Ординатура / Офтальмология / Английские материалы / Pediatric Ophthalmology for Primary Care 3rd edition_Wright, Farzavandi_2008

Figure 2 5.

Pathology of amblyopia (striate cortex). Histopathology of monkey striate cortex (visual cortex). A, Well-defined cortex columns are seen in the normal specimen. B, Cortex columns are underdeveloped in specimen from the amblyopic monkey. From Horton JC, Hocking DR. Timing of the critical period for plasticity of ocular dominance columns in macaque striate cortex. J Neurosci. 1997;17:3684–3709.

Table 2-3. Classification of Amblyopia

A.Strabismic Amblyopia

1.Congenital esotropia

2.Acquired esotropia in childhood

B.Monocular Blurred Image

1.Anisometropia (difference in refractive error)

a.Hypermetropic

b.Myopic

c.Astigmatic

2.Media opacity

a.Unilateral cataract

b.Unilateral corneal opacity (eg, Peter anomaly)

c.Unilateral vitreous hemorrhage or vitreous opacity

C.Bilateral Blurred Image

1.Refractive error

a.Bilateral high hypermetropia

b.Astigmatism

2.Media opacity

a.Bilateral congenital cataracts

b.Bilateral corneal opacities (eg, Peter anomaly)

c.Bilateral vitreous opacity (hemorrhages)

better visual acuity reading single optotypes, which are figures or letters that measure visual acuity, than reading multiple optotypes in a row (linear opto types). Often, patients with amblyopia will perform 1 or 2 Snellen lines’ bet ter when presented with single optotypes versus linear optotypes. Eccentric fixation is a characteristic of severe amblyopia. Patients with mild amblyopia (20/40–20/100) fixate so close to the fovea, they appear to fixate centrally. Severe amblyopia, usually 20/200 to count fingers, is associated with para foveal viewing or eccentric fixation. When the good eye is occluded and the amblyopic eye is forced to view, patients with eccentric fixation demonstrate roaming eye movements and cannot visually lock on the target. The pres ence of eccentric fixation is a clinical sign of severe amblyopia and has a poor visual prognosis. Bilateral severe amblyopia (eg, from untreated bilat eral congenital cataracts) can cause sensory nystagmus and searching eye movements (see Bilateral Blurred Image Amblyopia on page 32). Unilateral amblyopia is not associated with manifest nystagmus.

Even though amblyopia disrupts binocular vision, amblyopia does not interfere with learning or reading as long as there is a good eye.

Amblyogenic Period

Children are most susceptible to amblyopia during the critical period of visual development, which is the first few months of life. Stimulation by a severely blurred retinal image during the critical period of visual develop ment can result in dense, often irreversible amblyopia. This is why visually significant congenital cataracts must be operated and visually rehabilitated within the first few weeks of life for best visual results. Amblyopia can occur, however, in older children. Acquired strabismus, or an acquired media opac ity such as a cataract, can cause some amblyopia, even at up to 7 or 8 years of age, albeit of lesser severity. Table 2 4 lists the major periods of visual plas ticity and susceptibility to amblyopia.

Table 2-4. Visual Development and Amblyopia

Treatment of Amblyopia

The treatment of amblyopia depends on the type but in general the first step is to provide a clear retinal image. Refractive errors are corrected with spectacle or contact lenses and visually significant opacities such as cataracts must be surgically removed. The next step is to correct ocular dominance in patients with unilateral amblyopia by occluding the good eye. Occluding the dominant eye stimulates development of visual areas in the brain that cor relate to the amblyopic eye. Young children, especially younger than 1 year, may develop occlusion amblyopia of the good eye from over patching. Part time occlusion is usually suggested for children younger than 1 year.

The earlier the intervention, the better the prognosis for amblyopia. Children with visually significant congenital cataracts are best treated during the first weeks of life, while delaying treatment past 2 months of age carries a relatively poor visual prognosis. Patients with less severe forms of amblyopia, such as anisometropic amblyopia (difference in refractive error), will have a better prognosis even when treated between 3 and 5 years of age. After 8 to 9 years of age, however, the chance of significantly improving the ambly opia is small. Even so, patients who present late with amblyopia are often treated. Even older children with presumed congenital cataracts may show some visual improvement with aggressive amblyopia management. Adults with amblyopia, who lose vision in their good eye, will show some limited improvement of vision in the amblyopic eye.

