Neuro-Ophthalmology: Afferent Visual System

Examination of Visual Acuity

Visual acuity is the spatial resolution of vision. Visual acuity should always be measured in each eye individually and with the best possible optical correction (i.e., with the patient’s glasses); other optical means such as a pinhole device or refraction may be needed if optical correction is not available. The resulting measure, called best-corrected visual acuity , is the only universally interpretable measurement of central visual function. Ideally, visual acuity should be measured both at distance (usually 20 feet or 6 m) and near (usually 14 inches or 0.33 m). The notation 20/20 (6/6) indicates that the patient (numerator) is able to see the optotypes seen by a normal person at 20 feet (denominator). A visual acuity of 20/60 (6/18) indicates that the patient sees an optotype at 20 feet that a normal person would see at 60 feet.

A disparity between the distance and near visual acuities is often indicative of a specific problem. For example, the most common cause of better distance than near acuity is uncorrected presbyopia. Common causes of better near than distance acuity include myopia and congenital nystagmus. In the latter disorder, convergence needed for near vision dampens the nystagmus.

When measuring near vision, the reading card should be held at the specified distance of 14 inches (or 0.33 m) to control for variation in image size on the retina. The medical record should clearly specify if a nonstandard distance is used. Two types of near cards are readily available; one has numbers, and the other has written text ( Fig. 16.4 ). In neurological practice, a near card with text measures visual acuity as well as reading ability to some degree. A disparity between the measurements from the two types of near card might suggest a disturbance of some other cortical function, such as language function (see Chapter 13 ).

Contrast Vision Testing

Contrast vision , the ability to distinguish adjacent areas of differing luminance, can be evaluated by assessing the perception of lines or optotypes of different sizes (spatial frequencies) with varying degrees of contrast. Contrast vision can be impaired in numerous diseases of the eye (e.g., cataract) and retrobulbar visual pathways (e.g., optic neuropathies). Special charts—the Pelli–Robson chart (sensitivity) and Sloan chart (acuity)—are required to assess contrast vision.

Light-Stress Test

In some disorders of the macula, abnormalities are not apparent with the ophthalmoscope. The light-stress (or photo-stress) test is a useful method for determining whether reduced central vision is a consequence of macular dysfunction. Prior to the test, the best-corrected visual acuity is measured in each eye. Then, with the eye with decreased vision occluded, the other eye is exposed to a bright light for 10 seconds. Immediately thereafter, the patient is instructed to read the next largest line on the eye chart, and the recovery period is timed. The same procedure is followed for the eye with decreased vision, and the results are compared. Fifty seconds is the upper limit of normal for visual recovery, although most normal subjects recover within several seconds. In patients with macular disease, the recovery period often takes several minutes.

Color Vision Testing

Dyschromatopsia, especially if asymmetrical between the eyes, is an indication of optic nerve dysfunction but can also occur with retinal disease ( ). Symmetrical acquired dyschromatopsia might indicate a retinal degeneration, such as a cone–rod dystrophy. Congenital dyschromatopsia occurs in about 8% of men and 0.5% of women.

Techniques for assessing color vision range from the simple to the sophisticated. A gross color vision defect is identifiable at the bedside by assessing for red desaturation. The clinician holds a bright red object in front of each of the patient’s eyes individually and asks for a comparison of both brightness and color intensity. Asking for a comparison of red saturation on each side of fixation sometimes detects a subtle hemianopia. Formal measurements of color vision can be obtained with pseudoisochromatic color plates (e.g., Ishihara or Hardy–Rand–Rittler plates) or with sorting tests (e.g., Farnsworth–Munsell test).

Examination of the Pupils

Examination of the pupils involves assessing pupil size and shape, the direct and consensual reactions to light, and the near response. The examination should also include an assessment for a relative afferent pupillary defect (RAPD). If a difference in pupil size (anisocoria) is noted, look for ptosis and ocular motility deficits, keeping in mind the possibility of Horner syndrome or third cranial nerve palsy. Record findings in an easily understood format ( Table 16.1 ).

