Sensory System

 

Pathophysiologic manifestations
	Vision
	Hearing
	Taste and smell
Disorders
	Cataract
	Glaucoma
	Macular degeneration
	Meniere's disease
	Otosclerosis

 

T hrough the sensory system, a person receives stimuli that facilitate interaction with the surrounding world. Afferent pathways connect specialized sensory receptors in the eyes, ears, nose, and mouth to the brain ― the final station for continual processing of sensory stimuli. Alterations in sensory function may lead to dysfunctions of sight and hearing, as well as smell, taste, balance, and coordination.

PATHOPHYSIOLOGIC MANIFESTATIONS

Alterations can occur in all the senses.

Vision

Disorders of vision include alterations in ocular movement, visual acuity, accommodation, refraction, and color vision.

Ocular movement

The eyes constantly move to keep objects being viewed on the fovea, which is a small area of the retina that contains only cones and is responsible for the best peripheral visual acuity. The six extraocular muscles that move each eye are innervated by the oculomotor (III), trochlear (IV), and abducens (VI) cranial nerves. (See Extraocular control of eye movement .) Alterations in ocular movement include strabismus, diplopia, and nystagmus.

Strabismus. Strabismus occurs when one eye deviates from its normal position due to the absence of normal, parallel, or coordinated movement. The eyes may have an uncoordinated appearance, and the person may experience diplopia.

In children, types of strabismus are:

• concomitant, in which the degree of deviation doesn't vary with the direction of gaze
• nonconcomitant, in which the degree of deviation varies with the direction of gaze
• congenital (present at birth or during the first 6 months)
• acquired (present during the first 2? years)
• latent (phoria; apparent only when the child is tired or sick)
• constant.

Tropias are categorized into four types: esotropia (inward deviation), exotropia (outward deviation), hypertropia (upward deviation), and hypotropia (downward deviation).

Strabismic amblyopia is characterized by the loss of central vision in one eye; it typically results in esotropia (due to fixation in the dominant eye and suppression of images in the deviating eye). Strabismic amblyopia may result from hyperopia (farsightedness) or anisometropia (unequal refractive power).

Esotropia may result from muscle imbalance and may be congenital or acquired. In accommodative esotropia, the child's attempt to compensate for the farsightedness affects the convergent reflex, and the eyes cross.

Strabismus is frequently inherited, but its cause is unknown. In adults, strabismus may result from trauma. The incidence of strabismus is higher in patients with central nervous system disorders, such as cerebral palsy, mental retardation, and Down syndrome.

EXTRAOCULAR CONTROL OF EYE MOVEMENT The six muscles that control the movement of each eye are innervated by three cranial nerves. <center> <a name="ch0015extraocularcontrolofeyemovement"></a> <a name="ch0015extraocularcontrolofeyemovement"></a> </center> MUSCLE DIRECTION OF MOVEMENT INNERVATION   Superior oblique muscle moves the eye down and in Trochlear (cranial nerve [CN] IV) Superior rectus muscle moves the eye up and out Oculomotor (CN III) Medial rectus muscle moves the eye inward Oculomotor (CN III) Lateral rectus muscle moves the eye outward Abducens (CN VI) Inferior rectus muscle moves the eye downward and outward Oculomotor (CN III) Inferior oblique muscle (not shown) moves the eye up and in Oculomotor (CN III)

Muscle imbalances may be corrected by glasses, patching, or surgery, depending on the cause. However, residual defects in vision and extraocular muscle alignment may persist even after treatment.

 

AGE ALERT In the absence of early intervention, children with strabismus may develop amblyopia as a result of cerebral suppression of visual stimuli. Deviation of an eye is the second most common symptom in a child with retinoblastoma. Therefore, acquired strabismus should always be checked.

Diplopia. Diplopia, or double vision, results when the extraocular muscles fail to work together and images fall on noncorresponding parts of the retinas. Diplopia usually begins intermittently or affects near or far vision exclusively. It can be classified as monocular (persisting when one eye is covered) or, more commonly, binocular (clearing when one eye is covered). Monocular diplopia may result from an early cataract, retinal edema or scarring, iridolysis (surgical lysis of adhesions of the iris), subluxated lens (partial dislocation of the lens of the eye), poorly fitting contact lens, or uncorrected refractive error. Binocular diplopia may result from ocular deviation or displacement, extraocular muscle palsies, or psychoneurosis, or after retinal surgery. Other causes of binocular diplopia include infection, neoplastic disease, metabolic disorders, degenerative disease, inflammatory disorders, and vascular disease.

Nystagmus. Nystagmus refers to involuntary oscillations or alternating movements of one or both eyes. These oscillations are usually rhythmic and may be horizontal, vertical, or rotary. They may be transient or sustained and may occur spontaneously or on deviation or fixation. Nystagmus may be classified as pendular or jerk. Nystagmoid movements usually have a fast and a slow component. The direction of the nystagmus is given by the fast component. (See Classifying nystagmus .)

