Eye and Visual System Anatomy

The anatomies of the eye and visual system are shown in Figs. 32.1 and 32.2 . The optic nerves, made up of the converging nerve fiber layer of the retina, have intraocular, intraorbital, intracanalicular, and intracranial portions. Partial decussation of the optic nerve fibers occurs in the chiasm, which gives binocular visual input to each side of the brain. The visual cortex is where the conscious process of seeing occurs.

Anatomy of the eye as seen in cross section.

(Modified from Reilly BM. Practical Strategies in Outpatient Medicine . 2nd ed. Philadelphia: WB Saunders; 1991:36.)

Anatomy of the visual pathways. The anatomy of the visual pathways appears at the top of the figure, the pink shading indicating how visual information from the left visual space eventually courses to the right brain. Visual field defects are at the bottom of the figure. Anterior defects (labeled 1 from disease of the optic nerve or retina) characteristically affect 1 eye and cause defects (red shading) that may cross the vertical meridian (i.e., the vertical meridian is the vertical line bisecting each visual field). Chiasmal defects (labeled 2 ) and postchiasmal defects (labeled 3 for a lesion in the anterior temporal lobe, 4 for the parietal lobe, and 5 for the occipital cortex) characteristically affect both eyes and respect the vertical meridian.

(Modified from McGee S. Evidence-Based Physical Diagnosis . 3rd ed. Philadelphia: Elsevier; 2012:514.)

Development of the Eye and Visual System

The eyes and vision of a newborn are immature and require several years to reach adult proportions and functional status. By the 9th month of gestation, the retinal vessels have reached the periphery of the retina (an important factor in the pathogenesis of retinopathy of prematurity [ROP]), the optic nerve has completed myelination, and the pupillary membrane has disappeared. Postnatal reorganization of neuron-to-neuron connections in the visual cortex improve the poor visual acuity and other visual processes, which are not fully developed at birth. The visual acuity of the newborn has been estimated to be 20/400-20/600 and may reach the normal 20/20 level as early as 6-12 months of age as tested with visual evoked cortical responses. Acuity of 20/20 is not reached with other types of testing such as preferential looking with Teller acuity cards until ages 3-5 years. Visual acuity measured with conventional letter or symbol recognition methods does not reach 20/20 until 6 years of age because of cognitive factors. Binocular vision, including establishment of normal ocular alignment and depth perception, and improved facility of accommodation, the ability to focus on images at different distances, develop rapidly in the 1st year of life. The rapid maturation of visual function in the 1st year of life accounts for the critical period of visual development and the extreme sensitivity of the visual system to abnormal visual input from strabismus or cataracts. Children deprived of vision in this critical period will have limited visual potential and may develop nystagmus (abnormal eye movements).

The majority of newborns are moderately hyperopic. Heredity contributes to the refractive status of the eyes and the environment and visual experience also plays a role.

Amblyopia and Vision Screening

Amblyopia is defined as a unilateral or, less commonly, bilateral reduction in visual acuity that cannot be immediately corrected with glasses or surgery. In children in whom visual acuity can be accurately measured, a practical definition of amblyopia is a 2-line or greater difference between the best-corrected visual acuity of the eyes. For preverbal children, differences between the eyes in fixation and following behavior or fixation preference are used to diagnose amblyopia. Automated photoscreeners can aid in the diagnosis of risk factors for amblyopia and strabismus, particularly in preverbal children who perform poorly on subjective testing. Amblyopia results from abnormal visual experience early in life during the critical period for visual development. The sensitive period for amblyopia starts in early infancy and continues to at least the age of 6 years and often beyond the age of 8 years. There is a suggestion of cortical plasticity in adults that may allow for some vision improvement into adulthood. The prevalence of amblyopia in the North American population is 2-4%.

Unilateral amblyopia results from 3 types of abnormal visual experience: strabismus, anisometropia (unequal refractive errors), and monocular visual deprivation (e.g., cataract, corneal opacity, hemangioma (severe ptosis). Bilateral amblyopia results from bilateral media opacities or significant bilateral refractive errors (ametropia). Nearly all amblyopia is reversible if discovered at an early age and treated appropriately. Treatment effectiveness, however, declines after the age of 5 years.

Detection strategies for amblyopia can involve early recognition of factors that give rise to amblyopia or actual measurement of reduced visual acuity that may be caused by amblyopia. Most amblyopia risk factors can be detected through routine pediatric screening such as ocular history, red reflex evaluation, ocular motility, and vision assessment. Recommended screening and referral guidelines are shown in Table 32.1 . Whenever possible, a line of symbols or isolated symbols with surrounding crowding bars is recommended for screening ( Fig. 32.3 ). Isolated symbols can lead to overestimates of the visual acuity of an eye with amblyopia due to a crowding phenomenon or contour interaction in which symbols of a given size are more difficult to recognize if they are surrounded by similar symbols. Hence, visual acuity obtained with single optotypes without crowding bars can result in failure to detect amblyopia.

