Pupillary function testing is a series of examinations that evaluate pupil size, shape, light reflex, and near reflex. It is primarily used as an aid in diagnosing optic nerve and central nervous system diseases and is a representative bedside test that is simple, noninvasive, and can be performed immediately.
The main purposes of the examination are as follows.
Assessment of direct and consensual light reflex: Observe pupillary constriction in the stimulated eye and the contralateral eye to differentiate afferent and efferent pathway disorders.
Pharmacologic pupillary testing: Use of eye drops to confirm Horner syndrome or Adie pupil and to localize the lesion (preganglionic vs. postganglionic).
In acute conditions threatening vision, such as optic neuritis, traumatic optic neuropathy, and ischemic optic neuropathy, as well as in differential diagnosis in ophthalmology and neurology, the information from this test significantly influences clinical decisions.
QWhat kind of information does the pupillary function test provide?
A
The pupillary function test can help localize lesions in the afferent pathway (retina, optic nerve) and efferent pathway (oculomotor nerve) of the light reflex. In particular, RAPD is an important objective sign indicating unilateral optic nerve damage or extensive retinal disease 1). Comparison of anisocoria in light and dark conditions allows differentiation between sympathetic and parasympathetic disorders, and combining with pharmacologic testing can sometimes localize the lesion to preganglionic or postganglionic sites 2).
A positive RAPD indicates unilateral optic nerve damage or extensive retinal disease. The following table shows representative diseases with positive or negative RAPD1)3).
If RAPD is positive despite no obvious abnormality in the fundus (especially the macula), optic nerve disease should be strongly suspected. The fact that macular holes do not cause RAPD is an important distinguishing point.
In bilateral symmetric optic nerve disease, the afferent defects in both eyes cancel each other out, so RAPD is not detected. Post-chiasmal lesions (optic radiations, visual cortex) also do not produce RAPD.
This test is recommended for early screening of highly urgent diseases (CRAO, acute optic neuritis, traumatic optic neuropathy, etc.).
QWhat diseases cause a positive RAPD?
A
Diseases that cause a positive RAPD are broadly divided into optic nerve disorders and extensive retinal diseases. Representative optic nerve disorders include optic neuritis, traumatic optic neuropathy, ischemic optic neuropathy (e.g., NAION), and compressive optic neuropathy. Extensive retinal diseases include central retinal artery occlusion (CRAO), extensive retinal detachment, and AZOOR1). Macular holes and bilateral symmetric optic nerve diseases do not cause a positive RAPD. It is important to remember that post-chiasmal lesions (optic radiations, visual cortex) also do not produce RAPD.
3. Anatomy and principles of the pupillary light reflex
In humans, the magnitude of the direct and indirect pupillary light reflexes is almost equal. Therefore, even if the optic nerve on one side is damaged, anisocoria does not occur when both eyes are open. However, when the left and right eyes are alternately stimulated, the response differs between stimulation of the affected side and the healthy side 1).
Afferent pathway disorder (retina, optic nerve): When the affected side is illuminated, miosis in both eyes is insufficient, resulting in a positive RAPD.
Efferent pathway disorder (oculomotor nerve): The direct reflex on the affected side disappears, but the indirect reflex is preserved (ipsilateral pupillary sphincter muscle dysfunction).
The sympathetic pathway follows: hypothalamus → spinal cord (C8–T2 intermediolateral column) → superior cervical ganglion → long ciliary nerve → pupillary dilator muscle. In sympathetic nerve disorders (Horner syndrome), the dilator muscle does not function, so pupillary dilation in the dark is impaired.
ipRGCs contain melanopsin and play an important role in maintaining pupil constriction during sustained light stimulation. They are responsible for the sustained component of the pupillary light reflex after light adaptation 2).
Preparation: Perform in a dark room. Both eyes are open (do not cover one eye). Always perform before pupillary dilation with eye drops.
Procedure: Shine a penlight alternately into each eye for about 1–2 seconds. The duration and intensity of light should be equal for both eyes.
Interpretation: If dilation is observed when the penlight is moved to the other eye, that eye is considered to have a relative afferent pupillary defect (RAPD).
Example (left optic neuropathy): Right eye illumination → both pupils constrict. Move to left eye → dilation. Right eye → constriction. Left eye → dilation. This cycle repeats.1)
Quantification of RAPD Using ND Filters
Principle: A neutral density (ND) filter is placed in front of the healthy eye, and the RAPD is quantified by the filter density at which the RAPD disappears.
