Ординатура / Офтальмология / Английские материалы / Practical Ophthalmology A Manual for the Beginning Ophthalmology Residents 4th edition_Wilson_1996

108 Chapter 6: Ocular Motility Examination

Ocular Motility Examination

left gaze)

•Tertiary positions (the four oblique positions of gaze: up and right, up and left, down and right, and down and left)

•Cardinal positions (up and right, up and left, right, left, down and right, down and left)

•Midline positions (straight up and down from the primary position)

The presence of any nystagmus (involuntary rhythmic eye jerks) should be noted. Latent (occlusion) nystagmus (LN) is a congenital, conjugate, horizontal jerk nystagmus that occurs under monocular viewing conditions. In this condition, when one eye is occluded (as is done for cover testing) nystagmus develops in both eyes, with the fast phase directed toward the covered eye. Binocular vision is usually tested next, when possible.

The deviation in alignment is checked and quantified by means of the corneal light reflection test and/or the cover tests with the use of prisms. At this point, the patient's dominant eye can also be determined. To check for ocular dominance, a fixation object is held at approximately 50 cm in the midplane in front of the patient. The object is moved steadily toward the patient until one eye loses fixation and turns out. The eye that is able to maintain fixation is the dominant eye. Ocular dominance is additionally confirmed by determining the near point of convergence (see Clinical Protocol 4.4). The normal near point of convergence is between 6 cm and 10 cm and is relatively independent of patient age.

Accommodative convergence of the visual axes occurs as part of the near reflex. A fairly consistent increment of accommodative convergence (AC) occurs for each diopter of accommodation (A), and the relationship can be expressed as the accommodative convergence/ accommodation (AC/A) ratio. The normal AC/A ratio is 3A to 5A of accommodative convergence for each diopter of accommodation. Abnormalities of this ratio are common and are an important cause of strabismus. If the AC/A ratio is abnormally high, the excess convergence tends to produce esotropia during accommodation, or focusing on near targets. If the AC/A ratio is abnormally low, the eyes will tend to have an exodeviation when the individual looks at near targets. Two techniques of measuring the AC/A ratio are described in Clinical Protocol 6.2, but for other methods and for a more detailed discussion of the clinical implications of this interrelationship, refer to comprehensive texts such as the resources listed at the end of this chapter.

Tests of Binocularity and Fusion

Subjective testing of the patient's fusional ability is an important part "'<'*.<. of the ocular examination. Such tests evaluate both the patient's stereopsis, or fine depth perception (commonly called three-dimensional viewing), and gross binocular status. As noted above, these tests are

usually performed prior to dissociative tests that require occlusion.

Stereopsis

The term stereopsis refers to the binocular way in which we perceive details of depth, distinct from the monocular cues, such as motion parallax and light and shadow, that allow us to appreciate gross depth perception. In this way, we order things in space and judge relative nearness of objects to us. Fusion describes the ability of the brain to blend (fuse) separate images from the two eyes into a single image. Fusion is necessary to achieve high-grade stereopsis, defined as at least 40 seconds of arc. (Stereoacuity is always quantified in seconds of arc.) Patients without demonstrable fusion may use monocular cues for depth perception, such as differences in heights of objects. The ability

;to correctly identify stereoscopic targets does not automatically indicate

Other Tests of Binocular Status

Tests of Binocularity and Fusion HI

Worth four-dot test

The Worth four-dot test, performed at both distance and near, provides information about the patient's gross binocular status. The patient wears a pair of eyeglasses with a red filter over one eye and a green filter over the other eye (Figure 6.4). If there is a strong preference already noted by the examiner, the red lens is placed over the nondominant, or deviating, eye; otherwise the red lens is placed in front of the right eye. Red light is visible to the eye behind the red lens, but not green light because the red lens absorbs these wavelengths. Green light is visible to the eye behind the green lens, but not red light because the green lens absorbs these wavelengths. The patient looks at a target composed of four lighted dots: one red, two green, and one white. Looking at the target in the distance stimulates primarily central vision. When the patient views the test at 16 inches (40 cm), peripheral fusion is stimulated. Different responses may be obtained. Patients with normal ocular alignment will report seeing a total of four lights, indicating fusion. If the eye behind the green lens is suppressed, a total of two red lights is reported. Seeing three green lights indicates suppression of the eye behind the red lens. If the patient reports a total of five lights, two red and three green, determine it the lights are all seen simultaneously. If two red and three green are seen simultaneously, diplopia is present. If the red and green are seen in rapid succession, alternating suppression is present. If there is diplopia, note il it is crossed or uncrossed. Uncrossed images imply an esodeviation, whereas crossed imares indicate an exodeviation. The sinsrle w hire light may be noted as red or green if one eye is strongly dominant. It may also be reported as pink or as a mixture of the two colors it there is no strong preference for either eye.

Results are recorded (for both distance and near) either as orthophoria (or fusion), as suppression (state which eye), or as diplopia (clarify the type of diplopia when applicable).

Figure 6.4 TheWo'"; ••.ii,- dot test for near.The f;.-;-;h 'grit target, which illuminate^ the four dots, is held by tl v --:x,.im- iner. A larger, wall-mounrod illuminated target with a similar distribution of the foui lights is used for the a-snrre test (not shown).The oai.ent wears the same red-an i green glasses for born v .is.

112 Chapter 6: Ocular Motility Examination j

placed before the patient's preferred eye, and the patient is instructed to fixate on the light. Placing the striations horizontally allows the patient to see tlie line vertically; placing them vertically allows the patient to see the line horizontally. The patient is asked if a line is seen while looking at the fixation light. There are four possible responses:

•The line is not seen (indicates suppression of the eye that is looking through the Maddox rod)

•The line goes through the light (indicates fusion and suggests that orthophoria is present)

•The line is above or below the light (indicates vertical diplopia

and, concurrently, the presence of a vertical deviation)

•The line is to the left or to the right of the light (indicates horizontal diplopia and the presence of a horizontal deviation)

The examiner records the patient's responses for both distance and near as for the Worth four-dot test.

Tests of Alignment

Red Reflex Test

ter 6: Ocular Motility Examination

Figure 6.6 Hirschberg's method of performing the corneal light reflection test to estimate deviation. (A) Normal alignment. (B) 15° esotropia. (C) 30° esotropia.

(D) 45° esotropia.

Krimsky's method

Krimsky's method utilizes reflections from both corneas and is the preferred approach for assessing ocular deviation in a patient with poor vision in the deviating eye. Reflections from both eyes are produced by an appropriately placed penlight fixated by the patient's better eye. Prisms of increasing or decreasing power are placed in front of the fixating eye until the corneal reflection is centered in the deviating eye (Figure 6.7). Because of differences in prism power calibration, plastic prisms should be held with the rear surface in the frontal plane to achieve approximately the desired effect. Glass prisms should be held with the rear surface perpendicular to the eye's visual axis. The modified Krimsky's method of performing the corneal light reflection test is summarized in Clinical Protocol 6.4.