Diseases of the Visual System

Functional Evaluation

The most objective and common measure of ocular function is line letter acuity, with normal vision ( Table 391-1 ) defined as the ability to see at 20 feet what a normal person sees at 20 feet, known as 20/20 vision ( E-Fig. 391-1 ). Less than 20/20 vision can be caused by an abnormality anywhere from the tear film to the visual cortex of the occipital lobe (see Fig. 391-1 ). However, normal vision also includes other functions, such as the perception of color, motion, contrast, brightness, field, and depth, for which there is greater variation among individuals. Normal visual acuity is potentially achievable in essentially all individuals, either naturally or with the correction of the refractive error. Although effective treatments are available for refractive errors, cataracts, glaucoma, and other eye conditions, there is no evidence that routine screening for impaired visual acuity is associated with improved clinical outcomes.

Correction of vision is based on the refraction of light ( Fig. 391-2 ). The diopter (D) is the unit of measurement of the ability of an optical system to refract (bend) light. The normal human eye has a refractive capacity of approximately 60 D, which is accomplished by the cornea and the crystalline lens. This refractive capacity is needed to focus light onto the central retina, the macula. If the eye is too short, light will be focused behind the eye (hyperopia). If the eye is too long, light will be focused in the vitreous cavity in front of the retina (myopia). Normally, a person can involuntarily control the crystalline lens, alternating between near and distant tasks. During the fourth and fifth decades of life, the lens becomes less pliable and the eye loses its ability to alter its shape (presbyopia). Refraction is the method of determining the amount of optical correction (strength of glasses) needed to establish 20/20 vision.

Examination of pupillary response assesses whether neural function is intact (see Figs. 392-2 and 392-4 ). Confrontational visual field testing (see Fig. 392-1 in Chapter 392 ) should be performed in each eye to detect gross quadrantic defects. Extraocular motility should be assessed to exclude nerve or muscle abnormalities (see Table 392-6 ). Color vision testing plates are a sensitive indicator of optic nerve function and for detecting genetic variations that result in deficiencies of color perception.

Diagnostic testing during a routine eye examination also includes external examination of the lids and adnexa, applanation tonometry to determine intraocular pressure, biomicroscopy (slit lamp examination) of the anterior segment, and ophthalmoscopic examination of the ocular fundus (the retina and associated structures). Other tests include exophthalmometry (measurement of proptosis), visual field and electrophysiologic testing and imaging (fluorescein angiography, mainly in diabetic patients; optical coherence tomography [OCT] to investigate retinal disease and glaucomatous damage), and corneal topography.

The conjunctiva, cornea, lens, and anterior chamber are evaluated using a slit lamp biomicroscope. The slit lamp is composed of a binocular microscope with variable magnification (40× and 80×) in conjunction with adjustable light sources. An increased concentration of protein can be detected in the anterior chamber because of the Tyndall (flare) effect, indicating vascular incompetence associated with either inflammation or ischemia. Even individual inflammatory cells can be resolved with the slit lamp. A cobalt blue filter can be used to detect fluorescein dye that accumulates in regions of abnormal epithelium (dendrite of herpes simplex keratitis or a corneal abrasion). The slit beam is used to examine the crystalline lens to determine the depth of the anterior chamber and the degree of opacification from a cataract. By using a green filter with the 90-D lens, the retinal vessels and retinal vascular abnormalities such as microaneurysm can be seen.

Myopia

nearsightedness. Nearsightedness (myopia; see Fig. 391-2 ) is usually discovered during childhood when children cannot perform distant tasks during school (reading the blackboard) or during school screening. Myopia usually progresses until age 20 to 25 years. Rapidly progressing myopia at any time after age 25 years requires evaluation for diabetes mellitus (reversible metabolic changes in the crystalline lens), trauma (development of a cataract), or use of corticosteroids (development of a cataract).

Treatment and Prevention

Nonpathologic forms of myopia are usually fully correctable with eye glasses or contact lenses or can be corrected to a visual acuity of 20/40 or better in 95% or more of patients by various forms of laser-assisted in situ keratomileusis (LASIK). Complications of LASIK include glare symptoms, dry eye, and undercorrection or overcorrection. Rare but serious complications include epithelial ingrowth, diffuse keratitis, and flap dislocation. For the prevention of myopia, children can be exposed to repeated low-level red light or be given 0.05% atropine eye drops nightly.

pathologic myopia. Pathologic myopia is a partly heritable condition causing progressive weakening of the posterior sclera and resulting increases in the axial length of the eye. Localized areas of deformation arise where there is an abrupt increase in the posterior radius of curvature of the eye (posterior staphyloma). The axial length of a normal eye is typically between 20 and 24 mm, whereas it can increase to above 25 mm in pathologic myopia and may be as high as 40 mm in severe cases. Abnormal physical forces in pathologic myopia may lead to retinal hole formation, retinal detachment, choroidal neovascularization, or intraocular hemorrhage. Associated systemic conditions include Cornelia de Lange syndrome, Stickler syndrome, and Marfan syndrome. Clinical surveillance is important to identify treatable complications such as retinal holes and detachment.