Measurements of pupil size and light reaction are made in dim illumination with the patient fixating on an immobile distant target. If there is anisocoria, it is useful to measure pupil size in both darkness and bright light. Anisocoria due to oculosympathetic paresis (Horner syndrome) is greater in the dark because the affected pupil does not dilate well. Conversely, anisocoria due to parasympathetic denervation (e.g., Adie tonic pupil) is more evident in bright light because the affected pupil does not constrict well (see Chapter 17 ).

When measuring light reactions or assessing for an RAPD, the brightest light available should be used. The near reaction can be elicited by having the patient look at his or her thumb, positioned at a distance of 15–30 cm. With this method, a near reaction can be elicited even in a completely blind patient, owing to proprioceptive influences. The pupil shows light-near dissociation when the direct light reaction is less prominent than the near reaction. Light-near dissociation can be seen with parasympathetic denervation of the pupil (e.g., Adie tonic pupil), with dorsal midbrain lesions (e.g., as part of the dorsal midbrain syndrome), and in patients with severe bilateral optic neuropathies.

The presence of an RAPD (formerly called a Marcus Gunn pupil ) is an invaluable sign of a unilateral or asymmetric optic neuropathy. An RAPD is best detected by alternately illuminating the pupils by swinging a flashlight between them at a frequency of about once per second—hence the name, swinging flashlight test . The swinging flashlight test compares the direct and consensual light reactions in the same eye. Normally, these reactions are equal. However, in patients with a unilateral or asymmetric optic neuropathy, because of reduction in the direct reaction as compared with the consensual reaction, the pupil of the eye with decreased vision dilates when reilluminated (both pupils are actually “dilating” in that they are both resetting at the size commensurate with the amount of light transmitted back to the brain by the damaged optic nerve). Box 16.1 and Fig. 16.5 describe the method for detecting an RAPD. Two caveats exist. The test brings out an asymmetry of optic nerve conduction, so an RAPD is not present when both optic nerves are injured to the same extent. In addition, severe inner retinal pathology can produce an RAPD, but the abnormality is usually obvious on funduscopic examination. In contrast, an optic neuropathy with minimal loss of visual acuity often gives an obvious RAPD. The magnitude of an RAPD can be quantified using neutral density filters. See Chapter 17 for further discussion of pupillary abnormalities.

Light Brightness Comparison

Light brightness comparison is a subjective swinging flashlight test. The subjective appreciation of light intensity is often impaired in patients with optic neuropathies, but not in macular disease. The clinician shines a bright light into both eyes in succession and asks the patient to estimate the difference in brightness. For example, the clinician could ask, “If this light (normal eye illuminated) were worth $1 in terms of light brightness or intensity, what would this one be worth (abnormal eye illuminated)?”

Visual Field Testing

Evaluation of the visual fields is vital in patients with visual loss. Several techniques can be used for visual field examination, ranging from simple confrontation testing to sophisticated threshold static perimetry. Confrontation testing should be part of the routine neurological examination, although it is insensitive for detection of mild visual field loss ( ). For the purposes of this discussion, the emphasis is on simple and practical techniques, while more sophisticated methods are briefly summarized.

In the first assessment, the patient is asked to observe the clinician’s face with each eye in turn and to report if any part of the clinician’s face is missing, blurred, or distorted when the patient’s line of sight is directed to the nose. For example, a patient with a central scotoma may report that the eyes and nose are missing, a patient with an inferior altitudinal visual field defect may report that the lower half of the face is missing, while a patient with homonymous hemianopia may report that one side of the face is missing.

Confrontation testing should follow. Although many methods are available, a simple, thorough examination can be done by finger counting in all four quadrants, coupled with hand comparison. The steps are as follows:

• 1.

  The clinician has the patient occlude one eye and maintain fixation on the clinician’s nose.

• 2.

  Finger counting in the quadrants: The clinician holds up fingers sequentially in each of the four quadrants of the visual field and asks the patient to count the number seen.

• 3.

  Simultaneous finger counting using both hands: If step 2 is completed normally, the clinician asks the patient to count the number of fingers displayed with both hands, first in both of the upper quadrants of the patient’s visual field and then in the lower quadrants. Then, the patient is asked to add the total number of fingers shown with both hands. Visual inattention is often identifiable during this step of confrontation testing.

