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Chapter 203 – The Pupils

Chapter 203 – The Pupils











• Pupillary disorders may be classified into two major categories—afferent and efferent.

• Afferent pupillary defects interfere with the input of light to the pupillomotor system by light blockage or deficits in any of the retinal layers, into the optic nerve, chiasm, optic tract, or midbrain pretectal area. All of these result in a symmetrical decrease in the contraction of both pupils to light given to the damaged eye, compared with light given to the other less damaged or normal eye.

• Efferent pupillary defects interfere with contraction or dilatation of the pupil due to damage in the midbrain, in the peripheral nerve that supplies the iris muscles, or in the iris muscles themselves, often leading to asymmetrical pupils (anisocoria).




• Relative afferent pupillary defects cause a reduction in pupil contraction when one eye is stimulated by light compared with when the opposite eye is stimulated by light.

• Efferent pupillary defects cause anisocoria, a difference in pupil size between the right and left eyes, the extent of which depends on the condition of lighting or near effort.



• Relative afferent pupillary defects may be associated with visual field or electroretinographic asymmetries between the two eyes. Asymmetrical differences in retinal appearance or optic nerve appearance may occur.

• Efferent pupillary defects may be associated with either damage to the parasympathetic or sympathetic nerves that supply the iris or direct damage to the iris sphincter or dilator muscles that results in immobility of the pupil.





In this chapter the pupil is discussed from a practical, clinical standpoint. The focus is on features of pupil examination that enable effective diagnosis and management of a variety of diseases of the afferent visual system and of diseases that affect pupil size. The chapter is divided into two main portions—one on the use of pupil examination to assess afferent visual input and the second on the diagnostic implications of abnormal integration of the efferent output to the pupils. Abnormal integration may result in pupils of unequal diameter (anisocoria), pupils that do not dilate well in darkness, or a light–near dissociation, in which pupil contraction to a near reflex greatly exceeds the pupil constriction to a light reflex.


In general, the most important clinical use of the pupil is in the assessment of afferent input from the retina, optic nerve, and subsequent anterior visual pathways (chiasm, optic tract, and midbrain pathways). Because the pupillary light reflex represents the sum of the entire neuronal input (photoreceptors, bipolar cells, ganglion cells, and axons of ganglion cells), damage anywhere along this portion of the visual pathway reduces the amplitude of pupil movement in response to a light stimulus.[1] [2] Thus, the clinician can establish any asymmetrical damage between the two eyes by a simple comparison of how well the pupil contracts to a standard light shone into one eye compared with the same light shone into the other eye.[3] Observation of pupil movement in response to alternating the light back and forth between the two eyes is the basis for the alternating light test, or “swinging flashlight” test, used to assess the relative afferent pupillary defect (RAPD). [4] [5]

Another important aspect of pupil movement in response to light is that the pupillary light reflex summates the entire area of the visual field, with some increased weight given to the central 10°.[2] Thus, in general terms, the pupillary light reflex is roughly proportional to the amount of working visual field. Damage to peripheral portions of the retina and visual field defects outside the central field reduce the amplitude of the pupillary light reflex. Such damage may not be established by other objective tests of visual function, such as the electroretinogram and visual evoked potential.

Standard flash electroretinogram findings are affected very little by focal retinal damage that produces a visual field defect. For example, a patient who has a disciform scar caused by aged-related macular degeneration or a branch artery occlusion gives a normal flash electroretinogram result. However, in such an example, the pupillary light reflex is reduced compared with that of the other eye, and an RAPD is obvious. In addition, optic nerve disease or damage to the retinal ganglion cells is not detected by standard flash electroretinography. Assessment of the pupillary light reflex readily enables the detection of such damage.

Because the visual evoked potential primarily samples the occipital pole or tip, which represents the central 5–10° of visual field, it is not affected to any great extent by peripheral visual field defects. In addition, cooperation of the patient is required to fixate on the center of a computer monitor while the visual stimulus is presented. A patient who chooses not to fixate properly or who has media opacities that reduce the clarity of the checkerboard pattern may produce an abnormal result, even if the retina and optic nerve function are normal (i.e., a false-positive test result). Similarly, peripheral visual field defects caused by glaucoma or anterior ischemic optic neuropathy may yield a normal visual evoked potential, or false-negative result, but the pupillary light reflex is reduced. [6]

Therefore, the pupillary light reflex is one of the few objective reflexes that can be used as a clinical test for the detection and



quantification of abnormalities of the retina, optic nerve, optic chiasm, or optic tract. Because the amount of RAPD is correlated, to a large extent, with the amount of asymmetry of visual field deficit between the two eyes, it also may be used to help substantiate abnormal results of perimetric testing[7] [8] [9] [10] ; this often helps the clinician to determine whether a patient’s report of visual field defects is believable and trustworthy. The correlation between visual field asymmetry and RAPD also is a useful monitor of the course of disease for a worsening or improvement in function. RAPDs are, by definition, relative to the input of one eye compared with that of the other. Bilateral symmetrical damage does not produce RAPDs. Thus, a definite RAPD in one eye on the first visit but no RAPD on follow-up may represent improvement in the previously damaged eye or the development of damage in the previously better eye. Therefore, it is always important to remember that the RAPD is, indeed, relative to the other eye.

