Investigate technologies that are used to assist with the effects of a disorder, including hearing loss, vision loss and loss of kidney function, and explain how a named disorder is assisted by the use of named technologies

What this dot point is asking

NESA wants you to describe disorders affecting sense organs, explain the technologies used to assist with them, and link each technology to the specific biological problem it addresses. Hearing and vision are the main syllabus examples.

The answer

Hearing: how the ear works

Sound waves enter the outer ear and vibrate the tympanic membrane. The three ossicles (malleus, incus, stapes) in the middle ear amplify the vibration and transmit it to the oval window of the cochlea. In the cochlea, fluid waves bend the stereocilia of hair cells along the basilar membrane. Hair cells convert mechanical movement into electrical signals carried by the auditory nerve to the brain. The basilar membrane is tonotopic: the base responds to high frequencies, the apex to low frequencies.

Types of hearing loss

Conductive hearing loss

Sound transmission through the outer or middle ear is blocked or reduced. Causes include ear wax, otitis media, perforated eardrum and otosclerosis (ossicle stiffening).

Sensorineural hearing loss

Damage to the cochlear hair cells or auditory nerve. Causes include age-related (presbycusis), noise exposure, ototoxic drugs (some antibiotics, cisplatin) and genetic conditions.

Mixed hearing loss

Both conductive and sensorineural components.

Technologies for hearing loss

Hearing aids

Amplify incoming sound. Modern digital hearing aids have a microphone, amplifier, speaker (receiver) and battery, with software that selectively amplifies speech frequencies and suppresses background noise. Effective for mild to moderate hearing loss where hair cells still function.

Bone-anchored hearing aids (BAHA)

Used for conductive loss when the outer or middle ear is non-functional. A titanium implant in the skull conducts sound vibrations through bone directly to the cochlea, bypassing the middle ear.

Cochlear implants

Used for severe to profound sensorineural hearing loss. Components and mechanism are described in the past-question answer above. The implant bypasses damaged hair cells by directly stimulating the auditory nerve through an electrode array in the cochlea.

Middle ear implants

A small actuator attached to the ossicles vibrates them mechanically. Used when conventional hearing aids cause feedback or skin reactions.

Vision: how the eye works

Light enters through the cornea (the main refracting surface, providing about two-thirds of the eye's focusing power). The pupil controls light intake; the iris adjusts pupil diameter. The lens fine-tunes focus through accommodation (changing shape via the ciliary muscle). The retina contains photoreceptors (rods for low light, cones for colour). Photoreceptor signals travel via the optic nerve to the visual cortex.

For a sharp image, light must converge precisely on the retina. The total refractive power of the eye must match the eye's axial length.

Types of vision disorder

Myopia (short-sightedness)

Eye too long or cornea too curved; light focuses in front of the retina. Distant objects are blurred. Highly prevalent and rising; almost half of young adults globally.

Hyperopia (long-sightedness)

Eye too short or cornea too flat; light would focus behind the retina. Near objects are blurred.

Astigmatism

Irregular cornea curvature; light focuses at multiple points. Causes blurred or distorted vision.

Presbyopia

Age-related stiffening of the lens; the eye loses the ability to accommodate for near vision. Begins around age 40 to 45.

Cataract

Opacification of the natural lens, scattering light. Common with age, also caused by diabetes, UV exposure and steroid use.

Macular degeneration

Degeneration of central retinal photoreceptors. Leading cause of blindness in older Australians.

Technologies for vision disorders

Corrective lenses (spectacles)

External refracting lenses placed in front of the eye. A concave (negative power) lens diverges light to correct myopia; a convex (positive power) lens converges light to correct hyperopia; a cylindrical lens corrects astigmatism; multifocal or progressive lenses correct presbyopia.

Contact lenses

Sit on the tear film of the cornea, providing similar refractive correction in a smaller form factor. Soft (hydrogel) or rigid gas-permeable.

Laser refractive surgery (LASIK, PRK)

Reshapes the cornea by ablating tissue with an excimer laser. Permanently changes the cornea's refractive power. Suitable for stable mild to moderate myopia, hyperopia and astigmatism.

Intraocular lenses (IOLs)

Surgically implanted artificial lenses. Most commonly used in cataract surgery: the opaque natural lens is removed by phacoemulsification (ultrasound emulsification and aspiration) and a folded acrylic IOL is inserted through a 2 to 3 mm incision into the lens capsule. Power is calculated using corneal curvature and axial length measurements to correct any pre-existing refractive error simultaneously. Multifocal IOLs can also correct presbyopia.

Retinal implants

Experimental electronic arrays (Argus II, Australian Bionic Eye) implanted on or under the retina that convert images from an external camera into electrical stimulation of surviving retinal cells. Currently used for end-stage retinitis pigmentosa.

Examples in context

Example 1. Cochlear implant developed at the University of Melbourne. Professor Graeme Clark developed the multi-channel cochlear implant at the University of Melbourne in 1978, with the first commercial implant fitted in 1985. The device bypasses damaged hair cells in the cochlea entirely. An external microphone captures sound, a speech processor converts it into digital pulses, and a transmitter sends radio signals through the skin to an implanted receiver. The receiver delivers electrical pulses to 22 electrodes threaded through the cochlea, stimulating different regions of the auditory nerve according to sound frequency (tonotopic mapping). The Royal Institute for Deaf and Blind Children in NSW now refers around 80 children per year for cochlear implants under Medicare. Patients implanted before age 12 months develop near-normal spoken language.

Example 2. Haemodialysis at Westmead Hospital for end-stage renal disease. A patient with end-stage kidney disease attends Westmead Hospital three times a week for haemodialysis, each session lasting 4 to 5 hours. Blood is pumped from a surgically created arteriovenous fistula in the forearm, through a dialyser containing semipermeable hollow fibres surrounded by dialysis fluid. Urea, creatinine and excess potassium diffuse out of the blood down their concentration gradients, while bicarbonate and required electrolytes diffuse in. The dialysis machine functions as an external kidney, removing wastes and excess water that the failed organ cannot. Without dialysis or transplantation, end-stage renal failure is fatal within weeks. Around 14 000 Australians currently rely on regular dialysis according to ANZDATA registry data.

Try this

Q1. Distinguish between a hearing aid and a cochlear implant in terms of mechanism and the type of hearing loss each is suited for. [3 marks]

• Cue. Hearing aid amplifies sound for mild-to-moderate sensorineural or conductive loss with surviving hair cells. Cochlear implant electrically stimulates auditory nerve directly, for severe-to-profound sensorineural loss.

Q2. A patient with myopia has a near point of 8 cm and a far point of 30 cm. Calculate the power of corrective lens required to allow them to see clearly at infinity. [3 marks]

• Cue. Lens power P = 1/f, where f = -0.30 m (concave lens to diverge light); P = -3.33 dioptres.

Q3. Evaluate the use of dialysis for end-stage renal disease. (a) Describe how haemodialysis replaces kidney function. (b) Identify two limitations compared with kidney transplantation. (c) Justify whether dialysis or transplantation should be the first-line treatment for an otherwise healthy 40-year-old patient. [2+2+3 marks]

• Cue. (a) Semipermeable membrane allows diffusion of wastes from blood to dialysate. (b) Time burden (12-15 hours/week), no biological homeostatic regulation, infection risk. (c) Transplantation gives better survival and quality of life, but donor supply limits access.