Intraoperative
monitoring (IOM) measures small electrical signals produced within the
nervous system. These small signals can be on-going such as those measured
in an EEG or "evoked" responses produced by stimulating the
nervous system, similar to a reflex. In the hands of experienced technologists
or physiologists, IOM can reduce the incidence of permanent neurological
deficits following neurosurgical procedures such as microvascular decompression
surgery. IOM can detect subtle changes in nervous system function and
can provide an early warning to the surgical team so that permanent
damage is avoided. This information can guide the surgical team to modify
the procedure in order to improve outcome and reduce the risk of surgery.
In
MVD surgery, IOM personnel place recording electrodes, similar to EKG
electrodes, on the patient and connect these to a computerized workstation
that records and displays the results of the neurologic tests. Specifically,
we measure the Brainstem Auditory Evoked Response for the continuous
assessment of the cochlear (hearing) nerve. This nerve is very sensitive
to stretch or manipulation which may result in hearing loss postoperatively.
Monitoring of cochlear nerve function alerts the surgical team to potentially
injurious situations and allows them to make adjustments in order to
avoid permanent hearing loss.
In addition to monitoring
hearing during MVD surgery, we also examine other cranial nerves such
as the facial nerve. For hemifacial spasm we can continuously observe
the abnormal muscle activity in the facial muscles. This not only alerts
the surgical team of any threat to the facial nerve's function, but
can also indicate when the surgical procedure has relieved the cause
of the facial spasm.
Several other cranial nerves
are monitored during some operations. Also, additional neurological
systems and function may be assessed with somatosensory evoked potentials
(SSEPs) and motor evoked potentials (MEPs). While not all the IOM techniques
are necessary for every surgery, their routine use for specific operations
is advocated by many surgical teams including ours.
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