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

innervated by sympathetic nerves (Figure 1 13). In Horner syndrome (sym pathetic nerve palsy causing miosis, anhydrosis, and ptosis), the ptosis is secondary to denervation of the Müller muscle.

Tarsal plates are firm, dense, fibrous, connective tissue (not cartilage) structures that provide strong structural integrity of the upper and lower eyelids. The inside surface of the tarsal plate is lined with conjunctiva (tarsal conjunctiva). Within the tarsal plate are specialized sebaceous glands called meibomian glands. Blockage of the orifice of the meibomian gland can result in inspissation of the sebaceous material with secondary inflammation and is referred to as a stye (chalazion) (see Chapter 13). Meibomian glands are present in the upper and lower lids, and the orifices can be seen as they exit at the lid margins (Figure 1 13). Glands that are located anterior to

the tarsal plate include the glands of Zeis and Moll. An infection of the Zeis gland results in an external hordeolum. The upper and lower eyelids join nasally to form the medial canthal area and laterally to form the lateral can thal area.

Lacrimal System

Aqueous tears are secreted from the lacrimal gland soon after birth, at approximately 2 to 4 weeks of age. Tears exit the lacrimal gland, then course across the surface of the eye and exit through the upper and lower puncta located in the nasal aspect of the upper and lower lids (Figure 1 14). The puncta are connected to canaliculi that course through the upper and lower lids to come together as they enter the nasolacrimal sac. The nasolacrimal sac extends inferiorly to become the nasolacrimal duct. The nasolacrimal duct exits in the posterior aspect of the nose under the inferior turbinate. A series of small valves are present in the duct, with the most important being Hasner valve 8 at the distal aspect of the lacrimal duct. This valve is usually closed at birth but opens during the first few weeks of life. If the valve does not open, it will obstruct normal tear flow and will result in a nasolacrimal duct obstruction and infantile tearing (see Chapter 12).

The medial canthal area is the nasal attachment of the lid to the nose. It is important because the punctum and canaliculi are located here (Figure 1 15). A lid laceration in the medial canthal area can disrupt the canaliculi and result in tearing. Canalicular tears must be carefully sutured to avoid this complication.

Bibliography

1.Gordon RA, Donzis PB. Refractive development of the human eye. Arch Ophthalmol. 1985;103:785–789

2.Pierce EA, Foley ED, Smith LE. Regulation of vascular endothelial growth factor by oxygen in a model of retinopathy of prematurity. Arch Ophthalmol. 1996;114:1219–1228

3.Chow LC, Wright KW, Sola A, and the CSMC Oxygen Administration Study Group. Can changes in clinical practice decrease the incidence of severe retinopathy of prematurity in very low birth weight infants? Pediatrics. 2003;111:339–345

4.Wright KW, Sami D, Thompson LS, Ramanathan R, Joseph R, Farzavandi S. A physi ologic reduced oxygen protocol decreases the incidence of threshold retinopathy of pre maturity. Trans Am Ophthalmol Soc. 2006;104:78–84. Available at: www.aosonline.org/ xactions/2006/1545 6110_v104_p078.pdf. Accessed June 5, 2007

Chapter 2

Amblyopia and

Strabismus

Visual Development

Normal clear stereoscopic vision is ultimately the product of complex brain processing. The cornea and lens focus high resolution images onto the retina, and the retina in turn stimulates visual areas in the brain where the information is processed into what we perceive as sight. Three dimensional stereoscopic vision is derived from the brain receiving retinal input from each eye and integrating the images into a single image with depth. The cortical integration of 2 images into a single binocular stereoscopic image is called binocular fusion. For the images from each eye to be fused they must be similar, and this requires precise eye alignment and equal retinal image clarity. The primary brain processing centers are the lateral geniculate nucleus (LGN) and striate cortex. These brain centers process information from more than 1 million retinal axons from each eye. So excellent high resolution stereoscopic vision requires accurate eye alignment, clear retinal images, and highly developed visual centers in the brain.

At birth, our visual acuity is quite poor, in the range of 20/200 to 20/800 (legal blindness). This poor vision is primarily due to immaturity of the visual centers in the brain. The immature brain rapidly develops in response to visual stimulation from the retina. Normal visual brain development requires stimulation with clear, in focus, high resolution retinal images. A blurred retinal image stimulates abnormal development in visual centers

in the brain. The critical period of visual development is the first 2 to 4 months, but visual development continues up to 7 to 8 years of age. This critical period of visual development is when vision is most rapidly improv ing and when the visual system is most susceptible to the adverse effects of a blurred image. Disruption of normal brain development by stimulation with a blurred retinal image results in poor vision and is termed amblyopia (see Amblyopia on page 26). Figure 2 1 shows the exponential improvement in vision in the first months of life and the continued development of vision over many years.

Months

Figure 2 1.

Curve represents visual acuity development, with age on the horizontal axis and Snellen acuity on the vertical axis. Note the exponential improvement in visual acuity during the critical period of visual development (birth to 4 months).

