VISUAL ACUITY

A sizeable minority of patients who are referred to a neuro-ophthalmology clinic have a refractive error as the only cause for the visual symptoms. It is thus essential to assess visual acuity only after having ensured that any refractive error has been corrected. The patient should be assessed with spectacles on, and if the acuity is abnormal, the clinician should add a pinhole to the lens and repeat the assessment. A standard distance acuity should be assessed using a Snellen 6 m Chart placed 6 m from the patient in a brightly illuminated position. Low luminance reduces visual acuity because foveal ganglion cells have high light thresholds. LogMAR charts are similar but allow comparison of repeated measurements in a statistical way. Near acuity can be assessed using Jaeger charts held by the patient at whatever distance is comfortable. Patients with refractive, corneal, or lens problems may have better near than distance acuity, and patients with accommodative and convergence disorders may have better distance acuity than those with near acuity. A more sensitive assessment of visual acuity involves the use of contrast sensitivity threshold measurements using wall charts or computer programs. These measure the sensitivity of minimum spatial resolution of gratings. In patients with amblyopia, a neutral density filter placed in front of the affected eye will not cause a substantial further loss of visual acuity (a 2.0 log filter reduces acuity in normal people by a factor of 2.0), whereas patients with reduced acuity due to a problem causing delayed optic nerve conduction demonstrate a much greater fall in visual acuity such as 6/9 to 6/60).

Color vision is assessed using pseudo-isochromatic plates such as Ishihara, Hardy-Rand-Ritter, and Dvorine plates. These are all easy and quick to use, although they cannot be used to assess the severity of the loss of color vision and they do not provide an adequate assessment of blue-green disorders. More complicated measurements such as the Farnsworth Munsell 100 hue test and others are better but much more time consuming.

These tests are useful in the assessment of visual loss due to optic nerve and macular problems. Congenital color blindness occurs in 8% of males and is symmetrical; an asymmetrical loss of color vision is always acquired. The prevalence of color blindness is as follows:

Köllner’s rule states that loss of red-green discrimination occurs in optic nerve disorders and loss of blue/yellow discrimination arises in macular problems. There are exceptions to this rule, however, because optic neuropathies that involve disruption of the papillomacular bundle (e.g., optic neuritis) will cause red-green color deficits, whereas those that disrupt fibers arising from the perifoveal fields (e.g., glaucoma and papilloedema) will cause blue-yellow deficits (as well as a proportionately smaller reduction in visual acuity).

VISUAL FIELD EXAMINATION

The assessment of the visual field is of crucial importance to the examination of the visual system because it is the most helpful aspect of the examination in determining the anatomical substrate of the visual symptom. Confrontation methods are adequate only if carried out very carefully indeed. Wiggling fingers only detect field defects that are absolute; that is, there is no vision within that field, and some may have no vision within a hemianopic field but are nonetheless able to perceive movement. Finger counting is better; the patient must focus on the examiner’s eye and say or copy the number of fingers presented to the four quadrants and the central field. Use of a small target such as a hat pin is more accurate; a white hat pin plots out the peripheral field and the red is used for central defects, particularly optic nerve disorders in which, as noted earlier, red-green color deficits arise.

The principle behind dynamic perimetric methods of field analysis such as the Goldman is that the examiner is identifying successive boundaries of vision known as differential light sensitivities (DLSs). These are the thresholds within which it is possible for that part of the retina to identify when a light projected is more bright than the background. The fovea is most sensitive, and this reduces with distance from the fovea. The temporal field changes slowly with distance from the fovea, whereas the DLS on the nasal side reduces abruptly. A normal Goldman field is shown in Figure 21-1. The advantage of this method of field assessment is that the skilled examiner can plot very carefully visual field abnormalities and can return over and over again to check the boundaries of the field.

Figure 21-1 Normal Goldman field.

Automated static perimetry is available in all ophthalmic departments, can be carried out in around 10 minutes, is easy to administer, and does not require so much skill to perform. It is less sensitive a measure than the Goldman when the examiner is highly skilled in use of the latter. Automated static field tests identify the threshold of accurate vision within the four visual quadrants within a 10, 24-, 30-, or 70-degree field. Fields can be recorded for comparison with subsequent examinations; reliability can be assessed by noting fixation losses and false-positive and -negative errors. The gray scale indices note the mean or pattern deviation of the patient’s responses to those of age-matched normal controls (Fig. 21-2).

Figure 21-2 Normal Humphrey field, left, upper field; right, lower field.

Factors that influence the precision of these field examinations include cognitive function and tiredness, refractive errors, and ophthalmic disorders such as cataract. The field is plotted at a certain level of background illumination because the threshold varies with background luminance up to a certain point and then increases in a linear way with suprathreshold background luminance. Examples of common field defects that are seen are shown in Figures 21-3 to 21-8.

Figure 21-3 Upper quadrantinopia due to a left temporal infarction, left, upper field; right, lower field.

Figure 21-4 Congruous but incomplete hemianopia due to an infarction of the left occipital lobe, left, upper field; right, lower field.

Figure 21-5 Bitemporal hemianopia due to a craniopharyngioma, left, upper field; right, lower field.

Figure 21-6 Centrocecal scotoma due to toxic amblyopia.

Figure 21-7 Arcuate scotoma due to glaucoma.

Figure 21-8 Enlarged blind spots due to bilateral papilloedema.

The Amsler grid is useful for plotting central field distortions, such as macular disorders, but also very small central field abnormalities due to, for example, optic neuropathy. The patient plots out the abnormality on the grid himself (Fig. 21-9).

Figure 21-9 Amsler grid with central field defect due to age-related macular dystrophy.

Tangent screen testing is also easy and rapid; a 1-m screen can be attached to the wall of a clinic room, and a light source with varying target size and luminance can be used very accurately to plot out a visual field. The contour of light sensitivity to the target of the same size and luminance is termed an isopter. Different target sizes and luminances give rise to different isopters, and so the field is plotted.

PUPILS

Patients with recent-onset pupillary mydriasis may complain of blurring of vision and photophobia, but most patients have no symptoms.

The examination involves inspection of pupil size and shape at rest. Each should be round and of the same size. Pupil size can be measured using a ruler or more easily using a pupil gauge such as that seen on a hand-held pinhole occluder. The pupils should then be inspected in light and dark—particularly if there is inequality in room lighting. Physiological anisocoria is detectable in 20% of young people and increases in prevalence with age to 33% of people over the age of 60 years. The inequality increases in dark in the case of physiological anisocoria and to a greater degree in the case of Horner’s syndrome. It may also be affected by anxiety, which increases sympathetic drive, and by fatigue.