possible to avoid neurological injuries due to a change in the surgical attitude, before they occur.