Nystagmus is a supranuclear ocular palsy resulting from pathology in the visual perceptual area, vestibular system, or cerebellum. Causes of nystagmus include brain stem or cerebellar lesions, labyrinthine disease, stroke, encephalitis, Meniere's disease, multiple sclerosis, and alcohol and drug toxicity, including barbiturate, phenytoin (Dilantin), or carbamazepine (Tegretol) toxicity.

 

AGE ALERT In children, pendular nystagmus may be idiopathic or may result from early impairment of vision associated with such disorders as optic atrophy, albinism, congenital cataracts, and severe astigmatism.

Visual acuity

Visual acuity refers to the ability to see clearly. A lack of visual acuity is often associated with refractive errors. In nearsightedness, or myopia, the eye focuses the visual image in front of the retina, causing objects in close view to be seen clearly and those at a distance to be blurry. In farsightedness, or hyperopia, the eye focuses the visual image behind the retina, causing objects in close view to be blurry and those at a distance to be clear. Both these problems are caused by an alteration in the shape of the eyeball. Other causes of reduced visual acuity include aging, amblyopia, cataracts, glaucoma, papilledema, dark adaptation, and scotoma (an area of diminished visual acuity surrounded by an area of normal vision within the visual field).

Amblyopia is severely decreased visual acuity or virtual blindness in a structurally intact eye. It may be caused by toxins (including alcohol and tobacco) or may accompany such systemic diseases as diabetes mellitus or renal failure.

 

AGE ALERT With age, the pupil becomes smaller, which decreases the amount of light that reaches the retina. Older adults need about three times as much light as a younger person to see objects clearly.

Accommodation

Accommodation occurs as the thickness of the lens of the eye changes to maintain visual acuity. For near vision, the ciliary body contracts and relaxes the zonules, the lens becomes spherical, the pupil constricts, and the eyes converge. For far vision, the ciliary body relaxes, the zonules tighten, the lens becomes flatter, the eyes straighten, and the pupils dilate. The oculomotor nerve and coordinated brain stem pathways account for accommodation. Alterations in accommodation may be caused by pressure, inflammation, the aging process, or disorders affecting the oculomotor nerve. The result of impaired accommodation may be diplopia, blurred vision, or headache.

CLASSIFYING NYSTAGMUS Nystagmus is classified as pendular or jerk. Each type has further classifications. PENDULAR NYSTAGMUS   Oscillating: slow, steady oscillations of equal velocity around a center point; caused by congenital loss of visual acuity or multiple sclerosis.   Vertical or seesaw: rapid, seesaw movement in which one eye appears to rise while the other appears to fall; suggests an optic chiasm lesion. JERK NYSTAGMUS   Convergence-retraction: irregular jerking of the eyes back into the orbit during upward gaze; can reflect midbrain tegmental (roof of the midbrain) damage.   Downbeat: irregular downward jerking of the eyes during downward gaze; can signal lower medullary damage.   Vestibular: horizontal or rotary movements of the eyes; suggests vestibular disease or cochlear dysfunction.

Alterations in refraction

Refraction is the process of bending light rays so that they fall on the retina. As rays of light reach the surface of the cornea from all directions, the cornea directs them toward the lens. The lens further bends the light and directs the light rays to one spot on the retina. The greater the refractive power, the more the light rays are bent. Emmetropia is the condition in which light rays fall exactly on the retina. Alterations in refraction occur when light is not properly focused. Causes include abnormalities in curvature of the cornea, focusing of the lens, and eye length. Results include myopia, hyperopia, and astigmatism. (See Refractive errors .)

Myopia. Myopia, or nearsightedness, occurs when light rays are focused in front of the retina. Near objects can be seen clearly, and distant objects appear blurry. This condition may occur if the eye is too long or the refractive power of the cornea or lens is too great. Myopia may also occur if hyperglycemia in uncontrolled diabetes causes lens swelling. A concave lens that bends light rays outward is used to correct myopia.

Hyperopia. Hyperopia, or farsightedness, occurs when light rays are focused behind the retina. Distant objects appear clear, and nearby objects are blurred. This condition occurs when the eye is too short or the refractive power of the cornea or lens is too low, and may be corrected with a convex lens that bends light rays inward.

 

AGE ALERT Presbyopia is a form of hyperopia that begins in middle age as the lens becomes firm and loses its elasticity. As a result, the refractive power of the lens is reduced, the eye loses its ability to accommodate, and near objects appear blurred. This condition is treated with a convex lens that bends light rays in different directions so they focus in a single point.

Astigmatism. Astigmatism occurs when unequal curvature of the cornea or eyeball causes light rays to focus on different points on the retina, resulting in distorted images. Astigmatism may occur in conjunction with other refractive disorders.

Color vision

The cones of the retina are responsible for color vision. Each cone contains one of three different visual pigments (red, green, or blue) that absorb light waves of different wavelengths.

Color blindness is inherited on the X chromosome and therefore usually affects males. Acquired color blindness may also be due to diabetes, bilateral strokes affecting the ventral portion of the occipital lobe, or disease of the macula or optic nerve.

 

AGE ALERT Older adults often experience impaired color vision, especially in the blue and green ranges, because cones in the retina deteriorate. Yellowing of the aging lens also impairs color vision.

