Examination of Visual Acuity

Visual acuity is the spatial resolution of vision. Visual acuity should always be measured with each eye individually and with the best possible optical correction (i.e., with the patient’s glasses); other optical means such as the pinhole device or a refraction may be needed (Wall and Johnson, 2005). The resultant measure, called best-corrected visual acuity, is the only universally interpretable measurement of central visual function. Ideally, always measure vision both at a standard distance (usually 20 feet or 6 m) and at near (usually 0.33 m). The notation 20/20 (6/6 metric) means the patient (numerator) is able to see the optotypes seen by a normal subject at 20 feet (denominator). A vision of 20/60 (6/18 metric) means that the patient sees an optotype at 20 feet that a normal person would see at 60 feet.

A disparity between the best-corrected 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 are 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. 36.2). Both are useful, but in neurological practice, a near card with text measures not only visual acuity but also 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 11).

Fig. 36.2 A, Rosenbaum-style near vision card. B, Near vision card with written text.

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 neuropathy and Alzheimer disease). Special charts—the Pelli-Robson chart (sensitivity) and Sloan chart (acuity)—are required to measure contrast sensitivity and acuity.

Light-Stress Test

In some disorders of the macula, abnormalities are undetectable with the direct ophthalmoscope. The light-stress (or photo-stress) test is a useful method for determining whether reduced visual acuity is a consequence of macular dysfunction (Wall and Johnson, 2005). 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 larger 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 a good indication of optic nerve dysfunction, but it can also occur with retinal disease (Almog and Nemet, 2010). 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 then 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, direct and consensual reactions to light, near reaction, and the presence or absence of a relative afferent pupillary defect (RAPD). If a difference in pupil size (anisocoria) is noted, look for ptosis and ophthalmoplegia, keeping in mind the possibility of a Horner syndrome or third cranial nerve palsy. Record findings in an easily understood format (Table 36.1).

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 darkness and bright ambient light. Anisocoria due to oculosympathetic paresis (Horner syndrome) is often greater in the dark, because the affected pupil does not dilate well. Conversely, anisocoria due to parasympathetic denervation (e.g., Adie tonic pupil) is often more evident in bright light, because the affected pupil does not constrict well (see Chapter 16).

When measuring light reactions or looking for an RAPD, the brightest light available should be used. The best method for eliciting the near reaction is by having the patient look at their thumb, positioned at a distance of 15 to 30 cm. With this method, a near reaction is elicitable even in a completely blind patient, owing to proprioceptive influences. The pupil shows light-near dissociation when the direct light reaction is less 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 observation of an RAPD (formerly called a Marcus Gunn pupil) is an invaluable indication of a conduction defect in the optic nerve. A difference in pupillary reaction is best elicited 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 reactions in the same eye. Normally these reactions are equal. However, in patients with an optic neuropathy, because of reduction in the direct reaction as compared with the consensual reaction, the pupil dilates when re-illuminated. Box 36.1 and Fig. 36.3 describe the method for testing for an RAPD. Two caveats exist. The test brings out an asymmetry of optic nerve conduction, so with both optic nerves injured to the same extent, an RAPD is not present. In addition, a macular or retinal lesion can sometimes produce an RAPD, but abnormalities are usually evident on funduscopic examination. In contrast, an optic neuropathy with minimal loss of visual acuity often gives an obvious RAPD. See Chapter 16 for further discussion of pupillary abnormalities.

Fig. 36.3 Right relative afferent pupillary defect from a right optic nerve lesion. A, Poor direct and consensual reaction with illumination of the right eye. B, Excellent direct and consensual reaction with illumination of the left eye. C, Poor direct and consensual reaction, manifest as redilation of both pupils when the light is swung back to the right eye.

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 nerve disorders, 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 as a percentage. 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 unexplained visual loss. Several techniques are available for visual field examination, ranging from simple confrontation testing to sophisticated threshold static perimetry. Confrontation testing should be part of the routine neurological examination. For the purposes of this discussion, the emphasis is on simple and practical techniques, while complicated methods are briefly summarized.

The first assessment involves asking the patient to observe the clinician’s face with each eye in turn and to report whether anything is missing, blurred, or distorted. For example, a patient with a central scotoma may report the eyes and nose 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:

A major advantage of the finger counting method over kinetic (wiggling finger) methods 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 (Zeki and Ffytche, 1998). The Riddoch phenomenon can occur when homonymous hemianopia results from occipital 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 objects can be effective in detecting subtle defects. Confrontation testing is also useful for assessing patients with constricted visual fields. As the distance between the clinician and patient increases, the visual field should expand, producing a funnel. However, with nonorganic visual field constriction, the visual field often does not expand as the distance between the clinician and patient increases, thereby producing a tunnel (Fig. 36.4).

