Visual acuity V A It is the measurement

Visual acuity (V. A) - It is the measurement of spatial resolving capacity of the eye and is applied to central vision.

Importance of testing V. A Ø Testing V. A is very important in all of the cases because of: 1. It gives us an accurate diagnosis for the patient case and indicate the severity of the problem. 2. It may helps us to discover a new problem at the patient, which is not the main problem that the patient complaint form. 3. It is also help us in the following up the patient’s case, to compare between pre and post the treatment. Ø For all of these reasons V. A must be tested for all of the patients before visiting the ophthalmologists. Ø And this is the mission of the optometrists, who is the specialists examiner in these examinations.

Testing V. A in preverbal children 1. 2. 3. 4. 5. 6. Occlusion of one eye Fixation test ‘Hundreds and thousands’ sweet test Rotation test Optokientic nystagmus drum. Forced choice preferential looking tests

Occlusion of one eye Ø It is a simple method for testing the V. A in preverbal children and infants. Ø We cover one eye, and if strongly objected by the child, indicates poorer acuity in the other eye. Ø This method just indicate if the V. A is good or poor and does not give us an accurate measurements.

Fixation test Ø This is a very good method in testing V. A and it is performed as following: 1. A 16 ∆ base-down over one eye and the other eye is occluded. 2. The eye with prism is therefore forced to take up fixation. 3. Fixation is then graded as central or non-central; steady or unsteady. 4. The other is uncovered and the ability to maintain fixation is observed. 5. If fixation immediately returns to the uncovered eye, then V. A is impaired. 6. if fixation is maintained through a blink then V. A is good. 7. if fixation is alternate then V. A is equal. 8. Repeat the test with prism in front of the other eye.

Correlation between V. A and fixation patterns Gross eccentric fixation or affixation 1. 50/60; 5/200 Unsteady central fixation less than 10 seconds 4/60; 20/300 Steady central fixation, but will not hold fixation when cover is removed 6/60 -6/24; 20/10020/200 Steady central fixation will hold fixation with deviating eye when cover is removed, but prefers fixation with the other eye. 6/18 -6/9; 20/3020/70 Alternates spontaneously, hold well with both eyes, cross fixation 6/6; 20/20 B. E

'Hundreds and thousands' sweet test: Ø If the child pick small sweets; VA=6/24

Rotation test - Is a gross qualitative test for the infant to fixate with both eyes open. 1. The examiner holds the infant facing him and rotates briskly 360° 2. The child fixates moving targets behind the examiner, if vision is normal, the eyes will display Optokientic nystagmus. 3. When rotation stops, nystagmus stop 3 -5 seconds due to suppression of post-rotatory nystagmus by fixation. 4. If vision is severely impaired, no nystagmus but eye deviates to one side.

Optokientic nystagmus drum - Standardised drum that contain strips, which subtend small fractions of the infant’s visual field, are available. (alternating black and white strips with sharp, distinct interfaces). - They are spun at varying, uncelebrated rates and are bathed in variable illumination. - This method measures acuity by means of a motor response technique (eye movement).

Forced choice preferential looking - This behavioral technique is based on the observation that infants prefer to view a pattern stimulus than homogenous field. - The child must be alert and able to generate neck and eye movements like Optokientic nystagmus technique, vision is evaluated by means of a motor response. - Teller acuity cards: black strips of varying thickness. - Cardiff acuity cards: shapes of variable outline (bold lines- thin lines). - It used from early infancy.

Draw backs: - The strips in one orientation (vertical) only, children with uncorrected astigmatism may be erroneously estimated. - Children who have nystagmus unable to fixate on the targets accurately. - Children who have visual field defects may have difficulty finding the targets.