Eyes Ears Chapter Disorders Sensory Organs

Chapter 15
Disorders of the Eyes, Ears and Other
Sensory Organs

Sensory Receptors
 Classified into two major categories
 General senses
 Special senses
• Include the eye and ear
•Copyright © 2014, 2011, 2006 by Saunders, an imprint of Elsevier, Inc. •2

Sensory Receptors by Location
 Exteroceptors
 Located close to body surface (cutaneous
receptors)
• Examples—touch, pressure, temperature, pain
 Visceroreceptors
 Located internally around the viscera
 Proprioceptors
 Muscle sense

Sensory Receptors by Stimuli
 Mechanoreceptors
 Stimulated by mechanical force(s)
• Touch, pressure, equilibrium, hearing
 Chemoreceptors
 Change in chemical concentration
• Taste, smell
 Thermoreceptors
 Stimulated by change in the temperature
• Warm and cold receptors

Sensory Receptors by Stimuli
(Cont.)
 Photoreceptors
 Respond to light
• Rods and cones in the retina (eye)
 Nociceptors
 Respond to any tissue damage
• Results in pain
 Osmoreceptors
 Recognize changes in the osmolarity of body
fluids
• Concentrated in the hypothalamus

Eye
 Light rays enter the eye through the cornea.
 Pass through the lens to the receptor cells of
the retina
 Rods—black and white vision
 Cones—color vision
 Visual stimuli are conducted by the optic
nerve to the occipital lobe.
 Interpretation and processing
• Information sent to other appropriate areas of the brain

Protection for the Eye
 Protected by bony orbit of the skull
 Bony protection of the eye
 Eyelids and eyelashes
 Deflect foreign material from eyes
 Protect against excessive sunlight and drying
 Conjunctiva
 Mucous lining of eyelids
 Covers sclera
 Tears
 Produced by lacrimal glands
 Contain lysozyme—antibacterial enzyme

Muscles of the Eyeball
 Six extraocular skeletal muscles for
movement of the eyeball
 Muscles controlled by cranial nerves (CNs) III
and IV

Eyeball
 Outer layer
 Touch fibrous coat
 Posterior portion—sclera
• “White” of the eye
 Anterior portion—cornea
• Transparent portion
• Light rays pass and are refracted
• Does not contain blood vessels
• Nourished by fluids around it
• Oxygen diffusing from atmosphere

Eyeball (Cont.)
 Middle layer—uvea
 Choroid—dark vascular layer interior to the sclera
 Absorbs scattered light
 Located in anterior segment of the eyeball
• Choroid is specialized as ciliary body that controls shape
of the crystalline lens > accommodation for focus
• Iris is pigmented muscle of pupil
 Dilation occurs as a result of increased sympathetic
nervous system (SNS) activity.
 Constriction occurs as a result of increased
parasympathetic nervous system (PNS) activity.

Eyeball (Cont.)
 Inner layer—retina
 Contains no pain receptors
 Multilayered—in posterior two thirds of the eye
 Photoreceptor cells
• Rods—dim light, night vision (black and white)
• Cones—color vision
 Fovea centralis
• Cones for most acute vision

Structure of the Eye

Fluids in the Eye
 Posterior cavity
 Space between lens and retina
 Contains vitreous humor
• Formed during embryonic development
 Anterior cavity
 Between cornea and lens
 Divided into the anterior chamber and the
posterior chamber
 Filled with aqueous humor
• Amount formed should be equal to the amount
reabsorbed—maintenance of normal intraocular
pressure (IOP) below 24 mm Hg

Visual Pathway
 Light rays pass through cornea
 Refraction of rays
 Through aqueous humor and pupil
 To the retina (rods and cones)
 Nerve fibers form the optic nerve (CN II)
 Nerve fibers of the ganglion cells of the retina
 Optic chiasm
 Fibers cross
 Left occipital lobes receive images from right visual fields,
right occipital lobes from left visual fields
 Perception occurs in visual sensory and association
areas of the occipital lobes of the cortex.

Visual Pathway

Diagnostic Tests
 Snellen chart (or similar test)
 Measures visual acuity
 Visual field test
 Checks for central and peripheral vision
 Tonometry
 Assessment of IOP
 Ophthalmoscope
 Examines internal structures
 Gonioscopy—determines angle of anterior
chamber
 Muscle function and coordination tests

Structural Defects of the Eye
 Myopia
 Nearsightedness
 Image focused in front of the lens
 Hyperopia
 Farsightedness
 Eyeball is too short
 Image focused behind the retina
 Presbyopia
 Farsightedness associated with aging
 Loss of elasticity reduces accommodation

Refraction Defects in the Eye:
Myopia

Refraction Defects in the Eye:
Hyperopia

Structural Defects of the Eye
(Cont.)
 Astigmatism
 Irregular curvature in the cornea or lens
 Strabismus (squint or cross-eye)
 Results from deviation of one eye
 Double vision (diplopia)
 May be caused by weak or hypertonic muscle,
short muscle, neurological defect
 In children
• Must be treated immediately to prevent development of
amblyopia

Functional Changes
 Nystagmus
 Involuntary abnormal movement of one or both
eyes
 May result from neurological causes, inner ear or
cerebellar disturbances, drug toxicity
 Diplopia (double vision)
 May be caused by trauma to cranial nerves,
resulting in paralysis of extraocular muscles
 May occur in stroke
 Loss of depth perception occurs.

