EXAMINATION OF THE VISUAL SYSTEM

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CHAPTER 21 EXAMINATION OF THE VISUAL SYSTEM

Desmond Kidd

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:

Red-green males 8%
Red-green females 0.4%
Blue-yellow 0.005%
Achromacy 0.003%

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.

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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).

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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.

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

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

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

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Figure 21-6 Centrocecal scotoma due to toxic amblyopia.

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Figure 21-7 Arcuate scotoma due to glaucoma.

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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).

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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.

Pupillary Reaction to Light

The pupil should be assessed by a bright (but not too bright) light source such as a fully charged ophthalmoscope light source with low background illumination (just enough to see the pupil in darkness). The patient will be looking into the distance in order to prevent meiosis in accommodation. The light source is applied to the pupil for 2 or 3 seconds, and the response amplitude and reaction speed are noted. This should be repeated several times. The latency of redilatation should also be observed; early pupillary escape, in which redilation occurs earlier on one side and sometimes even before the light source is removed, implies a mild afferent pupillary defect.

Consensual Response

When light is applied to one eye, the pupillary response in the other should be equal in amplitude and synchronous because the decussation of pupillary fibers in the midbrain is 50%.

Near Response

The patient is instructed to look into the distance and then at a target held at the nearest point of distinct vision (25 cm in emmetropic people). A brisk and symmetrical meiosis should follow as convergence occurs. The speed and amplitude of the near response are noted and compared with the direct light response; light-near dissociation may arise in upper midbrain lesions due to compressive, infiltrative, or inflammatory causes (including, of course, neurosyphilis), severe bilateral optic neuropathies, and isolated parasympathetic nerve disorders.

Pupillary Dilatation

The pupil returns to its size appropriate for low levels of background illumination 12 to 15 seconds after a bright light source is removed from the eye. In patients with Horner’s syndrome, there is a dilatation lag, in which the affected side dilates more slowly than the normal side, and an increase in anisocoria is seen.

Relative Afferent Pupillary Defect

Everyone knows about this test and medical students believe that they should also perform it, but it is very complicated and requires skill and experience in order to perform it well. The eyes may be covered and uncovered in turn or the swinging light test may be applied. In the latter, the light source is applied in turn to each eye for 3 to 5 seconds repeatedly. The trick is to vary the time taken to move from one eye to the other; often, the relative afferent pupillary defect (RAPD) can be brought out thus. Great care should be taken to not apply the light source to one eye for longer than the other, to apply the light source to the same amount of retina in each eye (this is particularly important if there is ocular misalignment), and to ensure that there is no accommodative meiosis. Provided the test is performed properly, the examiner is able to see that there is pupillary dilatation on the side of a unilateral optic neuropathy when the light source returns to that side. Neutral density filters can be used, first, to measure the severity of the RAPD and, second, to bring it out if by regular testing the result is equivocal. The grading of RAPD is as follows:

Grade 1: weak initial contraction followed by a greater redilatation 0.4 log unit
Grade 2: a slight stall in movement followed by dilatation 0.7 log unit
Grade 3: immediate pupillary dilatation 1.1 log units
Grade 4: pupillary dilatation during prolonged illumination of the good eye for 6 seconds 2.0 log units
Grade 5: no signs of constriction no light transmission

Pupillography, in which infrared cameras are used to measure pupil size and shape in darkness and light, can be used to measure pupillary reaction times, amplitude, and latency.

Pharmacological Testing

Horner’s syndrome consists of meiosis and ipsilateral partial ptosis, apparent enophthalmos, and absence of sweating of the face ipsilaterally. The anisocoria is more evident in dark than in the light. The direct responses are normal. There is a failure of the affected pupil to dilate with 10% cocaine solution. Hydroxyamphetamine 1% dilates the affected pupil if the lesion is central or preganglionic, and no response occurs if the lesion is postganglionic (Fig. 21-10).

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Figure 21-10 Pupillary responses in Horner’s syndrome (A) in light, (B) in dark, (C) following instillation of 10% cocaine, and (D) following instillation of 1% hydroxamphetamine; the lesion is postganglionic.

