Pre-intervention considerations for Cochlear Implant (CI) placement:

Preoperative radiological assessment is critical in cochlear implantation, enabling a thorough evaluation of the cochlea's structure (normality, patency, malformations), as well as the cochlear nerve’s status (normality, hypoplastic, absent) and a comprehensive examination of the auditory pathway up to the temporal cortex (normal, malformations, parenchymal lesions). This detailed analysis is crucial for surgical planning, guiding decisions on the most appropriate surgical technique (closed mastoidectomy with posterior tympanotomy vs. petrosectomy) and the optimal approach to the cochlea (typically via the round window membrane vs. through a cochleostomy in the basal turn). To achieve these objectives effectively, it's imperative to utilize both CT and MRI, as each modality offers unique and complementary information.

The Standard surgical approach consists of a closed mastoidectomy, with posterior tympanotomy (drilling of the facial recess) and introduction of the elèctrode through the round window membrane. In other cases, the introduction of the CI through subtotal petrosectomy (drilling of temporal pneumatic cells, with elimination of the middle ear by eversion of the EAC).

• Integrity of the auditory pathway. The presence of infarcts, sequelae of traumatic brain injuries (TBI) or meningitis sequelae.
• Cochlear status. Whether the cochlea is normal and pathological, considering the presence of malformations, otosclerosis foci, or cochlear fibrosis. These factors are decisive in selecting the candidate and the most appropriate surgical technique, as well as choosing the type of electrodes (straight or perimodiolar, among others).
• State of the cochlear nerve. MRI is essential to determine its existence, thickness, and normal appearance.
• State of the temporal bone. To determine the degree of pneumatization, the presence of osteodystrophies of the temporal bone, anatomical variants (aberrant internal carotid artery or facial nerve path, prominent jugular bulb, prominent sigmoid sinus, low middle meningeal artery, emissary veins, low-lying temporal pole) and to ensure the normal anatomy of the temporal bone.

Anatomical variants that complicate cochlear implantation:

• Hypoplastic mastoid. Minimal pneumatization and prominent sigmoid sinuses, resulting in a risk of bleeding during mastoidectomy. In such cases, a petrosectomy technique may be preferred to ensure safe access to the cochlea. FIG. 1.
• Prominent jugular bulb. A cochleostomy approach is preferred, instead of the typical round window entry for electrode insertion.   FIG. 2.
• Aberrant carotid artery. FIG.3.
• Aberrant course of cranial nerve VII. Facial nerve injury is rare because it is evaluated by CT beforehand and monitored in the operating room. FIG. 4.

Inner ear malformations that complicate CI placement:

Cochlear implantation in patients with inner ear malformations presents unique challenges but can still be an effective intervention for many cases. While some malformations significantly complicate the surgical procedure and may lead to poorer outcomes, others allow for successful implantation with good results. FIG. 5.

• Cochlear aplasia.
• Common cavity. Associated with GRB1L gene mutation (AD inheritance pattern). FIG. 6.
• Incomplete partition type I (IP-I). Cystic appearance of the cochlea due to the absence of the modiolus and interscalar septa. It is associated with hypoplasia or aplasia of the cochlear nerves in 25% of cases. This finding may influence the decision between cochlear implantation and auditory brainstem implantation. FIG. 7.
• Incomplete partition type II (IP-II) + enlarged vestibular aqueduct (Mondini). Cochlea with a normal basal turn and cystic apex, accompanied by a minimally dilated vestibule and enlarged vestibular aqueduct. FIG. 8.
• Cochlear hypoplasia.

Pathologies that worsen the performance of CI or increase the risk of malpositioning:

• Pericochlear otosclerosis. The preferred entry would be through cochleostomy. Fig. 9.
• Ossifying labyrinthitis. Can significantly complicate or contraindicate CI. Involves abnormal bone growth within the perilymphatic spaces of the cochlea, often secondary to meningitis. FIG. 10.
• Osteopetrosis. FIG. 11.

