Ophthalmic System and Vision

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Preterm infants are more likely than term infants to have significant abnormalities of all parts of the visual system, leading to reduced vision (Repka, 2002). The optic vesicles that will become the eyes form during the fifth and sixth weeks after conception (Back, 2005). The eyeball is well formed by the lower limit of viability (22 to 25 weeks of gestation). However, a pupillary membrane covers the anterior vascular capsule of the lens and gradually disappears between 27 and 34 weeks of gestation (Hittner et al., 1977). The retina is a vascular layer in the back of the eye that translates light into electrical messages to the brain. The retina is the one of the last organs to be vascularized in the fetus (Madan and Good, 2005). Blood vessel-forming cells originate near the optic disc (where the optic nerve enters the retina) from spindle cell precursors at 16 weeks of gestation and gradually spread across the surface of the retina, from the center to the periphery. Vessels cover only 70 percent of the retina by 27 weeks of gestation, but in most cases the retina is completely vascularized to the nasal side by 36 weeks of gestation and to the temporal side by 40 weeks of gestation (Palmer et al., 1991).

The visual system functions very early, with the preterm infant blinking in response to bright light by 23 to 25 weeks of gestation and with papillary constriction in response to light by 29 to 30 of weeks gestation (Allen and Capute 1986; Robinson, 1966). By 30 to 32 weeks of postmenstrual age, the preterm infant begins to differentiate visual patterns (Dubowitz 1979; Dubowitz et al., 1980; Hack et al., 1976, 1981; Morante et al., 1982). Visual acuity progressively improves with increasing postmenstrual age. The full-term neonate sees shapes (approximate visual acuity of 20/150) and colors and has a fixed focal length of 8 in. (anything closer or farther away becomes more blurry).

ROP is the most common eye abnormality in preterm infants. It is a neovascular retinal disorder, and its incidence increases with decreasing gestational age and decreasing birth weight. It is multifactorial in etiology, with the primary determinant being immaturity with an avascular retina (Madan et al., 2005). Environmental factors, including hypoxia, hyperoxia, variations in blood pressure, sepsis, and acidosis, may injure the endothelia (the cells that line) of the immature retinal blood vessels. The retina then enters a quiescent phase for days to weeks and forms a pathognomonic ridge-like structure of mesenchymal cells between the vascularized and the avascular regions of the retina by 33 to 34 weeks of postmenstrual age. In some infants, this ridge regresses, and the remaining retina is vascularized. In other infants, abnormal blood vessels proliferate from this ridge; and progressive disease can cause exudation, hemorrhage, and fibrosis, with subsequent cicatrical scarring or retinal detachment (i.e., the retina is pulled off the back of the eye). The presence of plus disease, in which dilated and tortuous blood vessels occur in the posterior pole of the eye, is especially ominous for an adverse visual outcome.

ROP occurs in 16 to 84 percent of infants born with gestational ages of less than 28 weeks, 90 percent of infants with birth weights of less than 500 or 750 g, and 42 to 47 percent of infants with birth weights of less than 1,000 or 1,500 grams (CRPCG, 1988, 1994; Fledelius and Greisen, 1993; Gibson et al., 1990; Gilbert et al., 1996; Lee et al., 2000; Lefebvre et al., 1996; Lucey et al., 2004; Mikkola et al., 2005; Repka 2002). Fortunately, severe ROP requiring therapy is less common, occurring in 14 to 40 percent of infants with gestational ages of less than 26 weeks, 10 percent of infants with gestational ages of less than 28 weeks, 16 percent of infants with birth weights of less than 750 grams, and 2 to 11 percent of infants with birth weights of less than 1,000 or 1,500 grams (Coats et al., 2000; Costeloe et al., 2000; Hintz et al., 2005; Ho and Saigal, 2005; Lee et al., 2000; Mikkola et al., 2005; Palmer et al., 1991). ROP resolves without significant visual loss in the majority (80 percent) of infants (CRPCG, 1988; O'Connor et al., 2002). Repka and colleagues (2000) found that involution occurred in 90 percent of infants with ROP by 44 weeks of postmenstrual age.

Treatments have improved the visual outcomes for children with severe ROP (i.e., threshold or plus disease, stages 3 and 4). The ablation of abnormal peripheral vessels with cryotherapy (in earlier studies) and laser therapy (in the last decade) have lead to favorable visual outcomes in at least 75 percent of infants with severe ROP (CRPCG, 1998, 2001; Repka, 2002; Shalev et al., 2001; Vander et al., 1997). Continuing improvements in treatments and more timely treatments of severe ROP have served to reduce the proportion of children with severe visual impairment or blindness from 3 to 7 percent down to 1.1 percent in children with birth weights of less than 1,000 or 1,500 grams (Doyle et al., 2005; Hintz et al., 2005; Tudehope et al., 1995; Wilson-Costello et al., 2005) (see Chapter 11). Severe visual impairment or blindness occurs in 0.4 percent of children with gestational ages of 27 to 32 weeks, 1 to 2 percent of children with gestational ages of less than 26 or 27 weeks, 4 percent of children with gestational ages of 24 weeks, and 8 percent of children with gestational ages of less than 24 weeks (Marlow et al., 2005; Vohr et al., 2005).

Timely diagnosis and the prompt treatment of ROP are essential for improving visual outcomes. Screening ophthalmologic examinations require an experienced examiner and careful examination of the retina to the periphery with an indirect ophthalmoscope and lid speculum after dilation of the pupils. Revised guidelines for screening preterm infants for ROP have recently been published, with recommendations for which infants should be screened (infants with birth weights of less than 1,500 grams or gestational ages of less than 32 weeks or selected other pre-term infants with an unstable clinical course), the timing and frequency of screening examinations, and the indications for treatment (Section on Ophthalmology, 2006 Pediatrics 117:572).

Although visual outcomes have improved, ROP continues to be a major problem, especially in the most immature infants. The primary method of prevention is to prevent preterm births. Prevention of wild swings in blood pressure, blood oxygen and carbon dioxide levels, and acidosis has been recommended (Madan et al., 2005). There is much interest in the role of oxygen in ROP, but the optimal blood oxygen levels and oxygen saturation levels remains controversial (Saugstad, 2005; STOP-ROP, 2000; Tin 2002a,b). Although the intravenous administration of high doses of vitamin E appears to reduce the incidence of severe ROP and blindness in infants with birth weights of less than 1,500 grams, it also appears to increase the incidence of sepsis and IVH (Brion et al., 2003).

Other ophthalmologic complications of prematurity include refractive disorders (especially myopia), strabismus (i.e., ocular misalignment), amblyopia (i.e., visual loss associated with reduced development of the visual cortex), optic nerve atrophy, cataracts, and cortical visual impairment (Repka, 2002) (see Chapter 11). Late ophthalmologic problems include angle closure glaucoma (i.e., increased pressure in the eye), retinal detachment, and phthisis (i.e., shrinkage and disorganization of the eye severely affected by ROP); but fortunately, these are rare. Severe threshold ROP is associated with other complications of prematurity, including BPD/CLD and IVH.

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