The Lea symbols in chart format (middle) and the Lea symbols and HOTV tests with crowding bars (right and left). All tests should be administered at a distance of 10 feet.

The treatment of amblyopia involves eliminating amblyopia risk factors, providing a focused retinal image with appropriate optical correction, and forcing use of the amblyopic eye through occlusion of the sound eye or blurring the image it receives. For patients with visual deprivation amblyopia, the depriving factor must be addressed medically or surgically. Optical correction including a bifocal is required for patients who have had cataract surgery. Intraocular implants are not generally placed in children under 1 year of age due to the high postoperative complication rates. For patients with anisometropic amblyopia, optical correction usually involves spectacles or, less commonly, contact lenses.

An adhesive patch worn over the sound eye most commonly achieves enforced use of the amblyopic eye. Occlusive devices can be attached to glasses but may be less reliable since the child can peek over the glasses or around the occlusive device. The use of the potent cycloplegic agent atropine sulfate may also be used to encourage use of the amblyopic eye. A drop of atropine is applied to the sound eye each day; this temporarily impairs its accommodative ability and, in the presence of sufficient hyperopia, prevents that eye from obtaining a clear retinal image. Atropine “penalization” for amblyopia works best in hyperopic patients with mild to moderate amblyopia (visual acuity of 20/100 or better). Close follow-up of patients being treated for amblyopia is important for monitoring compliance with treatment and for preventing the development of iatrogenic reverse amblyopia in the sound eye from excessive occlusion or penalization.

Visual Fields

Quantitative testing of the visual field of most children is difficult before the age of 10 years (see Fig. 32.2 ). Confrontation field tests can be performed to detect gross abnormalities of the visual field (hemianopsias) but even these are not reliable and need to be confirmed at an older age. Visual field defects in children are uncommon despite parental concerns about a child who seems to bump into objects frequently. Unilateral retinal or optic nerve disease can produce unilateral visual field defects, but these are almost always associated with reduced visual acuity in the involved eye. Bilateral visual field defects, particularly if symmetric (homonymous), indicate disease of the optic radiations or visual cortex. Visual acuity may be entirely normal. Causes of bilateral visual field defects in children include cerebrovascular accidents, pituitary or hypothalamic tumors, or congenital central nervous system (CNS) abnormalities.

Strabismus

Strabismus is derived from the Greek word strabismos , “to squint to look obliquely or askance.” It implies misalignment of the eyes in such a way that they are not simultaneously viewing the same object. Terms to describe eye alignment and movement are noted in Table 32.2 . Strabismus can be constant or intermittent and can be the same in all directions of gaze (comitant) or greater in 1 direction of gaze than in others (incomitant). Furthermore, it can be categorized as congenital or acquired, monocular or alternating. The direction of misalignment can be vertical or horizontal. Vertical strabismus is referred to as a hypertropia of the higher eye. Horizontal strabismus can be convergent (esotropia) or divergent (exotropia). The importance of strabismus detection derives primarily from the fact that it is the leading cause of amblyopia. Other reasons for detecting strabismus are the possibility of being able to restore normal binocular use of the eyes, improving depth perception, and minimizing the social and economic drawbacks to strabismus in society.

Strabismus detection can be simple, as in patients with a large angle of deviation ( Fig. 32.4 ), or difficult, as in patients with more subtle deviations or no deviation at all (pseudostrabismus) ( Fig. 32.5 ). Evaluation of the symmetry of the corneal light reflexes from a penlight directed at the eyes can reliably detect many cases (see Fig. 32.4 ). With smaller angles of strabismus or when the results of the corneal light reflex are in doubt, the cover test should be performed ( Fig. 32.6 ). It is important to provide attractive fixation targets for the child to view during the test.

Corneal light reflex test reveals an asymmetrically placed reflex that is laterally displaced in the right eye. This indicates an inward deviation of the eye (esotropia).

(From Lavrich JB, Nelson LB. Diagnosis and treatment of strabismus disorders. Pediatr Clin North Am. 1993;40:739.)

A child with pseudoesotropia. Note that the wide nasal bridge and prominent epicanthal folds create the illusion of an esotropia. The corneal light reflexes are centered in each eye; therefore, the eyes are straight.

(From Lavrich JB, Nelson LB. Diagnosis and treatment of strabismus disorders. Pediatr Clin North Am. 1993;40:741.)

