Ultrasound, A-scan(redirected from Amplitude modulation scan)
Synonym/acronym: Amplitude modulation scan, A-scan ultrasound biometry.
To assess for ocular tissue abnormality related to lens replacement in cataract surgery.
Area of applicationEyes.
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.
This procedure is contraindicated for
- Determination of power requirement for replacement intraocular lens in cataract surgery.
- Normal homogeneous ocular tissue
Abnormal findings related to
Factors that may impair the results of the examination
- Inability of the patient to cooperate and remain still during the procedure may interfere with the test results.
- Rubbing or squeezing the eyes may affect results.
- Improper placement of the probe tip to the surface of the eye may produce inaccurate results.
Nursing Implications and Procedure
- Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
- Patient Teaching: Inform the patient this procedure determines the strength of the lens that will be replaced during cataract surgery.
- Obtain a history of the patient’s complaints, including a list of known allergens, especially topical anesthetic eyedrops.
- Obtain a history of the patient’s 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.
- 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 online at DavisPlus).
- 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 discomfort will be experienced during the test but that some discomfort may be experienced after the test when the numbness wears off from the anesthetic drops administered prior to the test. Inform the patient 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.
- 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.
- 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.
- Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
- Instill topical anesthetic in each eye, as ordered, and provide 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.
- Ask the patient to place the chin in the chin rest and gently press the forehead against the support bar. When the US probe is properly positioned on the patient’s surgical eye, a reading is automatically taken.
- Multiple measurements may be taken in order to ensure that a consistent and accurate reading has been achieved. Variability between serial measurements is unavoidable using the applanation technique.
- 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, anticipated loss of driving privileges, or the possibility of requiring corrective lenses (self-image). Discuss the implications of test results on the patient’s lifestyle. Reassure the patient regarding concerns related to the impending cataract surgery. Provide teaching and information regarding the clinical implications of the test results, as appropriate.
- Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. 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 tests include refraction and slit-lamp biomicroscopy.
- Refer to the Ocular System table at the end of the book for related tests by body system.
Handbook of Laboratory and Diagnostic Tests, © 2013 Farlex and Partners