38.1: Introduction to the Eyes
By the end of this section, you should be able to:
- 38.1.1 Describe the structures and functions of the eyes.
- 38.1.2 Differentiate between glaucoma and other ocular disorders.
Eye Structures and Functions
The human eye is an extension of the brain. Diseases of the brain are sometimes diagnosed by abnormalities seen in the eye. There are eye symptoms that can be benign or progress with age, such as cataracts , whereas other ocular symptoms can be red flags for neurological conditions.
The eye consists of internal and external structures (see Figure 38.2). Internal ocular structures include (Garrity, 2022b; Rehman et al., 2022):
- Pupil: Allows light to enter the eye, sends neural signals, and begins the process of sight. The pupil is the round black opening in the middle of the iris that responds to light by constricting when light enters and dilating when light is reduced.
- Iris: The color of the eye surrounding the black pupil. The iris controls the size of the pupil based on the amount of light. This is an essential nursing assessment because deviations from normal size or unequal size pupils can indicate pathology.
- Sclera: The “white of the eye.” The sclera helps maintain the eye’s shape and functions as a protective layer from injury.
- Anterior chamber: Located between the back of the cornea and front of the lens, this chamber contains aqueous humor , which is the tissue fluid of the eye. The fluid provides continuous nourishment to the lens and cornea because they have no capillaries of their own.
- Posterior chamber: An extension of the anterior chamber located between the lens and retina that contains vitreous humor. This has a gel-like consistency that can thin as one ages and lead to a retinal breach (American Society of Retina Specialists, 2023).
- Macula: Responsible for central vision and produces the clearest visual detail.
- Fovea: A small depression in the macula and the area for central vision where visual acuity is the highest.
- Lens: A transparent structure that lies directly behind the pupil and transmits and focuses light to form images on the retina and for accommodation purposes. It is attached to the ciliary muscle.
- Optic nerve: The second cranial nerve; it has sensory functions and transmits electrical impulses from the retina to the brain’s visual cortex, which displays them as a visual representation.
- Retina: The innermost nerve layer, containing microscopic photoreceptor cells to detect light. These receptors are called rods and cones. Rods are the receptors for night vison and help with peripheral vision. Cones are for daytime vision; they help with central vision and can detect color as well. There are three distinct types of cones, each sensitive to a different color: red, green, or blue.
Intraocular pressure (IOP) is the fluid pressure caused by the amount of aqueous humor in the anterior chamber. A normal IOP is 10–21 mm Hg (Begum, 2023). The pressure is maintained by having the same amount of fluid come in as the amount of fluid leaving the front of the eye. Excess IOP is an important risk factor for glaucoma. Untreated elevated IOP can lead to blindness. Abnormally low IOP, known as hypotony, can lead to eye wasting.
External ocular structures (see Figure 38.3) include (Garrity, 2022a):
- Eyelids and eyelashes: These protect the eye, acting as barriers against foreign bodies, bright light, and small irritants, such as dust.
- Conjunctiva: The mucous membrane lining the eyelids. When this becomes inflamed, it is referred to as conjunctivitis , which is caused by allergies, certain bacteria, or viruses.
- Cornea: An avascular tissue barrier that protects the eye and will result in blurry vision if scratched. The cornea is the main structure that bends light entering the eye to allow the eye to focus clearly on an image. Refraction errors can manifest as nearsightedness, farsightedness, and astigmatism (described further later in this section).
- Extrinsic muscles of the eye: There are a total of six, and they are attached to the bony orbit and to the surface of the eye. There are four rectus muscles (lateral, medial, superior, and inferior) that move the eye up and down or side to side. The two oblique muscles (superior and inferior) rotate the eyes.
Fluid is produced by the lacrimal gland located at the upper, outer corner of the eye. This fluid flows continuously, but the amount increases due to irritation and certain emotions. Small lacrimal ducts take the fluid to the anterior surface of the eye. When a person blinks, the fluid spreads, which moistens and washes the surface of the eye. The fluid also contains an enzyme called lysozyme that can inhibit growth of bacteria on the surface of the eye.
There are two small openings at the medial canthus, which is the corner of the eye near the nose. These take the fluid to the lacrimal sac that leads to the nasolacrimal duct. Here the fluid empties into the nasal cavity. This is the reason that noses run when a person cries. If the increase in fluid exceeds the capacity of the drainage system, the excess fluid overflows the eyelids and becomes tears. This is known as lacrimation.