Hearing

Sound waves normally enter the external auditory canal, then travel to the tympanic membrane in the middle ear, causing it to vibrate. This vibration causes the malleus to move, setting in motion the incus and, in turn, the stapes. The malleus, incus, and stapes are collectively referred to as the ossicles. The stapes presses on the oval window of the inner ear, setting in motion the fluid of the cochlea and stimulating hair cells. The hair cells carry impulses through the cochlear division of the auditory cranial nerve (VIII) to the brain. This type of sound transmission to the inner ear, called air conduction, is normally better than transmission through bone (bone conduction).

Alterations in hearing are classified as conductive or sensorineural. Mixed hearing loss combines aspects of conductive and sensorineural hearing loss.

Conductive hearing loss

Conductive hearing loss results from disorders of the external and middle ear that block sound transmission. Causes include obstruction of the external auditory canal, tumors or fluid in the middle ear, perforation of the tympanic membrane, trauma or infection that affects the ossicular chain, and fixation of the ossicles. Treatment includes hearing aids, tympanoplasty for chronic otitis media and trauma, and stapedectomy for otosclerosis.

REFRACTIVE ERRORS <center> <a name="ch0015refractiveerrors"></a> <a name="ch0015refractiveerrors"> <br /> <br /> <br /> <br /> <br /> <br /> </a> </center>

TYPES OF PRESBYCUSIS Presbycusis, sensorineural hearing loss affecting cochlear hair cells and nerve fibers, occurs in four known types: sensory, neural, metabolic, and cochlear conduction. Sensory presbycusis begins in middle age and progresses slowly; loss of cochlear neurons parallels loss in the organ of Corti. Neural presbycusis develops later in life; loss of cochlear neurons occurs without loss in the organ of Corti. It progresses rapidly and may be accompanied by other signs of central nervous system decline, including intellectual deterioration, memory loss, and loss of motor coordination. Metabolic presbycusis tends to run in families, usually begins in middle age, and progresses slowly. Cochlear-conduction presbycusis also starts in middle age and reflects changes in the motion mechanism of the cochlear duct.

Sensorineural hearing loss

Sensorineural hearing loss results from damage to the hair cells of the organ of Corti or cranial nerve VIII by very loud noises; infection; such ototoxic drugs as aminoglycoside antibiotics, aspirin, diuretics including ethacrynic acid (Edecrin) and furosemide (Lasix); tobacco and alcohol; meningitis; cochlear otosclerosis; Meniere's disease; or aging. Sensorineural hearing loss may be helped by hearing aids and cochlear implantation.

 

AGE ALERT The most common sensorineural hearing loss in older adults, presbycusis affects the cochlear hair cells and nerve fibers. It begins with the loss of high-frequency sounds and may progress to middle and low frequencies. (See Types of presbycusis .)

AGE ALERT Peripheral or central hearing disorders in children may lead to speech, language, and learning problems. Early identification and treatment of hearing loss is crucial so that early intervention may be started. (See Congenital hearing loss .)

Taste and smell

The senses of taste and smell are also subject to alterations.

Taste

The sensory receptors for taste are the taste buds, concentrated over the surface of the tongue and scattered over the palate, pharynx, and larynx. These buds can differentiate among sweet, salty, sour, and bitter stimuli. Taste and olfactory receptors together perceive more complex flavors. Much of what is considered taste is actually smell; food odors typically stimulate the olfactory system more strongly than related food tastes stimulate the taste buds.

A factor interrupting the transmission of taste stimuli to the brain may cause taste abnormalities. (See Taste pathways to the brain .) Taste abnormalities may result from trauma, infection, vitamin or mineral deficiencies, neurologic or oral disorders, and the effects of drugs. Moreover, because tastes are most accurately perceived in a fluid medium, mouth dryness may interfere with taste. Two major pathologic causes of impaired taste are aging, which normally reduces the number of taste buds, and heavy smoking (especially pipe smoking), which dries the tongue.

Alterations in taste may include:

• ageusia, a complete loss of taste
• hypogeusia, a partial loss of taste
• dysgeusia, a distorted sense of taste
• cacogeusia, an unpleasant or revolting taste of food.

 

AGE ALERT Young children are frequently unable to differentiate between an abnormal taste sensation and a simple taste dislike.

CONGENITAL HEARING LOSS Hearing loss may be transmitted genetically as an autosomal dominant, autosomal recessive, or X-linked recessive trait. In neonates, it may result from trauma, toxicity, or infection during pregnancy or delivery. Predisposing factors include: family history of hearing loss or hereditary disorders (such as otosclerosis) maternal exposure to rubella or syphilis during pregnancy exposure to ototoxic drugs during pregnancy prolonged fetal anoxia during delivery congenital abnormality of ears, nose, or throat. Premature or low-birth-weight infants are likely to have structural or functional hearing impairments. Infants with a serum bilirubin level more than 20 mg/dl also risk hearing impairment from bilirubin toxicity to the brain. In addition, trauma during delivery may cause intracranial hemorrhage and damage to the cochlea or acoustic nerve.

