The Eye

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Chapter 4 The Eye

Salvatore Mangione, MD

“The eye is the light of the body.”

Matthew 6:22

“To drive away inflammation of the eyes, grind the stems of the juniper of Byblos, steep them in water, apply to the eyes of the sick person and he will be quickly cured. To cure granulations of the eye, prepare a remedy of cyllyrium, verdigris, onions, blue vitriol, powdered wood, mix and apply to the eyes.”

The Ebers Papyrus, 1500 BC

A. Generalities

The eye is often bypassed or cursorily examined. Yet from measurement of visual acuity to funduscopy, all components of the exam can unlock important secrets, not only of the eye but also of the body. Some findings are so important that they should be recognized by all practicing physicians.

1 Who should have visual acuity testing?

Anyone. Measuring vision is like taking the vital signs of the eye: nothing intelligent can be said without it. Since normal vision requires not only intact ocular function but also integrity of its vascular and neurologic supply, visual acuity is an excellent tool for the evaluation of ophthalmologic complaints. And since it can detect painless loss of vision in one or both eyes, it should be part of the screening exam of all adults (visual loss is in fact painless, except for disorders presenting with a red eye). Note that, in children, vision should be measured early (usually after the third birthday) to allow early detection of amblyopia.

2 What is needed to measure visual acuity?

Snellen chart, pinhole occluder, and pocket-size near-vision test card. All are readily available.

3 What is a Snellen chart?

A standard, wall-mounted eye chart imprinted with lines of black characters (letters, numbers, or illiterate Es), ranging in size from smallest (bottom) to largest (top).

4 Who was Snellen?

Hermann Snellen (1834–1908) was a Dutch ophthalmologist who graduated from (and later taught at) the University of Utrecht. He created his famous chart in 1862 as a first attempt to standardize visual acuity measurements.

5 How is the Snellen chart used?

By asking the patient to cover one eye at a time and then read from a well-illuminated chart at a distance of 20 feet (6 m). Testing starts by convention with the right eye and is carried out with reading or distance glasses. Visual acuity is the smallest line on which the patient can distinguish >50% of all letters. The number of letters missed on the same line should also be recorded, such as 20/20 − 1 or 20/40 − 2.

6 What does 20/20 mean?

That the patient can read at a distance of 20 feet a letter that was indeed designed to be read at 20 feet; conversely, 20/40 means that the patient can distinguish at 20 feet a letter that could normally be read at 40 feet, and so on. The numerator of the visual acuity fraction represents the distance at which the chart is placed, whereas the denominator represents the distance at which the letter could be recognized by a person with normal acuity. The denominator also identifies the smallest line on the chart where the patient is able to read >50% of all letters.

7 Can visual acuity be better than 20/20?

Yes. Although 20/20 is normal, most people without ophthalmic disorders can do better than that. Hence, their vision should be recorded accordingly, as 20/15, 20/12, and so on.

8 What is a pinhole vision?

It is vision through a pinhole occluder. If visual acuity is less than 20/20 (i.e., the patient cannot read the 20/20 line), measurement should be repeated after placing a pinhole occluder over the tested eye. This is a device with a very small opening (pinhole), which admits only axial light rays while eliminating instead all the peripheral rays that blur vision through either refractive errors or imperfections/opacities of the cornea and crystalline (see also strabismus and diplopia). Hence, the occluder improves vision in refractive or ocular media problems, but does nothing for retinal or neurovisual disorders. Once distant visual acuity has been measured in both eyes, near visual acuity should be evaluated, too. This can be done with a modified Snellen chart, such as the Rosenbaum Pocket-Size Visual Screener. Hold the card 14 inches (35 cm) away from the patient’s eyes, one at a time, covering the other with your hand. Reading/distance glasses should be kept on. Visual acuity is the fraction notated at the right of the smallest legible line.

9 How do you measure vision in patients who cannot read any letters on the chart?

By reducing the distance between patient and chart. This should be recorded as the numerator of the new visual acuity measurement (e.g., 5/70), and the fraction then translated into a more conventional recording (e.g., 20/280). If the patient cannot read the largest letters on a Snellen chart at 3 feet, vision may then be measured in terms of counting fingers (the examiner records the distance at which counting is accurate). If even this measurement is not possible, vision may be measured as ability to see hand motions (HM) or a flashlight (light perception [LP]), or finally as no light perception (NLP).

10 How is vision measured in illiterate patients or children?

By using the single “E” chart and asking patients to designate the strokes’ direction.

11 How is vision measured in bedridden patients?

By using the Rosenbaum card, which can be found at any medical supply store.