Method: Filters are added in increments of 0.3 log units (e.g., 0.3, 0.6, 0.9, 1.2 log units), and the swinging flashlight test is repeated.
Clinical application: The ND filter density at which RAPD disappears serves as an indicator of the magnitude of the RAPD. This is useful for quantitative evaluation of treatment progress.1)
Clinical photograph of anisocoria showing unequal pupil diameters between the left and right eyes.
Russavia. Anisocoria in a 25-year-old male of unknown cause with early facial nerve paralysis. Wikimedia Commons. 2012. Figure 1. Source ID: commons.wikimedia.org/wiki/File:Anisocoria.jpg. License: CC BY 3.0.
External photograph of a 25-year-old male showing obvious anisocoria with unequal pupil diameters. This corresponds to the differentiation between sympathetic and parasympathetic nerve disorders by comparing pupil size in dark and light conditions, as discussed in the section “Evaluation of Anisocoria.”
By comparing the difference in pupil size in dark and light conditions, sympathetic and parasympathetic nerve disorders can be differentiated.
Greater anisocoria in the dark (affected side smaller): Suggests sympathetic nerve disorder (Horner syndrome). In the dark, only the healthy eye dilates, so the difference increases.
Greater anisocoria in the light (affected side larger): Suggests parasympathetic nerve disorder (e.g., oculomotor nerve palsy). In the light, only the healthy eye constricts, so the difference increases.
Physiological anisocoria: Within 1 mm. The difference is constant in dark and light conditions, and the pupillary light reflex is normal.
Pharmacologic eye drops are used to localize the lesion or differentiate specific diseases2)4).
0.1% pilocarpine eye drops: In Adie pupil (postganglionic ciliary ganglion lesion), denervation supersensitivity causes more pronounced miosis in the affected eye than in the healthy eye. Normal eyes show no miosis.
4% cocaine eye drops: Inhibits norepinephrine reuptake; in eyes with normal sympathetic innervation, dilation occurs. In Horner syndrome, the affected eye does not dilate or dilates insufficiently.
1% hydroxyamphetamine eye drops: Promotes norepinephrine release from postganglionic neurons. In postganglionic lesions (third-order neuron lesions), dilation is insufficient in the affected eye, showing a different pattern from preganglionic lesions.
QHow is the swinging flashlight test performed?
A
In a dark room with both eyes open, alternately shine a penlight on each eye for about 1–2 seconds with equal duration and intensity. Perform this test before using any dilating drops. If dilation is observed in the illuminated eye, that eye has a relative afferent pupillary defect (RAPD). For example, in left optic neuropathy: when the right (healthy) eye is illuminated, both eyes constrict; when the light moves to the left (affected) eye, dilation occurs1). It is recommended to repeat this cycle 2–3 times to confirm6).
5. Interpretation of Test Results and Clinical Application
If the fundus (especially the macula) is normal: Strongly suspect optic neuropathy. Proceed with evaluation for optic neuritis, ischemic optic neuropathy, compressive optic neuropathy, etc.
The presence or absence of RAPD is important information in the follow-up of the optic nerve (recovery from optic neuritis, evaluation after trauma) and preoperative assessment. Quantification using ND filters is useful for objective evaluation of treatment effects 1).
Argyll Robertson pupil: Characterized by loss of light reflex, preserved near reflex, bilateral, small irregular pupils. It is a characteristic finding in neurosyphilis 2)
Adie pupil (Holmes-Adie syndrome): Caused by postganglionic fiber lesion of the ciliary ganglion. Sector palsy (sectoral paralysis of the pupillary sphincter), light-near dissociation, and supersensitive miosis to 0.1% pilocarpine instillation are diagnostically valuable 4)
Oculomotor nerve palsy: Direct light reflex is absent in the affected eye, and indirect reflex is absent when the healthy eye is illuminated. If accompanied by eye movement disorder or ptosis, posterior communicating artery aneurysm must be rapidly ruled out 3)
6. Pathophysiology of the Pupillary Light Reflex Pathway
The afferent pathway consists of axons of retinal ganglion cells (optic nerve) → optic chiasm → optic tract → midbrain (anterior part of Edinger-Westphal nucleus). Differences in responses depending on the type of disorder are as follows.