Estimation of the amount of RAPD in log units (asymmetry between the two eyes) provides an idea of how much visual field damage is present and whether it is consistent with the results of the visual field test. In addition, the amount of RAPD may indicate whether the cause of damage is consistent with the results of the pupil examination. For example, a patient affected by a small amount of macular degeneration in one eye and not the other is expected to have only a 0.3–log unit RAPD, but if that patient has a 1.0–log unit RAPD, then some other cause of visual loss is likely, such as a previous branch retinal artery occlusion or optic neuropathy ( Table 203-1 ).

In general, with unilateral visual loss, loss of the central 5° of the visual field results in an RAPD of approximately 0.3 log





Log Unit

Relative Afferent

Pupillary Defect

Influencing Factors

Intraocular hemorrhage

Anterior chamber or vitreous (dense)


Density of hemorrhage


Anterior chamber (diffuse)


Density of hemorrhage


Preretinal (central vein occlusion or diabetic)


Preretinal location does not significantly reduce light

Diffusing media opacity

Cataract or corneal scar

0.0–0.3 in opposite eye

Dispersion of light produces increase in light input

Unilateral functional visual field loss

None (nonorganic)


No real visual field loss

Central serous retinopathy or cystoid macular edema

Retina (fovea)


Area of retina involved, depth of scotoma

Central or branch retinal vein occlusion

Inner retina

0.3–0.6 (nonischemic)

=0.9 (ischemic)

Area of visual field defect and degree of ischemia

Central or branch retinal artery occlusion

Inner retina


Area and location of retina involved

Retinal detachment

Outer retina


Area and location of detached retina (e.g., 0.6–0.9 log units for macula +0.3 log units for each quadrant)

Anterior ischemic optic neuropathy

Optic nerve head


Extent and location of visual field defect

Optic neuritis (acute)

Optic nerve


Extent and location of visual field defect

Optic neuritis (recovered)

Optic nerve


No visual field defect, residual relative afferent pupillary defect

Compressive optic neuropathy

Optic nerve


Extent and location of visual field defect, other eye involvment

Chiasmal compression

Optic chiasm


Asymmetry of visual field loss, unilateral central field involvement

Optic tract lesion

Optic tract

0.3–1.2 in the eye with temporal field loss

Incongruity of homonymous field defect, hemifield pupillomotor input asymmetry

Postgeniculate damage

Visual radiations

Visual cortex


Stimulus light size (no residual relative afferent papillary defect but definite pupil perimetry defects)

Midbrain tectal damage

Olivary pretectal area of pupil light input region of midbrain


Similar to optic tract lesions, but no visual field defect

The expected magnitude of defect is given as well.



units. Loss of the entire central area of field (10°) causes an RAPD of 0.6–0.9 log units. Each visual field quadrant outside of the macula is worth about 0.3 log units, but the temporal field loss seems to result in more loss of pupillary input compared with loss in the nasal field quadrants. The correlation between the relative afferent defect and the area and extent of visual field loss, however, is only approximate. Differences between the two may be important clues as to the cause and extent of damage to the anterior visual system.

Studies that used computerized pupillography to quantify the RAPD more precisely showed that some subjects who have normal visual fields and examination results can have a small (0.3–log unit) RAPD.[11] [12]

The amount of pupillomotor input asymmetry (the RAPD) may be estimated roughly using the alternating light test (without any neutral density filters) and the subjective grades +1, +2, +3 or +4 for asymmetry of pupillary response. This subjective grading also may be categorized according to the amount of “pupil escape,” or dilatation of the pupils, as the light is alternated between the eyes.[13] However, most subjective grading of RAPDs has serious limitations, such as some large-scale errors that arise from age variations in pupil size and pupil mobility. For example, a patient who has small pupils and small pupillary contractions to light may have a large RAPD, but this may appear deceptively small on the basis of small differences in pupil excursion observed as the light is alternated between the two eyes. However, the amount of neutral density filter needed to dim the better eye until the small contractions are equal represents substantial input damage. To estimate the size of RAPDs



without using filters is very much like the estimation of an ocular deviation “by Hirschberg” without a prism cover test. More accurate quantification of RAPDs is accomplished by determination of the log unit difference needed to “balance” the pupil reaction between the two eyes.[4] [5] Photographic neutral density filters (49?mm, screw mount, 0.3 log, 0.6 log, and 0.9 log) often are available through local photography stores.

Measurement of the Relative Afferent Pupillary Defect

Measurement of RAPDs is the most important part of the pupil examination, because it may give the most valuable clinical information. The alternating light test for an afferent defect is based on the assumption that the irises are a matched pair—each has sphincter and dilator muscles of good shape and properly innervated—so that the light reactions can be compared. Therefore, it is important to first establish whether an anisocoria is present, which may indicate an efferent defect.