Holmes-Adie syndrome consists of subacute severe mydriasis, which partially resolves over many months. It is associated with absent reflexes and rarely autonomic failure (Ross syndrome). The anisocoria is more marked in light than in dark. Pupils are tonic; denervation is rarely complete so vermiform movements (movements of the parts of the iris that have not been denervated) can be seen on slit lamp examination. Pilocarpine 0.1% constricts the affected pupil more than the normal owing to denervation supersensitivity (Fig. 21-11). Tonic pupils also occur following damage due to trauma and more commonly to inflammation due to viral infections and uveitis.

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Figure 21-11 Pupillary responses in tonic pupil (A) in light, (B) in dark, and (C) following instillation of 0.1% pilocarpine solution.

A partial third nerve palsy manifested only as mydriasis does not show denervation hypersensitivity and so does not constrict with 0.1% pilocarpine but does with 1.0%.

Finally, a pharmacologically mediated mydriasis will fail to constrict with 1% pilocarpine.

In essential anisocoria, the pupils are usually the same size in light and dark; this may be more apparent in dark but not to the same degree as with Horner’s syndrome. There are normal responses to cocaine (Fig. 21-12).

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Figure 21-12 Pupillary responses in essential anisocoria (A) in light, (B) in dark, and (C) following instillation of 10% cocaine.

EYELIDS

The eyelids are held open predominantly by levator palpebrae superioris, innervated by fibers of the superior division of the oculomotor nerve, which arise in the dorsocaudal nucleus in the midbrain. Hence, lid function may be spared or, alternately, the only presenting manifestation of differently situated midbrain lesions; a lesion of the dorsocaudal nucleus itself causes bilateral complete ptosis. Müller’s muscle is a thin sheet of smooth muscle fibers that attaches the levator to the upper tarsus and is innervated by the sympathetic nerve. The two eyelids are yoked, resulting in synkinetic and symmetrical movement alongside change in gaze, although compensatory lid retraction on the nondiseased side is not common.

When the patient is looking forward, the palpebral fissures should be the same and 12 to 15 mm in vertical length. Levator function is assessed by asking the patient to look into extreme downgaze and then extreme upgaze, and the excursion of the eyelid is measured; normal levator function is 12 to 17 mm of movement. Diminished levator function is not due to levator disinsertion or an acquired aponeurotic defect of eyelid control but only to neurogenic or myopathic processes.

In aponeurotic ptosis, the levator function is normal and the height of the lid crease is often noticeably greater on the ptotic side. The severity of the ptosis tends to increase in downgaze. It is common in the elderly and in patients following ophthalmic surgery such as cataract extraction, glaucoma procedures, and orbital or eyelid surgery.

Neurogenic Ptosis

Central Lesions

Cortical lesions are usually unilateral and contralateral, usually temporal and frontal lesions, and occasionally bilateral.

Midbrain lesions of the dorsocaudal nucleus are usually bilateral and complete and are associated with other signs of mesencephalic dysfunction. When entirely lone, they may be associated with Cogan’s twitch.

Oculomotor Fascicle or Nerve Lesions

Fascicle lesions occur with other central signs such as contralateral hemiparesis or cerebellar signs. Pupils are often but not always affected.

Nerve lesions usually but not always are associated with other signs, although these may be minimal even when aneurysm or pituitary adenoma is the cause.

Lesions of the Oculosympathetic Pathway

These lesions cause ipsilateral Horner’s syndrome.

Neuromuscular Ptosis

Examples include myasthenia gravis and, much less frequently, Eaton-Lambert syndrome.

Myopathic Ptosis

Congenital myopathic ptosis is only rarely neurogenic and is more often associated with a developmental abnormality of the levator muscle. There is a family history in 15% of cases and 20% of cases are bilateral. Often, there are other ocular problems, particularly superior rectus underaction.

Acquired myopathic ptosis includes chronic progressive external ophthalmoplegia (CPEO), myotonic dystrophy, and oculopharyngeal dystrophy.