Infections and Trauma
 Stye
 Infection involving a hair follicle on the eyelid
 Usually caused by staphylococci
 Swollen, red mass forms on eyelid
 Purulent exudate
 Other infections
 Conjunctivitis, or “pink eye”
 Trachoma
 Keratitis

Conjunctivitis
 Superficial inflammation or infection
 Allergens, irritating chemicals, bacteria, viruses
 Redness, itching, excessive tearing
 Involves the conjunctiva lining of eyelids
 Pink eye—Staphylococcus aureus
 Frequently occurs in children
 Sclera and eyelid appear red; purulent discharge
 Spread by fingers or contaminated towels
• Occurs with contact lens use, contaminated makeup,
contaminated medication
 Antibiotic treatment to prevent damage to cornea

Conjunctivitis (Cont.)
 Other causes
 Chlamydia trachomatis and Neisseria
gonorrhoeae
 Both cause infections in the reproductive tract.
 May infect eyes of newborns
 May be transferred by self-inoculation

Gonococcal Conjunctivitis

Pseudomonas aeruginosa
Around Soft Contact Lens

Bacteria Recovered from Contact
Lenses and Solutions

Trachoma
 Caused by Chlamydia trachomatis
 Follicles develop on inner surface of eyelids
 Can occur in any age group
 “Scratchy” eye
 Antibiotic treatment
 Globally, most common cause of vision loss
where water is scarce and inadequate
hygiene occurs
 Scarring of lid leads to eyelashes abrading
cornea → loss of transparency

Keratitis
 Severe pain and photophobia
 Develops when cornea is infected or irritated
 Herpes simplex can be cause
• Transfer from herpes lesion around mouth
• Transfer by fingers, dental office, spray of contaminated
saliva
 Keratitis—increased risk of ulceration eroding the
cornea
 Scar tissue formation interferes with vision.
 Trauma to cornea
 Damage from chemicals, splashes, fumes

Glaucoma
 Result of increased IOP caused by excessive
accumulation of aqueous humor
 Most common and preventable loss of vision
in developed countries
 May be acute or chronic
 Signs and symptoms
 Halos around lights at night
 Loss of peripheral vision
 Pain may occur if IOP is greatly increased, as in
acute form

Acute Glaucoma
 Acute or narrow-angle glaucoma
 Angle between cornea and iris is decreased.
 May be caused by aging, developmental
abnormalities, or scar tissue from trauma or
infection
• Iris pushed forward and to side
• May block the outflow of aqueous humor
• Sudden marked increase in IOP
• May be triggered by pupil dilation
• Treatment may include surgery.

Glaucoma (Cont.)
 Chronic or open-angle glaucoma
 Higher incidence after age 50 years
 Thickening of trabecular network, which allows for
resorption of fluid
 Has insidious onset
 Pressure increases over time
• May cause ischemia and damage to the retinal cells
• If pressure continues to increase, damage to the optic
nerve occurs.
• Irreversible, eventually can cause blindness
 Treated by regular administration of eye drops

Glaucoma: Normal Flow of
Aqueous Humor

Chronic Glaucoma

Acute Glaucoma

Cataracts
 Progressive opacity or clouding of the lens
 Interferes with light transmission
 Size, site, and density of clouding vary
among individuals.
 May be different in individual’s two eyes
 Changes may be
 Age-related or caused by metabolic abnormalities
 Excessive exposure to sunlight
 Congenital
 Traumatic

Appearance of Eye with Cataract

Cataracts (Cont.)
 Blurred vision over visual field
 Becomes darker with time
 Night driving especially difficult
 Rate of impairment varies.
 One eye’s cataract may develop faster than
the other’s.
 May interfere with person’s ability to function
or work
 Outpatient surgery involves lens replacement.

Detached Retina
 Acute emergency
 Retina tears away from underlying choroid
 Retinal ischemia can lead to irreversible loss of
receptors.
 No pain or discomfort
 Visual field contains areas of blackness
(scotomas), as if a curtain has fallen over the eye.