Pupillary inequality usually results from an iris innervation problem, so to evaluate anisocoria the iris sphincter and dilator muscles must be checked. In the office, the best way to decide whether the sphincter muscle or dilator muscle is weak is to compare the amount of anisocoria in darkness and in light, which can be carried out without any special equipment. The examiner must be able to change the lighting and still view the pupils. Of course, usually no anisocoria is found in darkness or in light, in which case the efferent arm of the light reflex arc is presumed intact, and the examiner proceeds to check for an afferent defect. When anisocoria is present, the examiner needs to establish whether it increases in darkness or in light. If one sphincter is weak, the investigator may still check for an afferent defect in the pupil that still works by comparison of its direct and consensual reactions. If no asymmetry of input is apparent, and hence no RAPD, this impression may be confirmed using the “tilt test” described below ( Fig. 203-1 ).

Anisocoria may influence the estimate of pupillary input asymmetry. Small pupils allow less light to pass and large pupils more. However, if neither pupil is less than 3.0?mm wide in light, then any anisocoria less than 2.0?mm difference may be disregarded—at least with respect to a false afferent defect induced by the pupillary inequality. Only very large anisocorias cause enough difference in retinal illumination between the two eyes to produce an apparent asymmetry of pupillomotor input.


To check for RAPD, the light is alternated from one eye to the other ( Fig. 203-2 ). If the light is too bright, the pupils do not redilate promptly, and very little pupil movement is seen as the light is alternated to the other eye. The problem may be solved by a direct reduction in stimulus intensity or if the light is moved 3–4 inches (8–10?cm) away from the eyes and alternated between them.



Figure 203-1 Checking for an RAPD using the “tilt test.” If there seems to be no input asymmetry, the “tilt test” can confirm this by inducing an RAPD of the same magnitude in each eye by holding a 0.3–log unit neutral density filter over one eye during the alternating light test and then repeating it with the filter over the other eye.

Observe the Illuminated Eye.

If the pupils react relatively weakly when one eye is stimulated and better when the other is stimulated, an afferent defect relative to the better eye (RAPD) has been identified.

Balance the Responses Using Filters.

To balance the response, a filter is held over the good eye and the alternating light test repeated. If the input asymmetry is still visible, the density of the filter over the good eye is increased until the amplitudes of the direct light reactions of the two eyes are balanced. To be certain of the measurement, the balance point may be overshot deliberately and then back titrated. When a dense filter is used, it may be necessary to look behind the filter to see the pupil ( Fig. 203-3 ).


If a very small asymmetry is suspected, such as a defect of less than 0.3 log units in the left eye, it may be just the result of noise in the system (e.g., “hippus”). An effort must made to confirm the asymmetry by “tilting” the RAPD to the right and to the left using a 0.3–log unit filter (see Fig. 203-1 ). If no RAPD exists, the examiner should be able to induce the same amount of input asymmetry by holding a 0.3–log unit filter over the right eye during the alternating light test and then repeating the test with the filter switched to the left eye. If a small RAPD is present, it will become more apparent when the filter is held over that eye.


Unfortunately the pupillary response to a repeated light stimulus is far from constant; it changes from moment to moment.[11] [12] A common error is to judge the apparent asymmetry of the light reflex too quickly. It is important to alternate the light back and forth at least 3 times to obtain a mental average of any asymmetry. In this way, moment-to-moment fluctuations in the pupillary response are “averaged out.” Despite such efforts, the afferent asymmetry may still fluctuate slightly with time, even when carefully recorded using computerized pupillography.


Infants and small children may appear to have weak pupillary responses to light, largely because of the excitement



Figure 203-2 Demonstration of a large afferent defect in the right eye. This is best demonstrated when the light is alternated from eye to eye at a steady rate. The light is kept just below the visual axis and 1–2 inches (3–5?cm) from each eye. Each eye is illuminated for about 1 second and then the light switched quickly to the other eye; this allows comparison of the initial direct pupil contraction with light in each eye.



Figure 203-3 Balancing pupillary response using filters. A dark iris appears even darker behind the filter so it may be difficult to view the pupil, in which case it helps to peek behind the filter to obtain a better view of the iris.



and apprehension that inhibit the pupillary light reflex at the supranuclear level in the midbrain. Usually, after the light stimulus has been repeated several times, the light reaction begins to improve and “loosen up,” especially as the child becomes less anxious. A baby’s pupils are checked at about 3?ft (1?m) away with a direct ophthalmoscope. In a dark room, the brightest light and smallest spot are used, focus is on the red reflex, and the light is alternated from eye to eye. The baby usually is fascinated, and a filter sometimes can be placed in the beam to one eye.


