Nerve Fiber Analysis

General

Synonym/Acronym:
NFA.

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

Patient Preparation
There are no food, fluid, activity, or medication restrictions unless by medical direction. Ensure that the patient who will have a dilated test understands that they must refrain from driving until the pupils return to normal (about 4 hr) after the test and has made arrangements to have someone else be responsible for transportation after the test.

Normal Findings

  • Normal nerve fiber layer thickness.

Critical Findings and Potential Interventions
N/A

Overview

(Study type: Sensory [ocular]; related body system: Nervous system.)

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. Nerve fiber layer measurements are also useful in assessing the severity of diabetic retinopathy; optic nerve thickness diminishes as the disease progresses. 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 [OCT]) 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.

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.

Interfering Factors

  • 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.

Potential Medical Diagnosis: Clinical Significance of Results

Abnormal Findings In:

  • Diabetes related to thinning of the retinal nerve layers
  • Glaucoma or suspicion of glaucoma
  • Ocular hypertension
  • Optic nerve disease

Nursing Implications, Nursing Process, Clinical Judgement

Before the Study: Planning and Implementation


Teaching the Patient What to Expect

  • Review the procedure with the patient.
  • Discuss how this procedure can assist in diagnosing eye disease.
  • Explain that the procedure evaluates both eyes and takes about 10 to 15 min. No pain will be experienced during the test, but there may be moments of discomfort after the test when the numbness wears off from anesthetic drops administered prior to the test.

Procedural Information

  • The patient is seated comfortably during the test.
  • Topical anesthetic drops are instilled in each eye and allowed time to take effect.
  • The ordered mydriatic is administered to each eye if dilation is to be performed. The patient is advised there may be a brief stinging sensation when the drop is put in the eye, but no discomfort will be experienced during the examination. Remind the patient that the dilating effects of ordered mydriatic drops may last up to 4 to 6 hr.
  • The equipment used to perform the test determines whether dilation of the pupils is required (OCT) or avoided (GDx).
  • The patient is asked to look straight ahead, keeping the eyes open and unblinking, at a fixation light with the chin in the chin rest and forehead against the support bar. The patient is reminded of the importance that there be no movement of the eyes or blinking during the test as the measurement is taken.
  • Once the examination is completed, the patient is advised to gently rinse the face, especially eyelids and corners of the eye, to remove any crystallized residue of the dilation drops. Gentle rinsing and drying by dabbing without rubbing is crucial to avoid causing a corneal abrasion. Rinsing helps decrease the time the eyes remain dilated by removing residue that otherwise would be washed back into the eye when the patient blinks.
  • Baseline data are stored, and the mean image from current and previous data can be retrieved as the computer makes a comparison against previous images.

Potential Nursing Actions

  • Investigate 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.
  • Investigate history of narrow-angle glaucoma, known or suspected visual impairment, changes in visual acuity, and use of glasses or contact lenses.
  • Advise the patient to remove contact lenses or glasses, as appropriate.

After the Study: Implementation & Evaluation Potential Nursing Actions


Treatment Considerations

  • Discuss the use of any ordered medications, usually eyedrops.
  • Explain the ocular adverse effects associated with the prescribed medication and encourage a review of corresponding literature provided by a pharmacist. Discuss the importance of adhering to the therapy regimen, especially since glaucoma does not present symptoms.
  • Be supportive of impaired activity related to vision loss or anticipated loss of driving privileges.

Clinical Judgement

  • Consider ways to facilitate activities designed to assist with adaptation to any vision loss.

Followup Evaluation and Desired Outcomes

  • Acknowledges contact information provided for the Glaucoma Research Foundation (www.glaucoma.org).

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