Exophalmos, arising, for example, due to dysthyroid eye disease or to an orbital mass, can be assessed by comparing the supraorbital ridges of the patient from above and more precisely using an exophthalmometer.

OCULOMOTOR SYSTEM

Fixation

The patient is asked to fixate on a target such as the top letter of the Snellen chart and the efficiency of fixation with both eyes open; then each eye is observed with the other covered. Latent nystagmus and a change of fixation due to phoria can also be seen thus. Further examination of the eyes with fixation eliminated using Frenzel goggles is helpful because vestibular nystagmus is always accentuated under these circumstances. Standard clinical tests of vestibular function such as Hallpike’s maneuver and caloric testing are also important.

Smooth Pursuit

The patient is asked to fix on a target such as a pen top or hat pin while it is moved slowly and at steady speed from side to side. Those with abnormalities of smooth pursuit show corrective “catch up” saccades and the pursuit becomes fragmented. An optokinetic drum also shows up abnormalities on one side (there will be fewer corrective saccades on that side). In patients with nystagmus, pursuit can be tested by testing for suppression of the vestibulo-ocular reflex; the patient is asked to fixate on the thumb with the arm outstretched while he rotates the head. With abnormalities, there are corrective saccades because the eyes will be taken off track by the slow phase of the vestibulo-ocular reflex.

Vestibulo-ocular Reflex

Vigorous head shaking from side to side and then up and down with Frenzel goggles shows abnormalities were there to be a unilateral peripheral vestibular disorder; there may be a brief run of jerk nystagmus with the slow phase in the direction of the abnormality. This may not arise in acute lesions in which complete canal paresis occurs or in bilateral disorders or in central disorders. In central disorders, horizontal head shaking may induce vertical nystagmus.

The vestibulo-ocular reflex may be tested at the bedside when the patient fixes on a target and the examiner moves the head rapidly in one or another sideway direction. When the vestibulo-ocular reflex is normal, gaze is held steadily; with a lesion on one side, a corrective saccade arises at the end of the head movement.

Visual symptoms due to vestibular problems can also be tested by asking the patient to fix on a Snellen chart while the head is moved from side to side and then up and down at a steady state at about 2 cycles per second: with abnormalities of vestibular gain, a deterioration in acuity of several lines ensues.

Examination of Saccades

The patient is asked to fix alternately on two objects (a finger and a nose, a finger and a thumb) placed in horizontal and then vertical planes. The examiner initiates the movement with a command. Attention is made to the character of the movement and its conjugacy, velocity, and accuracy. Slowing can readily be appreciated in this setting or when using a hand-held optokinetic drum.

Saccadic latency (the time taken after instruction to initiating the saccade) may be abnormal in reduced levels of consciousness or disorders of attention. It also occurs in Huntington’s disease, Parkinson’s disease, ocular motor apraxia, conditions such as Balint’s syndrome, or the congenital type.

Saccadic dysmetria occurs when there is inaccuracy in fixation; hypermetria occurs when there is an overshoot; and hypometria occurs when there is an undershoot. Dysmetric saccades are usually followed by a corrective saccade. Saccadic dysmetria arises in brainstem and cerebellar pathologies, in drowsiness and drug-induced states, and in field defects due to visual pathway lesions.

Saccadic Intrusions

Squarewave jerks are involuntary saccades in which the eye moves from its position of fixation and then returns to the correct position after a normal intersaccadic interval (of 130 to 200 milliseconds). When these are small, they are not abnormal, but larger squarewave jerks (of 1 to 5 degrees) are more frequent and are readily seen during a clinical examination of fixation. These arise in cerebellar disorders, progressive supranuclear palsy, and Huntington’s disease. Much larger intrusions, so-called macrosquare wave jerks (of 10 to 40 degrees), have an intersaccadic latency of 100 milliseconds. These arise most commonly in the cerebellar form of multiple system atrophy (MSA-C) and multiple sclerosis. When large saccadic intrusions occur back-to-back in multiples (three to five), without an intersaccadic interval, a condition known as ocular flutter arises; if the condition is manifest as a series of saccadic intrusions in all directions, it is known as opsoclonus. These conditions occur in a variety of brainstem lesions and are due to abnormalities of the pause cells within the pons.