Fig. 36.4 A, The normal visual field enlarges with an increase in testing distance; it “funnels.” B, The constricted visual field from organic disease also proportionally enlarges. C, The constricted visual field from nonorganic disease does not usually enlarge as testing distance increases; rather, it “tunnels.”

(Adapted from Trobe, J.D., Glaser, J.S., 1983. The Visual Fields Manual: a Practical Guide to Testing and Interpretation. Gainesville, Triad p. 135.)

The central 20 degrees of the visual field (in each eye separately) can be assessed using the Amsler grid (Fig. 36.5). With the chart held in good light at a distance of 30 cm from the eye, the patient wearing their reading glasses if needed, the following questions are asked:

Fig. 36.5 Amsler grid. Upper left, Paracentral scotoma. Lower right, Metamorphopsia (straight lines appear wavy).

If the patient indicates an abnormality, the clinician should ask the patient to draw the abnormal areas on the chart. The chart can then be kept in the patient’s medical record. Patients with a central scotoma often report that the center of the grid is missing or blurred; those with hemianopic defects say that half of the grid is missing; and those with macular disease might report that the lines are wavy or distorted (i.e., metamorphopsia) (see Chapter 14).

Numerous other methods for examining the visual fields are available (Wall and Johnson, 2005), but these are beyond the scope of this chapter and the expertise of most neurologists. Examination of the entire visual field requires a perimeter; the tangent screen measures only the central 30 degrees of 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 available today are automated and driven by computer. Static perimeters can determine the actual visual threshold at defined points in the visual field (threshold static perimetry) or may test these points using stimuli of set luminance (suprathreshold static perimetry). The Goldmann perimeter is the most commonly used kinetic perimeter, while the Humphrey and Octopus perimeters are the most commonly used static perimeters. On theoretical grounds, threshold static perimeters are the most sensitive and quantitative, but the examination is more time consuming and tiring for the patient. To gain useful information from static perimetry, intense concentration and alertness are required. Many patients with neurological disorders are unable to sit and concentrate for an examination that can take as long as 15 minutes per eye, so static perimetry findings can be unreliable in these patients. Recent refinements in testing strategy have made it possible to reduce testing time and increase reliability, but many neuro-ophthalmologists continue to use Goldmann perimetry to assess the visual fields in selected patients.

Rule 1

Optic nerve lesions can produce prechiasmal visual field abnormalities that are characteristic. Nonarteritic anterior ischemic optic neuropathy often produces an inferior altitudinal defect (Fig. 36.6, A), optic neuritis often produces a central or cecocentral scotoma (see Fig. 36.6, B), and compressive optic neuropathies often produce abnormalities in both the peripheral and central portions of the visual field (Fig. 36.7).

Fig. 36.6 Inferior altitudinal visual field defect in a 67-year-old woman with anterior ischemic optic neuropathy of the right eye. A, Threshold static measurement of the central 30 degrees of the right visual field (Octopus 1-2-3 perimeter; G1X program). Upper left, Grayscale measurement; darker areas correspond to areas of visual field loss (displays measured and interpolated data). Area of blind spot was not investigated and therefore is reproduced as white, not black. Upper right, Mean sensitivity (MS) display in decibels. The decibel (dB) is a logarithmic relative scale to quantify differential light sensitivity (dB = 0.1 log-unit of stimulus intensity). Lower right, Mean deviation (MD) display (MD = mean of the deviation of measured dB from age-matched normal values). Positive values indicate decreased sensitivity. Lower left, The cumulative defect curve (Bebie curve) arranges test locations with the least difference on the left side. This curve helps differentiate local from diffuse damage. B, Kinetic perimetry of the right visual field using the Goldmann perimeter demonstrates a cecocentral scotoma in a patient with optic neuritis.

Fig. 36.7 Constriction of the left visual field with a cecocentral scotoma due to a compressive optic neuropathy, with a normal right visual field.

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 hemianopias (Fig. 36.8) can result from papilledema, ischemic optic neuropathy, glaucoma, optic nerve head drusen, optic nerve pits, optic nerve hypoplasia, and sectoral retinitis pigmentosa. Less often, hydrocephalus, ectatic parasellar arteries, and basal tumors can produce binasal field defects. Binasal field defects that are organic in etiology do not respect the vertical meridian, whereas nonorganic binasal defects may.

