Anatomy and Physiology of the Eye and Ocular System
The vision system in infants and children can be thought of as a combination of the eye and extraocular orbital structures as well as connections to the vision-related portions of the brain. A basic understanding of the normal anatomy, embryology, developmental biology, and physiology of these structures is necessary to comprehend the abnormalities that result in disease and the appropriate means to diagnose and treat them. The development of good visual function requires an interaction of the cornea, lens, retina, optic nerve, cranial nerves, and autonomic nervous system, along with the brain, to allow a properly focused, clear, single image to fall on the retina of each eye and then be converted into electrical signals that are transmitted to the appropriate areas of the brain.
Babies are not born with normal 20/20 visual acuity or normal binocular vision. They must develop these by having a clear, focused image fall on the appropriate area of the retina of each eye during the period from birth to 6 to 9 years of age, sometimes called the “period of vision development.” Failure of a focused image to fall on the retina during any part of this critical period, due to uncorrected refractive error, eye misalignment (strabismus), or ocular media opacity (eg, cataract, vitreous, corneal opacity), can result in poor vision development, known as amblyopia. For example, infants with bilateral congenital cataracts must have their cataracts removed and appropriate optical correction applied as early as possible in order to optimize visual outcome and prevent irreversible deprivation amblyopia in 1 or both eyes. Infants with infantile esotropia must be aligned before 2 years of age in order to demonstrate at least some binocular vision. Therefore, it is mandatory that ocular abnormalities in infants and children are diagnosed early and treated in a timely fashion.
Six of the 12 cranial nerves are involved in ocular physiology and mechanics of vision. The second cranial nerve, the optic nerve, allows afferent input for visual stimuli as well as pupillomotor reactivity. Electrical impulses from the retina travel via the optic nerve through the optic chiasm and the temporal and parietal lobes of the brain, ultimately arriving at the occipital cortex to begin the process of translation into vision. The third cranial nerve, the oculomotor nerve, innervates the medial rectus (adduction), superior rectus (elevation), inferior rectus (depression), and inferior oblique (elevation, abduction, and extorsion) muscles, as well as the levator palpebrae superioris muscle (elevation of the upper eyelid). Additionally, the oculomotor nerve carries the parasympathetic efferent input of the sphincter muscle of the pupil. The fourth cranial nerve, the trochlear nerve, innervates the superior oblique muscle (depression, abduction, and intorsion). This is the most likely nerve to be affected after trauma due to its long, unprotected intracranial course over the dorsal aspect of the superior ...