C

Blind eye

Bitemporal hemianopsia

Left homonymous hemianopsia

Left upper homonymous quadrantanopsia

Left lower homonymous quadrantanopsia

Lower occipital cortex Upper occipital cortex

Lower occipital radiation Upper occipital radiation

Figure 10-11 Visual field defects.

The condition of a deviated, or crossed, eye is strabismus, or tropia. Strabismus is the non-alignment of the eyes in such a way that the object being observed is not projected simultaneously on the fovea of each eye. Esotropia is deviation of an eye nasally; exotropia is deviation of an eye temporally; hypertropia is deviation upward. An alternating tropia is the term used to describe the condition in which either eye deviates. Figure 10-12 shows a patient with a left exotropia.

Perform the Cover Test

The cover test is useful for determining whether the eyes are straight or a deviated eye is present. The patient is instructed to look at a distant target. One eye is covered with a 3 x 5 inch (7.6 x 12.7 cm) card. The examiner should observe the uncovered eye. If the uncovered eye moves to take up fixation of the distant point, that eye was not straight before the other eye was covered. If the eye did not move, it was straight. The test is then repeated with the other eye.

Evaluate the Six Diagnostic Positions of Gaze

An important cause of a deviated eye is paretic (weak), or paralyzed, extraocular muscle(s). Paralysis of these muscles is detected by examination of the six diagnostic positions of gaze.

Because the rotational actions of the oblique and vertical rectus muscles cannot be easily assessed, the eye must be moved into six diagnostic gaze positions that best isolate the vertical actions of these muscles to test their innervations. The oblique muscles are tested in adduction to maximize their vertical action. In contrast, the vertical rectus muscles are tested in abduc-tion;in these positions, the superior rectus acts now as a pure elevator and the inferior rectus as a pure depressor. These diagnostic positions of gaze are the testing positions for the muscles, and the abduction/adduction testing movements are different from the normal actions of the muscles as indicated in Table 10-1.

Hold the patient's chin steady with your left hand, and ask the patient to follow your right hand as it traces a large "H" in the air. Hold your right index finger about 15 to 18 inches (38 to 46 cm) from the patient's nose. From the midline, move your finger about 12 inches (30 cm) to the patient's left and pause;then up about 8 inches (20 cm) and pause, as shown in Figure 10-13; down about 16 inches (40 cm) and pause;up about 8 inches (20 cm);and then slowly back to the midline. Switch your hands, now holding the patient's chin with your right hand. Cross the midline and repeat the finger movements on the other side. These are the six diagnostic positions of gaze. Observe the movement of both eyes, which should follow the finger smoothly. Look for the parallel movements of the eyes in all directions.

On occasion, when looking to the extreme side, the eyes develop a rhythmic motion called end-point nystagmus. There is a quick motion in the direction of gaze, which is followed

Figure 10-13 A and B,

Technique for testing ocular motility.

Figure 10-13 A and B,

Technique for testing ocular motility.

Superior rectus Inferior obliques Superior rectus

Superior rectus Inferior obliques Superior rectus

Inferior rectus Superior obliques Inferior rectus

RIGHT EYE LEFT EYE

Figure 10-14 Diagnostic positions of gaze.

Inferior rectus Superior obliques Inferior rectus

RIGHT EYE LEFT EYE

Figure 10-14 Diagnostic positions of gaze.

by a slow return. This test differentiates end-point nystagmus from pathologic nystagmus, in which the quick movement is always in the same direction, regardless of gaze.

If the eye and eyelid do not move together, lid lag is present.

The six diagnostic positions of gaze, with the related muscles, are illustrated in Figure 10-14, and Table 10-6 summarizes the abnormalities in ocular motility that are caused by paretic muscles.

The images projected on the retina may be interpreted by the brain in one of three ways: fusion, diplopia, or suppression. Fusion and diplopia have already been discussed. In children, strabismus leads to diplopia, which leads to confusion and then suppression of the image and finally to amblyopia. Amblyopia is the loss of visual acuity secondary to suppression. Amblyopia is reversible until the retinas are fully developed, at about the age of 7 years. Amblyopia is a phenomenon that occurs only in children. An adult who acquires strabismus secondary to a stroke, for example, cannot suppress the deviated eye's image and will have diplopia.

Evaluate the Pupillary Light Reflex

Ask the patient to look in the distance while you shine a bright light in the patient's eye. The light source should come from the side, with the patient's nose acting as a barrier to light to the other eye. Observe the direct and consensual pupillary responses. Then repeat the test on the other eye.

The swinging light test is a modification for testing the pupillary light reflex. This test reveals differences in the response to afferent stimuli of the two eyes. The patient fixates on a distant target while the examiner rapidly swings a light from one eye to the other, observing for constriction of the pupils. In some conditions, there is a paradoxical dilatation of the pupil on which the light is shined. This condition, called a Marcus Gunn pupil, is associated with an afferent limb defect in the eye being illuminated.

The most extreme example of an eye displaying the Marcus Gunn phenomenon is a blind eye. When light is shined into the blind eye, there is neither a direct nor a consensual response. When the light is moved to the other eye, there is both a direct and a consensual response because both afferent and efferent pathways are normal. When the light is swung back to the

Table 10-6 Paretic Muscles Causing Abnormal Ocular Motility

Position to Which Eye Paretic Muscle Will Not Turn

Medial rectus Nasal

Inferior oblique Up and nasal

Superior oblique Down and nasal

Lateral rectus Temporal

Superior rectus Up and temporal

Inferior rectus Down and temporal blind eye, no impulses are received by the retina (afferent), and the pupil of the blind eye no longer remains constricted; it dilates. There are different degrees of severity of Marcus Gunn pupils, depending on the involvement of the optic nerve.

Evaluate the Near Reflex

The near reflex is tested by having the patient look first at some distant target and then at a target placed about 5 inches (13 cm) away from his or her nose. When the patient focuses on the near target, the eyes should converge, and the pupils should constrict.

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