12 What is the significance of reduced visual acuity?

It may indicate any of the following ocular processes:

Refractive and correctable errors (myopia, astigmatism, and presbyopia)

Treatable and reversible blinding disease (cataracts or uveitis)

Manifestations of systemic disorders, progressive if untreated (diabetes or hypertension)

Vision-impairing and possibly life-threatening CNS disease (multiple sclerosis, gliomas)

Congenital disorders (rubella or toxoplasmosis)

Infectious diseases (cytomegalovirus, retinitis, or toxoplasmosis)

13 How can one confirm the cause of reduced visual acuity?

Uncorrected refractive errors of the eye are usually suspected whenever vision improves with a pinhole occluder.

Opacities of the light-transmitting media are diagnosed by either ophthalmoscopy or by simply examining the red reflex.

Neurologic or retinal disorders are revealed by ophthalmoscopy, the swinging flashlight test, or visual field testing.

Amblyopia is suggested by a childhood history of reduced visual acuity often accompanied by strabismus.

14 What are the refracting problems that can be corrected by glasses?

Hyperopia (in which the axial length of the eye is too short)

Myopia (in which the axial length is too long)

Astigmatism (in which the refracting power of the eye is different in one meridian than another)

15 When should you refer a patient with subnormal visual acuity?

Presence of visual symptoms

No symptoms, but visual acuity of 20/40 (or worse) in one or both eyes

A visual acuity difference between eyes of two lines or more

Middle-aged or elderly patients with presbyopia, even if distance visual acuity is preserved (for prescription of reading glasses)

Presence of an afferent pupillary defect

Glaucoma screening

B. Color Vision

16 What are color vision screening plates?

They are pseudoisochromatic tables (i.e., pseudo-similarly colored) that present numbers or figures against a background of colored dots. This color combination is confusing and illegible to a person with abnormal color discrimination.

17 How is color vision tested?

By consecutively presenting color plates to the patient, one eye at a time, and by then recording results as a fraction, with the numerator indicating the number of correct responses and the denominator the number of total plates presented. In testing for unilateral dyschromatopsia, a simpler but still reliable method is to ask the patient to cover the affected eye and look at a red object, such as the cap of a mydriatic solution bottle. Then the patient looks at the same object with the other eye (while the unaffected one is covered) and reports whether the red color appears the same. In dyschromatopsia, the affected eye will see the color red as being gray or washed-out.

18 What is dyschromatopsia?

From the Greek dys (abnormal), chroma (color), opsis (vision), this is an acquired deficiency in color vision, as opposed to congenital color blindness.

19 What causes dyschromatopsia?

Optic nerve diseases or toxic/degenerative processes of the macula.

C. Visual Fields

20 Why is visual field testing important?

Because it may reveal significant neurologic or ocular diseases. Terminology and interpretation, however, are often confusing.

21 Describe the normal anatomy of the visual pathways.

This is quite complex and yet fundamental for the understanding of visual field defects (Fig. 4-1). Our visual world (i.e., visual field) is divided into right and left hemifields. Each eye gets information from both.

Figure 4-1 Neurologic visual pathways, and diseases originating from their interruption.

(From Mihailoff G: Crash Course Nervous System. St. Louis, Mosby, 2005.)

After originating in the photoreceptors of the retina, vision is transmitted via the optic nerve axons. These partially cross at the optic chiasm and then emerge as optic tracts, wrapping around the midbrain to finally reach the lateral geniculate nucleus (LGN), where they stop for synapsis. From the LGN, axons fan out through the deep white matter of the brain (parietal and temporal lobe) as optic radiations, ultimately reaching the primary visual cortex in the occipital lobes. The crucial aspect of this entire pathway is its inversion of images, insofar as the nasal retina of the right eye sees the right half of the world (i.e., the temporal half of the visual field), whereas the temporal retina of the right eye sees the left half of the world (i.e., the nasal half of the visual field). As a result, the right nasal retina and left temporal retina see very much the same aspect of the outside world: the right hemifield (i.e., if you drew a line through the world at your nose, they would see everything to the right of that line).

Because of all these crossings, a prechiasmal lesion will be very much akin to losing one eye (i.e., it will affect only one eye but both hemifields). Conversely, chiasmal lesions will affect parts of both eyes and both hemifields (i.e., inputs from the nasal retinas will be lost, thus causing loss of peripheral vision on both sides). Finally, postchiasmal lesions will affect parts of both eyes, but only one hemifield.

