Structures and Functions of Visual System

The visual system consists of the external tissues and structures surrounding the eye, the external and internal structures of the eye, the refractive media, and the visual pathway. The external structures are the eyebrows, eyelids, eyelashes, lacrimal system, conjunctiva, cornea, sclera, and extraocular muscles. The internal structures are the iris, lens, ciliary body, choroid, and retina. The entire visual system is important for visual function. Light reflected from an object in the field of vision passes through the transparent structures of the eye and, in doing so, is refracted (bent) so that a clear image can fall on the retina. From the retina, the visual stimuli travel through the visual pathway to the occipital cortex, where they are perceived as an image.

Refractive errors

Refraction is the bending of light rays; any problem associated with eye length or refraction can lead to refractive errors.

Myopia (nearsightedness): Refractive ability of the eye is too strong for the eye length; images are bent and fall in front of, not on, the retina.

Hyperopia (farsightedness): Refractive ability of the eye is too weak; images are focused behind the retina.

Presbyopia: Loss of lens elasticity because of aging; less able to focus the eye for close work, and images fall behind the retina.

Astigmatism: Occurs because of the irregular curvature of the cornea; image focuses at two different points on the retina.

Diagnostic

The Weber test has been mainly used to establish a diagnosis in patients with unilateral hearing loss, to distinguish between conductive and sensorineural hearing loss. It is a useful, quick, and simple screening test for evaluating hearing loss. The test can detect unilateral conductive and sensorineural hearing loss. The outer and middle ear mediate conductive hearing. The inner ear mediates sensorineural hearing. The Weber test is often combined with the Rinne test to detect the location and nature of the hearing loss. This activity describes the technique of the Weber test and its clinical relevance, and highlights the interprofessional team’s role in the management of patients with deafness.

Weber test: Place the base of a struck tuning fork on the bridge of the forehead, nose, or teeth. In a normal test, there is no lateralization of sound. With unilateral conductive loss, sound lateralizes toward affected ear. With unilateral sensorineural loss, sound lateralizes to the normal or better-hearing side.

Rinne test: Place the base of a struck tuning fork on the mastoid bone behind the ear. Have the patient indicate when sound is no longer heard. Move fork (held at base) beside ear and ask if now audible. In a normal test, AC > BC; patient can hear fork at ear. With conductive loss, BC > AC; patient will not hear fork at ear.

The Snellen eye chart is commonly used to assess visual acuity, a type of vision chart that tests the clarity and sharpness of your eyesight, also known as visual acuity. A Snellen Chart may be used during a routine eye test to determine how well your eyes see the details of an object at a distance of 20 feet, or 6 metres, away.

Interventions

Eye drop administration

• Have the client sit down and tilt his or her head slightly backward.
• Gently pull tissue underneath eye downward to expose lower conjunctival sac.
• Hold the dropper and stabilize hand on the client’s forehead.
• Have the client look up and instill the number of prescribed drops.
• Have the client gently close the eye and move it around.
• Gently press on the lacrimal duct for 1 minute.

Irrigation 

• Fill a syringe with warm irrigating solution.
• Place the tip of the syringe at an angle in the external canal.
• Apply gentle but continuous pressure to the syringe plunger.
• Watch for fluid return and signs of cerumen.
• If cerumen does not appear, wait 10 minutes and repeat the irrigation.
• After completion of the irrigation, have the client turn the head to the side to facilitate drainage.

Common pathologies 

Diabetic retinopathy occurs due to microvascular damage to the retina resulting from chronic hyperglycemia (eg, diabetes mellitus). Diabetic retinopathy can lead to retinal detachment, which is a separation of the retina from the underlying epithelium.  Diabetic retinopathy is a complication of diabetes mellitus that occurs due to microvascular damage to the retina resulting from chronic hyperglycemia.  

The early stage of Non-proliferative Diabetic Retinopathy characterized by – Microaneurysm formation (capillary leak and later become occluded)

• Intraretinal hemorrhages (dot-and-blot hemorrhages)
• Hard exudates
• Cotton-wool spots (infraction of the nerve fiber layer)
• Edema
• Clinically significant macular edema (CSME) if the center of the macula (fovea) is involved
• Non-clinically significant macular edema (NCSME)

Retinal detachment is separation of the retina from the underlying epithelium that allows fluid to collect in the space. 