When the input defect is in an eye that has an injured iris or a dilated pupil, the pupillary responses of the uninjured eye must be observed. The direct and consensual responses of the working pupil may be compared by alternation of the light from one eye to the other. While a measurement is made, the good eye is behind the filter and it may be hard too see the pupil. Sometimes it is necessary to use a side light on the healthy iris in order to see its consensual pupil reaction. Because this may corrupt the measurement, an infrared video system is used, if possible.


Instruments are available that give a more precise evaluation of the pupillary light reflex; these include infrared video recording equipment and a computerized interface to present controlled light stimuli and quantify the dynamics of pupil movement in response to each light stimulus.

Infrared Videography.

Sometimes it is important to view the magnified movement of both pupils at once. Infrared videography ( Fig. 203-4 ) enables the examiner to see both pupils clearly in the dark, which is particularly helpful when difficult afferent pupillary defects need to be checked for (e.g., one pupil is fixed, or both irises are pigmented very darkly). Because melanin reflects infrared light, dark irises appear light and so the black pupils stand out in contrast and are viewed easily. Videography also is used to establish the dilatation lag of a Horner’s pupil, to catch the brief paradoxical constriction in patients who have some retinal abnormalities (found when lights are turned out), to transilluminate the iris in pigment dispersion syndrome,[14] and in Adie’s syndrome.

Computerized Pupillometry.

Various computerized infrared-sensitive pupillometers are available commercially and can record precisely the dynamics of pupil movement in the light or in the dark; the results are analyzed by sophisticated software. Such systems provide quantitative information about the pupillary light reflex and, in the future, may help to automate the clinical determination of pupillary input deficits that result from retinal and optic nerve diseases.[11] [12] [15] [16] [17] In addition, information obtained using computerized pupillography provides evidence



Figure 203-4 The infrared video-pupillometer. The infrared sources (clusters of light-emitting diodes) are mounted in gooseneck lamps. The double base–out prisms bring the pupils close together on the screen, allowing increased magnification from the infrared videocamera and telephoto lens.

that a number of different types of visual stimuli can produce changes in the pupillary light reflex, related to color, form, movement, and acuity.[18] [19] [20] [21] [22] [23] [24]

Pupil Perimetry.

An automated perimeter may be modified to record pupillary responses. A video camera is pointed at the pupil and the amplitude of each light reaction is measured and stored in the computer, which is helpful as an objective form of perimetry and to localize lesions in the pupillary pathways.[25] [26] Pupil perimetry is also useful in cases of nonorganic, functional visual loss to show objectively that messages are, indeed, going normally into the brain from parts of the visual field in which the patient claims to see nothing.



As discussed above, a pupillary inequality usually indicates that one of the four iris muscles, or its innervation, is damaged ( Fig. 203-5 ). To establish which is the weaker muscle, it is useful to know how the anisocoria is influenced by light. An anisocoria always increases in the direction of action of the paretic iris muscle, just as an esotropia increases when gaze is in the direction of action of a weak lateral rectus muscle.

Pupillary Inequality That Increases in the Dark

In patients who have a pupillary inequality that increases in the dark ( Fig. 203-6 ), the problem is to differentiate Horner’s syndrome from a simple anisocoria (or physiological anisocoria)—in which the inequality also is greater in dim light. A simple anisocoria may vary from day to day, or even from hour to hour, and is visible in about one fifth of the normal population; it is not related to refractive error. Clinically, Horner’s syndrome is recognized by associated signs such as ptosis, “upside-down ptosis” of the lower lid and, in an acute case, conjunctival injection and lowered intraocular pressure.

Simple anisocoria is common (about 10% of normal subjects, examined in room light, have an anisocoria of 0.4?mm or more) and is not associated with disease. Simple anisocoria also, like Horner’s syndrome, decreases slightly in light, but it does not show a dilatation lag of the smaller pupil. It is believed that simple anisocoria most likely arises from asymmetrical inhibition at the Edinger–Westphal nucleus in the midbrain. Normally, during wakefulness some inhibition from the reticular activating formation keeps the pupils midsize or larger. During sleep, this inhibition fades and allows the neurons in the Edinger–Westphal nucleus to discharge, which results in miotic pupils. If, during wakefulness, the inhibition is greater to the right Edinger–Westphal nucleus than the left, the right pupil is larger, especially in dim light. When light is added or a near reflex is generated, this inhibition is overcome and the pupils become smaller and any asymmetrical inhibition diminishes. A reduction of the anisocoria results as the pupils become smaller.


The characteristic “dilatation lag” of the pupil in Horner’s syndrome is seen easily in the office using a handheld light shone from below. The room lights are switched off and the smaller pupil examined for an apparent reluctance to dilate. Pupil dilatation is normally a combination of sphincter relaxation and dilator contraction, a combination that produces a prompt dilatation. The patient who has Horner’s syndrome has a weak dilator muscle in one iris and, as a result, that pupil dilates more slowly than the normal pupil. If the sympathetic lesion is complete, the affected pupil dilates only by sphincter relaxation. The resultant asymmetry of pupil dilatation produces an anisocoria that is largest 4–5?sec after the lights have been turned out—the process is much slower than is generally thought. At 10–20?sec after the lights have been put out, the anisocoria lessens as the sympathectomized pupil gradually catches up, a process referred to as dilatation lag. The test is a quick and simple way to differentiate Horner’s syndrome from simple anisocoria, and it does not require





Figure 203-5 Parasympathetic and sympathetic innervation of the iris muscles.

pupillary drug tests. It works well most of the time, particularly in young people who have mobile pupils, but if the dilatation lag is inconclusive, cocaine eye drops may be used to confirm the diagnosis of Horner’s syndrome.