The anatomic distribution of the optic radiations follows a wide three-dimensional arc, which first dives into the temporal lobe (Meyer’s loop), then heads back though the parietal lobe, and finally terminates into the occipital lobe. Lesions involving Meyer’s loop (i.e., temporal lobe) cause loss of vision in the upper visual world, but only in the left hemifield. Lesions involving the parietal portion of the optic radiations cause loss of vision in the lower visual world, but only on one side (the left hemifield in this case). Finally, lesions affecting the occipital (visual) cortex cause loss of vision in one hemifield, but with macular sparing (i.e., notched hemifield). This sparing results from either overlapping blood supply or large cortical representation of the fovea.

22 How do you test visual fields?

By using either a static or a dynamic technique. Diagnostic accuracy for visual field defects is similar, with high specificity but low sensitivity, especially for anterior or chiasmal lesions (see later).

Static testing: The examiner sits 3 feet away from the patient, who covers one eye with the palm of one hand while fixating on the examiner’s opposite eye. While the patient is doing so, the examiner outstretches an arm and briefly holds up one or two fingers. These are then displayed in each quadrant, and the patient is asked to report how many fingers are seen. In fact, the maneuver is often carried out by testing two quadrants simultaneously (by presenting both examiner’s hands to the two quadrants at the same time). This may unmask a parietal lesion that would otherwise allow a patient to see a single object (but not a double) by relying on the contralateral field. The test is then repeated for the opposite eye.

Dynamic testing: This is carried out very much like its static counterpart, except that the examiner continuously wiggles his or her fingers while gradually (and very slowly) bringing them into the patient’s visual field from a peripheral angle. The patient is then asked to report when (s)he sees the fingers.

After detecting the field defect, the examiner should then identify whether this defect respects the vertical or horizontal meridian of the visual field (see later).

23 What are visual field defects?

They are abnormalities of peripheral vision, reflecting lesions at various levels of the visual pathways. They are described as hemianopias (from the Greek hemi, half; an, lack of; and opsis, vision) if the loss of vision involves one half of the visual field of each eye; quadrantanopias if it instead involves one fourth of the visual field of each eye.

24 How are visual fields defects classified?

As anterior or posterior:

Anterior (or prechiasmal) defects are ocular lesions, since they involve the retina and optic nerves.

Posterior defects (chiasmal and postchiasmal) are instead neurologic lesions, since they involve optic tracts, radiations, and the cortex.

25 What is the difference between anterior and posterior visual field defects?

Anterior defects are unilateral (monocular)—typically crossing the vertical meridian.

Posterior defects are instead bilateral, with borders aligned to (and not crossing) the vertical meridian of the visual field. They are referred to as chiasmal if the defect is bitemporal and retrochiasmal if homonymous (see later).

26 What are the causes of visual field defects?

It depends on their location:

Prechiasmal defects are due to ocular lesions (glaucoma, retinal emboli, optic neuritis).

Chiasmal defects are due to pituitary lesions (mostly neoplastic).

Postchiasmal defects are due to cortical lesions (temporal, parietal, or occipital).

Optic tracts lesions are instead rare, representing only 5% of all postchiasmal defects.

27 What is the most important step after finding a monocular visual field defect?

To test the visual field of the other eye. Monocular field defects are usually due to problems in the affected eye (prechiasmal), whereas binocular defects are usually caused by intracranial (neurologic) processes, either chiasmal or postchiasmal.

28 What are the other characteristics of prechiasmal (ocular) defects?

They typically affect only one eye, compromise vision, and may have an abnormal pupillary/retinal exam (such as an afferent pupillary defect, cupping or atrophy of the disc, drusen, and retinal findings). In addition, prechiasmal visual defects typically cross the vertical meridian. This is because retinal nerve fibers draining the temporal retina arch across the vertical meridian in order to exit through the optic disc and nerve (which are located on the nasal side of the retina).

29 What are the main types of prechiasmal defects?

(1) “Arcuate defects” of the visual field, shaped indeed like an arch, and caused by small lesions of the nerve fibers, and (2) “altitudinal defects,” typically characterized by a sharp horizontal border in the visual field and caused instead by larger lesions of the nerve fibers.

30 What are severely constricted visual fields? What are their causes?

They also are a form of anterior defect, characterized by a concentric “cut” of the most peripheral vision (in this regard they are exactly the opposite of a central scotoma, which is characterized by loss of central vision with preservation of the peripheral one). Note that both these prechiasmal defects are monocular; typically cross the vertical meridian; and are usually due to advanced glaucoma, retinitis pigmentosa, and other more uncommon conditions (even though they also may be related to malingering.) A central scotoma is instead usually caused by damage to nerve fibers originating in the macula.