Treatment

Pathologic myopia is treated palliatively with eye glasses or contact lenses. Refractive procedures are less successful in pathologic myopia because of the severity of the refractive errors and the presence of posterior segment abnormalities. Surgical and laser procedures may be required to treat retinal and choroidal lesions.

Hyperopia

In hyperopia (farsightedness; see Fig. 391-2 ), in contrast to myopia, the eye tends to have a shorter than average axial length. Shorter axial lengths are associated with narrow-angle glaucoma and angle-closure glaucoma. Compensatory mechanisms of the crystalline lens may functionally correct small degrees of hyperopia until age 40 years, when the crystalline lens loses its pliability. The initial pair of glasses may have to correct for both distance and near tasks (bifocals). Refractive surgical procedures can correct up to 5 D of hyperopia.

Presbyopia

Presbyopia typically presents in the fourth to sixth decades of life and manifests as a progressive decrease in the ability to focus clearly on near objects. Presbyopia results when the crystalline lens loses its pliability, thereby preventing light rays from close objects from focusing onto the retina. The lens is suspended by fibers (zonules) attached to the ciliary body. In youth, the musculature of the ciliary body, acting via the zonules, alters the shape of the lens to provide increased near focusing power. As the lens hardens and presbyopia ensues, symptoms include difficulty reading small print, eyestrain, headaches, and the inability to read material unless it is farther away. Treatment is typically with eye glasses, such as bifocals, to provide adequate magnification to replace the lost ability of the eye.

Astigmatism

Astigmatism derives from changes in the radius of curvature of the cornea and from the lens. Symptoms are predominantly blurry vision with “shadowing” and difficulty seeing fine detail. Regular astigmatism is not a pathologic state but rather a variation in anatomy; most people have some degree of regular astigmatism that can be corrected with eye glasses or contact lenses. Irregular astigmatism is usually from changes on the cornea (e.g., from scarring owing to infections or trauma) or from adjacent tissue on the cornea (e.g., pterygia or limbal masses that induce changes in the corneal curvature). Some forms of regular astigmatism can be corrected with corneal laser ablation, but irregular astigmatism often requires correction with rigid contact lenses.

Keratoconus

Keratoconus is an acquired thinning of the cornea that induces irregularities of the corneal surface and irregular astigmatism; its cause is controversial but probably is partly genetic. Chronic eye rubbing has been associated with progressive disease. Onset generally is during adolescence, and the process often evolves over 5 to 10 years. The prevalence in the United States is approximately 55 cases per 100,000, but this figure may underestimate subclinical cases. Corneal surface mapping allows earlier and more precise diagnosis.

Patients with keratoconus typically report a progressive worsening of vision, sometimes severe. The condition is usually bilateral but not symmetrical. Mild degrees of early keratoconus can be corrected with glasses or contact lenses. The condition is typically progressive, but corneal collagen cross-linking can halt the progression in early to moderate cases and provide a sustained improvement in vision. For severe cases with significant corneal scarring and intolerance to contact lenses, corneal transplantation (graft) is the primary therapy.

Strabismus

The control of simultaneous orientation of the two eyes to ensure that the visual axes of both eyes are aligned is not complete until 6 to 12 months following birth. Misalignments of the two eyes (strabismus) may be the result of abnormalities in the central oculomotor nuclei of the brain, malfunction of one or several oculomotor nerves, or intrinsic abnormalities of the rectus or oblique muscles (see Fig. 392-6 ). If the eyes are not simultaneously stimulated with the images of the same degree of clarity or complexity, there is a risk that only one eye will develop normal vision (amblyopia). In most cases, only the central vision is affected, so the peripheral vision in both eyes is likely to be normal. Binocular vision can be disrupted with amblyopia, with a loss in depth perception that may become permanent if the normal eye alignment is not restored.

Amblyopia also may be caused by a marked difference in refractive error between the two eyes (anisometropic amblyopia), congenital cataracts, or eyelid ptosis (deprivational amblyopia). Ptosis may be neurogenic or mechanical (e.g., congenital eyelid hemangioma). To avoid amblyopia, it is extremely important to refer a child with strabismus to an ophthalmologist as soon as the strabismus is noted.