The action of cocaine is to block the reuptake of norepinephrine (noradrenaline) normally released from the nerve endings. If, because of an interruption in the sympathetic pathway, norepinephrine is not released, cocaine has no adrenergic effect. The affected pupil in a patient with Horner’s syndrome dilates less with cocaine than does the normal pupil, regardless of the location of the lesion. Cocaine drops are placed in both eyes and after 60 minutes the anisocoria has increased clearly, because the normal pupil has dilated more than the Horner’s pupil.

The author recommends cocaine hydrochloride 10% in both eyes (not more than two drops) to ensure that even the darkest iris receives a full mydriatic dose; corneal epithelial defects will not result from this dose. Cocaine 2%, 4%, and 5% also have been used in diagnostic tests for Horner’s syndrome and work well. Anisocoria is measured after 50–60 minutes have elapsed. If very little dilatation of the pupil occurs, even though an oculosympathetic defect is suspected, and the pupil did not dilate well in darkness even after 30 seconds before the cocaine test, then a false-positive cocaine test result must be considered. A false-positive result may occur if the iris is held in a miotic state, through either scarring or aberrant reinnervation of the iris sphincter. In such cases, the addition of a direct-acting sympathomimetic agent to both eyes (e.g., 2.5% phenylephrine) at the conclusion of a positive cocaine test should dilate the suspected eye easily and eliminate the cocaine-induced anisocoria almost immediately. For the reasons stated above, pseudo–Horner’s syndrome results in inadequate dilatation to direct-acting sympathomimetic agents. The likelihood of Horner’s syndrome increases steadily as the degree of pupillary inequality (measured after the instillation of cocaine) increases. Unlike in the hydroxyamphetamine test, calculation of the change in anisocoria from before to after cocaine application is unnecessary. If there is at least 0.8?mm of pupillary inequality after cocaine administration, Horner’s syndrome is highly likely.[27]


Whether the damage to the sympathetic pathway is in the postganglionic neuron is a question of considerable clinical importance, because many postganglionic defects are caused by benign, vascular headache syndromes or carotid dissections, and a preganglionic lesion sometimes results from the spread of a malignant neoplasm.

Hydroxyamphetamine eyedrops help to localize the lesion in Horner’s syndrome. The clinician needs to know where the lesion is to direct the radiographic workup, for example, to the internal carotid artery rather than to the pulmonary apex. Horner’s syndrome sometimes manifests so characteristically that further efforts to localize the lesion are not needed, as with patients who have cluster headaches.

Hydroxyamphetamine releases norepinephrine from storage in the sympathetic nerve endings. When the lesion is postganglionic, the third order nerve is dead and no norepinephrine stores are available for release at the iris. When the lesion is complete, the pupil does not dilate at all. However, the dying neurons and their stores of norepinephrine may last for almost 1 week from the onset of damage. Therefore, a hydroxyamphetamine test administered within 1 week of a postganglionic lesion may give a false preganglionic localization if some of the norepinephrine





Figure 203-6 Diagnosis of pupillary abnormalities in which anisocoria increases in dim light. If the anisocoria is greatest in dim light and diminishes in bright light, then the pupillary inequality is either physiological (a simple anisocoria) or arises from the loss of sympathetic innervation to the dilator muscle (Horner’s syndrome). A few other conditions need to be considered, but this chart is concerned only with acute damage to a single intraocular muscle or its innervation. (Adapted from Thompson HS, Kardon RH: Clinical importance of pupillary inequality, Focal Points: Clinical Modules for Ophthalmologists, vol 10, no 10, December, 1992, American Academy of Ophthalmology.)

stores remain. When Horner’s syndrome is caused by preganglionic or central lesions, the pupils dilate normally, because the postganglionic third order neuron and its stores of norepinephrine, although disconnected, are still intact; when the lesion is in the preganglionic neuron, the involved pupil often becomes larger than the normal pupil after hydroxyamphetamine administration, apparently because of “decentralization supersensitivity.”


The test is simple—the pupil diameters are measured before and 40–60 minutes after hydroxyamphetamine drops have been placed in both eyes. The change in anisocoria in room light is noted. If the affected pupil—the smaller one—dilates less than the normal pupil, an increase in anisocoria occurs, and the lesion is in the postganglionic neuron. If the smaller pupil now dilates so much that it becomes the larger pupil, the lesion is preganglionic and the postganglionic neuron is intact. The examiner must wait at least 2 days after cocaine has been used before the administration of hydroxyamphetamine; cocaine seems to block its effectiveness.

