Structures and Functions of Vision

Eyeball.

The eyeball, or globe, is composed of three layers (Fig. 21-1). The tough outer layer is composed of the sclera and the transparent cornea. The middle layer consists of the uveal tract (iris, choroid, and ciliary body), and the innermost layer is the retina. The anterior cavity is divided into the anterior and posterior chambers. The anterior chamber lies between the iris and the posterior surface of the cornea, and the posterior chamber lies between the anterior surface of the lens and the posterior surface of the iris. The posterior cavity lies in the large space behind the lens and in front of the retina.

Visual Pathways.

Once the image travels through the refractive media, it is focused on the retina (Fig. 21-2). From the retina, the impulses travel through the optic nerve to the optic chiasm where the nasal fibers of each eye cross over to the other side. Fibers from the left field of both eyes form the left optic tract and travel to the left occipital cortex. The fibers from the right field of both eyes form the right optic tract and travel to the right occipital cortex. This arrangement of the nerve fibers in the visual pathways allows determination of the anatomic location of abnormalities.

External Structures and Functions

The eyebrows, eyelids, and eyelashes serve an important role in protecting the eye. They provide a physical barrier to dust and foreign particles (Fig. 21-3). The eye is further protected by the surrounding bony orbit and by fat pads located below and behind the globe, or eyeball.

The upper and lower eyelids join at the medial and lateral canthi. Blinking of the upper eyelid distributes tears over the anterior surface of the eyeball and helps control the amount of light entering the visual pathway. The eyelids open and close through the action of muscles innervated by cranial nerve (CN) VII, the facial nerve.

The conjunctiva is a transparent mucous membrane that covers the inner surfaces of the eyelids and also extends over the sclera, forming a “pocket” under each eyelid. Glands in the conjunctiva secrete mucus and tears. The sclera is composed of collagen fibers meshed together to form an opaque structure commonly referred to as the “white” of the eye. The sclera forms a tough shell that helps protect the intraocular structures.

The transparent and avascular cornea allows light to enter the eye (see Fig. 21-1). The curved cornea refracts (bends) incoming light rays to help focus them on the retina. The cornea consists of five layers: the epithelium, Bowman’s layer, the stroma, Descemet’s membrane, and the endothelium. The lacrimal system consists of the lacrimal gland and ducts, lacrimal canals and puncta, lacrimal sac, and nasolacrimal duct (see Fig. 21-3). In addition to the lacrimal gland, other glands provide secretions to make up the mucous, aqueous, and lipid layers of the tear film. The tear film moistens the eye and provides oxygen to the cornea.

Each eye is moved by three pairs of extraocular muscles: (1) superior and inferior rectus muscles, (2) medial and lateral rectus muscles, and (3) superior and inferior oblique muscles. Neuromuscular coordination produces simultaneous movement of the eyes in the same direction.

Internal Structures and Functions

The iris provides the color of the eye. The iris has a small round opening in its center, the pupil, which allows light to enter the eye. The pupil constricts via action of the iris sphincter muscle (innervated by CN III [oculomotor nerve]) and dilates via action of the iris dilator muscle (innervated by CN V [trigeminal nerve]) to control the amount of light that enters the eye.

The lens is a biconvex, avascular, transparent structure located behind the iris. The primary function of the lens is to bend light rays so that they fall onto the retina. Accommodation occurs when the eye focuses on a near object and is facilitated by contraction of the ciliary body, which changes the shape of the lens. The ciliary body consists of the ciliary muscles, which surround the lens and lie parallel to the sclera. The ciliary processes lie behind the peripheral part of the iris and secrete aqueous humor. The choroid is a highly vascular structure that nourishes the ciliary body, the iris, and the outer portion of the retina. It lies inside and parallel to the sclera (see Fig. 21-1).

