The retinal is a multilayered structure that lines the inside of the back of the eye. Light rays are focused on the retina by the cornea and lens. When they reach the retina, they cause chemical reactions in the deepest layer of the retina, the rods and cones (photoreceptors). This creates an impulse that is transmitted through the middle layer of the retina (the bipolar cells) to the inner portion (the ganglion cells) (Figure 31–1). The ganglion cells then travel and coalesce in the posterior portion of the eye to form to the optic nerve, which transmits the impulses to the brain. The retina is analogous to the film in a camera, in that it senses and changes in reaction to light.
The cones are responsible for discriminating fine visual detail and color. They are concentrated in the macula, which is the central portion of the retina between the vascular arcades. At the center of the macular is the fovea, visible as a focal area of increased pigment (Figure 31–2). This is the area of highest visual discrimination. It is used when reading or watching objects. The rods are more sensitive to dim light. They are concentrated in the peripheral retina. They are primarily responsible for peripheral vision.
On a molecular level, the perception of light is based on chemical reactions that occur in the outer layer of the photoreceptors. In the rods, photons are absorbed by rhodopsin molecules. This causes a reaction that results in release of glutamate (a neurotransmitter), which initiates a sequence of cellular connections that ultimately stimulates the ganglion cells and is transmitted to the brain via the optic nerve. The cones have a similar response, but there are 3 separate opsin molecules that respond to different wavelengths of light. The apparent color of an object results from central processing of the relative inputs from these 3 types of cones.
The retina is highly metabolically active. The rods and cones are nourished by the retinal pigment epithelium, which lies between the retina and the choroidal blood vessels. The inner portion of the retina receives its blood supply from the blood vessels that line the retina. The retina itself is transparent. The red reflex that is visible during ophthalmoscopy and in photographs results from light reflecting off the blood supply within the choroid.
The vitreous is the normally clear substance that fills the posterior portion of the eye between the retina and the lens.
The retina develops from neuroectodermal cells in the inner layer of the optic cup. This process begins during the first month of development. The outer layer of the cup becomes the retinal pigment epithelium, and the inner layer develops into the neurosensory retina. Within the retina itself, the inner ganglion cells develop first. Beginning at the sixth week of gestation, they migrate to the optic stalk to form the optic nerve. The rods and cones develop from the outer layer of neuroblastic tissue, with the outer segments of the cones differentiating at 5 months and the rods at 7 months. Overall, the differentiation of the retina begins in the central, posterior portion of the retina, then spreads to the peripheral portion. The process of retinal cell maturation continues after birth. This is why retinoblastoma tumors, which affect developing retinal cells, form in the posterior retina in newborns, and in the peripheral retina as children age.
The vitreous goes through 3 stages of development. The primary vitreous forms between the first and second months of gestation. It consists of vascular and mesenchymal cells between the lens vesicle and inner surface of the optic cup. The secondary vitreous develops between the second and third month. It is an avascular structure that gradually replaces the primary vitreous. The tertiary vitreous forms during the third and fourth month by condensation of fibrils in the area of the future lens zonules. The vitreous is supplied by the hyaloid artery during development. This structure normally regresses by the time of birth.
Retinal disorders are uncommon in children. Many are untreatable or only partially treatable, and therefore they are an important cause of visual morbidity. Several retinal abnormalities have specific findings that help in establishing a diagnosis (Table 31–1). These are described later the chapter.
| • Egg yolk—Best disease |
| • Lacunae—Aicardi syndrome |
| • Cherry-red spot—metabolic diseases |
| • Retinal necrosis and hemorrhage—Cytomegalovirus |
| • Astrocytic hamartoma—Tuberous sclerosis |
The degree to which retinal abnormalities affect vision varies depending on the disorder, but many cause marked visual loss. Similar to most other ocular problems in children, unilateral abnormalities may be difficult to detect, because the children function well if the vision is good in the other eye. The vision loss may not be noticed until the child has a vision test or if the child occludes the normal eye (e.g., when rubbing the eye). Strabismus may develop in eyes with unilateral vision loss, and this may be the first sign of a problem. In some lesions, such as retinoblastoma or large colobomas, an abnormality of the red reflex may be noted. If the vision loss is bilateral, decreased vision will usually be evident. In infants, significant bilateral vision loss initially manifests as nystagmus that develops within the first 2 to 3 months of life.
