Chapter 579. 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 the 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, but 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 or 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 afferent input of the dilator muscle of the pupil. The fourth cranial nerve, the trochlear nerve, innervates the superior oblique muscle that depresses, abducts, and intorts the eyes. 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 brainstem to exit the cranial vault inferior to the brainstem. The fifth cranial nerve, the trigeminal nerve, is responsible for sensory input from the cornea and eyelids. In addition, the sympathetic nerve supply to the eye, involved in vasomotor function and pupil constriction, travels via the trigeminal nerve. The sixth cranial nerve, the abducens nerve, provides innervation to the lateral rectus muscle (abduction). It also has a long course beneath the brainstem along the floor of the cranium, which makes it vulnerable to ischemic injury and palsy caused by brain swelling. Lastly, the seventh cranial nerve, the facial nerve, has both motor and sensory functions. This nerve carries the afferent limb of the corneal sensory reflex and provides the motor innervation to the eyelid orbicularis muscles, which close the eyes.

The most external portion of the ocular system is the eyelid. The muscular component of the eyelid includes the orbicularis muscles, Muller muscle, and the levator palpebrae superioris. These muscles all act in opening and closing the eyelid. Proper closure of the eyelid is important to protect the anterior external structures of the eye, including the cornea and conjunctiva. Additionally, these muscles act as a “lacrimal pump,” which aids in tear drainage via the nasolacrimal duct system. Improper position of the eyelid can provide clues to potential problems with these muscles and the nerves that supply them, as well as the more general sympathetic and parasympathetic nervous system. The orbicularis and levator muscles are innervated by voluntary motor nerves whereas Muller muscle is innervated by the sympathetic autonomic system. It is the latter that results in mild ptosis of the upper lid and “upside-down ptosis” of the lower lid (ie, lower lid position higher) in Horner syndrome.

The eyelid margins contain the eyelashes and glands. Each lid has 20 to 30 meibomian glands that secrete the most external layer of the tear film. Failure of these glands to function normally may result in hordeolum and blepharitis. Lashes must be positioned normally to avoid scratching the ocular surface (trichiasis or ingrown eyelash). The other 2 layers of the tear film, which constantly covers the ocular surface, are the middle layer, secreted by the lacrimal gland (located under the superior temporal portion of the upper lid), and the inner layer, which is secreted by conjunctival goblet cells. If the tear film is inadequate, dryness of the ocular surface can result. This can also occur if the lids are not blinking and closing properly, for example when a patient is in the intensive care unit.

Also deep within the lids is the periorbital septum, a membrane that connects to the bony orbital rims and creates a barrier that separates the orbital from the periorbital spaces. Infection external to the septum is termed periorbital or preseptal cellulitis, as distinguished from infection deep to the septum, which is termed orbital cellulitis. The septum is also an important landmark when repairing lid lacerations as suture bites that go too deep and catch the septum can result in lid deformity.

Normal anatomy of the eyeball (Fig. 579-1) is essential to the development of vision and proper ocular function. The conjunctiva is a thin vascularized membrane that covers the white sclera, a rigid collagen framework for the eyeball. Light enters the eye through the cornea, a clear 5-layer structure (corneal epithelium, Bowman membrane, stroma, Descemet membrane, and endothelium), which runs confluent with the sclera and allows for refraction, or bending, of light to focus an image onto the retina. In fact, the cornea provides over two thirds of the refractive power of the eye. If the cornea becomes cloudy as a result of glaucoma, metabolic disease, trauma or infection, vision is impaired.

The anterior chamber of the eye is bounded by the posterior (inner) surface of the cornea and the colored iris, the pupil (which is simply a hole in the iris through which light can pass), and the lens of the eye, which is located directly behind the pupil. The anterior chamber is filled with aqueous humor, a clear liquid that is produced by the ciliary body, located posterior to the iris and from which the lens zonules (suspensory fibers for the lens) arise. Aqueous humor flows through the pupil into the anterior chamber and drains out via the trabecular meshwork in the anterior chamber angle (where the peripheral iris meets the peripheral cornea) and into Schlemm’s canal. This aqueous humor provides nourishment for the cornea and lens. The relationship between its production and drainage is responsible for maintaining the normal intraocular pressure in the eye. Increased production (rare) or, more commonly, reduced drainage (outflow) of aqueous humor from the eye results in elevated intraocular pressure known as glaucoma. Hyphema is the collection of blood in the aqueous humor in the anterior chamber.