The cover test. In each instance, the occluder is placed over the right eye while the patient is viewing a fixation target and the examiner is watching for movement of the patient’s left eye. If the left eye is not aligned, it will need to move to look at the fixation target. If there is no movement of the left eye, the test needs to be repeated by occluding the left eye and watching for movement of the right eye.

Infantile esotropia is defined as convergent strabismus with onset within the 1st 6 months of life (see Fig. 32.4 ). Transient crossing or divergence of the eyes is common in newborns and is probably not significant unless it persists beyond 3 months of age. In the classic form of infantile esotropia, there is a large-angle, constant deviation. The child may alternate fixation (cross fixate) in which case the visual acuity is usually good in both eyes. Cross fixation may mimic bilateral 6th nerve palsy. If the crossing is present in only 1 eye, amblyopia occurs. The cause of infantile esotropia is not known, but hereditary factors play a definite role. The incidence of infantile esotropia is less than 1% among neurologically normal infants.

Early correction of infantile esotropia may result in full or nearly full restoration of normal binocular function, a result not believed to be obtainable with correction of misalignment at older ages. The ideal timing of surgery for infantile esotropia is not known, although usually done between 6 and 24 months to optimize binocularity. However when early surgery is done, there is a higher chance of needing further surgery. Early detection and prompt referral of infants with suspected esotropia are indicated.

A second category of esotropia occurs in children whose eyes are initially straight but start to cross, usually intermittently at first, at 1-3 years of age. These children have excessive hyperopia and an abnormal relationship between accommodation and convergence. This type of esotropia is called accommodative esotropia . Amblyopia frequently develops. Treatment consists of correcting amblyopia and providing spectacles to correct hyperopia, thereby modulating the amount of accommodation required by the child ( Fig. 32.7 ). Bifocal spectacles may also be necessary for some forms of accommodative esotropia.

Accommodative esotropia (top). The deviation is completely controlled with glasses at both distant (middle) and near (bottom) fixation distances.

Esotropia caused by paralysis of a lateral rectus muscle, a 6th cranial nerve palsy, occurs much more frequently in children than in infancy ( Fig. 32.8 ). Approximately 30% of children will have an intracranial lesion. Other causes include head trauma or a recent viral illness. The 6th nerve palsy may resolve spontaneously if the cause is benign. An older child may present with complaints of diplopia or a face turn or closure of 1 eye to avoid diplopia, whereas a younger child may present with only the esotropia because of rapid development of suppression to eliminate diplopia. Neurologic investigation is indicated if the history does not support a benign etiology or the paralysis does not spontaneously abate in a few weeks (a so-called benign 6th nerve palsy believed to be postviral in origin) or if the child demonstrates other neurologic impairment or has papilledema.

Left 6th cranial nerve palsy in a 4-year-old. A large manifest esotropia is present when the child looks straight ahead (top). The left eye does not abduct beyond the midline in gaze to the left (middle). Gaze to the right is normal (bottom).

Infantile exotropia is much less common than infantile esotropia. Infantile exotropia presents as a large deviation of the eyes prior to 6 months of age. Infantile exotropia is uncommon in otherwise healthy infants. It is, however, commonly associated with craniofacial disorders or neurologic impairment. Surgery may be done early in life, but these patients are less likely to obtain good binocular vision than infantile esotropes.

Intermittent exotropia is the most common type of exotropia. This usually manifests by 5 years of age. Parents will notice that the eyes deviate out at times and yet not at others. The deviation is most likely to occur when the child is tired or ill. Because the child maintains the ability to keep the eyes aligned part of the time, amblyopia is uncommon. Diplopia is prevented by active cortical suppression of input from the portion of the retina of the deviated eye that overlaps the central view of the fixating eye. When the eyes are straight, the child generally maintains normal binocular function, including stereopsis. The clinical management of intermittent exotropia is not clear. Treatment options include part-time patching, additional minus power spectacles in patients with myopia, orthoptic exercises, and surgery.

Primary vertical strabismus is far less common than horizontal strabismus. A small vertical deviation in association with a larger amount of horizontal strabismus, however, is common, and is managed in conjunction with the horizontal deviation. A common cause of hypertropia in children is congenital paralysis of the superior oblique muscle, a 4th cranial nerve paralysis. In some children, the “paralysis” is actually caused by an anatomic abnormality of the superior oblique tendon. Acquired causes of a superior oblique palsy include trauma, central nervous system abnormalities, or brain tumors. Children with a superior oblique paralysis of any cause frequently present with a head tilt and face turn toward the side opposite the paralyzed superior oblique muscle. If there is a question as to the timing of the onset of the superior oblique palsy, a review of pictures at a younger age may be helpful in determining chronicity. Superior oblique paralysis is 1 of the more common causes of ocular torticollis. An eye muscle disorder needs to be ruled out in any child with a chronic abnormality of head position. The anomalous head position and hypertropia caused by a superior oblique paralysis can be improved by eye muscle surgery in most instances. An approach to the evaluation of strabismus is noted in Fig. 32.9 and less common forms of strabismus are listed in Table 32.3 .

