I was given this assignment and i found it challenging so i decided to add to my blog. i hope optometry students would find it useful.
NAME: UDABOR EROMOSELE G.
DEPARTMENT OF OPTOMETRY, UNIVERSITY OF BENIN, BENIN CITY, NIGERIA
SUMMARY OF GRADIENT AC/A RATIO PROCEDURE USING WWW.123 (BAZUAYE TRIAD)
W1 (WHERE): It is usually carried out in the examination room
(WHICH EYE): The patient uses his two eyes to accommodate and converge. The examiner uses his two eyes to observe and measure the vergence using prism cover test.
ACQUAINTANCE WITH INSTRUMENT: The instrumentation for this test is relatively simple and instrument needed are lens (+/- 1.00DS to 3.00DS) and prism bar.
ADVANTAGES OF GRADIENT AC/A METHOD:
-It is quick and easy to carry out.
-It gives the clinician immediately feedback.
-Proximal and fusional vergence are eliminated, therefore they do not aster the final result.
INDICATIONS OF GRADIENT AC/A METHOD:
-Accommodative esotropia with convergences excess.
-Intermittent distant exotropia to differentiate true and stimulated distant exotropia.
-Measurement can be useful in all deviation in which there is a significant difference between the near and far deviation.
BENEFIT OF GRADIENT AC/A METHOD:
AC/A ratio can be useful when determining lens power for the optical correction got convergence problem
SIGNIFICANCES OF GRADIENT AC/A METHOD:
–High AC/A indicates convergence excess problem or latent hyperopia which signifies that patient depends on his accommodation for convergence
–A low AC/A ratio mane indicative of a convergence insufficiency or a convergence weakness problem. This means that the patient does not depend on his accommodation for convergence.
CONTRAINDICATION OF GRADIENT AC/A METHOD:
-The AC/A ratio obtained using conventional far gradient method is significantly biased by the lag lf accommodation and thus does not always represent the actual relationship between accommodation and vergence control system.
LIMITATION OF GRADIENT AC/A METHOD:
-It is sometimes difficult for the patient to exert accommodation to clear the letter or picture at 6cm through concave lenses. It is not always possible to increase the lens strength to 3.00DS even in children and he ratio must be measured using -1.00DS or -2.00DS lenses instead.
APPROPRIATENESS OR INSTRUMENTS USED FOR GRADIENT AC/A:
–The lens and prism used should be clean and of high optical standard
–concave lens is used when carry out the test at 6cm
–convex lens is used when carrying out the test at 33cm
Also the room, local and target illumination should be such that it is similar to the patient’s habitual illumination level.
The patient is seated on the examination chair at a height which is easy for the examiner to manipulate the trial frame.
The patient looks straight ahead at the target because for clinical purpose deviation is measured at primary position of gaze.
Also the patient is asked to focus on the target and ensure that the target is seen clearly throughout the procedure.
Gradient AC/A ratio should be carried out when symptom of abnormal AC mechanism is present e.g. blur vision, diplopia, ocular discomfort during or immediately after near work, frontal headache, nausea, general fatigue, patient with refractive error must be corrected.
EXPLAINING REASON FOR TEST AND WHY:
Accommodation is increase in the optical power of the eye in order to maintain clear images as object are moved closer.
Convergence is the inward movement of the two eyes towards each other. Convergence and accommodation works harmoniously for clear single binocular vision to be achieved.
When accommodation is exerted, the eyes are induced to converge which is known as AC. The muscles involved in the two situations get their nerve supplies from oculo-motor nerve CNII.
Here the change in stimulus to accommodate is produced by means of ophthalmic lens, not change in viewing distance. For a given fixation distance minus lenses placed before the eyes increase the requirement for accommodation and plus lens relax accommodation.
We assume that -1.00DS lens produce equivalent of 1D of accommodation whereas +1.00DS of lens relax accommodation by 1D and that the accommodative convergences response to the lens is measured.
DISCUSSIONS AVOUT INSTRUCTIONS ON HOW PATIENT OUGHT TO RESPOND:
–The patient is asked to fixate at a target with details ensure it is seen clearly throughout the procedure. if the patient is too young for fixation, picture should be used and the child asked about the details on it. Time is allowed for accommodation to take place as soon as the patient reports seeing the target clearly, prism cover test is carried out.
2. BASIC ORIENTATION IN GRADIENT AC/A METHOD:
–The patient is seated on an examination chair.
-The target is either at 6m or 33cm from patient.
The examiner is by the side of the patient.
-The trial frame is worn by the patient (where plus or minus lens are placed)
The prism used to measure the vergence is either placed on the trial frame or hand held at about 14mm from the eyes.
–The test is carried out at the primary position of gaze.
3. BEGINNING THE TASK OF EXAMINATION
PRESENTATION OF FIXATION TARGET
Two target distances can be used depending on the type of lens used. Concave lens used when using 6m, convex lens used when using at 33cm.
Whatever the distance of the target from the patient, what is important is that the patient sees it clearly and the target (snellen’s letter) has accommodative properties i.e. details (e.g. letter E).
PERFORMING TASK (GRADIENT AC/A):
This is performed with the determination of the difference in the deviation of the eyes by the prism and alternate cover test for a given distance of fixation (with the use of a target which excites the appropriate amount of accommodation). After placing concave or convex sphere lens in front of the eyes so that there is a change in accommodation i.e. induce or relieve respectively, a change in convergences (positive or negative). In this way the AC/A ratio is the difference between the deviation after placing the spherical lens and the original deviation in prism diopters, with a division of the differences by the power of the lens used in determination.
USING CONCAVE LENS FOR GRADIENT AC/A :
-The patients wear his refractive correction
-The deviation is measured by prism cover test at 6m, using a distant fixation target.
-Concave lenses are inserted into trial frame in strength up to 3.00DS; Time allowed for accommodation to take place, as soon as target is seen clearly, the prism cover test is repeated.
-For young patient an amblyoscope may be necessary to measure the deviation.
USING CONVEX LENS FOR GRADIENT AC/A:
-The deviation is measured using an accommodative target e.g. near VA card at 33cm.
-Convex lenses are inserted into trial frame in strength up to +3.00DS.
-The patient must relax his accommodation in to see the target clearly.
-The prism cover test is then repeated.
-This method is particularly applicable in differentiating true and stimulated distance exotropia.
CLINICAL READINGS AND MEASUREMENTS FOR GRADIENT AC/A:
Gradient AC/A ratio is calculated with this formula
AC/A = d1-d2 /D
d1 = deviation with supplementary lens in prism dioptres.
d2 = deviation without supplementary lens in prism dioptres.
D = power of supplementary lens in dioptres sphere.
(NB the magnitude of exodeviation is calculated with negative sign while esodeviation is calculated with positive sign)
FREQUENCY OF PERFORMING GRADIENT AC/A:
Borderline patient should have their gradient AC/A ratio checked through two lenses for accurate diagnosis and management.
CLINICAL RESULT AND GRADING FOR GRADIENT AC/A:
Expected AC/A ratio is 4/1 with standard deviation of +/-2 i.e. between 2/1 and 6/1 is normal (Morgan 1944).
If AC/A is greater than 6/1 it is considered high.
If AC/A is less than 2/1 it is considered low.
-It is recorded as AC/A or AC:A in figures.
-It should be recorded clearly and legibly.
- AMAECHI, O.U. and OBIORA, I.====== JNOA-VOL11, 2004
- CATHERINE L. HEYMAN, O.D.
- KENNETH WYBAR
BY UDABOR EROMOSELE G.