Smell

As air travels between the septum and the turbinates of the nose, it touches sensory hairs (cilia) and olfactory nerve endings in the mucosal surface. The resultant stimulation of cranial nerve I sends impulses to the olfactory-receiving area, primarily in the frontal cortex. (See Olfactory perception .) Temporary impairment in the sense of smell can result from a condition that irritates and causes swelling of the nasal mucosa and obstructs the olfactory area in the nose, such as heavy smoking, rhinitis, or sinusitis. Permanent alterations in the sense of smell usually result when the olfactory neuroepithelium or a part of the olfactory nerve is destroyed. Permanent or temporary loss can also result from inhaling irritants, such as cocaine or acid fumes, that paralyze nasal cilia. Conditions such as aging, Parkinson's disease, Alzheimer's disease, or Kallmann syndrome (a congenital disorder) may also alter the sense of smell. Because combined stimulation of taste buds and olfactory cells produces the sense of taste, the loss of the sense of smell is usually accompanied by the loss of the sense of taste.

Alterations in smell include:

• anosmia, a total loss of the sense of smell
• hyposmia, an impaired sense of smell
• parosmia, an abnormal sense of smell.

DISORDERS

The most common disorders of vision are cataract, glaucoma, and macular degeneration. Other common sensory disorders include Meniere's disease and otosclerosis.

Cataract

A cataract is a gradually developing opacity of the lens or lens capsule. Light shining through the cornea is blocked by this opacity, and a blurred image is cast onto the retina. As a result, the brain interprets a hazy image. Cataracts commonly occur bilaterally, and each progresses independently. Exceptions are traumatic cataracts, which are usually unilateral, and congenital cataracts, which may remain stationary. Cataracts are most prevalent in people aged older than 70 as part of the aging process. The prognosis is generally good; surgery improves vision in 95% of affected people.

Causes

Causes of cataracts include:

• aging (senile cataracts)
• congenital disorders
• genetic abnormalities
• maternal rubella during the first trimester of pregnancy
• traumatic cataracts
• foreign body injury
• complicated cataracts
• uveitis
• glaucoma
• retinitis pigmentosa
• retinal detachment
• diabetes mellitus
• hypoparathyroidism
• myotonic dystrophy
• atopic dermatitis
• exposure to ionizing radiation or infrared rays
• drugs that are toxic to the lens―see below
• prednisone (Deltasone)
• ergot alkaloids
• dinitrophenol
• naphthalene
• phenothiazines
• pilocarpine
• exposure to ultraviolet rays.

TASTE PATHWAYS TO THE BRAIN <center> <a name="ch0015tastepathwaystothebrain"></a> <a name="ch0015tastepathwaystothebrain"></a> </center>

OLFACTORY PERCEPTION The exact mechanism of olfactory perception remains unknown. The most likely theory suggests that the sticky mucus covering the olfactory cells traps airborne odorous molecules. As the molecules fit into appropriate receptors on the cell surface, the opposite end of the cell transmits an electrical impulse to the brain by way of the olfactory nerve (cranial nerve I). <center> <a name="ch0015olfactoryperception"></a> <a name="ch0015olfactoryperception"></a> </center>

Pathophysiology

Pathophysiology may vary with each form of cataract. Congenital cataracts are particularly challenging. (See Congenital cataracts .) Senile cataracts show evidence of protein aggregation, oxidative injury, and increased pigmentation in the center of the lens. In traumatic cataracts, phagocytosis of the lens or inflammation may occur when a lens ruptures. The mechanism of a complicated cataract varies with the disease process; for example, in diabetes, increased glucose in the lens causes it to absorb water.

Typically, cataract development goes through four stages:

• immature: the lens is not totally opaque
• mature: the lens is completely opaque and vision loss is significant
• tumescent: the lens is filled with water; may lead to glaucoma
• hypermature: the lens proteins deteriorate, causing peptides to leak through the lens capsule; glaucoma may develop if intraocular fluid outflow is obstructed.

CONGENITAL CATARACTS Congenital cataracts may be caused by: chromosomal abnormalities metabolic disease (such as galactosemia) intrauterine nutritional deficiencies infection during pregnancy (such as rubella). Congenital cataracts may not be apparent at birth unless the eye is examined by funduscope. If the cataract is removed within a few months of birth, the infant will be able to develop proper retinal fixation and cortical visual responses. After surgery, the child is likely to favor the normal eye; the brain suppresses the poor image from the affected eye, leading to underdeveloped vision (amblyopia) in that eye. Postoperatively, in the child with bilateral cataracts, vision develops equally in both eyes.

Signs and symptoms

Possible signs and symptoms of cataracts include:

• gradual painless blurring and loss of vision due to lens opacity
• milky white pupil due to lens opacity
• blinding glare from headlights at night due to the inefficient reflection of light rays by the opacities
• poor reading vision caused by reduced clarity of images
• better vision in dim light than in bright light in patients with central opacity; as pupils dilate, patients can see around the opacity.