The retina is the innermost layer of the eye that extends and forms the optic nerve. Neurons make up the major portion of the retina. Therefore retinal cells are unable to regenerate if destroyed. The retina lines the inside of the eyeball, extending from the area of the optic nerve to the ciliary body (see Fig. 21-1). It is responsible for converting images into a form that the brain can understand and process as vision. The retina is composed of two types of photoreceptor cells: rods and cones. Rods are stimulated in dim or darkened environments, and cones are receptive to colors in bright environments. The center of the retina is the fovea centralis, a pinpoint depression composed only of densely packed cones.¹ This area of the retina provides the sharpest visual acuity. Surrounding the fovea is the macula, an area less than 1 mm², which has a high concentration of cones and is relatively free of blood vessels.

Assessment of Visual System

Assessment of the visual system may be as simple as determining a patient’s visual acuity or as complex as collecting complete subjective and objective data pertinent to the visual system. To do an appropriate visual evaluation, determine which parts of the data collection are important for each individual patient.

Case Study

Patient Introduction

F.M. is an 81-year-old Hispanic woman who comes to the emergency department with complaints of visual disturbances. F.M. states that her vision “looks like everything is covered with a spider web.” She also reports seeing periodic light flashes and small white spots “floating” in the air.

Critical Thinking

As you read through this assessment chapter, think about F.M. with the following questions in mind:

1. What are the possible causes of F.M.’s visual disturbances?

2. What type of assessment would be most appropriate for F.M.: comprehensive, focused, or emergency?

3. What questions would you ask?

4. What should be included in the physical assessment? What would you be looking for?

5. What diagnostic studies might you expect to be ordered?

Nutritional-Metabolic Pattern.

High doses of vitamins containing antioxidants (vitamins C and E and beta-carotene) may be important to ocular health. Some patients with AMD may benefit from supplements of these vitamins.

Elimination Pattern.

Straining to defecate (Valsalva maneuver) can raise the intraocular pressure. After eye surgery, many surgeons do not want the patient to strain. Assess the patient’s usual pattern of elimination and determine whether there is the potential for constipation in the patient who has had ophthalmic surgical procedures.

Activity-Exercise Pattern.

The patient’s usual level of activity or exercise may be affected by reduced vision, symptoms accompanying an ocular problem, or activity restrictions after a surgical procedure. Inquire about leisure activities during which the patient may incur an ocular injury. For example, gardening, woodworking, and other craft activities can result in corneal or conjunctival foreign bodies or even penetrating injuries of the globe. Sports activities such as racquetball, baseball, and tennis carry risks for blunt trauma to the eyes. Protection goggles should be worn for these sports.

Sleep-Rest Pattern.

In the otherwise healthy person, lack of sleep may cause ocular irritation, especially if the patient wears contact lenses. Normal sleep patterns may be disrupted in the patient with painful eye problems such as corneal abrasions.

Cognitive-Perceptual Pattern.

The entire assessment of the ophthalmic patient focuses on the sense of sight, but do not overlook other cognitive or perceptual problems. For example, the functional ability of a patient with a visual deficit will be further compromised if the patient also has hearing problems. The patient who cannot see to read has increased difficulty following postoperative instructions if he or she also has trouble hearing or remembering verbal instructions. Eye pain is always an important symptom to assess. If eye pain is present, question the patient about treatment and response.

Self-Perception–Self-Concept Pattern.

The loss of independence that can follow a partial or complete loss of vision, even if the condition is temporary, can have devastating effects on the patient’s self-concept. Carefully evaluate the potential effect of vision loss on the patient’s self-image. For instance, disabling glare from a cataract may prevent nighttime driving. In today’s highly mobile society, the loss of ability to drive can represent a significant loss of independence and self-esteem.

Role-Relationship Pattern.

Ocular problems can negatively affect the patient’s ability to maintain the necessary or desired roles and responsibilities in the home, work, and social environments. For example, AMD may decrease visual acuity so that the patient can no longer adequately function at work. Many occupations place workers in conditions in which eye injury may occur. For example, factory workers may be at risk from flying metal debris. Eye-safety practices, such as the use of goggles or safety glasses, are now a legal requirement in the workplace. The patient with diabetes may not be able to see well enough to self-administer insulin. This patient may resent dependence on a family member who takes over this function. Sensitively inquire if the ocular problem has affected the patient’s preferred roles and responsibilities.

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