In about one half of ambulatory patients with Horner’s syndrome, the location of the lesion is identified satisfactorily by the nature and location of the injury or disease. The other half of these patients offer no clues as to the location of the damage— a pharmacological localization of the lesion in these patients can be most helpful.

The author has attempted to apply the results of hydroxyamphetamine mydriasis in those patients with a known lesion location to those in whom the lesion location is unknown. It appears that postganglionic lesions (along the carotid artery) can be separated from the nonpostganglionic lesions (in the brainstem, spinal cord, upper lung, and lower neck) with a degree of certainty that varies with the amount of anisocoria induced when the drops are placed in both eyes.[28]


When a child is observed to have a unilateral ptosis and miosis, the first question is





Figure 203-7 Horner’s syndrome clearly acquired in infancy must be evaluated for neuroblastoma, a treatable tumor. This baby, with a right ptosis and miosis, developed a flush during cycloplegia that made the vasomotor abnormality very clear—the Horner’s side remained pale. The baby had no sign of Horner’s syndrome during her first 8 months, but at 16 months Horner’s syndrome is obvious (ptosis, miosis, and upside-down ptosis). Because the syndrome was acquired, a chest radiograph was ordered; it showed a mass in the pulmonary apex. Magnetic resonance imaging confirmed the lesion. Surgery showed it to be a neuroblastoma.

to ascertain whether Horner’s syndrome is present. The ptosis of Horner’s syndrome is moderate, never complete. Sometimes the elevation of the lower lid is helpful. A child who has congenital Horner’s syndrome and naturally curly hair has, on the affected side of the head, hair that seems limp and lank. The shape of the hair follicles apparently depends on intact sympathetic innervation, as does the iris pigment. A child who has blond, straight hair and very pale, blue eyes does not have any visible hair straightness or iris heterochromia.

A weaker solution of cocaine (two drops of cocaine 2% in each eye) is used in children. The most telling symptom is the hemifacial flush (blanch on the affected side) that occurs with nursing or crying. Generally, the affected side is pale. In an air conditioned office, it may be hard to decide whether decreased sweating on the affected side is present. A cycloplegic refraction sometimes produces an atropinic flush everywhere except on the affected face and forehead and, thus, provides additional evidence toward diagnosis because of lack of sympathetic innervation to the skin vasculature.

In infants, hydroxyamphetamine drops do not help to localize the lesion, because orthograde transsynaptic dysgenesis takes place at the superior cervical ganglion after early interruption of the preganglionic oculosympathetic neuron. Fewer postganglionic neurons result, even though no direct postganglionic injury has occurred, which produces weak mydriasis and ambiguous results in children.[29] A patient with Horner’s syndrome clearly acquired in infancy must be evaluated for neuroblastoma—a treatable tumor ( Fig. 203-7 ).

Pupillary Inequality That Increases With Light

For a patient who has pupillary inequality that increases with light, several problems must be addressed ( Fig. 203-8 ).


Trauma to the globe usually results in a torn sphincter and an iris border that transilluminates at the slit lamp. The pupil often is not round and other evidence of ocular injury may be present. Naturally, such a pupil does not constrict well to light. The residual reaction often is segmental in a traumatic iridoplegia. An atrophic sphincter caused by previous herpes zoster iritis also may reveal transillumination defects, as seen with the slit lamp, that arise from previous ischemic insults to the iris. If, however, the iris looks normal, further investigation is required, as outlined below.


If no residual light reaction is present, the possibility of pharmacological mydriasis must be explored.[30] However, a completely blocked light reaction sometimes may occur when the sphincter is denervated by either a preganglionic lesion (third cranial nerve palsy) or a postganglionic lesion (acute, complete tonic pupil), in acute angle closure (iris ischemia), or with an intraocular iron foreign body (iron mydriasis). If the dilated pupil still has some response to light, the dilatation may result from partial denervation of the sphincter, incomplete atropinization, or adrenergic mydriasis. When the light reaction is poor because the dilator muscle is in spasm (as a consequence of adrenergic mydriatics such as phenylephrine), then the pupil is very large, the conjunctiva is blanched, and the lid is retracted. In such cases, any decrease in the amplitude of accommodation is minor and is the result of spherical aberration and a shallow depth of field—both optical results of the dilated pupil, or from the small inhibitory effect of sympathetic receptor activation or accommodation.


When some residual light reaction occurs, the iris sphincter is examined for sector palsy using the slit lamp. When the dilator is in a drug-induced adrenergic spasm or when the cholinergic receptors in the iris sphincter are blocked by an atropine-like drug, the entire sphincter muscle (all 360°) is less effective. This does not happen when postganglionic parasympathetic nerve fibers have been interrupted. In patients with Adie’s syndrome, all pupils that have a residual light reaction (about 90%) show segmental contractions of the sphincter (so-called vermiform movements). Thus, a pupil that has a weak light reaction and no segmental palsy usually indicates a drug-induced mydriasis, but signs of a third cranial nerve paresis (preganglionic parasympathetic nerve) must also be sought.