32 What is a bitemporal hemianopia?

It is the loss of vision of one half of the visual field of both eyes (from the Greek hemi, half; an, lack of; and opsis, vision), more specifically, the temporal fields of both eyes (bitemporal). This is typical of chiasmal tumors (gliomas and craniopharyngiomas in children, pituitary adenomas in adults).

33 What are postchiasmal defects?

Homonymous hemianopias or quadrantanopias. These typically involve both eyes but preserve vision and pupillary and retinal exam (see below).

34 What is homonymous hemianopia?

A visual defect affecting the same side (homonymous) of visual space in both eyes. For example, if the defect in the left eye involves the temporal region (the left portion of the left eye’s visual field), the defect in the right eye will affect the nasal region (the left portion of the right eye’s visual field). Homonymous hemianopia is typical of retrochiasmal lesions (optic tracts, optic radiations, and especially, the visual cortex).

35 What are the causes of inferior and superior quadrantanopias?

Diseases of either the parietal (causing inferior quadrantanopia) or temporal cortex (causing instead superior quadrantanopia). Still, lesions of the temporal and parietal cortex cause more commonly dense hemianopias. Note that the most common cause of an isolated homonymous hemianopia is a lesion to the occipital cortex, usually an ischemic stroke. In patients with other neurologic findings (hemiparesis, aphasia and, especially, an asymmetric optokinetic nystagmus), the most common cause is a parietal lesion.

36 What is an asymmetric optokinetic nystagmus?

A horizontal nystagmus that can be elicited by having patients look at a vertically striped tape that rapidly moves in front of their eyes, first toward one direction and then toward the other. Amplitude of horizontal nystagmus should be similar in the two directions. If not, it indicates a parietal lobe injury, since this typically reduces the nystagmus amplitude triggered by tape moving toward the same side of the lesion.

D. Pupils

37 Why is examination of the pupils important?

Because attention to pupillary shape, size, and response to external stimuli provides extremely valuable clinical information. Figures 4-2 and 4-3 summarize the most common pupil abnormalities.

Figure 4-2 Pupil abnormalities.

(From Liporace J: Crash Course Neurology. St. Louis, Mosby, 2006, Fig. 6.3.)

Figure 4-3 Additional pupil abnormalities.

(From Mihailoff G: Crash Course Nervous System. St. Louis, Mosby, 2005.)

38 What is the size of a normal pupil?

It depends on age. Pupils are larger in children (7 mm of diameter at 10 years of age), but progressively smaller with aging (5 mm at 50, 4 mm at 80). In addition to being important for amateur astronomers and late-evening bird watchers, this pupillary characteristic also may explain why children are “cute,” something cunningly exploited by Japanese cartoonists, who always draw their characters with big eyes and humongous pupils. Big and cute pupils are also nature’s way to prevent us from killing our young, especially when the young are crying, incontinent, and totally dependent. Conversely, crying, incontinent and totally dependent octogenarians have small pupils. Hence they are not cute, and that is why we put them in nursing homes. Italian Renaissance women also were quite aware of this important property of the eye, since they used to dilate their own pupils with atropine (aptly named belladonna [i.e., nice-looking woman]) before going out on dates. This, however, often backfired since loss of accommodation led to embarrassing accidents and, ultimately, to the discontinuation of the practice.

39 How does one examine the pupils?

In a darkened room. Ask the patient to fixate the gaze on something in the distance while you shine a penlight from below to ascertain that the pupils are round and equal.

40 What major features should be identified in the pupil?

(1) Shape, (2) size, and (3) responses to external stimuli. These, in turn, include response to accommodation and light, both direct and consensual. “Direct reaction” is the ipsilateral response to a light stimulus; “consensual reaction,” the contralateral response.

41 What is unique about the light reflex?

It contains crossed pathways, which means that both pupillary sphincters receive the same input from the midbrain.

42 What is anisocoria?

Pupillary asymmetry—from the Greek an (lack of), iso (equal), and core (pupils). Anisocoria reflects asymmetric diseases of either iris of efferent connections (i.e., third nerve or sympathetic nerves). The latter causes different signals to reach the two pupillary sphincters. A flip side to this rule is that anisocoria is absent in afferent disorders (i.e., affecting the optic nerve or retina) because crossed efferent pathways supply both eyes with the same signal.

43 What is accommodation?

A reflex that allows the subject to focus on a near object by activating three effectors: (1) the pupillary sphincters (causing constriction of the pupils); (2) the medial recti muscles (causing convergence of the eyes); and (3) the ciliary bodies (causing accommodation of the lens). It is typically spared in the Argyll-Robertson pupil.