Atropine Drops for the Treatment of Amblyopia (Penalization)

Topical atropine 1% drops dilate the pupil and paralyze accommodation (focusing). Atropine drops can be used in patients with amblyopia to blur the vision of the good eye to force use of the amblyopic eye; this is called penalization. The blurring effect of atropine is greatest for near vision and eyes that are hyperopic. Atropine will not significantly blur the vision if the eye is myopic or if there is no refractive error. Thus, atropine penalization will not work unless the good eye is significantly hyperopic. Most patients with amblyopia are best treated with occlusion of the good eye, but in selected cases atropine may be useful.

Clinical Types of Amblyopia

Strabismic Amblyopia

This is amblyopia associated with strabismus where there is strong fixation preference for the dominant eye and constant suppression of the nondomi nant eye, which is always deviated. Patients who alternate fixation do not have amblyopia because each eye receives visual stimulation. Strabismic amblyopia is treated by occluding the dominant eye to stimulate cortical development of the brain centers of the amblyopic eye. Strabismic patients may have a refractive error and if present, should be treated (eg, glasses) in conjunction with occlusion therapy. The end point to occlusion therapy is alternating fixation or when the patient holds fixation well with the ambly opic eye. Occlusion therapy rarely corrects the strabismus and its role is to improve vision. Strabismus surgery corrects the eye alignment but does not improve the amblyopia. Most patients with strabismic amblyopia will require both occlusion therapy and strabismus surgery. Vision therapy and eye exer cises have no therapeutic role.

Anisometropic Amblyopia

This is one of the most common types of amblyopia and is caused by a dif ference in refractive errors that results in a monocular image blur. Most patients with anisometropic amblyopia have straight eyes that appear nor mal, so the only way to identify these patients is through vision screening.

Stereo acuity testing has had limited value in screening for anisometropic amblyopia because most patients have relatively good stereopsis (between 70 and 3,000 seconds’ arc). Myopic anisometropia is generally less ambly ogenic than hypermetropic anisometropia. As little as +1.00 hypermetropic anisometropia and 2.00 myopic anisometropia can be associated with amblyopia. Astigmatic anisometropic amblyopia does not occur unless there is a unilateral astigmatism greater than 1.50 diopters. Myopic anisometropic amblyopia is often amenable to treatment even in late childhood, whereas hypermetropic amblyopia is often difficult to treat past 4 or 5 years of age.

This is probably because high myopia is usually acquired after the critical period of visual development, and the more myopic eye is in focus for near objects (a baby’s world is up close). In contrast, patients with hypermetropic

anisometropia always use the less hypermetropic eye because it requires less accommodative effort. This causes the more hypermetropic eye to become blurred and be constantly suppressed. The treatment is to prescribe full opti cal correction via glasses or contact lenses. If vision does not improve with optical correction, part time occlusion of the dominant eye is indicated. In many cases early treatment with glasses alone will suffice.

Monocular Media Opacity Amblyopia

A media opacity is an obstruction or clouding of otherwise clear structures of the eye, including the cornea, lens, and vitreous. A significant opacity in the visual axis will distort the retinal image and can cause amblyopia in chil dren. If the opacity is dense and present at birth it should be cleared as early as possible, even in the first week of life, for best visual results. The visual prognosis is guarded in most cases when the surgery is performed after age 2 months. Infants with dense congenital cataracts are best treated during the first weeks of life. Treatment is to remove the opacity, correct the refractive error, and occlude the dominant eye to stimulate cortical development of the amblyopic eye.

Bilateral Blurred Image Amblyopia

Bilateral amblyopia occurs when there is bilateral, symmetrical retinal image blur and no strabismus. Clinically, the effects of pure image blur are seen

in cases of bilateral high hypermetropia (>6.00 diopters) or bilateral sym metrical astigmatism (>3.00 diopters) or with bilateral severe ocular opaci ties such as dense bilateral congenital cataracts. The severity of the visual deficit depends on the extent of the image distortion. If severe image blur occurs during the neonatal period so that essentially no pattern stimulation is provided, extremely poor vision and sensory nystagmus develops. Bilateral amblyopia and sensory nystagmus will occur in cases of dense bilateral con genital opacities. Other causes of sensory nystagmus include organic causes of congenital blindness such as macular or optic nerve pathology (bilateral choroidal colobomas or bilateral optic nerve hypoplasia). Sensory nystagmus does not occur with cortical blindness because extra striate visual pathways anterior to the occipital cortex supply the fixation reflex. Acquired opacities after 6 to 12 months of age usually do not cause sensory nystagmus because the motor component of fixation has already been established. The presence of sensory nystagmus indicates a poor prognosis. The treatment of bilateral

symmetrical amblyopia is to clear the retinal image. Occlusion therapy is not needed as there is no strabismus, no cortical suppression, and the vision is equally reduced. Children with large refractive errors such as high hyperme tropia and large astigmatism will require full correction.