Esotropia is a deviation of one eye inward (“crossed eyes”). Although intermittent esotropia may be a normal finding in infancy, it should not be present beyond 6 months of age. Delay in facial maturation (underdeveloped nasal bridge) may give the appearance of esotropia, even though the visual axes are correctly aligned. In true strabismus, the corneal light reflex will be in the center of one cornea and decentered in the other.

Exotropia is deviation of one eye outward. Exotropia is often intermittent and less likely to result in amblyopia compared to esotropia. Like esotropia, intermittent exotropia may also be a normal developmental finding in infancy.

Treatment

Treatment options for amblyopia include patching or atropine eye-drops (1%, typically daily but effective if used as infrequently as twice per week) to blur the better-seeing eye, thereby strengthening the amblyopic eye. These therapies have been shown to be equally effective in providing good vision if patients are treated before age 7 years. The outcome of treatment is more favorable if amblyopia is detected before age 2 to 3 years but may be occasionally successful into the teenage years depending on the severity. Success is directly related to compliance and the timeliness of diagnosis.

In some cases, esotropia or exotropia may be treated by using appropriate corrections of refractive error with glasses (occasionally bifocals). It is critically important to recognize that strabismus may be the presenting sign of vision loss due to other causes (e.g., retinoblastoma). Extraocular muscle surgery may be necessary to correct alignment.

Diplopia (Double Vision)

Acute onset of diplopia is an ominous sign suggestive of a cranial nerve palsy (i.e., III, IV, or VI) or pathology in the orbit ( Chapter 392 ). Diplopia of any kind is usually an intolerable symptom, which is often worse with vertical deviations than horizontal deviations.

Color Vision Change

Most cases of congenital color blindness are genetically determined and go undetected for many years. Acquired color deficiency at any age may be caused by a cataract or optic nerve disease.

Change in Vision

If only one eye has a change in vision, the problem, such as a cataract or retinal detachment, is most likely in that eye. If both eyes have a change in vision, the problem generally is outside of the eye, such as homonymous hemianopia ( Chapter 392 ). Improvement of near vision in middle age may be a sign of cataract (“second sight”) or hyperglycemia. Transient complete or partial unilateral or bilateral loss of vision may be caused by vascular abnormalities inside or outside of the eye ( Table 391-2 ).

Pain

The most severe eye pain ( Table 391-3 ), typically associated with a red eye, is caused by acute angle-closure glaucoma. Sharp, intermittent pain is usually caused by ocular surface abnormalities (e.g., corneal foreign body). Burning pain that clears with blinking generally relates to tear film abnormalities (dry eyes). Deep boring pain most often is associated with an ocular abnormality (e.g., uveitis).

Red Eye

A red or inflamed eye can be caused by conjunctivitis (allergic, inflammatory, or infectious), iritis (anterior uveitis), acute glaucoma, ocular trauma, or infection ( Table 391-4 ). All of these causes are typically painful to varying degrees.

Distorted Vision

Distorted vision (metamorphopsia), which is the perception that straight lines are distorted or bowed, results from macular dysfunction. Causes include fluid under the retina; macular degeneration, which tends to elevate the retina; and an epiretinal membrane, which tends to contract the retina.

Night Blindness

Retinitis pigmentosa, vitamin A deficiency, and systemic medications such as phenothiazines can cause true night blindness, in which patients have difficulty seeing any stars in the sky on a clear night and may be unable to ambulate without assistance in a dark environment. Patients with cataracts may have difficulty driving at night because of excessive glare and visual distortion. Gene therapy using adeno-associated virus vectors carrying the RPE65 gene has been shown to increase retinal sensitivity and to improve night vision in patients with retinitis pigmentosa.

Sensation of Flashing Lights

The sudden onset of flashes (photopsia) in the peripheral visual field suggests traction of the vitreous on the peripheral retina, sometimes with a resulting retinal tear. The flashes, which may be more pronounced in the dark and with rapid eye movement, may be associated with the sudden onset of floaters, which can indicate debris or blood in the vitreous cavity. Because a tear in the retina can lead to a retinal detachment, urgent consultation with an ophthalmologist is required.

Flashing light with a migraine ( Chapter 367 ) is described as scintillations or zigzagging lights that march across the visual field for a few minutes or as long as 30 minutes, sometimes associated with transient visual field loss. Headache is not universal.