Symptoms of retinal detachment include flashes of light, floaters, or black spots across the field of vision, the sense of a curtain being drawn over the eye, and loss of a portion of the visual field.  Retinal detachment requires emergency surgery to reattach the retina in an attempt to restore vision.  Without rapid intervention, it can lead to permanent blindness.

Types of retinal detachment:

• Traction: means the retinal is pulled up
• Rhegmatogenous: hole in the retina
• Combined traction-rhegmatogenous
• Serous: fluid behind the retina

Blunt-force trauma to the head is associated with potentially severe complications (eg, brain damage and herniation, retinal detachment, seizures).  Prompt recognition of potential complications is essential to prevent irreversible changes to the client’s neurological status and level of function.

The signs/symptoms include sudden onset of light flashes, floaters, cloudy vision, or a curtain appearing in the vision.

After repair, interventions focus on promoting retinal reattachment.  Postoperative teaching should include:

• Avoiding activities that increase intraocular pressure (eg, rubbing the eye, straining)
• Reporting sudden pain, flashes of light, vision loss, or bleeding, which may indicate detachment or infection, to the health care provider 
• Avoiding focused activities (eg, reading, writing, sewing), which can cause rapid eye movements and increase the risk for detachment 
• Wearing an eye patch or shield as directed to prevent rubbing/scratching of the eye and minimize eye movement
• Ensuring appropriate positioning as instructed by the surgeon because clients may receive intravitreal oil or gas, which holds the retina in a specific position to allow healing

Meniere disease (endolymphatic hydrops) results from excess fluid accumulation inside the inner ear. Clients have episodic attacks of vertigo, tinnitus, hearing loss, and aural fullness. The vertigo can be severe and associated with nausea and vomiting. Clients report feelings of being pulled to the ground (drop attacks). 

During an attack, the client is treated with vestibular suppressants, including sedatives (eg, benzodiazepines such as diazepam), antihistamines (eg, diphenhydramine, meclizine), anticholinergics (eg, scopolamine), and antiemetics. The nurse’s priority is to plan for client safety with fall precautions given the severe vertigo and use of sedating medications. Fall precautions include adjusting the bed to a low position with side rails up and instructing the client to call for help before getting up. Vertigo may be minimized by staying in a quiet, dark room and avoiding sudden head movements. The client should reduce stimulation by not watching television or looking at flickering lights. The client’s diet should be salt restricted to prevent fluid buildup in the ear.

Clients have episodic attacks of vertigo, tinnitus, hearing loss, and feelings of fullness or pressure in the ear.  The disorder typically affects only one ear and can lead to permanent hearing loss.  Attacks of Meniere disease can result in a total loss of proprioception, and clients often report feeling “pulled to the ground” (drop attacks), making client safety a priority.  Vertigo can be severe and is associated with nausea, vomiting, and feelings of anxiety.  Self-care for Meniere disease may include:

• Consuming a low-sodium diet to decrease the potential for fluid excess within the inner ear.  Intake of potassium and other electrolytes does not need to be restricted.
• Limiting or avoiding aggravating substances (eg, nicotine, caffeine, alcohol) and stimuli (eg, flickering lights, watching television) 
• Adhering to prescribed therapies for relief of symptoms (eg, antiemetics, antihistamines, sedatives, and mild diuretics)
• Avoiding sudden changes in the position of the head (eg, bending over) during vertigo spells
• Participating in vestibular rehabilitation therapy
• Implementing safety measures during attacks (eg, assistance with walking, bed rest) 

Increased intraocular pressure can cause damage to the blood vessels and retina and cause potential permanent vision loss. Coughing, vomiting, straining to lift objects (>5 lb), and bending at the waist temporarily increase intraocular pressure and must be avoided after eye surgery. Antiemetic medication is administered as needed following ophthalmic surgery to prevent vomiting.

Glaucoma, acute angle-closure glaucoma, is a form of glaucoma that requires immediate medical intervention. Glaucoma disorders are characterized by increased intraocular pressure (IOP) due to decreased outflow of the aqueous humor, resulting in compression of the optic nerve that can lead to permanent blindness. In acute angle-closure glaucoma, IOP increases rapidly and drastically, which can lead to the following manifestations:

• Sudden onset of severe eye pain
• Reduced central vision
• Blurred vision
• Ocular redness
• Report of seeing halos around lights

Gradual loss of peripheral vision and difficulty adjusting to different lighting are manifestations of chronic open-angle glaucoma. Although further evaluation and treatment are necessary, this condition develops slowly and is not considered an emergency situation. Primary open-angle glaucoma (POAG) is an eye condition characterized by an increase in intraocular pressure and gradual loss of peripheral vision (ie, tunnel vision). The signs/symptoms of POAG develop slowly and include painless impairment of peripheral vision with normal central vision, difficulty with vision in dim lighting, increased sensitivity to glare, and halos observed around bright lights.  POAG can lead to blindness if left untreated.