Bibliography

1.Horton JC, Hocking DR. Timing of the critical period for plasticity of ocular dominance columns in macaque striate cortex. J Neurosci.1997;17:3684–3709

2.Sondhi N, Archer SM, Helveston EM. Development of normal ocular alignment. J Pediatr Ophthalmol Strabismus. 1988;25:210–211

3.von Noorden GK, Crawford ML, Levacy RA. The lateral geniculate nucleus in human anisometropic amblyopia. Invest Ophthalmol Vis Sci. 1983;24:788–790

4.Wiesel TN, Hubel DH. Ordered arrangement of orientation columns in monkeys lacking visual experience. J Comp Neurol. 1974;158:307–318

5.Wright KW, Matsumoto E, Edelman PM. Binocular fusion and stereopsis associated with early surgery for monocular congenital cataracts. Arch Ophthalmol. 1992;110:1607–1609

Chapter 3

Ocular Examination

and Vision Screening

Early detection and treatment of pediatric ocular disease is critical. Dis eases such as congenital cataracts, retinoblastoma, and congenital glaucoma require early treatment during infancy. Delay in diagnosis may result in irre versible vision loss and in the case of retinoblastoma, potentially death. It is therefore imperative to provide effective vision screening of all children from newborns to older children. Including the newborn nursery, every well baby check should include a red reflex test.

Vision Screening

Vision screening examinations should start at birth and continue as part of routine checkups. The acronym I ARM (Inspection, Acuity, Red reflex, Motility) can be a helpful reminder of the essential parts of a pediatric screening examination. Table 3 1 summarizes the I ARM screening eye

Table 3-1. Screening Eye Examination: I-ARM

examination for neonates, babies, and children. The most important test for the newborn is the red reflex test. If an abnormal red reflex is present, an immediate referral to an ophthalmologist is required. Infant screening examinations take less than a minute, but these brief examinations are quite powerful and if performed properly, can detect the vast majority of eye pathology including the important diagnoses mentioned previously. Equip ment needed for the I ARM screening examination includes the direct oph thalmoscope and a visual chart such as the E game, Wright figures (Figure 3 1), or Snellen letters (for literate children).

Figure 3 1.

Wright figures for visual assessment of preverbal children. The figures are made up of white and black bars, with the overall footprint approximately 2 times the size of a normal Snellen letter, but the resolution equivalent to Snellen letters. (Copyright © 2000 Kenneth W. Wright, MD)

Inspection

Simple inspection of the eyes and lids for abnormalities of symmetry can be very helpful. Figure 3 2 shows a patient with a left orbital dermoid cyst. At first glance, the patient appears to have no gross abnormality. However, as one compares the right eye to the left eye, it becomes obvious that this patient has a left ptosis and the left eye is displaced down. Also, inspect for a face turn or head tilt because this can be a compensatory mecha nism to reduce strabismus or dampen nystagmus. If the onset of an ocular

Figure 3 2.

Inspection. Look for symmetry. Thirteen-year-old boy. Note the left ptosis and downward displacement of the left eye. A dermoid cyst located on the roof of the left orbit is causing the inferior displacement of the left eye.

abnormality is in question, look at family photographs for documentation. Table 3 2 outlines the 3 major categories for ocular inspection.

Table 3-2. Ocular Inspection

1.Symmetry—compare fellow eyes, look at pupils, eyelids, and lid fissures.

2.Check for face turn or head tilt.

3.Ocular irritation (pink eye, squinting).

Acuity

Visual acuity testing for preverbal and verbal children is described in the following paragraphs. Table 3 3 shows the normal visual acuity expected at specific ages and the guidelines for referral.

Preverbal Children

It is important to try to obtain a visual acuity on every child, even infants. From birth to approximately 2 months of age, there may only be sporadic fix and follow. Between 2 and 6 months of age, patients should have the ability to fix and follow on a small toy or the human face. Cover one eye and move a target (eg, your face, a toy) right, left, up, and down to observe if the eyes will accurately follow the motion. It is important to test each eye individually because with both eyes open, the eyes will track together even if one eye is