If weak pilocarpine (about 0.1%) or weak methacholine (2.5%) is applied to both eyes (with both corneas healthy and untouched), and the affected (dilated) pupil constricts more than the normal pupil to become the smaller pupil, that iris sphincter is denervated. It seems likely that with a postganglionic denervation (ciliary ganglion to the eye), the sphincter will show a little more supersensitivity than in the preganglionic case (third cranial nerve palsy); however, the differences are not great. Cholinergic supersensitivity of the iris sphincter is considered now to be only a weak sign of Adie’s syndrome. As the iris sphincter is reinnervated by cholinergic accommodative fibers and becomes smaller over time, supersensitivity can be lost.[31]

Ptosis or diplopia must be re-evaluated, because it is very rare for an ambulatory patient to have an isolated sphincter palsy from damage to the intracranial third nerve. If the normal pupil constricts a little and the dilated pupil not at all, the mydriasis may result from a local dose of an anticholinergic drug such as atropine. A stronger concentration of pilocarpine is needed to establish this.


If, on application of pilocarpine 1% in each eye, the affected pupil reacts little or not at all and the unaffected pupil constricts normally, the pupil was not dilated because of innervation problems but because of a problem in the sphincter muscle, itself. Non-neuronal causes of mydriasis are:

• Anticholinergic mydriasis (e.g., scopolamine [hyoscine], cyclopentolate, atropine)

• Traumatic iridoplegia (sphincter rupture, pigment dispersion, angle recession)

• Angle-closure glaucoma (ischemia of the iris sphincter)

• Fixed pupil after anterior segment surgery

• Bound down iris (synechia) after iritis

The cause for complete loss of function of the iris muscles after anterior segment surgery is unknown. Sometimes an excessive rise in intraocular pressure during or after surgery can cause ischemic damage to the iris sphincter.

Tonic Pupil of Adie’s Syndrome

Young adults (more women than men) may discover that one pupil is large or that they cannot focus up close with one eye. Slit-lamp examination usually shows segmental denervation of the iris sphincter. Within the first week, supersensitivity to cholinergic substances may be demonstrated. After about 2 months, nerve regrowth is active and fibers originally bound for





Figure 203-8 Diagnosis of pupillary abnormalities in which anisocoria increases in bright light. Initial pupillary inequality greater in bright light than in the dark indicates that the sphincter of the large pupil is weak or that a parasympathetic lesion is present on that side.

the ciliary muscle (they outnumber the sphincter fibers by 30:1) start to arrive (aberrantly) at the iris sphincter, which produces the characteristic light–near dissociation of Adie’s syndrome. Eventually, the affected pupil becomes the smaller of the two pupils, especially in dim light, because of the aberrant reinnervation by accommodative fibers (“little old Adie’s pupil”). The segmental palsy of the iris sphincter is seen particularly well using infrared video recording of transillumination of the iris.[31]

Fixed, Dilated Pupil

When a pupil is dilated by an atropinic medication, the resultant condition can be differentiated with confidence from an innervational palsy by its tendency to resist the miotic effects of cholinergic drops such as pilocarpine. Pilocarpine 1% is a sufficient miotic dose for any eye, but a sphincter with all its cholinergic receptors blocked by atropine or tropicamide does not constrict with pilocarpine 1%. If the anticholinergic drug starts to wear off such that a small light reaction begins to return, pilocarpine 1% may only cause minimal constriction compared with the normal pupil.

Third Cranial Nerve Palsy

An old clinical rule of thumb states that if the pupillary light reaction is spared, the third cranial nerve palsy probably does not result from compression or injury, but more likely from small-vessel disease such as might be seen in diabetes. The rule still applies,



but a very small number of pupil-sparing third cranial nerve palsies arise from midbrain infarcts.

Aberrant Regeneration of the Third Cranial Nerve

The third cranial nerve carries instructions to several different muscles, so when the nerve is injured and the fibers regrow, they may grow into the wrong place. This most commonly occurs when the glial scaffolding, which normally segregates nerve bundles, is disrupted by trauma or external compression by a tumor. For example, the eye may inappropriately turn in when the patient is trying to look down, or the pupil may inappropriately constrict with depression, adduction, or supraduction of the globe.


The pupillary contractile response to a near effort is observed as a standard part of the pupil evaluation. If the light reaction seems a little weak, the examiner must check whether the pupils constrict better to near than to light. If they do, this is called light–near dissociation, the causes of which are summarized in Table 203-2 .

How to Test for a Pupillary Near Response

The pupillary near reaction usually is weak when tested in dim light or in the dark. The patient needs a clear view of an object before it can be brought into focus. The examiner must not induce a near response at the slit lamp unless magnification is required to view segmental contractions of the iris sphincter; the room usually is dark and too much equipment is close to the patient’s face in this situation.