EXAMINATION OF DIPLOPIA

The patient is asked to follow a target through its range of movements in the nine directions of gaze. One eye is tested at a time (ductions) and then both are tested together (versions). The patient is asked to comment on the presence and severity of diplopia during version testing, and the eyes are examined for evidence for paresis. The patient notes that the diplopic image is displaced in the direction of the paresis when the direction of gaze is that of the paresis. Covering one eye with a red filter often helps to determine which image is which. Orthoptists use other tests such as the Maddox rod or the Hess chart.

Alternate Cover Test

This is most helpful in subtle abnormalities of vergence because when one eye and then the other eye are covered, the patient fixing on a target, the uncovered eye will be required to perform a corrective saccade in order to regain fixation, and so small movements in both horizontal and vertical directions may be seen. Esotropia and exotropia refer to outward and inward movements of the uncovered eye, respectively, when there is a horizontal deviation and hypotropia and hypertropia when there is a vertical one. When the eyes appear not to be misaligned but nonetheless a refixation movement occurs at the alternate cover test, the movement is termed a phoria.

The extent of the tropia can be measured in diopters using a prism to correct the diplopia.

Bielschowsky’s Head-Tilt Test

This test is undertaken in four stages:

1. Identification of the site of the hyperphoria
2. Identification of whether the deviation is greater in the right or left gaze
3. Identification of whether the deviation is greater in the up or down gaze
4. Measurement of the size of the deviation with the head tilted to the right or to the left

In the first stage, the patient is examined with the alternate cover test in the primary position of gaze and this reveals a right-over-left hyperphoria; either the depressors of the right (superior oblique and inferior rectus) or the elevators of the left (inferior oblique and superior rectus) are weak (Fig. 21-13). The second stage, in which the patient is examined again with the alternate cover test but in the left and then right gaze, the right-over-left deviation increases on the left gaze. Hence, the oblique muscles, which exert a greater influence on vertical eye movements in adduction, and the recti, which exert a greater influence in abduction, can be differentiated. In this case, the right hyperphoria increases in left gaze, so either the right superior oblique or the left superior rectus must be weak.

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Figure 21-13 Diagram of the Bielschowsky maneuver. This patient has a right IV neuropathy.

In the third stage, the patient is asked to look up then down in left gaze. If the hyperphoria increases in downgaze, then the oblique is weak; if the hyperphoria increases in upgaze, the rectus must be weak. In this case, the diagnosis is a right superior oblique palsy. The fourth stage measures its severity, the degree of head tilt to the left required to correct the vertical diplopia.

This works well for acute palsies, but in longstanding cases, changes in the tone of the reciprocally innervated muscles may give differing abnormalities.

KEY POINTS

• The neuro-ophthalmological examination is intricate and requires care and practice to perform it well.
• All of the examination can readily be performed at the bedside or in the office with only a few pieces of equipment.

The neurological examination of the visual system begins only after the ophthalmic examination of the visual system has been completed. Corneal abnormalities, lens opacities, and retinal problems can all cause blurring or distortion of vision, which are entirely unrelated to the structure and function of those parts of the nervous system that are responsible for the appreciation of vision.

Suggested Reading

Digre KB. Principles and techniques of examination of the pupils, accommodation and the lacrimal system. In: Miller NR, Newman NJ, editors. Walsh and Hoyt’s Clinical Neuro-ophthalmology. 5th ed. Baltimore: Williams and Wilkins; 1998:933-961.

Johnson CA, Keltner JL. Principles and techniques of examination of the visual sensory system. In: Miller NR, Newman NJ, editors. Walsh and Hoyt’s Clinical Neuro-ophthalmology. 5th ed. Baltimore: Williams and Wilkins; 1998:153-237.

Leigh RJ, Zee DS. The Neurology of Eye Movements, 2nd ed. Philadelphia: FA Davis, 1991.

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