Pupillary Response: The pupil of the eye is actually a hole surrounded by a circular muscle (the iris). The iris controls the amount of light entering the eye by making the pupil larger (dilation) or smaller (constriction) . Since it is an automatic response, doctors often use the pupillary response test to check whether the automatic nervous system is working. The pupillary response test involves shining a light at the child's face (never directly into the eyes) and watching what the irises do; they should make the pupils smaller when the light is shining on the face; they should make the pupils larger when the light is taken away. In infants, the pupillary response is less pronounced than in older children. (The dilator muscle does not reach full maturity until the child is about 5 years old.) Medications that relax muscles can also make the iris less responsive, and the pupillary response an unreliable test. In children with dark irises, it may be difficult to differentiate pupils from the irises.
Blink Response: The eyelids automatically close when an object approaches unexpectedly and rapidly; this reflex is a protective behavior. When used as a diagnostic tool , it is usually after other tests have been unsuccessful ; since the blink response is an automatic reaction, it only indicates that the eyes are reacting to a gross threat. If used in a vision screening protocol, it should be a final measure (i.e., not among the initial procedures), since it may be perceived as threatening by the child. The probability is that other screening tests will have elicited enough information about visual functioning that the blink response measure will be unnecessary.
Convergence: When visual attention shifts from distance to nearpoint, the eyes automatically converge (move slightly towards the nose). (This assumes that the eyes are normally aligned and automatic brain control is operational . ) Convergence can be elicited by having a child maintain attention on an object as the object is brought towards the face. The eyes should move inwards until the object is about 3"-5" from the face. Since ocular muscles are not usually fully coordinated until about 6 months of age, convergence testing is inappropriate before that time. After 6 months, the ability to converge should be present, but eliciting the behavior may depend more on whether the child is interested in the object and will attend long enough to demonstrate convergence
Muscle Balance: The two eyes are meant to operate in alignment (moving together and in the same positions). If they are not in alignment (not "straight"), or are unable to move at the same speed and direction at the same time ' the brain receives images from the two eyes that are too dissimilar to fuse into a single image, and double vision" (dyplopia) occurs. There are several ways of testing for muscle balance:
- Corneal Light Reflex: When a penlight is held about 12" from a child's face and pointed at his/her forehead, the reflection of the light should be at exactly the same location in each eye. If it is not, the eyes are not "straight" and referral is indicated. (This test is most reliable for children 6 months of age or older.)
- Cover Test: As the child looks at an object (or light), held about 12" from the child's face, quickly cover - then uncover - one eye. If there is a latent muscle imbalance, the covered eye will "wander" while covered, and move back into alignment when uncovered. Repeat with the other eye. If in doubt, repeat the test. Not many children fail this test, but it is worth performing if only one child is discovered who needs referral
- Tracking: Although this procedure is usually listed by itself, it is actually a measure of how well the eyes work together. Since eye muscles move the eyes, it is indirectly a measure of ocular muscle balance. The child looks at an object (or a light, or a face) and follows it with his/her eyes as the object is moved to the left, to the right (crossing midline), up, and down. Infants younger than 6 months of age may not be able to perform this entire test satisfactorily, since their eye muscles may not be fully coordinated yet, but they may be able to follow a slowly moving target for short distances. Between 6 months and a year, smoother tracking may be elicited, however many children cannot yet separate eye and head movements. The examiner should watch the child's eyes and note whether they seem to be moving in a coordinated manner.
Fixation: This term refers to the ability of the child to look at an object for at least several seconds (2-3 seconds). The variable factors are the size of the object and the distance at which it is viewed. Objects should be silent. Two viewing distances are used: Nearpoint (8" 18") and distance (10'). Three sizes of objects are used: a 4" object for both nearpoint and distance; a I " object at nearpoint, and a smaller object (e.g., a raisin or Cheerio) at nearpoint. A child less than 6 months may be unable to do all of these tests satisfactorily, since acuity may not be good enough; using a black and white target may improve the chances for success. After about 6 months, a child should be able to locate and look at all three targets at nearpoint, and should be able to attend to larger objects at 10'. Around a year, the child should be able to complete all tests for fixation satisfactorily. Shifting Gaze is an extension of fixation screening. It simply requires the child to look from one target to another of equal size and complexity at about 12" from the child, and about 12" apart.
Eye Preference: This is a negative screening procedure, since the child should not be using one eye in preference to the other; he/she should be using both eyes simultaneously. If there is evidence of a preferred eye (noticeable head turning or head tilting, or consistently holding a toy to one side to look at it), the child should be referred.
Fields Testing: This procedure requires the child -to maintain attention to an object or face "front and center" while another object is brought from behind the child slowly into the line of vision in an arc from the left, from the right, from above, and from below. When the child notices the moving object, a head turn is almost automatic. Obviously, the test is difficult for young children who have difficulty maintaining the "front and center" orientation for any period of time. In addition, some children will attend to the examiner's hand, rather than the "front and center" head position, and results will be unreliable.
General Rules: Screening procedures should be practiced before applied. 7bere are ways of combining procedures so that entire screening time is less than 15 minutes. More important than the actual screening procedures is the examiner's ability to observe visual behaviors. It may be possible to collect information through observation that will provide indicators of a visual impairment before screening occurs, thus shortening the time needed to do the screening. For example, a child who brings all objects to his/her face for viewing will be unlikely to perform distance tasks; or, the child whose eyes are never steady will have difficulty tracking and will probably fail the corneal reflection test.