Floaters

Floaters, which are caused by small aggregates or opacities in the vitreous cavity, result from the normal aging of the vitreous. The acute onset of vitreous floaters may be associated with uveitis or with the sudden onset of bleeding in the vitreous cavity owing to diabetes or vascular occlusions. Acute floaters, however, particularly if associated with flashing lights, may indicate a posterior vitreous detachment and/or a retinal tear with an impending retinal detachment. Urgent ophthalmic referral is essential.

Photophobia

Photophobia, particularly if associated with eye pain, redness, and decreased vision, is a symptom of intraocular inflammation (uveitis) from trauma or other causes. Photophobia is also typical of acute migraine and meningeal irritation. Prompt ophthalmologic referral is prudent.

Halos Around Lights

Patients with cataracts commonly see halos around lights, particularly when driving at night. Episodic decreased vision, redness, and halos around lights may be symptoms of impending angle-closure glaucoma owing to increased intraocular pressure with resulting corneal edema and loss of corneal clarity. Halos also can occur as a complication of refractive eye surgery.

Foreign Body Sensation

A foreign body sensation is commonly caused by ocular surface diseases such as dry eye syndrome (see later). Entropion (see Fig. 391-3 ) or misdirected lashes (trichiasis) also can cause a foreign body sensation. Most corneal abrasions cause severe pain, but minor corneal abrasions may be associated with a foreign body sensation rather than the severe pain. An arc welder burn causes a punctate corneal keratopathy, and foreign body sensation may be a prominent symptom. A true conjunctival or corneal foreign body also may be present.

Excessive Tearing (Epiphora)

Tearing can occur because of the overproduction of tears or impairment of tear drainage. Any abnormalities in upper or lower lid laxity, position, or closure can impair the lacrimal pump system. The most common cause of epiphora is chronic or acute irritation that results in the overproduction of reflex tearing from ocular dryness (dry eye syndrome). Ocular surface irritants, such as trichiatic eyelashes touching the cornea, also can stimulate excessive tear production. Obstruction of the nasolacrimal tear drainage system owing to age, inflammation, infection, trauma, or neoplasm can prevent appropriate tear drainage. Treatment depends on the underlying condition.

Eyelid Twitching

Any irritation of the conjunctiva or cornea can cause the eyelids to twitch. Occasional twitching of the lids usually is associated with stress or adrenergic stimulation. Benign essential blepharospasm is severe spasm of the lids leading to functional impairment. Multiple sclerosis ( Chapter 380 ) also can cause lid spasm.

Conjunctivitis

Any ocular inflammation, including corneal ulcers, angle-closure glaucoma, endophthalmitis, and uveitis, can be associated with secondary conjunctival hyperemia. Conjunctivitis usually involves the entire conjunctiva, is associated with a discharge, and may or may not be associated with pain (see Table 391-4 ).

Ptosis (Droopy Eyelid)

Ptosis ( Fig. 391-4 ) is typically due to age-related degeneration and even disinsertion of the levator palpebrae superioris muscle. However, it also can be caused by a third nerve palsy, Horner syndrome, myasthenia gravis ( Chapter 390 ), or other neurologic conditions. Third nerve palsy is usually associated with diplopia and a large pupil. Horner syndrome (see Fig. 392-5 ) is associated with a small pupil and ipsilateral anhydrosis. With myasthenia gravis ( Chapter 390 ), other typical features of muscle weakness and fatigability are usually present or can be elicited. Non-neurologic causes of ptosis include age-related dehiscence of the levator muscle.

Proptosis (Exophthalmos)

Proptosis, or a prominent globe, can be a manifestation of thyroid eye disease, which in turn is associated with thyroid abnormalities, especially Graves disease ( Chapter 207 ). In this setting, proptosis can be subacute and asymmetrical. Any orbital inflammation, infection, or neoplasm can cause unilateral proptosis; in the case of inflammation or infection, proptosis is typically acute in onset. All orbital processes can affect orbital cranial nerves, with associated diplopia or optic neuropathy. Orbital infection, or cellulitis, typically presents with concurrent severe eye redness, pain, sinusitis, and an elevated white blood cell count ( Fig. 391-5 ). Idiopathic orbital inflammation can cause acute, usually unilateral proptosis, with severe pain, particularly with eye movement, and often with decreased vision. An optic nerve tumor causes chronic, unilateral proptosis associated with a slow onset of visual field loss. Acute cellulitis can be associated with unilateral proptosis, severe redness, and moderate to severe pain, commonly with sinusitis and an elevated white blood cell count.

