Visual acuity is a measurement of the individual's capacity for visual discrimination of fine details of high contrast. ^ Best corrected visual acuity should be tested for each eye. Distance vision is assessed with a standard Snellen chart and near vision with a hand-held card. If the patient does not bring corrective lenses for the examination, a pinhole can correct most refractive errors.
Acuity is most often recorded as, for example, 20/40, in which the numerator refers to the distance (in feet) from which the patient sees the letters and the denominator the distance from which a patient with normal vision sees the same letters. Visual acuity with the near card is often recorded using the standard Jaeger notation (J1, J3, etc). If a patient is unable to read the largest Snellen letters (20/200 or 20/400), the acuity should be characterized by the ability to count fingers (CF) (and at what distance), detect hand motions (HM), or have light perception (LP). An eye that is blind has no light perception (NLP). Contrast sensitivity testing with sine-wave gratings is a useful adjunct in the evaluation of visual acuity. In younger preverbal patients, assessment of fixing and following in most instances is sufficient. When more accurate visual acuities are required, preferential looking tests (Teller's acuities) may be used. U These tests are based upon the principle that a child would rather look at objects with a pattern stimulus (alternating black and white lines of specific widths) than at a homogeneous field. The smallest pattern that the child seems to prefer is an indicator of best visual acuity.
Visual acuity can be altered by macular, optic nerve, or chiasmal lesions. Disturbances that are posterior to the chiasm (retrochiasmal, that is, tract, optic radiations, and occipital lobe) can affect visual acuity only if they are bilateral.
Color vision can be tested with standard pseudoisochromatic Ishihara or Hardy-Rand-Ritter plates, both of which contain numbers or geometrical shapes that the patient is asked to identify among different colored dots. y Qualitative inter-eye differences in color perception can be tested by comparing a red bottle top, for example, with each eye. A patient with monocular "red desaturation" may state that with the affected eye the red bottle top appeared washed out, pink, or orange.
Color vision can be altered by macular, optic nerve, or chiasmal lesions. Retrochiasmal disturbances can produce abnormal color vision in the defective visual field. A lesion of the lingual and fusiform gyri can cause defective color vision in the contralateral hemifield.
Testing the patient's visual fields can be accomplished at the bedside by finger confrontation methods in all four quadrants of each eye by asking the patient to "count my fingers" or "tell me when you first see my finger wiggling." In a patient who is aphasic, uncooperative, intubated, sedated, or very young, responses such as finger mimicry, pointing to targets presented, visual tracking, or reflex blink to visual threat allow for a gross assessment of visual field integrity. Color or subjective hand comparison is a useful
adjunct to elicit defects respecting the vertical or horizontal meridians. y To test central or macular vision, an Amsler grid (similar to a piece of graph paper with a central fixation point) can be viewed by the patient. Patients may perceive abnormal areas on the grid that may correspond with visual field deficits.
Accurate documentation of visual fields requires kinetic testing with a tangent screen, Goldmann's perimeter, or automated threshold perimeter. The Goldmann's kinetic technique is useful when evaluating patients with significant neurological impairment, because it is shorter and involves interaction with the examiner. Threshold computerized perimetry of the central 30 degrees of vision is a more sensitive and reproducible test for patients with optic neuropathies and chiasmal disturbances. When visual fields are recorded, they are presented so that the right field is to the right and the left field to the left. The blind spot, corresponding to the optic nerve, is located approximately 15 degrees temporal to fixation and is recorded as an area without vision. y
The visual field can be altered by lesions anywhere in the afferent visual pathway, and the specific patterns in relationship to neuroanatomical structures are discussed in more detail later.
It is often difficult to separate a dense left hemianopia from dense neglect in a patient with a large right parietal lesion. In instances when the deficits are more subtle, the examiner can screen for visual inattention by presenting visual stimuli, such as a fingers, separately in each hemifield, then together on both sides of midline (double simultaneous visual stimulation). People with subtle visual inattention but intact fields see the stimulus when presented separately but not when shown simultaneously. Other bedside tests include letter cancellation, in which the patient is asked to find a specific letter or shape within a random array. Patients with left visual neglect may find the specified letter only when it appears on the right side of the page. When patients with left neglect are asked to bisect lines drawn randomly on a page, they may tend to "bisect" the lines to the right of their center.
In patients suspected of agnosias, informal tests of visual recognition can be performed at the bedside using common objects such as a pen, cup, or book. An inability to recognize faces (prosopagnosia) or interpret complex scenes (asimultagnosia) can be tested with magazine or newspaper photos and advertisements. Standardized facial and object recognition tasks are available during more formal neuropsychological testing.