 

AGE ALERT Elderly patients with reduced vision may become depressed and withdraw from social activities rather than complain about reduced vision.

Complications

Complications of cataracts include:

• blindness
• glaucoma.

Surgical complications may include:

• loss of vitreous humor
• wound dehiscence from loosening of sutures and flat anterior chamber or iris prolapse into the wound
• hyphema, which is a hemorrhage into the anterior chamber of the eye
• vitreous-block glaucoma
• retinal detachment
• infection.

Diagnosis

Diagnosis is based on the following tests:

• physical examination (shining a penlight on the pupil to show the white area behind the pupil, which remains unnoticeable until the cataract is advanced)
• indirect ophthalmoscopy and slit-lamp examination to show a dark area in the normally homogeneous red reflex
• visual acuity test to confirm vision loss.

Treatment

Cataract treatment may include:

• extracapsular cataract extraction to remove the anterior lens capsule, and cortex and intraocular lens implant in the posterior chamber, typically performed by using phacoemulsification to fragment the lens with ultrasonic vibrations, then aspirating the pieces (See Comparing methods of cataract removal .)
• intracapsular cataract extraction to remove the entire lens within the intact capsule by cryoextraction (the moist lens sticks to an extremely cold metal probe for easy and safe extraction; rarely performed today)
• laser surgery after an extracapsular cataract extraction to restore visual acuity when a secondary membrane forms in the posterior lens capsule that has been left intact
• discission (an incision) and aspiration may still be used in children with soft cataracts
• contact lenses or lens implantation after surgery to improve visual acuity, binocular vision, and depth perception.

COMPARING METHODS OF CATARACT REMOVAL Cataracts can be removed by extracapsular or intracapsular techniques. EXTRACAPSULAR CATARACT EXTRACTION The surgeon may use irrigation and aspiration or phacoemulsification. To irrigate and aspirate, he makes an incision at the limbus, opens the anterior lens capsule with a cystotome, and exerts pressure from below to express the lens. He then irrigates and suctions the remaining lens cortex. In phacoemulsification, he uses an ultrasonic probe to break the lens into minute particles and aspirates the particles. INTRACAPSULAR CATARACT EXTRACTION The surgeon makes a partial incision at the superior limbus arc. He then removes the lens using specially designed forceps or a cryoprobe, which adheres to the frozen lens to facilitate its removal.

Glaucoma

Glaucoma is a group of disorders characterized by an abnormally high intraocular pressure (IOP) that damages the optic nerve and other intraocular structures. Untreated, it leads to a gradual loss of vision and, ultimately, blindness. Glaucoma occurs in several forms: chronic open-angle (primary), acute angle-closure, congenital (inherited as an autosomal recessive trait), and secondary to other causes. Chronic open-angle glaucoma is usually bilateral, with insidious onset and a slowly progressive course. Acute angle-closure glaucoma typically has a rapid onset, constituting an ophthalmic emergency. Unless treated promptly, this acute form of glaucoma causes blindness in 3 to 5 days.

NORMAL FLOW OF AQUEOUS HUMOR Aqueous humor, a transparent fluid produced by the ciliary epithelium of the ciliary body, flows from the posterior chamber through the pupil to the anterior chamber. It then flows peripherally and filters through the trabecular meshwork to the canal of Schlemm, through which the fluid ultimately enters venous circulation. <center> <a name="ch0015normalflowofaqueoushumor"></a> <a name="ch0015normalflowofaqueoushumor"></a> </center>

In the United States, approximately 2.5 million people have been diagnosed with glaucoma; another 1 million people have the disease but are undiagnosed. Glaucoma accounts for 12% of all new cases of blindness in the United States. The prognosis is good with early treatment.

 

CULTURAL DIVERSITY Blacks have the highest incidence of glaucoma, and it's the single most common cause of blindness in this group.

Causes

Risk factors for chronic open-angle glaucoma include:

• genetics
• hypertension
• diabetes mellitus
• aging
• black ethnicity
• severe myopia.

Precipitating factors for acute angle-closure glaucoma include:

• drug-induced mydriasis (extreme dilation of the pupil)
• emotional excitement, which can lead to hypertension

 

CULTURAL DIVERSITY The risk for acute angle-closure glaucoma is greater in persons with narrow iridic angles, such as those of Asian and Eskimo descent.

Secondary glaucoma may result from:

• uveitis
• trauma
• steroids
• diabetes
• infections
• surgery.

Pathophysiology

Chronic open-angle glaucoma results from overproduction or obstruction of the outflow of aqueous humor through the trabecular meshwork or the canal of Schlemm, causing increased IOP and damage to the optic nerve. (See Normal flow of aqueous humor .) In secondary glaucoma, conditions such as trauma and surgery increase the risk for obstruction of intraocular fluid outflow caused by edema or other abnormal processes.

Acute angle-closure glaucoma results from obstruction to the outflow of aqueous humor. Obstruction may be caused by anatomically narrow angles between the anterior iris and the posterior corneal surface, shallow anterior chambers, a thickened iris that causes angle closure on pupil dilation, or a bulging iris that presses on the trabeculae, closing the angle (peripheral anterior synechiae). Any of these may cause IOP to increase suddenly. (See Congenital glaucoma .)

