Cochlear implants and the Radiologist

9 yr old girl  shows presence of cochlear implant in situ, shows  pneumocochlea, pneumocephalus, and subtle  fluid density in to the middle  ear location, possibly perilymph fistula,  through the  iatrogenic, intended cochleostomy. The electrodes appear close to cochlea particularly apical region and are normal. There is no evidence of infection on this. Treatment of such moderate air without parenchymal presence is usually conservative. Case and discussion by Dr MGK Murthy

Indication

Nonfunctional  (ganglion cells ineffective)cochlea on  both sides leading to bilateral sensorineural  deafness with intact auditory nerve function. Usually in children, but of late the adult varieties are receiving the implant

Contraindications

·         Obliterative labyrinthine ossification, severe cochlear or fenestrative otosclerosis, congenital cochlear  malformations(mondini deformity, ossified cohlea etc), severe bilateral temporal bone fractures, Infected middle ear

·         Usually HRCT temporal bone would evaluate all , but some prefer MRI to evaluate 8th nerve, speech and language functional MRI as well

Normal hearing

Sound from the environment—via the external and middle ear(including tympanic membrane and ossicles)--- Cochlea spiral ganglion cells----auditory nerve---brainstem.

Implant  has two parts

External-microphone  to pick the environment sound

-Speech processor-digitizes the signal

-Transmitter-converts to FM type  of Radiosignals

Internal –kept aligned well by the magnets , and placed underskin behind ear

  -Receiver/stimulator(disk shape)-converts FM signals to electric signals

- through a wire connected to Electrodes(usually 24)

-stimulate the spiral ganglion cells (apical better because sound will be more natural)------auditory nerve---brain. Sound produced is different from ordinary and robotic, needs adjustment, because 24 electrodes cannot match 15000 haircells normally present

Surgery

Making communication between mastoid/ middle ear and mastoid / cochlea . Intra -operative Radiographs help in correct positioning of the electrodes

Complications

Extra cochlear placement of the electrodes including in the semicircular canals, breakage facial palsy, infection, pneumocephalus, fluid drainage, meningitis, cochlear damage during insertion of electrodes, osteogenesis, vestibular symptoms etc

MR Safety

Previously thought to be Unsafe (because of the magnets with in the gadgets), recent models are classified as conditional (can be performed with specific recommendations of the manufacturer  including the magnetic strength)

HRCT (if needed with contrast) is adequate for post operative evaluation including electrodes malpositioning