Lesson 12F: Dysfunction of Special Senses
Dysfunction of Special Senses
The ideas below are not module objectives, but rather these are concepts that you will hopefully feel more comfortable with after reviewing this lesson:
Explain examples of dysfunction associated with the five special senses.
Special Sense Dysfunction
Because smell and taste are closely linked, we'll talk about their abnormalities together. Anosmia is loss of olfaction (smell), and ageusia is loss of gustation (taste). They are often misdiagnosed as each other because olfaction and gustation work together to allow one to experience flavor. Both anosmia and ageusia can be due to hormonal imbalances, medication, trauma of oral or nasal cavities, injury to nerves/brain regions associated with sensation among other causes. Anosmia and ageusia both impact enjoyment/ disgust of food since most of our sensation of 'taste' is from olfaction not gustation; if one's olfaction is lost, food may not have the same flavor.
Vertigo is abnormal sense of equilibrium that occurs when one perceives movement that isn't happening. Vertigo can be linked to abnormalities in inner ear regions (especially semicircular canals), middle ear pressure, multiple sclerosis, medications, head trauma, alcohol/drugs or trauma to brain regions associated with equilibrium. Vertigo leads to dizziness, nausea, issues with locomotion among other problems. Even though it's 'just' a motion problem, it can be debilitating because dizziness worsens with movement, so one may be confined to stillness.
Tinnitus is abnormal perception of sound or 'ringing in ears'; it is sometimes called 'phantom noise' because it involves sensation of sound that isn't there. Tinnitus can be due to aging, meningitis, medications, otitis, cardiovascular issues, head trauma, to name a few causes. Like vertigo, it's not 'just' sensation issue and can be debilitating because ringing impacts sleep. Tinnitus and vertigo may present together since ear is used to pick up both special senses.
Otitis is inflammation of the ear, and can be subdivided based on region with infection: otitis externa affects outer ear (pinna, auditory canal, tympanic membrane), otitis media affects middle ear (cavity and structures between tympanic membrane and membranous windows, auditory tube), and otitis interna affects inner ear (sensory organs). Different forms of otitis can be caused by congenital issues, infection, medication, stress and/or external factors (water or pressure). If left untreated, it may lead to pain/hearing loss. Otitis may be associated with vertigo since ear is used for both special senses.
Pathological nystagmus is involuntary and/or unstimulated tracking eye movements caused by inappropriate equilibrium information. It can be associated with vertigo because the rapid vision change that occurs with nystagmus influences equilibrium. Pathological nystagmus may be due to genetic mutation of inner ear structures, ear/brain damage during surgery, alcohol/drugs, head trauma among other causes. Note that physiological nystagmus, tracking eye movement following spinning activity, is normal and occurs because crista ampullaris of semicircular canals continue to be stimulated even though one stops moving. Don't be concerned if your kid briefly experiences nystagmus after twirling around - it's simply due to endolymph inertia!
The eye can experience a number of dysfunctions; we'll cover two disorders/diseases (cataract that impacts the anterior region and glaucoma that affects the posterior region) and some common optical abnormalities (astigmatism, hyperopia, myopia and presbyopia).
Cataract is clouding of lens leading to blurry/loss of vision. Aging, diabetes, smoking, and UV light are all risk factors. A cataract can be treated with visual aids or by surgically removing lens. Glaucoma is death of retinal neurons. It occurs when a sinus canal that usually recycles aqueous humor is blocked. This increases pressure in anterior pole leading to high intraocular pressure in posterior pole, which presses on retina killing neurons required to sense changes in light. Treatment is available to slow or lessen pressure, but once vision is lost, it's irreversible.
Optical abnormalities include four common visual defects of image formation, in which light rays do not converge at the appropriate focal point on the retina. First, astigmatism is caused by a misshapen cornea. The eye is unable to focus light rays that enter the eye on different planes. Astigmatism can be corrected with cylindrical lenses that bend light more in one direction than the other. A second common defect is hyperopia , which is caused by an eyeball that is too short. This visual defect causes difficulty in viewing objects that are close by and is referred to as 'farsightedness'. Hyperopia can be corrected with convex lenses that bend light rays inward (i.e. convergence) before they enter the eye. Third, myopia is caused by an eyeball that is too long. This visual defect causes difficulty in viewing objects that are far away and is referred to as 'nearsightedness'. Myopia can be corrected with concave lenses that bend light rays outward (i.e. divergence) before they enter the eye. Lastly, presbyopia is caused by a decreased flexibility of the lens that happens as we age. This visual defect causes reduced ability to accommodate for near vision, such as reading. Presbyopia can be corrected with bifocal lenses or reading glasses.