Small Pupil

A unilateral small pupil is best detected in dark conditions. Causes include Horner syndrome, associated with ptosis on the same side; the bilaterally small, poorly reacting pupils of tertiary syphilis (Argyll Robertson pupils), which accommodate with normal constriction to a near object; miotic drops (e.g., pilocarpine); traumatic iritis; uveitis; and opioids.

Large Pupil

Any α-adrenergic or anticholinergic agent placed into the eye can cause a large pupil. With eye trauma, the iris sphincter muscle can be damaged, and an abnormally large pupil can result. Tears in the iris sphincter can sometimes be appreciated on slit lamp examination. Third nerve palsy may cause a dilated pupil associated with ptosis and decreased elevation, depression, and medial eye movement. Adie pupil (see Fig. 396-3 ) is an idiopathic, unilateral large pupil that is hypersensitive to weak cholinergic stimulation. Recent eye surgery with pharmacologic dilation, uveitis, an acute angle-closure glaucoma attack, and traumatic iritis can cause a large pupil.

Leukocoria

Leukocoria (white pupil) in a young child is critically important, because it may be a sign of retinoblastoma. Some of the more common non-retinoblastoma conditions presenting with leukocoria ( Table 391-5 ) include cataracts, retinal detachment, persistent fetal vasculature (a developmental anomaly of the vitreous resulting in intraocular fibrosis and retinal detachment), Coats disease (a developmental vascular malformation of the retina leading to exudative retinal detachment), and ocular toxocariasis (a parasitic intraocular nematode infection that leads to intraocular scarring and retinal detachment).

Ectropion and Entropion

An ectropion is an out-turning of the eyelid, typically the lower lid ( Fig. 391-6 ), with exposure of the undersurface of the lid. Causes include aging, scarring, a mass on the eyelid, and seventh nerve (Bell) palsy. Common symptoms, which derive from corneal exposure and drying, include burning, itching, tearing, and the sense of a foreign body. Treatment is surgical, although mild symptoms may be managed conservatively with lubrication.

An entropion, which is an in-turning of the eyelid (see Fig. 391-3 ), is usually age related and associated with irritation, burning, and a foreign body sensation because the eyelashes touch the cornea. If it leads to trichiasis, in which the eyelashes rub or abrade the cornea, the lashes can be removed with forceps or surgery to correct the eyelid malposition. Botulinum toxin can be injected into the orbicularis muscle to temporarily correct the entropion.

Chalazion

A chalazion ( Fig. 391-7 ) is a localized lipogranulomatous inflammation owing to an occluded sebaceous (meibomian) gland. The retained, lipid-rich sebaceous material acts as a foreign material that stimulates a lipogranulomatous foreign body inflammatory reaction. A painless or slightly tender, nonmobile nodule forms under the eyelid skin. Most lesions resolve over days to weeks with warm compresses or without specific treatment. If the chalazion persists, it may be surgically débrided. Some individuals may have recurrent chalazia. A recurrent chalazion in the same spot may point to an underlying carcinoma.

Hordeolum (Stye)

A hordeolum (stye) ( Fig. 391-8 ) is a painful abscess in a hair or eyelash follicle or in a sebaceous gland. Styes are usually self-limited infections that respond to warm compresses and topical antibiotics (e.g., bacitracin or erythromycin ointment or moxifloxacin or gatifloxacin drops). An ophthalmologist may perform incision and drainage if symptoms do not improve within 48 hours.

Blepharitis

Blepharitis ( Fig. 391-9 ) is a common, nonspecific inflammation of the eyelid margin or skin. The condition is usually bilateral and symmetrical. Rosacea ( Chapter 406 ) is the most common associated cutaneous condition, and Staphylococcus aureus is the most common infectious agent. If untreated, blepharitis becomes chronic and may lead to corneal and conjunctival inflammation (blepharoconjunctivitis). Effective supportive care for blepharitis includes warm compresses and maintenance of good lid hygiene. Ophthalmic antibiotic ointment (e.g., bacitracin or erythromycin) is more efficacious than eye-drops, but systemic antibiotics (e.g., minocycline, 50 to 100 mg; doxycycline, 100 mg, once daily; tetracycline, 250 mg twice daily; or erythromycin, 250 mg three times daily) infrequently may be recommended, particularly if there is any evidence of inflammation of the cornea or conjunctiva.

In seborrheic blepharitis, exfoliated keratinous debris accumulates along the eyelid margin, particularly at the follicles of the eyelashes, and irritates the conjunctiva. Treatment of this chronic condition is directed at mechanically removing the keratinous debris by scrubbing the eyelid and eyelashes daily with a mild detergent (“baby shampoo”) in warm water applied with a soft cloth.