Demyelination (optic neuritis): Axons are relatively preserved but conduction delay occurs, and RAPD is observed. RAPD often diminishes with recovery.
Axonal damage (ischemic or traumatic optic neuropathy): The signal itself is attenuated, causing RAPD. Axonal loss is irreversible, and RAPD tends to persist.
Extensive retinal disease: Photoreceptors and ganglion cells are damaged, resulting in positive RAPD.
The efferent pathway is Edinger-Westphal nucleus → oculomotor nerve → ciliary ganglion → pupillary sphincter. In efferent pathway disorders, the direct light reflex of the stimulated eye (affected eye) is absent, but the indirect reflex when the contralateral eye is stimulated is also absent in the affected eye. This pattern differs from RAPD (afferent pathway disorder).
Waster (uploaded by Monopol). Miosis in Horner syndrome, from: Nautiyal A, et al. Painful Horner Syndrome as a Harbinger of Silent Carotid Dissection. PLoS Med. 2005. Figure 1. Source ID: commons.wikimedia.org/wiki/File:Miosis.jpg. License: CC BY 2.5.
External eye photograph of a patient with Horner syndrome, clearly showing miosis due to sympathetic dysfunction in both eyes. This corresponds to the impaired dilation in darkness due to dilator muscle dysfunction discussed in the section “Sympathetic Pathway Disorder (Horner Syndrome)”.
The sympathetic pathway follows the route: hypothalamus → spinal cord C8–T2 intermediolateral column → superior cervical ganglion → long ciliary nerve → pupillary dilator muscle. Lesion sites are classified into first-order, second-order, and third-order neurons, and are differentiated by the hydroxyamphetamine eye drop test. Lesions of first-order (central) and second-order (preganglionic) neurons cause mydriasis, but lesions of third-order (postganglionic) neurons do not cause mydriasis2).
ipRGCs (intrinsically photosensitive retinal ganglion cells) express melanopsin and respond directly to light. They are responsible for the sustained component of the pupillary light reflex (intrinsic light reflex) and contribute to persistent miosis after intense light stimulation. ipRGCs are sometimes relatively preserved in various retinal diseases, which is one reason why the pupillary light reflex may remain even in cases of severe degeneration of cones and rods 2).
Lesions posterior to the optic chiasm (optic radiations, visual cortex) are outside the pupillary light reflex pathway, so RAPD does not occur. This characteristic provides important information for differentiating whether the cause of visual loss is in the anterior visual pathway (retina, optic nerve) or the posterior visual pathway.
The main precautions for the examination are listed below.
Perform in a dark room: Sufficient darkness is necessary to improve RAPD detection accuracy.
Both eyes open: Do not occlude either eye.
Use the same light intensity for the penlight: Maintain a constant flashlight intensity.
Equal stimulation time: Make the illumination time equal for each eye (alternating about 1–2 seconds each).
Perform before pupillary dilation: Cannot be evaluated after mydriatic eye drops.
Portable and tabletop automated pupillometers enable objective and quantitative RAPD assessment. Devices capable of precise quantification in 0.01 log unit increments, eliminating inter-examiner variability, have been developed 5). Since they can record temporal changes in pupil diameter (latency, constriction amplitude, and redilation velocity), they are attracting attention as a technology that complements subjective judgment in the swinging flashlight test.
By using different color light stimuli (red and blue light), the functions of cones, rods, and ipRGCs (melanopsin cells) can be selectively evaluated. Research is progressing to separately assess the degree of damage to each cell type in age-related macular degeneration, glaucoma, and diabetic retinopathy2). This may provide information that cannot be obtained with standard white-light pupillary light reflex testing.
Development of portable pupillometers using smartphone cameras and LED illumination is advancing. They are expected to be applied in low-cost and telemedicine settings, and are considered promising as RAPD assessment tools in environments without specialized ophthalmic equipment, such as emergency rooms, obstetrics, and ICUs 5). However, verification of accuracy and reproducibility is still ongoing.
Application in Structure-Function Correlation in Glaucoma
Studies on the correlation between structural damage (OCT-RNFL thinning) and pupillary function (RAPD, chromatic pupillometry) in glaucomatous optic neuropathy are accumulating. Knowledge of the extent to which ipRGCs are damaged in glaucoma may provide new indicators for functional monitoring of glaucoma2).