Eye Disorders
Healthy vision requires three basic components: formation of retinal images, stimulation of rods and cones, and nerve impulse conduction to the brain. Malfunction of any of these can disrupt vision. Eye disorders range from common refractive errors that can be treated with the use of eyeglasses, contact lenses, or surgery to more serious conditions such as glaucoma, macular degeneration, and ocular inflammation and infection.
Topical ophthalmic agents play a prominent role in managing many eye disorders. These drugs are intended for direct administration into the conjunctiva of the eye. This route limits systemic absorption, thereby decreasing the risk of adverse effects. Various eye disorders will be addressed in this section. The following section will discuss relevant drugs for specific eye conditions.
Refractive Errors
Refraction of light rays is the deflection or bending of rays as they pass through one object and into another of greater or lesser density. Refractive errors occur when the shape of the eye keeps light from focusing directly on the retina, thereby distorting how objects are seen. When looking at a distant object, the ciliary muscle is relaxed, causing the lens to have a flat shape. If looking at a near object, the ciliary muscle contracts, causing the lens to have a convex shape. The lens should be able to quickly adjust from far to near vision and vice versa. It is the only adjustable structure within the refractory system.
Errors in refraction can be addressed with surgical and nonsurgical interventions. In some cases, a vision prescription can correct how light is refracted on the retina (National Eye Institute, 2022). There are four main refractive errors:
- Hyperopia , referred to as farsightedness, is characterized by seeing close objects out of focus; however, distant objects are clearly seen. The light refracts behind the retina to cause this distortion. This is because the eyeball is too short or the lens is too thin.
- Myopia , referred to as nearsightedness, is characterized by seeing distant objects out of focus; however, close objects are clearly seen. The light refracts in front of the retina to cause this distortion. This is because the eyeball is too elongated or the lens is too thick.
- Astigmatism occurs when light is focused on multiple points along the retina, rather than just a single point. This occurs when the cornea or the lens has a different shape than normal, often shaped more like a football than a baseball (Boyd, 2022a).
- Presbyopia occurs as the eye ages and is characterized by lens thickening and loss of elasticity. This change occurs around age 45. Clients may begin to use over-the-counter reading glasses for close-up reading (National Eye Institute, 2020).
Link to Learning
The American Academy of Ophthalmology has a video entitled “Vision Loss and Refractive Error,” presented by the University of Wisconsin Department of Ophthalmology and Visual Sciences. It discusses topics such as myopia, hyperopia, astigmatism, and presbyopia.
Glaucoma
Glaucoma is a group of diseases characterized by a decrease in peripheral vision due to an obstruction of aqueous humor that increases IOP, which can damage the optic nerve. This leads to diminished and/or distorted vision. Glaucoma can be an inherited condition or occur spontaneously. Black , Hispanic , and Asian clients have a higher risk for glaucoma. Also at higher risk are clients older than 40 years, those with thinner corneas, those with chronic eye inflammation, and those taking medications that are known to increase eye pressure (Boyd, 2022b).
Multiple parameters are assessed when screening for glaucoma, including:
- IOP: Average values are 10–21 mm Hg (Begum, 2023).
- Corneal thickness: Measured in microns with an average cornea thickness of 555 microns. Thinner corneas may result in artificially low pressure readings, potentially missing a glaucoma diagnosis. Similarly, thicker corneas may cause artificially elevated pressure readings, potentially creating a false glaucoma diagnosis (Glaucoma Research Foundation, 2022).
- Optic nerve: A healthy optic nerve is intact and generally does not show asymmetry or cupping (Waisberg & Micieli, 2021).
Clinical Tip
Anxiety Can Affect IOP
The nurse should be aware that a client’s anxiety can artificially increase an IOP reading. It is important to acknowledge the client’s wariness about the tonometer touching the cornea, but the nurse should reassure the client that a numbing drop is used alleviate any perceived discomfort.
Clients are considered “glaucoma suspects” if they have risk factors for glaucoma, including elevated IOP, optic nerve damage, visual field defect, or a strong family history of glaucoma (Ahmad, 2018).