Fig. 36.8 Binasal hemianopic visual field defects (e.g., due to glaucoma). Note that the vertical meridian is not respected.

Rule 2

True bitemporal hemianopias are the hallmark of chiasmal disease; common causes are discussed in Chapter 14. Less commonly, ischemia, radiotherapy, and demyelination can cause a chiasmal syndrome. Bitemporal defects that do not respect the vertical meridian (pseudobitemporal hemianopias) are almost always due to congenital rotation or tilting of the optic discs (Fig. 36.9) (see Chapter 15). Bilateral cecocentral scotomas can masquerade as bitemporal field defects, and the defining difference is whether or not the defect respects the vertical meridian of visual field.

Fig. 36.9 Pseudobitemporal hemianopia (e.g., due to tilted optic discs). Note that the vertical meridian is not respected.

Rule 3

A homonymous visual field defect is present in the same hemifield (i.e., right or left) or visual quadrant (i.e., upper or lower) of each eye. The only exception to this rule is with the monocular temporal crescent syndrome, in which only unpaired visual fibers residing in the contralateral anteromedial occipital lobe are affected.

Rule 4

Incongruous hemianopias tend to result from more anterior retrochiasmal lesions (e.g., those affecting the optic tract or temporal lobe) (Fig. 36.10) (Kedar et al., 2007). Optic tract lesions often produce a contralateral relative afferent pupillary defect, which is helpful for clinical localization.

Fig. 36.10 Incongruent left homonymous hemianopia from a right optic tract lesion.

Rule 5

Congruous homonymous hemianopias have patterns that are very similar or identical in the two eyes. Congruous hemianopias usually result from occipital lobe infarcts (Fig. 36.11) but can sometimes occur with more anterior retrochiasmal lesions (Kedar et al., 2007).

Fig. 36.11 Congruent paracentral right homonymous hemianopia from a left occipital pole lesion.

Rule 8

Even a complete unilateral homonymous hemianopia does not decrease visual acuity, because the macular cortex in the opposite hemisphere is still functioning. If input to both macular cortices is abnormal, central acuity is often diminished (cortical blindness), but the acuities should be equally diminished. If the visual acuities are not similar, the clinician should search for another (or additional) explanation for the asymmetry.

Diagnostic Techniques

A careful social and family history must be obtained when evaluating a patient with suspected nonorganic visual disturbance, especially regarding abuse, peer pressure, and visually impaired friends and family members. Different examination approaches are necessary for different nonorganic visual disturbances. For example, if a patient claims total blindness, the clinician should examine the pupils, check for optokinetic nystagmus using an optokinetic drum, and oscillate a large mirror in front of the patient to try inducing pursuit eye movements. If the patient claims visual loss in one eye, the clinician should examine the pupils looking for an RAPD, attempt the mentioned tests with the good eye occluded, and test stereopsis. Specialized ophthalmic techniques such as “fogging” can also be helpful in this setting but require the assistance of an ophthalmologist.

Testing of visual fields is extremely important in patients with suspected nonorganic visual loss and may reveal one of several patterns including tubular constriction, cloverleaf constriction, spiraling isopters, crossing isopters, or inverted isopters (Fig. 36.12). Confrontation or tangent screen testing done at different distances can be useful in evaluating a patient with visual field constriction, since the visual field area should increase as the distance between the patient and clinician increases. Lack of expansion of the visual field area with an increasing distance from the clinician suggests nonorganic visual field constriction (see Fig. 36.4). Cloverleaf constriction is best detected on automated static perimetry and cannot be produced by organic disease. Spiraling, crossing, and inversion of isopters can be detected using kinetic perimetry and cannot be produced by organic disease. Kinetic perimetry with both eyes opened in a patient with suspected nonorganic monocular visual loss may demonstrate nonphysiologic visual field constriction on the side of the reported visual loss.

Fig. 36.12 Two common visual field abnormalities with nonorganic visual loss. Left, Concentric (tubular) constriction. Right, Spiraling of a single isopter (seen only with kinetic visual field testing).

The use of visual-evoked potentials (VEPs) to diagnose nonorganic visual loss can be frustrating and unreliable. If the VEP is normal, useful information is gained, but factitious abnormalities in the VEP can be induced by normal persons who defocus their vision during the test. Therefore, an abnormal VEP is not always diagnostic of an organic visual disturbance.