Detached Retina (Cont.)
 Tear allows vitreous humor to flow behind
retina
 Increasing portion of the retina is lifted away form
the choroid
 Retinal cells cease to function.
 Surgical intervention
 Reattachment of retina

Macular Degeneration
 Age-related macular degeneration (AMD)
 Common cause of visual loss in older adults
 Combination of genetic factors and
environmental exposure
 Dry or atrophic
 More common—deposits form in retinal cells
 Wet or exudative
 Neovascularization
 Central vision becomes blurred, then lost
 New therapies being investigated

Normal Vision

Age-Related Macular
Degeneration

Parts of the Ear
 External ear
 Pinna and external auditory meatus (canal)
 Middle ear
 Tympanic membrane
 Bony ossicles
 Auditory tube connects to upper respiratory tract
 Inner ear
 Cochlea
• Organ of Corti—hearing
 Semicircular canals
• Balance and equilibrium

Structure of the Ear

Pathway of Sound
 Sound waves enter the external ear canals.
 Vibration of the tympanic membrane causes
the ossicles to vibrate.
 Motion of stapes against oval window initiates
movement of the fluid in the cochlea.
 Stimulation of hair cells in organ of Corti
 Initiation of nerve impulses
 Impulses conducted to the auditory area in
the temporal lobe of cerebral cortex for
interpretation of sound

Semicircular Canals
 Three structures at right angles with each
other
 Crista ampullaris of each semicircular canal
 Contain receptor hair cells
 Stimulated by motion of the endolymph in
response to head movements
 Stimuli conducted by vestibular branch of the
auditory nerve to cerebellum and medulla

Hearing Loss
 Two types
 Conduction deafness
• Sound is blocked in the external ear or middle ear.
• Accumulation of wax, foreign object, scar tissue
• Otosclerosis of the ossicles
 Sensorineural impairment
• Damage to the organ of Corti or auditory nerve
• Infection
• Head trauma
• Neurological disorders
• Ototoxic drugs
• Sudden very loud sounds or prolonged exposure to loud
noise
• Presbycusis and congenital defects

Hearing Loss (Cont.)
 Newborns are screened for hearing deficits.
 Audiologists and otolaryngologists for consultation
 Hearing aids
 Used if appropriate for individual hearing deficit
 Cochlear implants
 Used successfully in some cases of sensorineural
loss

Cochlear Implant

Terminology
 Deaf—congenital
hearing deficit
 Deafened—
acquired hearing
deficit
 “Hearing-impaired”
preferred to “hard
of hearing”

Ear Infections: Otitis Media
 Inflammation or infection of the middle ear
 Exudate builds up in cavity
 Causes pressure on tympanic membrane
 Auditory tube may be obstructed by inflammation
• May cause rupture of the tympanic membrane
 Prolonged infection is likely to produce scar tissue
and adhesion.
• Can lead to permanent conductive hearing loss or
speech problems
 Chronic infection may lead to mastoiditis.
• Infection involving mastoid cells of temporal bone

Ear Infections
 May spread along nasopharynx and
respiratory structures
 Caused by Haemophilus influenzae
 Particularly in young children
 Pneumococci, b-hemolytic streptococci,
staphylococci
 Viral infections also common
 Frequently complicated by secondary bacterial
infections

Ear Infections (Cont.)
 May be asymptomatic in chronic stages
 Most often—severe pain or earache
 Tympanic membrane red and bulging
 Mild hearing loss or feeling of fullness
 Fever, nausea might be present

Ear Infections (Cont.)
 Treatment
 Ibuprofen or acetaminophen to reduce discomfort
in first 48 hours
 Use of antibacterials if bacterial infection
 Decongestant may be useful in draining auditory
tube
 Surgery may be done to Insert temporary
tubes in tympanic membrane to allow for
drainage.

Normal Tympanic Membrane

Tympanic Membrane
with Otitis Media

Otitis Externa
 Also called swimmer’s ear
 Usually bacterial (Pseudomonas aeruginosa)
 May be fungal
 Infection of the external auditory canal and
pinna
 Often associated with swimming
 Irritation when cleaning ear
 Frequent use of earphones or earplugs
 Pain usually increased with movement of
pinna
 Purulent discharge and hearing deficit

Chronic Disorders of the Ear
 Otosclerosis
 Imbalance in bone formation and resorption
 Development of excess bone in middle ear cavity
• Stapes becomes fixed to oval window.
 Blockage of conduction sounds to cochlea
 May be caused by genetic or environmental
factors
 Treatment—surgical removal of stapes and
replacement prosthesis to restore hearing

Chronic Disorders of the Ear
(Cont.)
 Meniere’s syndrome
 Inner ear labyrinth disorder causing severe vertigo
and nausea
 Intermittent, with remissions and exacerbations
 Excessive endolymph produced
 Attack may last minutes or hours
• Change in barometric pressure may precipitate an attack
 Balance test, electronystagmography,
electrocochleography, MRI
 Treatment with drugs
• Home exercise programs to reduce sensitivity to motion

Eyes Ears Chapter Disorders Sensory Organs

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