Intraoperative neurophysiological monitoring (MNIO) is a subspecialty of the specialty of Clinical Neurophysiology dedicated to continuously assess the integrity of the neurological structures of the patient under anesthesia during surgery, alert if there is evidence of harm to try to correct the causal factor and, if they can not prevent damage, document when it happened. Overall, it has reduced postoperative morbidity in what refers to neurological deficit and allow performing more aggressive surgeries that otherwise would not be made by the risk involved. On the other hand, allows the location of the neurological structures using mapping techniques.
Advances in monitoring techniques have been parallel with the development of other techniques to evaluate the nervous system and have been applied to the peculiarities that requires surgery. Thus, monitoring of spinal surgery is performed years only techniques potential somatosensory only assess sensory pathways and, as were developed studies transcranial magnetic stimulation initially and subsequently transcranial electrical stimulation, could also monitor motor pathways, whose involvement can take place independently and not associated with the above. New technologies such as the application of stimuli trains versus single stimulus or development of multimodal neurological monitoring multiple variables monitored simultaneously allow continuous monitoring of the integrity of the nervous system.
Despite the progress represented in surgical safety, these techniques are not without complications, although not frequent. May produce bites tongue or, what is worse, the same tube endotracheal intubation, damage in the surgical field due to induced stimulation, seizures, burns in places where the electrodes or cardiovascular disorders movement .
However, the most important complication is not to detect damage due to not recognize all circumstances that may influence or may not have properly monitored the appropriate variables. Therefore, as in all, these techniques rely on the expertise of the person performing and good interaction of professionals involved in the surgical act.
Techniques used in intraoperative neurophysiological monitoring and what surgeries are used
Although the cornerstone of what constitutes MIO monitoring motor and sensory potential, there are many other neurophysiological techniques commonly applied simultaneously and will depend on the surgical procedure to be performed. We will mention some techniques and their application in different surgeries:
- Transcranial cortical SSEP. They represent reproducible cortical and subcortical structures electrical activity after stimulation of a peripheral nerve (commonly, median nerve or posterior tibial). a final cortical potentials known for its latency and polarity are obtained (N20 and P39 respectively). Evaluated sensitive long way, especially the laces post-riores. They are indicated in surgery at risk of direct mechanical damage to the sensitive to any level pathways and vascular surgical procedures that endanger irrigation pathway. They are very useful in monitoring ischemia. However, it is possible that, due to different irrigation spinal anterior and posterior, motor deficits may occur without alteration SSEP. On the other hand, because they have a very small amplitude, it is necessary to join by averaging techniques, so it is not possible to detect damage to the exact time when it occurs.
- Transcranial motor evoked potentials. PEMs are produced at the beginning depolarizing action potential in axons of pyramidal cells in response to stimulation transcranially applied. Transcranial electrical stimulation is achieved with short high voltage electrical stimuli via electrodes subdermal. The recorded responses can be obtained at the spinal level by epidural electrodes consist waves and D (direct) and a series of waves I (indirect) or also at muscle level. The muscular components of the PEM biphasic responses are recorded on the muscle belly. Motor value long routes (via corticospinal) to any central level. They would be given in surgeries that endanger the motor cortex, the spinal cords engines or any part of the route between the two places. They are very sensitive in the assessment of motor function and is coupled with the wave D, essential to predict prognosis in surgery intramedullary. However, they are very susceptible to small changes in anesthesia, and especially halogenated gases and relaxing muscle, making minor changes in it can derail monitoring. Furthermore, a single stimulus is unable to produce them, so we need a train of stimuli, causing some movement in the patient that may disturb the surgeon.
Other techniques depending on the type of surgery:- Supratentorial Surgeries: PEM and direct cortical stimulation PESS. In these cases the monitoring be conducted exclusively ipsilateral to the lesion.- Surgeries brain stem: PEM cortico-bulbar, blink reflex (Blink Reflex), Auditory brainstem response (ABR), electromyography (EMG).- Surgeries column with instrumentation. Free electromyography (free-running EMG) or stimulation of screws (triguered-EMG). More recently stimulation techniques pedicle journey by trains of stimuli and recording of PEM in distal muscles. Reflection bulb-Cavernous (RBC).- Surgeries intramedullary expansive processes. Onda D.- Epilepsy surgery, vascular surgery and anesthetic depth assessment. Electroencephalography (EEG).
- Surgeries supratentorial locate precise areas of eloquent cortex, either motor or language (cortical mapping), pre central sulcus (phase inversion technique SSEP), location in depth of the descending motor pathways (subcortical mapping) ...- Peripheral nerve surgeries. Peripheral nerve mapping.
What conditions are accurate to monitor?
In general, any surgery involving possible secondary neurological injury, whether of central origin as peripheral. These lesions are not always due to direct action of the surgical technique but can also be secondary to patient positioning during surgery, vascular disorders and / or the medullary blood flow and body temperature.
Differences between an awake patient and one anaesthetized
It is important to consider the problems in an operating room are totally different from those of the query.
Most anesthetics affect monitoring, altering both sensory and motor evoked responses so the choice of drugs may be limited, as they affect a greater or lesser extent evoked potentials. The anesthetic regimen that has been proven more effective is the combination of propofol (100-150 mg / kg / min) and remifentanil (1mg / Kg / h), with use of short-acting muscle relaxants during intubation only. In addition, keep in mind that you should avoid the administration of halogenated gases (for his performance at the level of central synapses) and muscle relaxants beyond the time of intubation (for its characteristic inactivation of peripheral synapses).
Intraoperative management includes maintaining stable physiological conditions involving haemodynamic parameters set, constant rheological blood to favor correct exchange of oxygen, ensure ventilation and prevent temperature variations.
The close collaboration between the anesthesiologist, surgeon and neurophysiologist ensure the success of the MNI, and neurological injuries will prevent a change in the surgical approach before they occur.