Evaluation of strabismus. CNS, central nervous system.

Refractive Errors

Refractive errors include myopia (nearsightedness), hyperopia (farsightedness), and astigmatism. Refractive errors may be similar (isometropia) or different (anisometropia) between the 2 eyes. Bilateral amblyopia may result from a high refractive error that is isometropic. Full-time spectacle correction will often correct bilateral amblyopia. Unilateral amblyopia may result from anisometropia. Patching or atropine penalization may be necessary in these children.

Vision Impairment in Children

Vision impairment is formally defined as best-corrected visual acuity of 20/70 or worse in both eyes. Impairment of vision exists as a continuum from 20/70 to no light perception. Unrestricted driver’s license has a requirement of best-corrected vision of 20/40 or better in 1 eye in all but 3 states (New Jersey and Wyoming have a requirement of 20/50 best-corrected vision in 1 eye and Georgia has a requirement of 20/60 best-corrected or better in 1 eye). Legal blindness is said to be present when best-corrected visual acuity is 20/200 or less in each eye. Constricted visual fields may also play a role in the diagnosis of vision impairment or legal blindness. An infant or child whose visual acuity and visual field cannot be quantitated may be judged visually impaired on the basis of inability to fixate on and follow movement of the examiner’s face or other objects or even, in severe instances, inability to perceive light. Milder vision impairment may be suspected on the basis of associated eye signs but can be difficult to confirm in a preverbal child because of compensatory behavior (holding objects close, a face turn) that allows the child to have relatively normal overall function and development. Observation of the child’s behavior in the examination room, examination of the eyes ( Table 32.4 ), and a detailed history ( Table 32.5 ) taken from the parents about the child’s visual behavior at home can be important in establishing the degree of impairment. Many visually impaired infants and children have objective signs such as nystagmus, sluggish pupillary light reflexes, or anatomic abnormalities such as optic nerve hypoplasia or chorioretinal scarring.

Visual inattentiveness in an infant deserves special attention because of the possibility that the child has a treatable but not obvious form of vision impairment such as bilateral congenital cataracts. Even if the cause of impairment is not remediable, early diagnosis is important for referral of the infant for physical and occupational therapy for visual impairment since these children have very specific needs. Vision impairment (monocular or binocular) acquired after infancy obligates the physician to search for a cause such as a retinal degeneration because some causes are treatable ( Tables 32.6 to 32.8 ).

Retinopathy of Prematurity

Premature infants are at risk for the development of ROP because the retinal vessels have not yet grown out to the periphery of the retina and are susceptible to a variety of postnatal influences, including oxygen that can adversely affect retinal vessel maturation. In advanced stages of ROP, retinal neovascularization and fibrosis may lead to traction on the retina and result in a retinal detachment, the most common cause of blindness in premature infants. In most cases fortunately, ROP spontaneously resolves.

The international classification system for the acute stages of ROP describes the location, extent, and stage of the disease according to the position of the advancing wave of retinal vessels ( Fig. 32.10 ). The retina is divided into zones I, II, and III. Zone I is centered on the optic nerve, from which the retinal arterioles emerge, and zone III exists as a crescent of retina on the temporal side; zone II occupies the midportion of the retina in all 4 quadrants. The severity is indicated by stages 1-5 with stage 1 representing mild ROP and stage 5 representing a total retinal detachment.

The international classification of retinopathy of prematurity (ROP). The stage of ROP is determined by the location, extent, and stage of disease according to the position of the advancing waves of retinal vessels. LE, left eye; RE, right eye.

The management of ROP begins with a systematic program of eye examinations at well-defined times in infants judged to be at risk for developing ROP. The frequency of examinations is dependent on the findings and progression of the disease. Infants with a birth weight of less than 1500 g are 1st examined 4-6 weeks after birth. Follow-up examinations are performed at regular intervals until the retina is fully vascularized. If treatment is required laser ablation of immature retina or intravitreal injections of anti-vascular endothelial growth factor (VEGF) medication such as bevacizumab are options. The choice of treatment depends on the zone and the rate of progression of disease. Insulin-like growth factor (IGF)-1 supplementation or alterations may be a means to prevent ROP in the future.