44 What are the most common abnormalities in pupillary shape?

The most common is probably the irregular, pear-shaped contour of pupils that have undergone an intraocular surgical procedure, such as cataract excision. There also are other causes of oval pupils, such as an incipient transtentorial uncal herniation (before the pupil becomes dilated, fixed, and fully round) and Adie’s pupil.

Blunt trauma to the eye. This may tear the iris sphincter, making the affected pupil larger and slightly irregular.

Inflammation of the iris (iritis). This may cause adhesions (synechiae) of the iris to the anterior capsule of the lens, thus making the pupil irregular.

A coloboma of the iris. This is a congenital defect characterized by incomplete closure of the embryonic fissure of the optic cup. The involved iris usually has a keyhole shape, with the defect being most commonly in an inferonasal position.

45 What is hippus?

A physiologic and synchronous oscillation in pupillary size. This may occur spontaneously or in response to light shined directly onto the eye. The term derives from the Greek hippos (horse), chosen to convey the up-and-down movements of a galloping horse. Note that hippus is not an afferent pupillary defect (i.e., a Marcus-Gunn pupil).

Pearl:

The response to a back-and-forth swinging of a penlight is initial dilation in afferent pupillary defect and initial constriction in hippus (see also question 60).

46 How is the pupillary diameter controlled?

By a dual mechanism: parasympathetic (through the third nerve) for the constrictor muscle, and sympathetic (through the cervical chain) for the dilator. Hence, parasympathetic denervation will cause dilation (mydriasis), whereas sympathetic denervation will cause constriction (miosis).

47 What are the most common causes of anisocoria?

Simple (physiologic) anisocoria: A normal variant characterized by a physiologic difference of at least 0.4 mm between the two pupils, due, in turn, to an imbalance in muscular tone of the right and left sphincters. It is the most common anisocoria, present at all times in 3% of the population and at some times in up to 20%, with presence or absence depending on day of observation. In contrast to pathologic anisocoria, the physiologic form is characterized by a pupillary difference that does not change with various levels of illumination. Moreover, physiologic anisocoria is chronic, rarely >1 mm and always isolated (i.e., never associated with ptosis double vision or light-near dissociation see later). Hence, presence of concomitant findings makes anisocoria a much more ominous condition.

Pharmacologic dilation: Another common and benign cause. This can be due to conscious or inadvertent instillation of mydriatic drops (like in Italian Renaissance women) or even to improper nebulization of anticholinergic agents. The latter may create a diagnostic dilemma in intensive care unit patients, whose mental status often waxes and wanes. A response to cholinergic eye drops (such as pilocarpine) may help separating pharmacologic paralysis (which will remain unresponsive) from either Adie’s pupil or a third nerve palsy (which, instead, will constrict in response to topical cholinergics—previously discussed).

Third cranial nerve palsy (i.e., parasympathetic denervation): This is characterized by (1) ipsilateral mydriasis plus; (2) ptosis (from paralysis of the levator palpebrae); and (3) weakness of all extraocular muscles, with the exception of the lateral rectus and superior oblique (which are the only ones not controlled by the third cranial nerve). Hence, the affected eye will be deviated outward and downward, thus resulting in diplopia. Note that the dilated pupil will still constrict if a cholinergic drop is instilled in the eye. Moreover, due to defective constriction, the anisocoria of third-nerve palsy is greater in bright illumination.

Horner’s syndrome (Fig. 4-4): First described in 1860 by the Swiss ophthalmologist Johann Friedrich Horner, this is characterized by miosis of the affected pupil (from paralysis of the pupillodilator muscle) plus dysautonomic findings, including (1) ipsilateral ptosis (from paralysis of the superior tarsal muscle), and (2) anhidrosis (from damage to sudomotor fibers). In contrast to physiologic anisocoria, the difference between pupils in Horner’s varies with illumination: greater in the dark (revealing defective dilation) and smaller in bright light (demonstrating intact constriction). Response to cocaine drops is also different: Horner’s worsens, whereas simple anisocoria improves. This has strong predictive value, with sensitivity and specificity > 95%, a positive likelihood ratio (LR) of 96.8, and a negative LR of 0.1. Horner’s should always prompt a search for lesions of (1) the first-order neuron (such as a brainstem stroke, the most common cause of Horner’s on a neurology service; hence, the need for a thorough neurologic exam, with special attention to a lateral medullary syndrome); (2) the second-order neuron (usually a tumor of lung or thyroid, the most common cause of Horner’s on a medical service; hence, the need for a thorough neck/supraclavicular/respiratory exam); or, finally (3) the third-order neurons. These are less common causes of Horner’s, usually due to vascular headache (migraine), trauma or inflammation of the orbit, and cavernous sinus syndrome. Lesions of third-order neurons may preserve facial sweating.