The lens, lens zonules, and ciliary body divide the eye into anterior and posterior portions or segments. The lens functions to further refract light, so its clarity is of utmost importance for clear vision. Any type of opacity in the lens of the eye is called a cataract. If the lens is out of position, it is called ectopia lentis (or lens subluxation). Additionally, the lens is able to change shape to clearly focus images on the retina that are different distances away. Focusing up close is called accommodation. The ciliary body contains the ciliary muscles, which contract to change the shape of the lens and allow for accommodation.

Posterior to the lens is the posterior segment of the eye, consisting of the vitreous, the retina, and the optic nerve. The vitreous humor is a clear gel that provides an internal structural framework for the posterior segment of the globe. It must be clear to allow for normal vision and may become cloudy in the case of trauma (vitreous hemorrhage), inflammation (vitreitis), or infection (endophthalmitis).

The inner surface of the posterior sclera is lined by the choroid and retina. The choroid is a vascular layer under the retina that supplies the retina with some of its oxygen and nutrition. The retina, the innermost layer of the posterior eye, is the “film of the eye,” where translation of the focused visual images into neural impulses takes place. The retina is composed of 10 layers. In order for a normal visual signal to be produced and sent to the brain, a clear, focused image must pass through all layers of the retina to stimulate the photoreceptors in the outer layer. The electrical signals created by the photoreceptors pass forward through the retina, eventually to the ganglion cells on the retinal surface, which join to create the optic nerve. The optic nerve provides the final connection to transport the electrical impulses from the eye to the brain, where the information is translated and interpreted.

The straight-back portion of the retina, just temporal to the optic nerve, is specialized for best vision. At the center of this region is the fovea, a dark spot that can best be viewed by having the patient look directly at the examiner’s light (eg, with a direct ophthalmoscopic view). The retina surrounding the fovea is called the macula. The macula plus the optic nerve is referred to as the posterior pole of the eye. Blood vessels course over the surface of the retina (except for the foveal region) and course into the eye with the optic nerve. At the surface of the optic nerve (the optic nerve head or disc) the canal within which the vessels flow presents as a white “cup” of variable size. An enlarged cup may be a sign that ganglion cell axons are being killed by a disease process such as glaucoma. When the optic nerve tissue is swollen, as in papilledema, the cup is obliterated and the edges of the disc become unclear.

The extraocular muscles are of special importance to the clinician examining the eye. There are 7 extraocular muscles: the medial, inferior, lateral and superior rectus muscles, the superior and inferior oblique muscles, and levator palpebrae superioris. Each extraocular muscle (agonist) has a specific force vector on the eyeball that determines its physiologic function or action in moving or stabilizing the position of the globe. Proper balance of the force vector for each muscle is required to align the visual axes of the eyes to allow for binocular vision and to keep the eyes aligned in different gaze positions. Table 579-1 delineates the primary (most important) and secondary/tertiary actions of the extraocular muscles when they contract.

The muscles originate at the back of the orbit (orbital apex), with the exception of the inferior oblique muscle, which originates from the anterior medial orbital wall. All muscles insert on the sclera anteriorly, under the conjunctiva, except for the inferior oblique muscle, which inserts on the posterior sclera.

The nasolacrimal system (Fig. 579-2) is responsible for draining the tears off the surface of the eye. Tears are made constantly throughout the day and night. As a result, failure of drainage will lead to tearing (epiphora). The drainage begins medially on the upper and lower lid margins through a small hole (puncta) sitting on a tiny elevation of the lid margin (papilla). Under the skin a channel (the canaliculus) then runs medially to empty into the lacrimal sac located medially between the eyeball and the side of the nose within the orbit. From the base of the lacrimal sac exits a long channel, the nasolacrimal duct, which runs lateral to the lateral wall of the nose, subcutaneously, and empties under the inferior turbinate within the nose. Increased tear formation, as with crying, results in nasal secretion. Failure of the lacrimal duct to empty into the nose is called nasolacrimal duct obstruction, commonly known as a “blocked tear duct.”

The bony orbit is a cone that extends posteriorly within the skull. It holds the eyeball anteriorly such that the bony orbital rims provide some protection. The optic nerve, autonomic nerves, extraocular muscles, and orbital fat are contained by the orbit behind the eye. Vascular and neurologic structures to and from the eye pass through foramina at the apex of the orbit. Hemorrhage or tumor in the orbit will push the eyeball anteriorly outward (proptosis). A broken orbital wall may allow the eyeball to “fall” backward into the orbit a bit, giving the eyeball a sunken appearance (enophthalmos).