Similar to iris colobomas, retinal colobomas occur as a result of incomplete closure of the embryonic fissure during formation of the eye. The fissure normally comes together in the inferonasal quadrant of the eye, and this is the location of most retinal colobomas. They appear as white defects, because the sclera is directly visible due to the absence of normal retina and retinal pigment epithelium. They can vary from small focal defects inferior to the optic nerve to large defects that encompass the optic nerve and posterior retina (Figure 31–3).
The visual prognosis for retinal colobomas is primarily dependent on whether the fovea is involved.1 If a clear foveal reflex is present, most patients will have good vision, even if the remainder of the coloboma is large. Conversely, even small defects may have significant effects on vision if they are located directly in the fovea.
Colobomas may occur as isolated findings or may be associated with systemic disorders, such as CHARGE (Coloboma, Heart defects, Atresia choanae, Retardation of growth, Genitourinary anomalies, and Ear anomalies) syndrome. Colobomas are not progressive. Retinal detachments may occur at the edge of the coloboma. These are difficult to treat.
Congenital retinal folds are rare anomalies in which a fold of retinal tissue extends from the optic nerve to the retinal periphery, usually in the inferotemporal portion of the eye (Figure 31–4). These may be inherited in an autosomal dominant fashion. They are usually associated with marked vision loss, and are not amenable to treatment.
Persistent fetal vasculature (PFV) (previously known as persistent hyperplastic primary vitreous) results from incomplete regression of the hyaloid blood vessel during embryological development. The most common ocular complication of PFV is a cataract (see Chapter 30). A stalk of tissue is often visible extending to the optic nerve and retina (Figure 31–5). If the stalk causes significant distortion of the retinal tissue, visual loss may occur. The cataract in most patients with PFV can be successfully removed, and the presence of retinal defects is therefore usually the most important factor in determining the patient’s visual prognosis.
Aicardi syndrome is an X-linked disorder that has very distinctive retinal changes. It typically occurs only in females, as it is lethal in males. There is often a history of maternal spontaneous abortions because of this. Systemic findings include absence of the corpus callosum, developmental delay, and seizures. Retinal findings consist of multiple round depigmented areas (lacunae), primarily in the posterior portion of the retina (Figure 31–6).
Patients with albinism have a number of ocular abnormalities, but the primary vision problems result from underdevelopment of the fovea. Albinism may be associated with systemic manifestations of decreased pigment (oculocutaneous albinism [OCA]), or the changes may be isolated to the eye (ocular albinism). Patients usually present in the neonatal period with decreased vision and nystagmus. Patients with OCA are easily identified by their decreased skin and hair pigment. Patients with ocular albinism often have slightly decreased, but not markedly abnormal, systemic pigment. Ocular examination findings in both forms of albinism consist of transillumination defects of the iris (Figure 31–7), decreased peripheral retinal pigmentation, and hypoplasia of the fovea (Figure 31–8).
The diagnosis of albinism can be established by genetic testing and a characteristic abnormality of visual evoked potentials. In normal patients, approximately half of the optic nerve fibers connect to each occipital lobe. In patients with albinism, a greater-than-normal number of fibers from each eye decussate and connect to the contralateral occipital lobe (Figure 2–9). Visual acuity in patients with albinism usually is in the 20/100 to 20/200 range.
The fovea is the structure at the center of the posterior retina that is responsible for fine visual discrimination. It is usually visualized as an area of increased pigment with a bright light reflex centered between the retinal vascular arcades. The retinal blood vessels arc around the fovea in an organized fashion, and the fovea itself is avascular. In foveal hypoplasia, the normal foveal reflex is missing and the retinal blood vessels are not organized around the fovea.