The two forms of glaucoma are open-angle glaucoma (the most common form) and closed-angle glaucoma. Open-angle glaucoma occurs when the trabecular meshwork becomes occluded and cannot drain the continuous production of aqueous humor. Often there are no early symptoms. Eventually, the client begins to lose peripheral vision (Mayo Clinic, 2022). Open-angle glaucoma typically progresses slowly with small increases in IOP over time. Treatments include ophthalmic drops as well as surgical treatments such as trabeculectomy , a glaucoma drainage implant that allows aqueous humor to drain through an artificial bleb (a surgically created hole) into the body for processing, and corneal transplant.
Closed-angle glaucoma progresses suddenly with a sudden rise in IOP. Clients can become blind with little warning. This condition is precipitated by either displacement of the iris or pupil dilation. Both cover the trabecular meshwork, preventing the exit of aqueous humor. IOP increases rapidly to dangerous levels when the angle between the cornea and iris becomes significantly narrowed. Symptoms can include sudden headache, eye pain, and/or blurred vision. Immediate treatment by an ophthalmologic specialist can limit ocular disability by widening the canal of Schlemm in a procedure known as canaloplasty .
Macular Degeneration
Macular degeneration occurs when the central portion of the retina inhibits central vision but maintains peripheral vision. Macular degeneration occurs in some clients as the macula ages. There are two types of degeneration:
- Dry macular degeneration (DMD) often has characteristic drusen, which consists of yellow lipid retinal deposits. There is no cure for DMD; however, there is some evidence that various nutritional and vitamin intakes, including vitamin C, lutein, and leafy green vegetables, can slow the progression (American Optometric Association, n.d.; Mrowicka et al., 2022).
- Wet macular degeneration (WMD) is characterized by the growth of new subretinal blood vessels, which are often fragile and leaky. This leakage lifts the macula from its normal place, quickly causing permanent injury. There is no cure for WMD, but it can be treated with laser technology and intraocular medication injections (National Retina Institute, 2023).
Ocular Inflammation and Infection
Ocular inflammation and infection occur when the eye has become inflamed or infected through either illness or injury. Conjunctivitis , referred to as pink eye, occurs when the thin layer of tissue in front of the eye becomes irritated and red, often producing a sticky coating on the eyelashes. These symptoms can increase tear production, produce a sensation of a foreign body in the eye, and cause the client to rub their eyes. Conjunctivitis can be caused by a bacterial or viral infection, allergy, or irritant, such as a stray eyelash or after swimming in a chlorinated pool. Bacterial conjunctivitis is often treated with ocular antibacterials, in the form of either an ointment or a solution (Centers for Disease Control and Prevention, 2019). Complications are rare; however, they can include eye scarring or a secondary infection such as meningitis (National Health Service, 2023).
There are numerous other infections and inflammatory conditions of the eye that occur at different ocular anatomical sites, including:
- Episcleritis occurs between the conjunctiva and sclera and has a similar presentation to conjunctivitis (Johns Hopkins Medicine, n.d.; Schonberg & Stokkermans, 2023).
- Keratitis is characterized by corneal inflammation that can cause ulceration. Keratitis is considered a medical emergency. The client will need a timely diagnosis and treatment plan because extensive ulceration can lead to blindness. Keratitis is associated with infectious and noninfectious causes (Singh et al., 2023).
- Uveitis is an inflammation of the middle part of the eye called the uvea. Though infection can be a factor, uveitis is largely considered autoimmune secondary to other systemic diseases such as ankylosing spondylitis and reactive arthritis (Rosenbaum, 2019). Uveitis is characterized by eye pain, eye redness, and photophobia . It is essential that the client seek timely diagnosis and treatment with ocular corticosteroids to avoid vision loss.
- A chalazion occurs when meibomian glands become occluded. It presents with edema and pain on the eyelid and is initially treated with warm wet compresses several times per day, but it may also require antibiotics and possible surgical intervention.
- A hordeolum ( stye ) is a red, painful bump near the edge of the eyelid that looks like a pimple or boil. A stye has an infectious cause and often responds to warm wet compresses. Oral antibiotics are sometimes needed to successfully treat a hordeolum (American Academy of Ophthalmology, 2022).
- Orbital cellulitis is an infection of the fat and muscles around the eye caused by pathogens, trauma, or spreading of infection from the sinuses. Orbital cellulitis can be serious and lead to brain abscess and vision loss. The client may present with a fever, eye pain, and a bulging eye. A dental history should also be reviewed because a dental abscess can track upward into the orbits. Treatment includes broad-spectrum antibiotics until the actual organism can be identified (Satar et al., 2021). Once the organism has been determined, appropriate antibiotics can be started.