• 4.

  Simultaneous hand comparison: Finally, the clinician holds both hands open, first in both upper quadrants and then both lower quadrants, and asks the patient to compare the quality of the images. For example, when shown hands on either side of the midline, a patient with a subtle bitemporal hemianopia may state that the hands held in the temporal hemifields are not as clear as those held in the nasal hemifields.

A potential advantage of the finger counting method over kinetic methods (e.g., wiggling fingers) is that it minimizes the potential for confounding by the Riddoch phenomenon , which refers to a dissociation between the visual perception of form and movement such that the patient can perceive moving but not stationary targets in half of the visual field ( ). The Riddoch phenomenon can occur when homonymous hemianopia results from visual cortex lesions. Accordingly, the clinician may miss a hemianopia when using only a moving target, such as wiggling fingers, in the far periphery.

Confrontation methods using colored (e.g., red) objects can be effective in detecting subtle visual field defects ( ). Confrontation testing is also useful for assessing patients with constricted visual fields. As the distance between the clinician and the patient increases, the visual field should expand, producing a funnel . However, with nonorganic (functional) visual field constriction, the visual field often does not expand as the distance between the clinician and the patient increases, thereby producing a tunnel ( Fig. 16.6 ).

The central 20 degrees of the visual field can be assessed in each eye separately using the Amsler grid ( Fig. 16.7 ). With the grid held in good light at a distance of 30 cm from the eye and the patient wearing his or her reading glasses, if needed, the following questions are asked:

• 1.

  Can you see the spot in the center of the grid?

• 2.

  While looking at the center spot, can you see the entire grid or are any sides or corners missing?

• 3.

  While you are looking at the center spot, are any of the lines in the grid missing, blurred, or distorted?

If the patient indicates an abnormality, the clinician should ask the patient to draw the abnormal areas on the grid. The grid can then be kept in the patient’s medical record. Patients with a central scotoma may report that the center of the grid is missing, those with a hemianopic defect may report that half of the grid is missing, and those with macular disease may report that the lines are wavy or distorted (i.e., metamorphopsia).

Interpretation of Visual Field Defects

Eight general rules for visual field interpretation are summarized in Box 16.2 . Comments relating to six of these general rules are presented here.

Rule 1

Optic nerve lesions can produce prechiasmal visual field abnormalities that are characteristic. Nonarteritic anterior ischemic optic neuropathy (NAION) often produces an inferior altitudinal defect, optic neuritis often produces a central or cecocentral scotoma, and compressive optic neuropathy often produces abnormalities in both the peripheral and the central portions of the visual field ( Fig. 16.8 ).

A lesion at the junction of the optic nerve and chiasm produces a junctional scotoma (i.e., ipsilateral cecocentral scotoma and contralateral temporal defect) due to involvement of both ipsilateral fibers and crossing fibers from the contralateral nasal retina. Binasal field defects ( Fig. 16.9 ) can result from papilledema, anterior ischemic optic neuropathy, glaucoma, optic nerve head drusen, optic nerve pits, optic nerve hypoplasia, and sectoral retinitis pigmentosa (RP). Binasal field defects that are organic in etiology do not respect the vertical meridian, whereas nonorganic binasal defects may.

Rule 2

True bitemporal hemianopias are the hallmark of chiasmal disease. Bitemporal field defects that do not respect the vertical meridian (pseudobitemporal hemianopias) are almost always due to congenital rotation or tilting of the optic discs ( Fig. 16.10 ). Bilateral cecocentral scotomas can masquerade as bitemporal field defects, and the distinguishing feature is whether the defect respects the vertical meridian of the visual field.

Rule 4

Incongruent hemianopias tend to result from more anterior retrochiasmal lesions (e.g., those affecting the optic tract or temporal lobe; Fig. 16.11 ; ). Optic tract lesions often produce a contralateral RAPD (i.e., in the eye with the temporal visual field defect), which is helpful for clinical localization.

Rule 5

Congruent homonymous hemianopias have patterns that are very similar or identical in the two eyes. Highly congruent hemianopias usually result from occipital lobe infarcts ( Fig. 16.12 ) but can sometimes occur with more anterior retrochiasmal lesions ( ).

