Chapter 015

Chapter 15

Disorders of the Eyes, Ears and Other Sensory Organs

•2•Copyright © 2014, 2011, 2006 by Saunders, an imprint of Elsevier, Inc.

Sensory Receptors

Classified into two major categories General senses Special senses

• Include the eye and ear


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, -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

Health & Medicine

Chapter 015