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.

Worked example

A 70-year-old man notices progressive blurring of vision and difficulty driving at night. Examination shows bilateral cataracts. He also has untreated long-sightedness.

Diagnosis. Bilateral cataract on background hyperopia.

Treatment. Phacoemulsification cataract surgery in each eye, with implantation of an IOL whose dioptric power corrects both the lens removal and the underlying hyperopia. After surgery he no longer needs distance glasses, although he may need reading glasses unless a multifocal IOL is used.

Mechanism. The IOL replaces the lost refractive power of the removed lens and adjusts for the patient's pre-existing refractive error, restoring a sharp image on the retina.

Common traps

Confusing hearing aids and cochlear implants. Hearing aids amplify sound; cochlear implants bypass damaged hair cells. They are not interchangeable.

Saying lenses "magnify" the image. Corrective lenses refract light to bring the focal point onto the retina; they do not magnify (except specialised low-vision aids).

Forgetting the cornea provides most refractive power. The cornea provides roughly two-thirds; the lens provides the remainder and fine-tunes through accommodation.

Treating presbyopia as a refractive error like myopia. Presbyopia is age-related loss of accommodation, not a refractive error of the eye's resting state.

In one sentence

Hearing loss is assisted by hearing aids (amplification for mild loss) and cochlear implants (direct auditory nerve stimulation bypassing damaged hair cells), while vision disorders are corrected by spectacles, contact lenses, laser refractive surgery and intraocular lenses, each restoring the correct convergence of light onto the retina or the conversion of vibration into neural signal.