The size of each pupil can be measured in light and dark using the pupil scale found on most near-acuity cards. Pupillary light reactivity is tested with a bright light, such as a halogen transilluminator, while the patient fixes on a distant object (to avoid accommodation). While the light is shining in one eye, the ipsilateral pupillary light reflex, termed the direct response, and the contralateral, termed the consensual response, should be noted. Transient fluctuations in pupillary diameter are normal and are called hippus. The swinging flashlight test involves alternating a bright light equally at each eye to compare each pupil's reactivity to light. Because light from one eye will reach both Edinger-Westphal nuclei (see Fig. 8-4 (Figure Not Available) ), normally both pupils react briskly when light is shined into just one eye, and the amount of constriction is the same regardless of which eye is stimulated.
If one eye has reduced vision owing to an optic neuropathy or large retinal process, light directed into that eye produces a relatively weaker pupillary response in both eyes (relative afferent pupillary defect [RAPD]) (see Fig 8-5 ).[1 Asymmetrical chiasmal disturbances may lead to an RAPD in the eye with worse acuity or greater field loss. Optic tract lesions can be associated with an RAPD in the contralateral eye, owing to interruption of the crossing nasal fibers, which outnumber the uncrossed temporal fibers. Because afferent pupillary fibers leave the afferent visual pathway before the geniculate, geniculocalcarine disorders are not associated with pupillary abnormalities.
The posterior pole of the eye can be viewed with a direct ophthalmoscope through an undilated pupil. Any
Figure 8-5 A, Normal swinging flashlight test, in which light directed in either eye elicits the same amount of pupillary constrictB, Swinging flashlight test revealing a left relative afferent pupillary defect (L RAPD) in the hypothetical setting of visual loss in the left eye due to an optic neuropathy. Pupillary sizes are equal at rest in ambient lighting (b-1). Light stimulation of the good right eye results in brisk bilateral pupillary constriction (b-2). Light stimulation of the defective left eye produces comparatively weaker pupillary constriction, and both pupils dilate (b-3).
Figure 8-6 (Figure Not Available) Photographleft) of a normal left fundus. The corresponding illustratiofright) identifies important structures/From Liu GT: Disorders of the eyes and eyelids. In Samuels MA, Feske S [eds]: The Office Practice of Neurology. New York, Churchill-Livingstone, 1996, p 41.)
media opacity due to corneal exposure, cataract, or vitritis, for instance, can create a hazy view. Posteriorly, the optic disc, retinal vasculature, macula, and peripapillary retina should be carefully examined (Fig. 8-6 (Figure Not Available) ). Important details of the optic disc that should be noted include its color and contour and cup-to-disc ratio. The retinal vasculature should be evaluated in detail, with particular attention to the caliber of arteries and veins, branching patterns, and, when suspected, possible emboli. The macula, best observed by asking the patient to look at the direct ophthalmoscope's light, is examined for evidence of degeneration, lipid deposition, detachment, edema, or change in pigment color. Thorough evaluation of the retinal periphery requires a pharmacologically dilated pupil and indirect ophthalmoscopy.
Optic disc swelling suggests an optic neuropathy or papilledema. Chronic optic nerve processes lead to disc atrophy. Macular disturbances are associated with central field loss and decrease in visual acuity. If large enough, other retinal abnormalities can also cause corresponding field loss. For instance, retinitis pigmentosa beginning peripherally causes field constriction, whereas inferior branch retinal artery occlusion is associated with a superior altitudinal field defect.
The evaluation of patients with disorders of the afferent visual pathways often is enhanced by consultation with an ophthalmologist or neuro-ophthalmologist, who has ancillary ophthalmological equipment available. The most common cause of blurry vision is refractive error, and an ophthalmologist is the individual best to make the required measurements. Applanation tonometry is a necessary screen for glaucoma. An exophthalmometer helps measure anterior protrusion of the eye, and it is indispensable when following proptosis, for example, in a patient with thyroid orbitopathy.
Slit lamp examination (biomicroscopy) uses what essentially is a horizontally mounted microscope and a special light source to visualize directly the cornea, anterior chamber, iris, vitreous, and posterior pole of the fundus (disc and macula, primarily). Indirect ophthalmoscopy, using a 20-diopter lens, allows more complete visualization of the posterior pole and peripheral retina. Both techniques follow pharmacological dilation of the pupil (usually 2.5 percent phenylephrine and 1 percent tropicamide topically), and each permits a stereoscopic view of the fundus, which is especially important when evaluating disc swelling, disc contour, cup-to-disc ratio, and macular edema. In addition, both slit lamp biomicroscopy and indirect ophthalmoscopy employ very bright light sources, allowing greater visualization of structures in the back of the eye when there is a media opacity such as a cataract.
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