The abnormal foveal anatomy causes decreased vision in patients with foveal hypoplasia. The most common disorder that causes foveal hypoplasia is ocular albinism (see above). It is also a feature of aniridia (see Chapter 29) (Figure 31–9). Rarely it may occur as an isolated abnormality. If this is the case, it is important to recognize foveal hypoplasia as the cause of decreased vision. Because the examination findings are fairly subtle, affected patients may be suspected of having amblyopia or factitious visual loss unless the retina is carefully examined.2
ROP is a disorder of retinal vascular development that occurs in premature infants. Normal retinal vascular development begins at 16 weeks of gestation. The vessels start in the posterior portion of the eye at the optic nerve and grow toward the retinal periphery. When infants are born prematurely, the blood vessels stop growing for a period. When growth resumes, most children develop normal vessels. In some infants, however, abnormal neovascular tissue develops. This tissue may proliferate and cause bleeding and traction, which can progress to retinal detachment. ROP is a major cause of morbidity in premature infants. The risk increases with decreasing birth weight and gestational age, occurring almost exclusively in infants born at 30 weeks gestation or less.
Regular ophthalmic monitoring of premature infants is required to identify and treat ROP when high-risk features are present. Guidelines have been published that describe the recommended age at initial screening and appropriate follow-up (Table 31–2).3 If retinal detachments occur, they are very difficult to treat and the visual outcome is usually poor. Therefore, the goal of treatment is to halt the progression of disease before this occurs. The presence of plus disease is a key indicator of risk. Plus disease describes increased tortuosity and dilation of the posterior blood vessels (Figure 31–10), which reflects the abnormal retinal vascular development.
In addition to plus disease, ROP is classified using a standardized system by stage and zone. The stage of ROP describes the type of abnormality present at the juncture of the posterior vascularized retina and the peripheral nonvascularized retina. Stage 1 is a demarcation line at this site. Stage 2 is an elevated ridge. Stage 3 is present when abnormal neovascular vessels begin to grow from the ridge (Figure 31–11). The presence of stage 3 (in addition to plus disease) is a high-risk factor. If the disease continues, patients may develop retinal detachments (stage 4—partial retinal detachment, and stage 5—complete retinal detachment).
The zone of ROP describes the location (Figure 31–12). Zone 1 is the most posterior portion of the retina, zone 2 is the middle portion, and zone 3 is the peripheral retina. The more posterior the abnormality, the higher the risk of vision loss due to the proximity of the abnormal vessels to the macula and fovea.
FIGURE 31–12
Standard diagram for documenting retinopathy of prematurity. Zone 1 is defined by a circle centered on the optic nerve, with a radius twice the distance from the optic nerve to the fovea. Zone 2 is defined by a circle centered on the optic nerve that extends to the nasal ora serrata (edge of retina). Zone 3 is the remaining temporal retina.
The aim of treatment is to prevent retinal distortion and detachment by stopping the abnormal vascular activity before these complications occur. Multicenter studies have identified the following high-risk factors in ROP: the presence of any plus disease or stage 3 disease in zone 1, or the presence of stages 2 or 3 and plus disease in zone 2. Treatment for patients with ROP is indicated when they develop these findings on examination.4
The peripheral retina in patients with ROP produces vascular endothelial growth factor (VEGF), which stimulates the growth of abnormal vessels. Treatment consists of ablation of the peripheral, nonvascularized retina with laser or cryotherapy (Figures 31–13 and 31–14). The goal is to decrease production of VEGF and induce regression of the abnormal vessels. Multicenter trials have shown that treatment is effective, but not all patients respond. Some patients develop scars in the fovea after treatment, which may impair visual acuity (Figure 31–15). If patients develop progressive retinal detachments, retinal surgery (such as vitrectomy) may be indicated. However, once the posterior retina has detached the visual outcomes are usually poor, even if the retina is successfully reattached. For this reason, efforts are made to treat the disorder before this occurs. Recently, anti-VEGF injections into the vitreous cavity have been reported to induce regression of ROP, but large-scale studies of this treatment have not been performed.
FIGURE 31–15
ROP. Scar in fovea following peripheral laser treatment.
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