Hearing Loss, Cochlear Implants and Bone Conduction

Q1.1 — Trend (2 marks)

As age at implantation increases, mean SLQ at age 5 decreases [1 mark]. Children implanted before 12 months scored 98, compared with 82 (12–24 months), 68 (24–36 months) and 51 (>36 months) — a drop of approximately 47 points across the full range [1 mark — reference to at least two data values].

Q1.2 — Critical period explanation (3 marks)

The auditory cortex undergoes experience-dependent synaptic development during a critical period from birth to approximately 3–3.5 years [1]. During this period, auditory input from a cochlear implant drives the formation and strengthening of synaptic connections in the auditory cortex and language areas (Broca’s and Wernicke’s areas) [1]. Without auditory input, cortical reorganisation occurs: other sensory modalities begin using auditory cortex, and the cortical infrastructure for language processing is progressively reduced. Earlier implantation provides auditory input during this critical window, supporting language cortex development and producing higher SLQ scores [1].

Q1.3 — Biological reason for incomplete match with hearing peers (2 marks)

Any one of the following for 2 marks: (1) A cochlear implant provides only 12–22 frequency channels compared with ~3,500 hair cell positions in the normal cochlea — the coarser spectral representation reduces the richness and accuracy of the auditory signal reaching the brain [1], which limits language and perception outcomes even with early implantation [1]. (2) Even at <12 months, there is a brief period of auditory deprivation before implantation during which some cortical reorganisation begins. (3) Electrode insertion may not stimulate all frequency regions optimally. Accept any biologically valid reason with mechanism.

Q1.4 — Evaluate the proposal (1 mark)

The proposal should be rejected [1]. Figure 1.1 shows a substantial drop in SLQ between the <12 month group (SLQ = 98) and the 12–24 month group (SLQ = 82). Delaying routine implantation from under 12 months to 18 months would shift children into the second group, reducing their expected language outcome by approximately 16 SLQ points — a clinically significant difference with lifelong consequences for language and education.

Q2 — Cause-and-effect chain

Effect 1: The sound wave is converted to an electrical signal by the microphone.

Effect 2: Coded electrical signals representing different frequency bands are produced and sent to the transmitter coil.

Effect 3: The internal receiver-stimulator receives the signals without wires penetrating the skin.

Effect 4: Precisely timed electrical pulses are delivered to the electrode array in the cochlea.

Effect 5: Auditory nerve fibres at that cochlear position are directly depolarised — action potentials are generated and travel along the auditory nerve to the brainstem and auditory cortex.

Overall outcome: …perceive sounds as electrical signals interpreted by the brain, bypassing non-functional hair cells, so they can access speech and environmental sounds despite profound sensorineural hearing loss. The perceived sound differs from natural hearing (fewer channels, different quality) but enables communication.

Q3A — Alinta (3 marks)

Technology: BAHA (bone-anchored hearing aid). Alinta has conductive hearing loss [1]: the ossicle chain is disrupted (stapes fused) but her cochleae and auditory nerves are fully functional [1]. BAHA transmits vibrations through the skull via osseointegrated titanium to bypass the immobile ossicular chain and deliver vibration directly to her functional cochleae [1]. Hearing aids cannot compensate because the ossicles cannot transmit the amplified sound; a cochlear implant is not indicated because the cochleae are functional and intact.

Q3B — Ben (3 marks)

Technology: bilateral cochlear implants. Ben has profound bilateral sensorineural hearing loss from CMV-destroyed cochlear hair cells [1]. Hearing aids are confirmed ineffective (failed trial) because amplification requires residual hair cells to transduce sound — which Ben lacks [1]. His auditory nerves are intact, fulfilling the essential eligibility criterion for cochlear implantation. The electrode arrays will thread into each cochlea and directly stimulate the auditory nerve fibres, bypassing the non-functional hair cells. At age 4 he is within the critical period but benefit would have been maximised with earlier implantation [1].

Q3C — Carlos (3 marks)

Technology: digital hearing aid with frequency-specific (high-frequency) amplification. Carlos has moderate-to-severe sensorineural hearing loss (presbycusis) but with significant residual hair cell function across all frequencies [1]. Because residual hair cells are present, amplifying the signal — particularly at 2+ kHz where his loss is greatest — allows remaining functional hair cells to respond to the louder signal [1]. Cochlear implant is not indicated: it is reserved for severe-to-profound loss with inadequate hearing aid benefit, and the irreversible surgery would destroy his residual hair cells. BAHA is inappropriate for sensorineural loss [1].

Q4 — Comparison table (sample answers)