Perimetry

Numerous techniques for examining the visual fields are available ( ), but a detailed discussion of these is beyond the scope of this chapter. Examination of the entire visual field requires a perimeter; the tangent screen measures only the central 30 degrees of the visual field at a distance of 1 m. Perimeters can be divided into those that use a moving (kinetic) stimulus and those that use a static stimulus. Most static perimeters are automated and driven by computer. Static perimeters can determine the visual threshold at defined points in the visual field (threshold static perimetry) or may evaluate these points using stimuli of set luminance (suprathreshold static perimetry). The Goldmann perimeter is the most commonly used kinetic perimeter (see Fig. 16.13 , A for a normal visual field obtained using the Goldmann perimeter), although kinetic perimetry can also be performed with the Humphrey and Octopus perimeters. The Humphrey and Octopus perimeters are the most commonly used static perimeters (see Fig. 16.13 , B for a normal visual field obtained using the Humphrey perimeter). Threshold static perimeters are the most sensitive and quantitative, allowing for a comparison of the patient’s responses with those of age-matched normal controls, but testing can be time consuming and tiring for the patient. To gain useful information from static perimetry, the patient must be alert, cooperative, and able to maintain steady central fixation. Many patients with neurological disorders are unable to concentrate for an examination that can take as long as 15 minutes per eye, and, thus, static perimetry findings may be unreliable in such patients. Recent refinements in testing strategy have made it possible to reduce testing time and thereby increase reliability, but many neuro-ophthalmologists continue to use Goldmann perimetry to assess the visual fields in selected patients.

Ophthalmic Imaging

Photographs of the ocular fundus may be obtained to identify and document ophthalmoscopic findings. Retinal vascular abnormalities, such as occlusions (e.g., central retinal artery occlusion [CRAO]) and microvascular disease (e.g., diabetic retinopathy), may be evaluated when red-free fundus photographs are taken following intravenous (IV) injection of fluorescein (fluorescein angiography) . Fundus autofluorescence photography allows for topographical mapping of lipofuscin in the retinal pigment epithelium layer. Lipofuscin is a fluorescent pigment that accumulates in retinal pigment epithelial cells following photoreceptor degradation, and, thus, autofluorescence may be used to detect subtle abnormalities in patients with retinal diseases ( ). Because optic nerve head drusen exhibit autofluorescence, they may be detected on fundus autofluorescence even when they are not visible on ophthalmoscopy ( ).

Optical coherence tomography (OCT) uses light waves to generate high-resolution cross-sectional images of the optic nerve and retina. As the retinal layers have differing optical reflectivity, they can be distinguished using OCT. The thickness of the layers can be determined from OCT and compared with age-matched normal controls. Measurement of the peripapillary retinal nerve fiber layer and macular ganglion cell layer thicknesses with OCT may help with the detection of mild optic neuropathy ( Fig. 16.14 ). Measurement of the thickness of retinal layers or identification of architectural changes on OCT can aid the diagnosis and management of retinal disease.

Electrophysiology

Electrophysiology may help in the investigation of unexplained visual loss or in identification of subclinical optic nerve dysfunction. Measurement of visual-evoked potentials (VEPs) has long been used for the evaluation of demyelinating optic neuropathies, which produce a delayed P-100. However, VEP findings can be misleading; a low-amplitude VEP could be misinterpreted as indicating optic neuropathy in a patient with retinal disease. Electroretinography (ERG) is useful for the evaluation of suspected retinal dysfunction, especially when ophthalmoscopic findings are subtle or absent. Full-field ERG evaluates the response of the entire retina to flashes of light. A variety of stimuli are presented in differing states of light adaptation, allowing for evaluation of different retinal elements, including the rod and cone photoreceptors. Because full-field ERG evaluates the response of the entire retina, it may not be abnormal in patients with focal retinal dysfunction (e.g., macular dysfunction). Multifocal ERG allows for the topographic evaluation of macula ERG responses and is more sensitive for detecting macular dysfunction ( ); multifocal ERG findings can be grossly abnormal even when ophthalmoscopic changes are absent or subtle.