Nerve Fiber Analysis

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Nerve Fiber Analysis

Synonym/acronym: NFA.

Common use

To assist in measuring the thickness of the retinal nerve fiber layer, to assist in diagnosing diseases of the eye such as glaucoma.

Area of application

Eyes.

Contrast

N/A.

Description

There are over 1 million ganglion nerve cells in the retina of each eye. Each nerve cell has a long fiber that travels through the nerve fiber layer of the retina and exits the eye through the optic nerve. The optic nerve is made up of all the ganglion nerve fibers and connects the eye to the brain for vision to occur. As the ganglion cells die, the nerve fiber layer becomes thinner and an empty space in the optic nerve, called the cup, becomes larger. The thinning of the nerve fiber layer and the enlargement of the nerve fiber cup are measurements used to gauge the extent of damage to the retina. Significant damage to the nerve fiber layer occurs before loss of vision is noticed by the patient. Damage can be caused by glaucoma, aging, or occlusion of the vessels in the retina. Ganglion cell loss due to glaucoma begins in the periphery of the retina, thereby first affecting peripheral vision. This change in vision can also be detected by visual field testing. There are several different techniques for measuring nerve fiber layer thickness. The equipment used to perform the test determines whether dilation of the pupils is required (by Optical Coherence Tomography) or avoided (by GDX™ Scanning Laser Polarimetry). One of the most common is Scanning Laser Polarimetry. The amount of change in polarization correlates to the thickness of the retinal nerve fiber layer.

This procedure is contraindicated for

    N/A

Indications

  • Assist in the diagnosis of eye diseases
  • Determine retinal nerve fiber layer thickness
  • Monitor the effects of various therapies or the progression of conditions resulting in loss of vision

Potential diagnosis

Normal findings

  • Normal nerve fiber layer thickness

Abnormal findings related to

  • Glaucoma or suspicion of glaucoma
  • Ocular hypertension
  • Optic nerve disease

Critical findings

    N/A

Interfering factors

  • Factors that may impair the results of the examination

    • Inability of the patient to fixate on focal point.
    • Corneal disorder that prevents proper alignment of the retinal nerve fibers.
    • Dense cataract that prevents visualization of a clear nerve fiber image.
    • Inability of the patient to cooperate or remain still during the test because of age, significant pain, or mental status.

Nursing Implications and Procedure

Pretest

  • Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
  • Patient Teaching: Inform the patient this procedure can assist in diagnosing eye disease.
  • Obtain a history of the patient’s complaints, including a list of known allergens.
  • Obtain a history of narrow-angle glaucoma. Obtain a history of known or suspected visual impairment, changes in visual acuity, and use of glasses or contact lenses.
  • Obtain a history of the patient’s known or suspected vision loss, including type and cause; eye conditions with treatment regimens; eye surgery; and other tests and procedures to assess and diagnose visual deficit.
  • Obtain a history of symptoms and results of previously performed laboratory tests and diagnostic and surgical procedures.
  • Obtain a list of the patient’s current medications, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values).
  • Instruct the patient to remove contact lenses or glasses, as appropriate. Instruct the patient regarding the importance of keeping the eyes open for the test.
  • Review the procedure with the patient. Explain that the patient will be requested to fixate the eyes during the procedure. Address concerns about pain and explain that no pain will be experienced during the test, but there may be moments of discomfort. Explain to the patient that some discomfort may be experienced after the test when the numbness wears off from anesthetic drops administered prior to the test. Inform the patient that a health-care provider (HCP) performs the test and that to evaluate both eyes, the test can take 10 to 15 min.
  • Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
  • Note that there are no food, fluid, or medication restrictions unless by medical direction.

Intratest

  • Potential complications: N/A
  • Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient.
  • Instruct the patient to cooperate fully and to follow directions. Ask the patient to remain still during the procedure because movement produces unreliable results.
  • Seat the patient comfortably. Instruct the patient to look straight ahead, keeping the eyes open and unblinking.
  • Instill topical anesthetic in each eye, as ordered, and allow time for it to work. Topical anesthetic drops are placed in the eye with the patient looking up and the solution directed at the six o’clock position of the sclera (white of the eye) near the limbus (gray, semitransparent area of the eyeball where the cornea and sclera meet). Neither the dropper nor the bottle should touch the eyelashes.
  • The equipment used to perform the test determines whether dilation of the pupils is required (OCT) or avoided (GDX).
  • Request that the patient look straight ahead at a fixation light with the chin in the chin rest and forehead against the support bar. The patient should be reminded not to move the eyes or blink the eyelids as the measurement is taken. The person performing the test can store baseline data or retrieve previous images from the equipment. The equipment can create the mean image from current and previous data, and its computer can make a comparison against previous images.

Post-Test

  • Inform the patient that a report of the results will be made available to the requesting HCP, who will discuss the results with the patient.
  • Recognize anxiety related to test results, and be supportive of impaired activity related to vision loss or anticipated loss of driving privileges. Discuss the implications of abnormal test results on the patient’s lifestyle. Provide teaching and information regarding the clinical implications of the test results, as appropriate. Provide contact information, if desired, for the Glaucoma Research Foundation (www.glaucoma.org).
  • Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Instruct the patient in the use of any ordered medications, usually eye drops. Explain the importance of adhering to the therapy regimen, especially because glaucoma does not present symptoms. Instruct the patient in both the ocular side effects and systemic reactions associated with the prescribed medication. Encourage him or her to review corresponding literature provided by a pharmacist. Answer any questions or address any concerns voiced by the patient or family.
  • Depending on the results of this procedure, additional testing may be performed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.

Related Monographs

  • Related tests include fundus photography, gonioscopy, pachymetry, slit-lamp biomicroscopy, and visual field testing.
  • Refer to the Ocular System table at the end of the book for related tests by body system.
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