Eye movement is a significant function of cranial nerves and utilizes three of the twelve to accomplish the task. Here is a brief on the neuroanatomy of eye movement.
Extraocular Eye Movement- Cranial Nerves
- Oculomotor CNIII: Controls most muscles of the eye
- Trochlear CNIV: Depression and elevation of the eye (up and down), and extorsion and intorsion movements that bring the eyes to the superior and inferior corners of our visual field and orbit.
- Abducens CNVI: Controls outward movement of the eye. The name here helps in memorization as to abduct is to move something away from the midline, or outward.
Testing of ocular movement includes testing the patient’s eye movement in the six cardinal gazes using your finger or a pen to create an H motion while the patient follows the pen or finger with their eyes while keeping the head in a front-facing position. The six cardinal gazes are described in the image below from Mosby’s Medical Dictionary, 9th edition.
To perform an exam of the cardinal gazes begin with your pen or finger directly in front of the person visual field bewteen the eyes aproximately six inches from the face. Instruct the patient to follow your finger movement with their eyes while keeping their head still. Observe the extraocular movements as you move your finger from the starting position directly to the right or left side until the eye has moved to the end of the abduction motion, then move striaght up and down to test the oblique muscles (trochlear CNIV). Return to the lateral position at the same level of the eye, and move across to the opposite eye’s abducted position. Move your finger up and then down. Bring your finger to the center plane and move back to the starting position and you have moved your finger in an H motion from start to finish. Observe the patient’s ability to reach each of these gazes and note the motion itself: is it smooth and continued, or does jerking or drifting occur. You may be able to observe nystagmus, appearing as jerking or “dancing” of the eye and indicative of both physiological and pathological conditions.
To note movement of the pupil, follow the test up with the use of a pen light to cause pupillary constriction. This will test an additional function of the oculomotor CNIII nerve. The first observation is of the pupil size, shape, and relative size compared to the opposite eye under normal lighting without the use of a penlight. This can tell you whether a predominately sympathetic or parasympathetic nerve response is occuring at that time. Parasympathetic innervation results in constriction, and sympathetic innervation in dilation. Next, have a patient look first at an object relatively far away from the patient which should cause dilation of the pupil in normal light, and then to look at an object closer to the patient which should cause constriction.
Use of a penlight or flashlight will aid in further pupillary responses. To perform make sure not to shine the light directly in the patient’s eye, instead bring the light from the outward side (lateral) toward the eye until the edge of light reaches the iris. A response should be observable before the light reaches the pupil itself. Note the size, shape, and whether the response is equal in both eyes. Make sure to observe the reaction of the opposite pupil when shining the light on one side. Pupil sizes are recorded in millimeters (mm) and the use of a pupil size chart can be very useful in aproximating size. We end our discussions with the below pupil size chart. Penlights can be purchased that have the pupil size chart on the side of the device for quick and easy reference since we often are not practiced in measurements involving the metric system. The pupil should constrict with light to aproximately 2 mm.