- Obstruction or inflammation of the lacrimal gland or the lacrimal sac can occur. When the lacrimal gland is inflamed, this is termed dacryoadenitis . If the lacrimal sac is inflamed, this is referred to as dacryocystitis . Treatment consists of warm compresses, ocular antibacterials, and oral antibiotics as needed. The Crigler massage is sometimes recommended to release material from the affected tear duct by pressing a clean index finger inward against the lacrimal sac and massaging downward toward the nose (Hu et al., 2022).
- Keratoconjunctivitis sicca is dryness of the conjunctiva and cornea. The dry eyes can be the result of too few tears being produced or tears that evaporate too quickly. This condition is characterized by inflammation of the ocular surface and lacrimal glands.
Administration of Ophthalmic Drugs
It is important for the nurse to communicate with the client prior to eye medication administration to set expectations regarding drug indications and administration technique. This helps ensure that the client can participate in ocular medication administration and support medication efficacy.
Administration of Solution or Drops
To properly administer solution or drops, the client should:
- Check the expiration date prior to administration, and discard if expired.
- Wash and dry hands thoroughly before administering eye medication.
- Shake the bottle gently to ensure the medication is evenly distributed, especially if the drug is a suspension.
- Tilt head back and stare upward so the medication does not directly hit the eyeball.
- Pull the lower eyelid away from the eye to form a pouch and instill the prescribed number of drops into the conjunctival sac.
- Release the eyelid slowly.
- Close the eye and look downward for 1–2 minutes.
- Apply gentle pressure to the corner of the eye where the upper and lower lids meet closest to the nose for 30–60 seconds to prevent systemic absorption.
The client should wait at least 2 minutes between drops if needing to administer more than one drop of the same medication; they should wait at least 5 minutes between drugs if a different ocular medication is prescribed.
The client should not :
- Allow the dropper to touch their eye or any other surface.
- Rub the eyes after administration.
- Rinse the eyedropper.
- Use eye drops that have changed color.
- Wear contact lenses before or several minutes after administration.
- Administer eye drops if they have injured their eye because the drops can cause pain and make the injury worse.
Clinical Tip
Geriatric and Pediatric Administration
Older adults or others needing assistance with administration should try a device that can attach to the medication bottle, making it easier to hold the bottle and administer the drops. These products are available at pharmacies and where medical supplies are sold. If the client’s hands shake, they can approach the eye from the side with the hand resting on the side of the face. They can also wrap a cloth or paper towel around the bottle and dropper to make it larger and easier to grip.
For pediatric clients , if possible, have a second person available to help position the child and keep them still during administration. If the child will not open their eyes, place the drops in the inner corner of the eye so when they open their eyes the medication will fall in.
Administration of Ointment
To properly administer ointment, the client should:
- Wash and dry hands thoroughly before administering eye medication.
- Hold the tube between their hands for 2–4 minutes to warm the ointment.
- Tilt their head back and stare upward.
- Pull the lower eyelid away from the eye to form a pouch.
- Hold the tube about 1 inch above the eye and place a thin layer of approximately 0.25–0.5 inches inside the lower lid.
- Close the eyes for 1–2 minutes and roll the eyes around in all directions so the ointment melts and gets dispersed over the entire eye.
- Wipe away any excess ointment from the eyes.
- Wait 30 minutes before applying a second ointment (if prescribed).
The client should not :
- Rub their eyes after administration.
- Wear contact lenses during treatment, unless told otherwise, because the contact lenses can become damaged.
- Use after the recommended time, to prevent an eye infection.
- Drive or operate heavy machinery. Ointments are thick and can cause transient blurred vision.
- Apply ointment if they have sustained an eye injury because the ointment can cause pain or make the injury worse.
Clinical Tip
Combination Prescription for Both Drops and Ointment
Some individuals may have to administer drops and ointment, depending on their prescribed medications. It is crucial the drops are instilled first. The client should then wait 5–10 minutes before applying the ointment. If they do it in the reverse order, the ointment may get washed away and will be ineffective.
Link to Learning
In the first link to learning, a video entitled “How to Safely Instill Eye Drops,” Dr. Brigitte Keener explains and demonstrates the proper way to instill eye drops to ensure safety and effectiveness. The video is presented by the Mayo Clinic.
In another video, a practicing pharmacist, Abraham Khodadi, describes and illustrates the correct way to use eye ointment.