Infant Vision
The visual system begins to develop early in the human embryo, but the most critical period of development of the visual system occurs late in gestation, from 28 to 40 weeks. At 36 weeks of gestational age, the pupillary responses to light are just becoming consistent, the photoreceptor to ganglion cell connections are continuing to mature, the lateral geniculate nuclei are developing their characteristic 6 layers, and the ocular dominance columns of the visual cortex are halfway through their formation. Development of the retina (especially the fovea) and visual cortex continues after birth. Postnatal visual stimulation is also essential for the development of normal vision, and this process takes place over the first 2 to 3 years of life. Early deprivation of visual stimulation secondary to severe ptosis, corneal opacity, cataract, or vitreous hemorrhage will hinder normal visual development.
NORMAL VISUAL BEHAVIOR IN AN INFANT
At birth, infants (defined as children from birth to 1 year of age) are able to see and blink in response to light. In the few weeks after birth, they are increasingly able to differentiate lines, patterns, movement, and finally, color.
At about 6 weeks of age, infants begin to make meaningful eye contact and display a social smile, both indicators of normal visual development. Smiling at this age is visually learned. The focal distance for infants is about 12 inches, and they are particularly attracted to faces and high-contrast visual targets. At this age, the infant has little regard for distance objects. It is estimated by sweep visual evoked potentials (VEP) that newborns have visual acuity of approximately 20/400. Infants at this age may fixate fairly well on a stationary object, but if the object is moved, the infant may not follow the target well. In addition, these infants often display decreased visual attention on monocular occlusion during the examination. Intermittent horizontal strabismus is very common and considered normal at this age.
At 3 months of age, the infant should be able to fixate and accurately follow a moving toy, tracking first horizontally and then vertically a few weeks later. At this age, the infant will fix and follow with an eye occluded.
By 6 months of age, infants will display more prolonged visual attention to near targets and will attend at greater distances. Stereovision is beginning to develop. At this age, all signs of intermittent strabismus should have resolved.
By 1 year of age, most infants will attend to objects at a 20‑foot distance.
ABNORMAL VISUAL BEHAVIOR IN AN INFANT
Abnormal vision in an infant ranges from no vision to decreased visual attention for age, presenting as a failure/delay in gaining the visual milestones noted earlier. The signs of abnormal vision may include:
poor eye contact with caregivers
poor fixation, and/or following, on a face, or later, toys
delayed onset of a social smile
nystagmus (if present, onset is typically between 1 to 4 months of age)
excessive light sensitivity (day blindness or hemeralopia)
staring at lights (ceiling lights, sun, sunny window)
pressing on the eye with a thumb, finger, or knuckle (oculodigital sign)
holding objects very close to the eyes
failure to develop attention to distant objects
Evaluation of the Visually Impaired Infant
Assessing the visually impaired infant may appear daunting, but employing knowledge of a few basic tenets (sidebar) and obtaining specific key historical and clinical examination information will lead the practitioner to the correct diagnosis in an efficient manner.
HISTORY
After the history of poor vision has been obtained, other historical information often supplies clues as to the etiology of the abnormal vision, especially if no etiology is readily apparent on the eye examination.
NEUROLOGIC DISORDERS. Decreased vision in an infant may be caused by cerebral insults or congenital cerebral maldevelopment, which is typically associated with other neurologic abnormalities such as delay in development of milestones (head control, rolling over, truncal control, crawling and walking, speech), microcephaly, and seizures (especially infantile spasms). These neurologic signs imply that the infant’s abnormal vision may be brain-based.
PRETERM BIRTH. Premature infants, especially those born at the lowest gestational ages, are at high risk for anterior and posterior visual pathway insults, which cause retinal, optic nerve, and/or brain damage, leading to visual loss.
FAMILY HISTORY OF BLINDNESS. A family history of blindness or very poor vision even after refractive correction, or a family history of albinism, nystagmus, or retinal dystrophy/degeneration, are critical historical elements and may lead to the diagnosis. A family history of consanguinity is important because it increases the likelihood of autosomal recessive disorders when the family history is negative.
UNUSUAL MANNERISMS. Light staring is a mannerism that is very common in children with brain-based causes of visual impairment. These children will often stare at a bright light, bright window light, and occasionally wave their hand or fingers in front of their faces as they gaze at the light.
The oculodigital sign (Figure 1) is a mannerism that poorly sighted children may display. It occurs more commonly in patients with retinal disease, particularly retinal dystrophies. This sign consists of repeated and forceful pressing or poking of the globe(s) with a fingertip or knuckle, typically through a closed eyelid. It is presumed that the blind child is creating a visual sensation by retinal phosphene production.
Day blindness, or hemeralopia, is the inability to see in bright light. This occurs in specific retinal diseases, namely cone retinal dystrophies such as achromatopsia. Affected patients are unable, or refuse, to open their eyelids in strong lighting conditions, such as outdoors. It has also been termed photo-aversion. Night blindness, or nyctalopia, occurs in retinal dystrophies that primarily affect the rods, but these conditions typically do not present in infancy.