Pathologies that increase the risk of Gusher/CSF fistula:

Gusher phenomenon consists of a profuse outflow of cerebrospinal fluid (CSF) when performing cochleostomy or opening the round window due to an abnormal communication between the subarachnoid and perilymphatic spaces through the cribiform plate (malformative or post-traumatic) and an abnormal diameter of the cochlear canal.

• IP-I. Significant Gusher phenomenon.
• IP-II. Mild Gusher phenomenon.

Lesions contraindicating cochlear implantation:

• Post-traumatic temporal encephalomalacia. Central hearing loss. FIG. 12.
• Cochlear hypoplasia with absence of the cochlear nerve. FIG. 13.

Correct positioning of the CI:

As a radiologist is fundamental to know how the CI should be placed inside the cochlear spiral depending on the surgical approach. FIG. 14. 

Post-surgical considerations:

Postoperative radiological evaluation serves two critical functions in cochlear implantation: verifying correct electrode positioning and detecting potential complications. The primary goal is to confirm that the electrode array is properly situated within the scala tympani of the cochlea. Additionally, imaging plays a vital role in identifying complications, which can vary in severity and management approach. These complications can be categorized into: minor complications, generally resolve with conservative treatment, and major complications, often necessitate surgical reintervention.

Minor Complications

Typically self-resolving or manageable with conservative treatments and device reprogramming. The most common minor complications include:

• Facial nerve stimulation. Involuntary movements of the face or neck when using the cochlear implant, or unwanted sensations in the face or other areas innervated by this nerve. It is managed with adjustments in implant programming. Is more frequent in patients with facial nerve dehiscence or otosclerosis in the spongiotic phase. FIG. 15.
• Wound complications. Resolves with the use of antibiotics or local care.
• Tinnitus, vertigo and instability. Usually transient.

Major Complications

Although less frequent, can have significant clinical consequences and often necessitate surgical intervention. The most serious complications include:

• Implant failure. Electrode breakage or device malfunction, requiring replacement through a new surgical intervention.
• Electrode array malposition. Scala dislocation (the electrode passes from the scala tympani to the vestibular scala), partial insertion into the cochlea, or extracochlear insertion (semicircular canal, internal auditory canal, electrode kinking). It is usually associated with straight electrodes. FIG. 16. 
• Facial paralysis. Usually transient, however, in cases of severity and persistence of symptoms, a CT study can evaluate the state of the facial canal.
• Severe infections. May require hospitalization, surgical drainage, or device removal. Chronic inflammation can cause implant extrusion or removal.FIG. 17.
• Pneumolabyrinth. Usually common and transient in the immediate postoperative period. However, persistent air presence can lead to decreased implant performance. FIG. 18.
• Meningitis / intracranial abscesses. A systematic pre-implantation pneumococcal vaccination has significantly reduced the rate of meningitis. However, there is a higher risk in patients with cochlear malformations, chronic otitis, or anatomical abnormalities. FIG. 19.
• Iatrogenic cholesteatoma. Occurs in cases where the posterior wall of the external auditory canal is too thin when drilled during surgery, favoring skin invagination through the bone defect. Can also be due to previous petrosectomy procedure, leaving epithelial remnants in the middle ear.

The study of cochlear implant complications requires both CT and MRI, with modern implants being increasingly MRI-compatible (1.5T and 3T). There are two critical steps to prevent magnet displacement during MRI: placing a magnetic splint directly over the subcutaneous implant magnet and applying a thick, two-layered compressive head bandage to stabilize the receiver-stimulator area.

Another adverse effect is demagnetization of the implant and local pain, which can lead to interruption of the study. The primary challenge is the metallic artifact produced by the implant, particularly pronounced in diffusion and T2 sequences. In some cases, it may be necessary to remove the implant's magnet.

A thorough radiological assessment is essential for identifying the various causes of cochlear implant failure and related complications.