Beyond a Hearing Aid
A cochlear implant (CI) is a surgically implanted neuroprosthetic device designed for individuals with severe-to-profound sensorineural hearing loss. Unlike conventional hearing aids, which simply amplify sound, a cochlear implant works by converting acoustic signals directly into electrical impulses that stimulate the auditory nerve (cranial nerve VIII), bypassing the non-functional cochlear hair cells entirely.
The result is an artificial sense of hearing that the brain learns, over time, to interpret as speech and environmental sound.
Two Parts, One System
The cochlear implant is divided into an external part worn on or behind the ear, and an internal part surgically placed under the skin and inside the cochlea.
Microphone
Captures ambient sound from the environment, mounted on the behind-the-ear unit.
Sound Processor
Analyses, filters, and converts the acoustic signal into a digital code using a speech processing strategy.
Transmitter Coil
Sends the coded signal transcutaneously (through the skin) via radio-frequency to the internal receiver.
Internal Receiver
Surgically fixed to the temporal bone; decodes the signal and converts it to electrical pulses.
Electrode Array
A thin flexible wire with 12–22 electrodes inserted into the scala tympani of the cochlea.
Auditory Nerve
The electrodes stimulate the spiral ganglion neurons, whose axons form the auditory nerve (CN VIII).
From Sound Wave to Perception
The journey from a sound in the environment to a perceived signal in the brain follows six key steps:
Sound captured by the microphone
The behind-the-ear microphone picks up environmental sound in real time.
Digital processing & coding
The sound processor applies a coding strategy (e.g., Continuous Interleaved Sampling) to extract the most perceptually relevant features of speech.
Transcutaneous transmission
The transmitter coil sends the coded signal across intact skin to the implanted receiver via magnetic radio-frequency coupling — no wires pierce the skin.
Electrical pulse generation
The internal receiver decodes the signal and generates precise electrical pulses for each electrode channel.
Direct neural stimulation
The electrode array, threaded into the cochlea, stimulates spiral ganglion cells at different positions corresponding to different frequency ranges (tonotopy).
Brain interpretation
Electrical impulses travel along the auditory nerve to the brainstem and auditory cortex, where they are interpreted as sound. Rehabilitation and brain plasticity refine this perception over weeks to months.
Scale & Outcomes
The cochlear implant is widely considered the most successful neuroprosthetic device in medical history. Children implanted before 18 months of age — during the critical period of auditory cortex development — often develop speech and language skills comparable to their hearing peers.
Who is a Candidate?
Candidacy is determined by a multidisciplinary team including an ENT specialist, audiologist, and speech-language therapist. Key criteria include:
Criteria in adults
- ● Bilateral severe-to-profound SNHL (≥70 dB HL)
- ● Limited benefit from optimised hearing aids
- ● Sentence recognition <50% in best aided condition
- ● No medical contraindications to surgery
- ● Realistic expectations and motivation for rehabilitation
Criteria in children
- ● Age ≥12 months (earlier in meningitis cases)
- ● Profound bilateral SNHL
- ● Insufficient progress with hearing aids over 3–6 months
- ● Supportive family environment for auditory-verbal therapy
- ● No cognitive barriers that would prevent auditory learning
Relative contraindications
- ● Cochlear ossification (post-meningitis)
- ● Auditory nerve aplasia / hypoplasia
- ● Active middle ear infection
Post-operative considerations
- ● MRI compatibility (device-dependent)
- ● Activation at 3–4 weeks post-surgery
- ● Ongoing audiological mapping & fitting
- ● Speech & auditory rehabilitation programme
The Operation
Cochlear implantation is performed under general anaesthesia and typically lasts 2–3 hours. The surgeon makes a post-auricular incision, performs a mastoidectomy and posterior tympanotomy to access the middle ear, then creates a cochleostomy or uses the round window membrane to insert the electrode array into the scala tympani.
The internal receiver is secured in a shallow well drilled into the squamous part of the temporal bone. Hospitalisation is typically 1–2 days. The device is activated ("switched on") 3–4 weeks later, allowing the surgical site to heal.
Bilateral implantation — either simultaneously or sequentially — is increasingly standard practice, particularly in children, as it improves spatial hearing, sound localisation, and performance in noise.