Another important requirement for normal development of visual cen ters in the brain is accurate alignment of the eyes. Proper eye alignment pro vides each retina with the same image, which allows cortical processing of the images into a single binocular stereoscopic image. Equal image clarity is also important to facilitate the development of binocular stereoscopic vision. Animal studies by Wiesel and Hubel (Nobel prize–winning laureates) have shown that binocular cortical connections are present from birth. Normally, around 70% of visual cortex neurons are binocular and respond to visual stimulation of both eyes. The minority of visual cortical cells are monocular, responding to only one eye. Even though binocular anatomy is present at birth, appropriate visual input from each eye is necessary to refine and main tain these binocular neural connections. The presence of strabismus (ie, ocular misalignment) or a unilateral blurred retinal image (eg, a congenital cataract or unilateral refractive error [need for glasses for one eye]) will dis rupt normal binocular visual development and cause a partial or complete loss of binocular fusion. Early treatment of strabismus, refractive errors, and ocular media opacities is critical to achieving binocular vision. A summary of the requirements for normal visual development are listed in Table 2 1.

Table 2-1. Requirements for Normal Visual Development

Clear retinal image

Equal image clarity

Proper eye alignment (no strabismus)

Visual Developmental Milestones

As seen in Figure 2 1, vision at birth is extremely poor but rapidly improves over time. Newborns show little in the way of visual behavior. Eye move ments are inaccurate, fast, and jerky. Over the next few months normal infants will start showing visual attentiveness and fixate and follow faces. By 4 to 6 months of age there should be central fixation with accurate, smooth pursuit eye moments to moving targets. A key developmental milestone for normal infants is the ability to accurately fixate on and follow small objects by 6 months of age. Normal infants may occasionally show delayed visual maturation; however, poor fixation past 6 months of age is usually pathologic and an ophthalmologic consultation is indicated. Visual acuity measured by figure optotypes can usually be obtained by 3 years of age. Table 2 2 outlines the visual developmental milestones from birth to visual maturity.

Eye alignment also changes after birth. At birth approximately 70% of infants will show a small variable exotropia, 30% have essentially straight eyes (orthotropia), and esotropia is rare. By 2 to 3 months of age, the majority of normally developing infants have established proper alignment. The persistence of a strabismus after 2 to 3 months of age may

Figure 2 3.

Infant with congenital esotropia and alternating fixation. In figure A, patient is fixing right eye. In figure B, patient has switched fixation to the left eye. Alternating fixation indicates equal visual preference, no amblyopia.

is constantly deviated. These patients will constantly suppress visual areas in the brain connected to the deviated eye. Constant cortical suppression of one eye in young children will degrade visual cortical connections and reduce visual acuity of the deviated eye. This is clinically termed strabismic amblyopia. Cortical suppression also degrades binocular fusion. Thus con stant strabismus in young children with strong fixation preference for the dominant eye results in loss of binocular fusion and vision of the deviated

eye (ie, strabismic amblyopia). Patients with alternating strabismus do not get amblyopia and develop equal vision as cortical visual areas from both eyes receive visual stimulation. Alternating strabismus, however, disrupts binocular fusion and stereoscopic vision.

Amblyopia

Amblyopia occurs in approximately 2% of the general population and is the most common cause of decreased vision in childhood. The term amblyopia is derived from the Greek language and means dull vision: amblys = dull, ops = eye. Generally speaking, amblyopia can refer to poor vision from any cause but in this text and in most medical literature, amblyopia now refers to poor vision caused by abnormal visual development secondary to abnor mal visual stimulation. The pathophysiology of amblyopia was discov ered by Wiesel and Hubel in the late 1960s. Their basic scientific research showed that in infant animals visual stimulation with a blurred retinal image resulted in loss of nuclear cells in the LGN, the first relay nucleus of the visual system (Figure 2 4). Normally, there are 6 nuclear layers of the

LGN—3 layers corresponding to the right eye and 3 layers corresponding to the left eye. A unilateral blurred retinal image destroys the 3 nuclear layers corresponding to the eye with the blurred image. Because of the increased visual stimulation from the clear image, the 3 layers corresponding to the clear image eye are darker stained and larger than normal (Figure 2 4).

Ocular dominance columns in the visual cortex are also damaged by a uni lateral blurred image during early development as shown in Figure 2 5. Thus, poor vision associated with amblyopia is caused by anatomic changes in the visual areas of the brain in response to abnormal visual stimulation during early visual development.

Clinically abnormal visual stimulation can be caused by a unilateral or bilateral blurred image (cataract or severe refractive error) or strabismus with strong preference for one eye and constant suppression of the non preferred eye. See Table 2 3 for a classification of amblyopia based on the type of abnormal visual stimulus.

Figure 2 4.

Pathology of amblyopia (lateral geniculate nucleus [LGN]). A, Cross section of LGN from a normal monkey. B and C, Vs amblyopic monkey caused by a unilateral blurred image. Note that

the normal LGN, A has 6 nuclear layers (darkly stained cell layer) and the amblyopic LGN, B and C only have 3 layers and are thicker than normal. From Wiesel TN, Hubel DH. Ordered arrangement of orientation columns in monkeys lacking visual experience. J Comp Neurol. 1974; 158:307–318.

Amblyopic Vision

For practical purposes, amblyopia is defined as at least 2 Snellen lines’ dif ference in visual acuity between the eyes. Amblyopia is truly a spectrum of visual loss, ranging from missing a few letters on the 20/20 line to hand motion vision. Mild image blur (ie, blur associated with a mild refractive error) causes mild amblyopia and allows for the development of some degree of binocular fusion and stereopsis (ie, peripheral fusion). A severely blurred image during infancy (ie, unilateral congenital cataract or corneal opacity), however, can result in profound vision loss, no binocular fusion, and strabismus.

The visual deficit associated with amblyopia has certain unique charac teristics, including the crowding phenomenon and eccentric fixation. The crowding phenomenon relates to the fact that patients with amblyopia have