13Intraoperative Monitoring and Verification

TGuarding the facial nerve

Continuous electromyographic facial-nerve monitoring, with needle electrodes in the orbicularis oris and orbicularis oculi, alerts the surgeon when drilling or dissection approaches the nerve, and is especially valued in malformed or ossified ears where the nerve may be aberrant or the dissection extensive. Monitoring depends on an intact neuromuscular junction, so long-acting paralytic agents are avoided after induction; if a relaxant is used for intubation, a short-acting drug is chosen so the muscle can respond when the case proceeds. The anaesthetist is asked to confirm that paralysis has worn off before any nerve-adjacent step, because residual blockade silences the monitor and creates false reassurance. Monopolar cautery must not be used once the implant is in place, so bipolar cautery is substituted, and the monitor is rechecked for baseline responsiveness at the start of cochlear work. A typical cochlear-implant procedure runs about 1.5 to 3 hours, during which the monitor provides continuous rather than only stimulated feedback.[2009][2014]

CIs the device alive and the array sealed

Impedance telemetry is the first electrical check after insertion, measuring the resistance to current flow at each contact to confirm electrical continuity of the array and integrity of the internal device. A true open circuit reads a near-infinite impedance, on the order of 20 to 30 kilohms or greater on modern devices, and points to a broken lead or a contact not in contact with fluid. A short circuit reads abnormally low impedance, on the order of about 1 kilohm or less, indicating current leaking between two contacts through a compromised insulating layer. Different coupling modes catch different faults, so monopolar testing detects an open circuit that common-ground testing can mask, and a sweep across all electrodes is run before closing. Impedances measured on the table also seed the initial map and give a baseline against which postoperative drift is tracked at activation.[2009][2013]

CIs the nerve responding

The electrically evoked compound action potential (ECAP) records the synchronous response of the auditory nerve to a stimulus on an adjacent contact, marketed as AutoNRT by Cochlear, NRI by Advanced Bionics, and ART by MED-EL. A present ECAP confirms a stimulable nerve and a functioning electrode, but it does not confirm correct array position, since measurable ECAPs have been recorded with extracochlear contacts and even with a folded tip. ECAP thresholds gathered intraoperatively provide a starting point for initial programming in very young children and others who cannot give behavioural feedback, even though their correlation with behavioural T and C levels is only modest. The electrically evoked stapedius reflex threshold (ESRT) is elicited by stimulating an electrode and watching for stapedial-tendon contraction, observed directly through the surgical microscope while the middle ear is still open. ESRT estimates the upper (comfort) level: with the round window still exposed the surgeon can confirm the reflex on representative electrodes, and at programming the maximum level is typically set a little below the ESRT.[2009][2014]

CIs the array where it should be

An intraoperative plain radiograph in a Stenvers (cochlear) or transorbital projection profiles the spiral of the array so the surgeon can confirm a smooth turn and full insertion before closing. The film's primary job is to exclude tip fold-over and to detect frank malposition such as an electrode that has passed into the IAC or a semicircular canal, findings that the electrical tests will miss. Because ECAP and NRT stay normal with a folded tip or an extracochlear array, imaging is the definitive arbiter of position; in published cases electrode malposition was identified by x-ray or CT, not by neural telemetry. Spread-of-excitation telemetry can flag tip fold-over electrically by showing two or more local maxima on the excitation curve, with suspected fold-overs then confirmed on a rotational radiograph. Any confirmed fold-over or malposition requires revision, ideally done immediately under the same anaesthetic by withdrawing the array and reinserting it or the backup device, which is why verification happens before the patient leaves the table.[2014][1997][2009]