Ultrasound, A-scan

Synonym/Acronym:
Amplitude modulation scan, A-scan ultrasound biometry.

Rationale
To assess for ocular tissue abnormality and to determine the power of the intraocular lens required for replacement in cataract surgery.

Patient Preparation
There are no food, fluid, activity, or medication restrictions unless by medical direction.

Normal Findings

  • Normal homogeneous ocular tissue.

Critical Findings and Potential Interventions
N/A

Overview

(Study type: Ultrasound; related body system: Nervous system.) Diagnostic techniques such as A-scan ultrasonography can be used to identify abnormal tissue. The A-scan employs a single-beam, linear sound wave to detect abnormalities by returning an echo when interference disrupts its straight path. When the sound wave is directed at lens vitreous, the normal homogeneous tissue does not return an echo; an opaque lens with a cataract will produce an echo. The returning waves produced by abnormal tissue are received by a microfilm that converts the sound energy into electrical impulses that are amplified and displayed on an oscilloscope as an ultrasonogram or echogram. The A-scan echo can be used to indicate the position of the cornea and retina. The A-scan is most commonly used to measure the axial length of the eye. This measurement is used to determine the power requirement for an intraocular lens used to replace the abnormal, opaque lens of the eye removed in cataract surgery. There are two different methods currently in use. The applanation method involves placement of an ultrasound (US) probe directly on the cornea. The immersion technique is more popular because it does not require direct contact and compression of the cornea. The immersion technique protects the cornea by placement of a fluid layer between the eye and the US probe. The accuracy of the immersion technique is thought to be greater than applanation because no corneal compression is caused by the immersion method. Therefore, the measured axial length achieved by immersion is closer to the true axial length of the cornea.

Indications

  • Determination of power requirement for replacement intraocular lens in cataract surgery.

Interfering Factors

Factors that may alter the results of the study

  • Rubbing or squeezing the eyes may affect results.
  • Improper placement of the probe tip to the surface of the eye may produce inaccurate results.

Potential Medical Diagnosis: Clinical Significance of Results

Abnormal Findings In:

  • Cataract

Nursing Implications

Before the Study: Planning and Implementation

Teaching the Patient What to Expect

  • Inform the patient this procedure determines the strength of the lens that will be replaced during cataract surgery.
  • Review the procedure with the patient. Address concerns about pain and explain that some discomfort may be experienced after the test when the numbness wears off from the anesthetic drops administered prior to the test.
  • Explain that a health-care provider (HCP) performs the test in a quiet, darkened room and that evaluation of the eye upon which surgery is to be performed can take up 10 min.
  • Explain to the patient that he or she will be comfortably seated for the procedure and instructed to look straight ahead, keeping the eyes open and unblinking.
  • Explain that a topical anesthetic is instilled in each eye and given time to work.
  • Explain that the chin is placed in a chin rest and the forehead gently pressed against the support bar. Once the US probe is properly positioned on the surgical eye, a reading is automatically taken.
  • Multiple measurements may be taken to ensure that a consistent and accurate reading has been achieved. Variability between serial measurements is unavoidable using the applanation technique.

Potential Nursing Actions

  • Investigate the patient's history of known or suspected vision loss; changes in visual acuity, including type and cause; use of glasses or contact lenses; eye conditions with treatment regimens; eye surgery; and other tests and procedures to assess and diagnose visual deficit.
  • Advise the patient that he or she will be asked to remove contact lenses or glasses, as appropriate.

After the Study: Potential Nursing Actions

Treatment Considerations

  • Reassure the patient regarding concerns related to potential impending cataract surgery.
  • Be supportive of impaired activity related to vision loss, anticipated loss of driving privileges, or the possibility of requiring corrective lenses (self-image).

Followup Evaluation and Desired Outcomes

  • Acknowledges that some temporary or permanent lifestyle changes may be required depending on procedure results. Additional testing may be performed to monitor disease progression and determine the need for a change in therapy.

Ultrasound, A-scanis the Nursing Central Word of the day!