The near response is examined in moderate room light, such that the patient’s pupils are midsize and the near object is clearly visible. The patient is given an accommodative target to view—something with fine detail on it. Sometimes a better response is obtained if some other sensory input is added to the stimulus, such as a ticking watch or clicking fingernails; or something proprioceptive for example, the patient’s own thumbnail can be brought into the patient’s view (perhaps, in the case of a child, with a little face drawn on it).

Convergence indicates how hard the patient is trying. The near response, although it may be triggered by blurred or disparate imagery, has a large volitional component, and the patient may need encouragement. If, for some reason, the patient has not made a near effort recently (for example, because stereopsis






Severe loss of afferent light input to both eyes

Anterior visual pathway (retina, optic nerves, chiasm)

Damage to the retina or optic nerve pathways

Loss of pretectal light input to Edinger–Westphal nucleus

Tectum of the midbrain

Infectious (Argyl Robertson pupils) or compression (pinealoma)

Adie’s syndrome

Iris sphincter

Aberrant reinnervation of sphincter by accommodative neurons

Third cranial nerve aberrant reinnervation

Iris sphincter

Aberrant reinnervation of sphincter by accommodative neurons or medial rectus neurons



is not achieved at near), then a few practice runs may be needed. Often, on the third or fourth try, a good near response is obtained. Sometimes the near effort must be generated for 5 to 10 seconds to see a good near reaction of the pupil. A lack of near response usually indicates that the patient (or the doctor) is not trying hard enough, or not enough time is given at the peak of convergence for the pupil to constrict maximally.

A patient who is completely blind and has no pupillary reaction to light sometimes provides a good near response when asked to “cross your eyes like you did when you were a child.” If a patient cannot produce a near response, the lid closure reflex is tried: the patient faces the examiner with eyes squeezed shut while the examiner tries, with both hands, to hold one of the patient’s eyes open. This often produces a surprisingly strong near response, and is called the eye closure pupil reaction.

Recognition of Light–Near Dissociation

Sometimes it is difficult to establish whether the near response is clearly greater than the light reaction. In such cases, when the examiner faces the patient with pocket light in hand, three levels of light are used:

• Darkness, with a light shining tangential on the pupils from below

• Room light

• Room light with an additional bright light shone in the eyes

With the patient looking in the distance, the bright light is shone in the eye for 1–2 seconds 3 or 4 times, which indicates how small the pupils may become using a light stimulus only.






Past inflammation or surgical trauma

Posterior iris surface or sphincter

Scarring or synechiae of the iris because of past iritis

Acute trauma


Prostaglandin release causes sphincter spasm

Adie’s syndrome tonic pupil

Third nerve aberrant reinnervation


Aberrant regeneration of iris sphincter by accommodative or extraocular motor neurons that are not inhibited in darkness

Pharmacologic miosis

Iris sphincter

Cholinergic influence

Unilateral episodic spasm of miosis

Postganglionic parasympathetic neuron

Uninhibited episodic activation of postganglionic neurons

Congenital miosis (bilateral)


Developmental abnormality

Fatigue, sleepiness

Edinger–Westphal nucleus

Loss of inhibition at midbrain from reticularactivating formation

Lymphoma, inflammation, infection

Periaqueductal gray matter

Interruption of inhibitory fibers to the Edinger–Westphal nucleus

Central-acting drugs

Reticular activating formation, midbrain

Narcotics, general anesthetics

Old age (bilateral miosis)

Reticular activating formation, midbrain

Loss of inhibition at midbrain from reticular activating formation

Oculosympathetic defect

Sympathetic neuron interruption

Horner’s syndrome





The near response must not be judged by the addition of a near stimulus to a bright light stimulus, which almost always produces an apparent light–near dissociation, because the near stimulus inevitably adds something to the light stimulus. A real light–near dissociation is present only if the near response (tested in moderate light) exceeds the best constriction that bright light can produce.


When one or both pupils stay small and miotic, even in darkness, a number of factors may be responsible ( Table 203-3 ). To better understand the different mechanisms possible it is important to understand the normal process in darkness that allows the pupil to dilate. When a light stimulus is terminated, two mechanisms cause the pupil to dilate. The greater part of pupil dilatation arises from inhibition to the Edinger–Westphal nucleus in the midbrain, which reduces the firing of the preganglionic parasympathetic neurons in the Edinger–Westphal nucleus and results in relaxation of the iris sphincter. Within a few seconds, sympathetic nerve firing increases, which augments the pupil dilatation by active contraction of the dilator muscle. The combined inhibition of the iris sphincter and stimulation of the iris dilator is a carefully integrated neuronal reflex. Therefore, inability of the pupil to dilate in darkness may occur because of a sympathetic nerve palsy, but also from mechanical limitations of the pupil (scarring), pharmacological miosis, aberrant reinnervation of cholinergic neurons to the iris sphincter that are not normally inhibited in darkness (accommodative or extraocular motor neurons), or inhibitory input signal not received by the Edinger–Westphal nucleus.





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