CONGENITAL GLAUCOMA Congenital glaucoma, a rare disease, occurs when a congenital defect in the angle of the anterior chamber obstructs the outflow of aqueous humor. Congenital glaucoma is usually bilateral, with an enlarged cornea that may be cloudy and bulging. Symptoms in a newborn, although difficult to assess, may include tearing, pain, and photophobia. Untreated, congenital glaucoma causes damage to the optic nerve and blindness. Surgical intervention (such as goniotomy, goniopuncture, trabeculotomy, or trabeculectomy) is necessary to reduce intraocular pressure and prevent loss of vision.

AGE ALERT In older patients, partial closure of the angle may also occur, so that two forms of glaucoma may coexist.

Signs and symptoms

Clinical manifestations of chronic open-angle glaucoma typically are bilateral and include:

• mild aching in the eyes caused by increased IOP
• loss of peripheral vision due to compression of retinal rods and nerve fibers
• halos around lights as a result of corneal edema
• reduced visual acuity, especially at night, not correctable with glasses.

Clinical manifestations of acute angle-closure glaucoma have a rapid onset, are usually unilateral, and include:

• inflammation
• red, painful eye caused by an abrupt elevation of IOP
• sensation of pressure over the eye due to increased IOP
• moderate papillary dilation nonreactive to light
• cloudy cornea due to compression of intraocular components
• blurring and decreased visual acuity due to aberrant neural conduction
• photophobia due to abnormal intraoccular pressures
• halos around lights due to corneal edema
• nausea and vomiting caused by increased IOP.

Complications

Glaucoma may be complicated by:

• blindness.

Diagnosis

Glaucoma may be diagnosed using the following tests:

• pressure measurement tonometry using an applanation, Schiotz, or pneumatic tonometer; fingertip tension to estimate IOP (on gentle palpation of closed eyelids, one eye feels harder than the other in acute angle-closure glaucoma)
• slit-lamp examination of anterior structures of the eye, including the cornea, iris, and lens
• gonioscopy, to determine the angle of the anterior chamber of the eye, enabling differentiation between chronic open-angle glaucoma and acute angle-closure glaucoma (normal angle in chronic open-angle glaucoma and abnormal angle in acute angle-closure glaucoma (See Optic disk changes in chronic glaucoma .)
• ophthalmoscopy, to show cupping of the optic disk in chronic open-angle glaucoma; pale disk suggesting acute angle-closure glaucoma
• perimetry or visual field tests, to detect loss of peripheral vision due to chronic open-angle glaucoma
• fundus photography, to monitor the disk for changes.

OPTIC DISK CHANGES IN CHRONIC GLAUCOMA Ophthalmoscopy and slit-lamp examination show cupping of the optic disk, which is characteristic of chronic glaucoma. <center> <a name="ch0015opticdiskchangesinchronicglaucoma"></a> <a name="ch0015opticdiskchangesinchronicglaucoma"></a> </center>

Treatment

Treatment of chronic open-angle glaucoma may include:

• beta-blockers, such as timolol or betaxolol (a beta 1-receptor antagonist), to decrease aqueous humor production
• alpha agonists, such as brimonidine or apraclonidine, to reduce intraocular pressure
• carbonic anhydrase inhibitors, such as dorzolamide or acetazolamide, to decrease the formation and secretion of aqueous humor
• epinephrine, to reduce IOP by improving aqueous outflow
• prostaglandins, such as latanoprost, to reduce intraocular pressure
• miotic eye drops, such as pilocarpine, to reduce IOP by facilitating the outflow of aqueous humor.

When medical therapy fails to reduce intraocular pressure, the following surgical procedures may be performed:

• argon laser trabeculoplasty of the trabecular meshwork of an open angle, to produce a thermal burn that changes the surface of the meshwork and increases the outflow of aqueous humor
• trabeculectomy, to remove scleral tissue, followed by a peripheral iridectomy, to produce an opening for aqueous outflow under the conjunctiva, creating a filtering bleb.

Treatment for acute angle-closure glaucoma is an ocular emergency requiring immediate intervention to reduce high IOP, including:

• I.V. mannitol (20%) or oral glycerin (50%), to reduce IOP by creating an osmotic pressure gradient between the blood and intraocular fluid
• steroid drops, to reduce inflammation
• acetazolamide, a carbonic anhydrase inhibitor, to reduce IOP by decreasing the formation and secretion of aqueous humor
• pilocarpine, to constrict the pupil, forcing the iris away from the trabeculae and allowing fluid to escape
• timolol, a beta-blocker, to decrease IOP
• narcotic analgesics, to reduce pain if necessary.
• laser iridotomy or surgical peripheral iridectomy, if drug therapy does not reduce IOP, to relieve pressure and preserve vision
• cycloplegic drops, such as apraclonidine, in the affected eye (only after laser peripheral iridectomy), to relax the ciliary muscle and reduce inflammation to prevent adhesions.