A cataract is an opacity in the lens of the eye causing cloudy vision with a glare. It is a nonemergency, age-related visual disorder. 

Following cataract surgery, the client will be instructed that for several days (or until approved by the surgeon), activities that may increase intraocular pressure should be avoided to decrease the risk of damage to sutures or surgical site. These include bending (eg, vacuuming floors, playing golf), lifting more than 5 lb, sneezing, coughing, rubbing or placing pressure on the eye, or straining during a bowel movement. Sleeping on 2 pillows will elevate the head of the bed and decrease intraocular pressure. The nurse should encourage this client to increase fluids and fiber in the diet as well as consider an over-the-counter stool softener or laxative. It is common for the client to experience itching (“sand” in the eye), photophobia, and mild pain for several days following surgery.  Purulent drainage, increased redness, and severe pain should be reported.

A cataract is cloudiness (ie, opacity) of the lens that may occur at birth or more commonly in older adults.

The signs/symptoms of a cataract include painless, gradual loss of visual acuity with blurry vision; scattered light on the lens producing glare and halos, which are worse at night; and decreased color perception.

Age-related macular degeneration is a degenerative eye disease that brings about the gradual loss of central vision, leaving peripheral vision intact.

Ocular chemical burns represent potentially blinding ocular injuries and constitute a true ocular emergency requiring immediate assessment and initiation of treatment. The majority of victims are young and exposure occurs at home, work place and in association with criminal assaults. Alkali injuries occur more frequently than acid injuries. Chemical injuries of the eye produce extensive damage to the ocular surface epithelium, cornea, anterior segment and limbal stem cells resulting in permanent unilateral or bilateral visual impairment. Emergency management if appropriate may be single most important factor in determining visual outcome. 

Chemical burns require emergency care to prevent permanent vision loss. Alkali burns (eg, ammonia, cement, lye-containing drain cleanser) are particularly dangerous as they will quickly penetrate deep into the eye, causing severe, irreversible damage. For all types of ocular chemical burns, copious eye irrigation with sterile saline or water should begin immediately to flush the chemical irritant out of the eye.

Before transport to an emergency care facility, tap water can be used for eye irrigation.  If transported by ambulance, emergency care personnel continue irrigation during transport with IV tubing or a Morgan lens.  Irrigation is continued until the pH of the eye returns to normal (pH 6.5-7.5), which typically requires 30-60 minutes depending on the type of chemical.

Conjunctivitis also known as “pink eye”, is a condition that occurs when the conjunctiva (a thin layer of cells covering the front of your eyes) becomes inflamed.

Useful information

Prioritization and Delegation

LPN/LVN who is functioning under the supervision of an RN:

• Administering sulfacetamide sodium 10% to a child with conjunctivitis.
• Reviewing hand-washing and hygiene practices with clients who have eye infections.
• Showing clients how to gently cleanse eyelid margins to remove crusting.
• Performing a routine check of a client’s visual acuity using the Snellen eye chart
• Irrigating the ear canal to loosen impacted cerumen.
• Administering amoxicillin to a child with otitis media.
• Reminding the client not to blow the nose after tympanoplasty.

Unlicensed assistive personnel (UAP) who is functioning under the supervision of an RN:

• Assisting the client with ambulating in the hall.
• Obtaining supplies for hygienic care.

Medications

Atropine for acute angle-closure glaucoma, suggested careful use of atropine in neurological operation to prevent elevated IOP and angle closure glaucoma in susceptible patients 

Gentamycin and ethacrynic acid medications may be causing or contributing to tinnitus.

Phenylephrine, an adrenergic agonist  for intraocular surgery,  this adverse physiologic response is the greatest concern cardiovascular response, such as hypertension or ventricular dysrhythmias.

Ranibizumab injected, for treatment of wet age-related macular degeneration. When the patient experiencing redness, light sensitivity, and pain, have to contact the health care provider immediately for possible eye infection.