How to verify electrode placement after cochlear implantation in a patient?

Verification of Cochlear Implant Electrode Placement

Postoperative plain film radiography (Stenver's view or digital radiography) is the essential standard for verifying electrode placement after cochlear implantation, as it reliably identifies critical complications including tip fold-over, electrode extrusion, and extracochlear placement. 1, 2

Imaging Modalities for Electrode Verification

Intraoperative CT Scanning (Preferred Method)

• Intraoperative CT (iCT) provides immediate feedback while the patient remains under anesthesia, allowing detection and correction of suboptimally placed electrodes before leaving the operating room. 1
• iCT successfully identified and enabled correction of tip fold-overs in 4 cases intraoperatively, preventing the need for revision surgery 1
• Surgeons can use iCT images to refine technique, such as adjusting precurved electrode arrays to improve perimodiolar positioning 1
• iCT is increasingly adopted, with utilization rates ranging from 14% to 100% among surgeons at academic centers (mean 60%) 1

Postoperative Digital Radiography (Standard Alternative)

• Digital radiography with intermittent fluoroscopy confirmation provides high diagnostic quality images enabling identification of each individual electrode, which is especially valuable for postoperative electrode mapping. 3
• This technique delivers lower radiation exposure (40-440 microGy) compared to conventional skull radiography (470 microGy) or CT scanning (950 microGy) 3
• Digital radiography is comfortable, easily reproducible, and now used routinely for all postoperative cochlear implant assessments at many centers 3

Plain Film Radiography (Minimum Standard)

• Intraoperative Stenver's view plain film radiography is the only imaging modality that consistently impacts surgical decision-making and prompts use of backup devices when complications are identified. 2
• Plain film radiography identified tip rollover and extracochlear electrode placement in all cases (5 of 277 patients, 1.8%) in one large series 2
• This modality remains essential when more sophisticated imaging is unavailable 2

What Electrode Verification Imaging Should Identify

Critical Complications Requiring Intervention

• Tip fold-over: Electrode array doubles back on itself within the cochlea 1, 2
• Extracochlear placement: Electrode exits the scala tympani 2
• Electrode migration: Displacement of the array after initial placement, occurring in approximately 9% of cases with suboptimal positioning 4
• Scalar deviation: Electrode crosses from scala tympani into scala vestibuli 1

Electrode Position Assessment

• Confirm intracochlear placement within scala tympani compartment 1, 5
• Document angular insertion depth (optimal target is 680° to cover neuronal distribution) 5
• Count number of active electrodes successfully inserted 3
• Assess for electrode kinking or slippage 3

Complementary Intraoperative Testing (Not Substitutes for Imaging)

Electrode Impedance Measurements

• Open or short circuits detected in 6% of patients, though half normalize upon remeasurement 2
• Impedance testing alone does not impact surgical decision-making regarding electrode repositioning or device replacement 2

Neural Response Telemetry (NRT)

• Absent responses on one or more electrodes occur in 14% of patients 2
• Complete lack of response is rare (1.4%) and does not correlate with dysfunctional devices 2
• NRT results do not reliably predict electrode malposition and should not be used as the primary verification method 2

Common Pitfalls to Avoid

• Do not rely solely on electrophysiologic testing (impedance or NRT) without radiographic confirmation, as these modalities fail to detect critical anatomical complications like tip fold-over 2
• Do not defer imaging to the postoperative period when intraoperative CT is available, as this eliminates the opportunity for immediate correction under the same anesthetic 1
• Be aware that electrode migration may be more common than historically recognized (9% suboptimal positioning rate), with true migration occurring exclusively in straight arrays in some series 4
• Recognize that most migrations (74%) occur in primary surgeries despite robust fixation techniques 4

Clinical Algorithm

1. During surgery: Obtain intraoperative CT if available, or intraoperative plain film radiography (Stenver's view) before closing 1, 2
2. If iCT/radiograph shows tip fold-over or extracochlear placement: Remove and reinsert electrode array or use backup device 1, 2
3. If no intraoperative imaging performed: Obtain postoperative digital radiography or CT within the early postoperative period 3
4. Document: Angular insertion depth, number of intracochlear electrodes, and absence of complications 3, 5
5. If suboptimal placement identified postoperatively: Consider revision surgery, particularly for tip fold-over, significant migration, or extracochlear placement affecting device performance 4