Macular degeneration

Macular degeneration ― atrophy or degeneration of the macular disk ― is the most common cause of legal blindness in adults. Commonly affecting both eyes, it accounts for about 12% of blindness in the United States and for about 17% of new blindness. It's also one of the causes of severe irreversible and unpreventable loss of central vision in the elderly.

Two types of age-related macular degeneration occur. The dry, or atrophic, form is characterized by atrophic pigment epithelial changes and most often gradually causes mild visual loss. The wet, exudative form rapidly causes severe vision loss. It's characterized by the subretinal formation of new blood vessels (neovascularization) that cause leakage, hemorrhage, and fibrovascular scar formation.

Causes

The causes of macular degeneration are unknown but may include:

• aging
• inflammation
• injury
• infection
• nutrition.

Pathophysiology

Age-related macular degeneration results from hardening and obstruction of retinal arteries, which probably reflect normal degenerative changes. The formation of new blood vessels in the macular area obscures central vision. Underlying pathologic changes occur primarily in the retinal pigment epithelium, Bruch's membrane, and choriocapillaris in the macular region.

The dry form develops as yellow extracellular deposits, or drusen, accumulate beneath the pigment epithelium of the retina; they may be prominent in the macula. Drusen are common in the elderly. Over time, drusen grow and become more numerous. Visual loss occurs as the retinal pigment epithelium detaches and becomes atrophic.

Exudative macular degeneration develops as new blood vessels in the choroid project through abnormalities in Bruch's membrane and invade the potential space underneath the retinal pigment epithelium. As these vessels leak, fluid in the retinal pigment epithelium is increased, resulting in blurry vision.

Signs and symptoms

Signs and symptoms of macular degeneration include:

• changes in central vision due to neovascularization, such as a blank spot (scotoma) in the center of a page when reading
• distorted appearance of straight lines caused by relocation of retinal receptors.

Complications

Possible complications are:

• visual impairment progressing to blindness
• nystagmus.

Diagnosis

Diagnosis is based on the following test results:

• indirect ophthalmoscopy, to show gross macular changes, opacities, hemorrhage, neovascularization, retinal pallor, or retinal detachment
• I.V. fluorescein angiography sequential photographs, to show leaking vessels as fluorescein dye flows into the tissues from the subretinal neovascular net
• Amsler's grid test, to show central visual field loss.

Treatment

Treatment includes:

• laser photocoagulation, to reduce the incidence of severe visual loss in patients with subretinal neovascularization (exudative form)
• currently no cure for the atrophic form.

Meniere's disease

Meniere's disease, a labyrinthine dysfunction also known as endolymphatic hydrops, causes severe vertigo, sensorineural hearing loss, and tinnitus. It usually affects adults between the ages of 30 and 60, men slightly more often than women, and rarely occurs in children. Usually, only one ear is involved. After multiple attacks over several years, residual tinnitus and hearing loss can be incapacitating.

NORMAL VESTIBULAR FUNCTION The semicircular canals and vestibule of the inner ear are responsible for equilibrium and balance. Each of the three semicircular canals lies at a 90-degree angle to the others. When the head is moved, endolymph inside each semicircular canal moves in an opposite direction. The movement stimulates hair cells, which send electrical impulses to the brain through the vestibular portion of cranial nerve VIII. Head movement also causes movement of the vestibular otoliths (crystals of calcium salts) in their gel medium, which tugs on hair cells, initiating the transmission of electrical impulses to the brain through the vestibular nerve. Together, these two organs help detect the body's present position as well as a change in direction or motion.

Causes

The cause of Meniere's disease is unknown. It may be associated with:

• family history
• immune disorder
• migraine headaches
• middle ear infection
• head trauma
• autonomic nervous system dysfunction
• premenstrual edema.

Pathophysiology

Meniere's disease may result from overproduction or decreased absorption of endolymph―the fluid contained in the labyrinth of the ear. Accumulated endolymph dilates the semicircular canals, utricle, and saccule and causes degeneration of the vestibular and cochlear hair cells. Overstimulation of the vestibular branch of cranial nerve VIII impairs postural reflexes and stimulates the vomiting reflex. (See Normal vestibular function .) Perception of sound is impaired as a result of this excessive cranial nerve stimulation, and injury to sensory receptors for hearing may affect auditory acuity.

Signs and symptoms

Signs and symptoms of Meniere's disease include:

• sudden severe spinning, whirling vertigo, lasting from 10 minutes to several hours, due to increased endolymph (attacks may occur several times a year, or remissions may last as long as several years)
• tinnitus caused by altered firing of sensory auditory neurons (may have residual tinnitus between attacks)
• hearing impairment due to sensorineural loss (hearing may be normal between attacks, but repeated attacks may progressively cause permanent hearing loss)
• feeling of fullness or blockage in the ear preceding an attack, a result of changing sensitivity of pressure receptors
• severe nausea, vomiting, sweating, and pallor during an acute attack due to autonomic dysfunction
• nystagmus due to asymmetry and intensity of impulses reaching the brain stem
• loss of balance and falling to the affected side due to vertigo.

