Optic Nerve (CN 2)
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I. Visual Acuity
- Visual acuity is the ability of the eye to identify a visual object. Visual acuity is most commonly assessed using the Snellen eye chart as follows:
- Corrective lenses/pinhole. To accurately test visual acuity, ensure that the patient’s visual acuity is not altered by refractive errors: errors that place the visual object in front of or behind the patient’s retina (such as myopia or hyperopia). Ask the patient to wear his/her corrective lenses (eyeglasses) during the test. If visual acuity is abnormal and the patient does not have corrective lenses, provide the patient with a pinhole occluder (an occluder with multiple 2.5 mm pinholes) through which to view the Snellen chart. The pinhole occluder focuses light on the patient’s retina and should improve the patient’s visual acuity by at least one line on the Snellen chart.
- Distance. For Snellen wall charts, the patient should be positioned 20 feet from the chart and for Snellen pocket cards, the patient should be positioned 14 inches from the card.
- Interpretation. Each eye is tested separately. The lowest line the patient can accurately read indicates the patient’s visual acuity. The lines on the Snellen chart contain “distant equivalent” values. For instance, the top line is 20/800, which means if that line is the lowest line the patient can read, the patient can read at 20 feet what a person with 20/20 vision can read at 800 feet. Each error the patient makes at their lowest (best) line of vision is subtracted from his/her distant equivalent score. Meaning if the lowest line the patient can read is 20/40 but the patient misses one letter at that line, his/her score is “20/40 – 1”.
- Visual field testing is commonly performed via confrontational testing as follows:
- Position yourself approximately 1 meter from the patient and ask the patient to cover one of his/her eyes and to fix his/her gaze at the bridge of your nose.
- Each eye is tested separately. Each eye’s entire area of visual perception is its visual field. The visual fields are divided into four quadrants. To test each quadrant, bring a red object from outside of the patient’s visual field quadrant into its midpoint. Ask the patient to report when the red object is first identified and also its color saturation (red desaturation may be a sign of visual field deficit). Rarely, patients will not see a stationary object but will perceive it when it is moved, called Riddoch’s phenomenon.
- Central vision is the area of highest visual acuity and corresponds to the macula of the retina (the fovea lies within the center of the macula). Amsler grid charts contain lines and patterns that qualitatively evaluate for maculopathies disorders of central vision. To test central vision, use one of a series of Amsler grid charts as follows:
- Each eye is tested separately. Position the chart 33 centimeters from the patient and ask the patient to fixate on a central spot on the chart. The patient should report any areas on the grid in which the lines or patterns are missing or abnormal. The grid will test vision 10 degrees in each direction of fixation and can accurately localize central vision abnormalities.
- The ophthalmoscopic exam is used to obtain a clear view of the patient’s fundus. The following techniques will help optimize the ophthalmoscopic exam:
- The room should be dark and the light source for the ophthalmoscope should be bright.
- The patient should be seated and should visually fixate on a specific point in the distance, which will eliminate pupillary constriction from accommodation.
- Initially position the ophthalmoscope approximately 15 cm in front of the patient’s eye. Use your right eye to examine the patient’s right eye and your left eye to examine the patient’s left eye.
- Direct the light beam slightly temporal into the patient’s pupil and then follow the red fundus reflex as you approach the patient’s eye. The closer you can get to the patient’s eye, the larger the area of fundus that will be available to visualize ideally you will be just 2-3 cm from the patient’s eye. Unless you or the patient have a severe refractive error, you should both remove your eyeglasses to allow you to get as close to the patient as possible.
- Adjust the ophthalmoscope’s Rekoss dial to adjust its refractive power. The dial can be used to calculate heights along the fundus: a 3-diopter change indicates a 1-mm change in height.
- Exam sequence: in systematic fashion, observe the patient’s optic disc, retinal vessels, macula, and any other visible areas of the retina.
- Interpretation of the normal optic disc. The optic disc sits approximately 3-4 mm nasal to the fovea (of the macula) and is roughly 1.5 mm in diameter. It has a slight depression in the center: the physiologic cup. The cup size relative to the rest of the disc (the cup-to-disc ratio) is normally less than 0.4. In normal conditions, the optic disc is paler than the rest of the retina: it is a yellowish orange to pink color. The temporal half of the disc is slightly lighter than the nasal half of the disc because it contains the maculopapillar bundle, which courses toward the temporal side of the disc and lacks larger vessels and capillaries. The optic disc should be flat with sharp margins.
- Papilledema. Papilledema is elevation or blurring of the optic disc secondary to increased intracranial pressure, which extends along the optic nerve via the subarachnoid space. In papilledema, the optic disc becomes hyperemic (from vein engorgement), venous pulsations are lost, the optic disc margins blur and the optic nerve head elevates, the physiologic cup is lost, and hemorrhages and exudates appear. Over time optic disc swelling lessens and disc pallor appears, which signifies optic nerve atrophy. Optic disc swelling that occurs in the setting of normal intracranial pressure is called disc edema (rather than papilledema).
- The optic nerve head is devoid of photoreceptors and, accordingly, lacks a visual field correlate: it forms the blind spot. Mapping of the blind spot requires formal perimetry testing, but a bedside exam can be performed as follows:
- Position yourself approximately 1 meter from the patient. Both you and the patient should cover your opposing eyes. For example, if the patient covers his/her right eye, you should cover your left eye.
- Hold a red object (such as the eraser on a pencil) straight ahead, equidistant between yourself and the patient.
- Slowly move the red object out laterally. Throughout the evaluation, the patient stares at your open eye (in this example, the patient uses his/her left eye to stare at your right eye) but both you and the patient should attend (with your peripheral vision) to the red object as it moves out laterally.
- The patient is asked to report when the red object disappears (enters the blind spot) and when it reappears (emerges from the blind spot), which provides an approximation of the size of the patient’s blind spot. If you have a normal blind spot, you can use your own identification of the red object as a reference as to whether the patient has an enlarged blind spot. When the optic disc swells, the blind spot will enlarge.