Prognosis

About half of patients with nonorganic visual disturbance improve with time and reassurance. Factors that indicate a good prognosis include youth and the presence of anxiety, whereas older age and depression are usually associated with a poor prognosis.

Optic Nerve Hypoplasia

Optic nerve hypoplasia is a congenital, nonprogressive condition that may result from a defect in differentiation of the retinal ganglion cell axons. The optic disc and scleral canal are of subnormal diameter, and a peripapillary ring of pigmentation is often present (the double-ring sign) (see Chapter 15). The hypoplasia can be diffuse or segmental, as in superior segmental hypoplasia, which is occasionally seen in children born to insulin-dependent diabetic mothers.

Optic nerve hypoplasia can be unilateral or bilateral. Visual acuity varies considerably in affected eyes and can range from 20/20 (6/6 metric) to light perception. Associated ocular conditions include microphthalmos, aniridia, and albinism. Astigmatism is common. Associated visual field defects are usually nasal or inferior altitudinal.

Patients with bilateral optic nerve hypoplasia often have strabismus, nystagmus, endocrine disturbances, and developmental delay. The common endocrine disturbances include hypothyroidism, growth hormone deficiency, diabetes insipidus, and neonatal hypoglycemia. Bilateral optic nerve hypoplasia in association with absence of the septum pellucidum and hypopituitarism is called septo-optic dysplasia or de Morsier syndrome; these patients often have associated nystagmus. Optic nerve hypoplasia is usually sporadic but may be associated with fetal alcohol syndrome, maternal diabetes, or maternal anticonvulsant ingestion during pregnancy. Mutations in the HESX1 homeobox gene may play a role in some cases of septo-optic dysplasia (Dattani et al., 2000).

Leber Congenital Amaurosis

Leber congenital amaurosis (LCA) is most broadly defined as a syndrome of bilaterally poor vision beginning in early childhood, which is associated with depressed or absent signal on the electroretinogram. It is estimated to be the cause of 10% to 18% of cases of childhood blindness and is not related to Leber hereditary optic neuropathy (see Chapters 14 and 15). Visual acuity in LCA is usually less than 20/200 (6/60 metric). The retinal pigment epithelium has a salt-and-pepper appearance, and retinal vascular attenuation is sometimes marked. Optic disc swelling may be present. Pathological changes in the retina are variable and can affect all layers or just the ganglion cell layer. Nystagmus occurs in approximately 75% of patients, and ocular motor apraxia or other eye movement problems may be present. Approximately 30% of patients have associated neurological abnormalities that include hyperkinetic behavior, poor coordination, spastic paraparesis, psychomotor retardation, and hydrocephalus. Some patients can exhibit the oculodigital phenomenon (eye gouging), which is thought to be an attempt to produce phosphenes from mechanical stimulation of the retina. The condition is usually sporadic but is inherited as an autosomal recessive trait in approximately a third of cases. Mutations in the retinal RPE65 gene appear to be the cause in some cases (Pang et al., 2005). Gene therapy shows promise for improving vision and alleviating nystagmus in patients with this syndrome (Bennicelli et al., 2008).

Albinism

Albinism is a genetic abnormality in melanin synthesis associated with congenital nystagmus, foveal hypoplasia, and impaired visual acuity. In contrast, albinoidism affects tissues derived from the neural crest (iris, skin, hair) and does not have associated visual abnormalities. True albinism may affect the skin and the eyes (oculocutaneous albinism) or may affect the eyes only (ocular albinism) and be less obvious to the clinician. The ocular fundus can be devoid of pigment or simply have a “blond” appearance. The degree of visual impairment is related to the degree of ocular pigmentation; tyrosinase-negative albinos have more marked reduction in acuity and more obvious nystagmus. Normal foveal development appears to be influenced by melanin in the retinal pigment epithelium in the macular area.

In albinos, approximately 20% of axons from the temporal retina cross at the optic chiasm, so the ratio of crossed axons to uncrossed axons is higher than in normal persons (Brodsky et al., 1996). This crossing asymmetry has been confirmed by VEP studies that demonstrate significant hemispheric asymmetry to monocular stimulation. Not surprisingly, many albinos have abnormal stereopsis. Albinism is also associated with Hermansky-Pudlak syndrome, in which there is associated hemorrhagic diathesis, and Chédiak-Higashi syndrome, in which there is associated immune system dysfunction with recurrent pyogenic infections.