Complications

A complication is:

• hearing loss.

Diagnosis

Diagnosis of Meniere's disease is based on:

• patient history of signs and symptoms
• audiometric testing showing a sensorineural hearing loss and loss of discrimination and recruitment
• electronystagmography showing normal or reduced vestibular response on the affected side
• cold caloric testing showing impairment of oculovestibular reflex
• electrocochleography showing increased ratio of summating potential to action potential
• brain stem evoked response audiometry test, to rule out acoustic neurinoma, brain tumor, and vascular lesions in the brain stem
• computed tomography scan and magnetic resonance imaging, to rule out acoustic neurinoma as a cause of symptoms.

Treatment

During an acute attack, treatment may include:

• lying down to minimize head movement, and avoiding sudden movements and glaring lights to reduce dizziness
• promethazine (Phenergan) or prochlorperazine (Compazine), to relieve nausea and vomiting
• atropine, to control an attack by reducing autonomic nervous system function
• dimenhydrinate (Dramamine), to control vertigo and nausea
• central nervous system depressants, such as lorazepam (Ativan) or diazepam (Valium) during an acute attack, to reduce excitability of vestibular nuclei
• antihistamines, such as meclizine (Antivert) or diphenhydramine (Benadryl), to reduce dizziness and vomiting.

Long-term management may include:

• diuretics, such as triamterene (Dyrenium) or acetazolamide (Diamox), to reduce endolymph pressure
• betahistine, to alleviate vertigo, hearing loss, and tinnitus
• vasodilators, to dilate blood vessels supplying the inner ear
• sodium restriction, to reduce endolymphatic hydrops
• antihistamines or mild sedatives, to prevent attacks
• systemic streptomycin, to produce chemical ablation of the sensory neuroepithelium of the inner ear, and thereby control vertigo in patients with bilateral disease for whom no other treatment can be considered.

In Meniere's disease that persists despite medical treatment or produces incapacitating vertigo, the following surgical procedures may be performed:

• endolymphatic drainage and shunt procedures, to reduce pressure on the hair cells of the cochlea and prevent further sensorineural hearing loss
• vestibular nerve resection in patients with intact hearing, to reduce vertigo and prevent further hearing loss
• labyrinthectomy for relief of vertigo in patients with incapacitating symptoms and poor or no hearing, because destruction of the cochlea results in a total loss of hearing in the affected ear
• cochlear implantation, to improve hearing in patients with profound deafness due to Meniere's disease.

Otosclerosis

The most common cause of chronic, progressive, conductive hearing loss, otosclerosis is the slow formation of spongy bone in the otic capsule, particularly at the oval window. It occurs in at least 10% of persons of European descent and is three times as prevalent in females as in males; onset is usually between the ages of 15 and 30. Occurring unilaterally at first, the disorder may progress to bilateral conductive hearing loss. With surgery, the prognosis is good.

 

CULTURAL DIVERSITY Otosclerosis occurs less frequently in Asians and blacks, but it is a common disorder in southern India.

Causes

Causes include:

• autosomal dominant trait
• pregnancy.

 

AGE ALERT Children with osteogenesis imperfecta, an inherited condition characterized by brittle bones, may also have otosclerosis.

Pathophysiology

In otosclerosis, the normal bone of the otic capsule is gradually replaced with a highly vascular spongy bone. This spongy bone immobilizes the footplate of the normally mobile stapes, disrupting the conduction of vibrations from the tympanic membrane to the cochlea. Because the sound pressure vibrations aren't transmitted to the fluid of the inner ear, the result is conductive hearing loss. If the inner ear becomes involved, sensorineural hearing loss may develop.

Signs and symptoms

Signs and symptoms of otosclerosis include:

• bilateral conductive hearing loss due to the disruption of the conduction of vibrations from the tympanic membrane to the cochlea
• tinnitus due to overstimulation of cranial nerve VIII afferents
• ability to hear a conversation better in a noisy environment than in a quiet one (paracusis of Willis) as a result of masking effects.

Complications

A complication of otosclerosis is:

• deafness.

Diagnosis

Diagnosis is based on:

• otoscopic examination showing a normal-appearing tympanic membrane; occasionally, the tympanic membrane may appear pinkish-orange (Schwartz's sign) as a result of vascular and bony changes in the middle ear
• Rinne test showing bone conduction lasting longer than air conduction (normally, the reverse is true); as otosclerosis progresses, bone conduction also deteriorates
• audiometric testing showing hearing loss ranging from 60 dB in early stages to total loss
• Weber's test to detect sounds lateralizing to the more affected ear.

 

AGE ALERT Audiometric testing should be performed in late adolescence when otosclerosis and noise-induced hearing may start to occur.

Treatment

Treatment of otosclerosis may include:

• stapedectomy (removal of the stapes) and insertion of a prosthesis to restore partial or total hearing
• stapedotomy (creation of a small hole in the footplate of the stapes) and insertion of a wire and piston as a prosthesis to help restore hearing
• hearing aid (air conduction aid with molded ear insertion receiver) if surgery is not possible to permit hearing of conversation in normal surroundings.