Residency · Residency · Neurology

Neuro-ophthalmology Essentials: Pupils, Visual Fields, and Eye Movements

Overview

Neuro-ophthalmology sits at the intersection of neurology and ophthalmology, focusing on the neural pathways that govern vision, pupillary function, and eye movements. Mastery of the afferent (visual) and efferent (oculomotor) systems enables precise localization throughout the entire neuraxis. The bedside examination of pupils, visual fields, and eye movements is among the most powerful localizing tools available to the neurologist.

The Afferent Visual Pathway

Anatomy

The visual pathway begins at the retina, where rods and cones transduce light into neural signals. Retinal ganglion cell axons form the optic nerve (CN II), which carries fibers to the optic chiasm. At the chiasm, nasal retinal fibers cross to the opposite side while temporal retinal fibers remain ipsilateral. Beyond the chiasm, the optic tracts carry fibers to the lateral geniculate nucleus (LGN) of the thalamus. From the LGN, optic radiations project to the primary visual cortex (V1) in the calcarine cortex. The superior retinal fibers, which carry information about the inferior visual field, travel in the parietal optic radiations (Baum loop). The inferior retinal fibers, carrying superior visual field information, sweep through the temporal optic radiations (Meyer loop).

Visual Field Defects and Localization

Visual field defects map predictably to the site of the lesion. Monocular visual loss indicates an optic nerve lesion, which may be due to optic neuritis, ischemic optic neuropathy, or compression. Bitemporal hemianopia results from chiasmal compression, classically from a pituitary adenoma, craniopharyngioma, or meningioma. A homonymous hemianopia indicates a lesion posterior to the chiasm on the contralateral side, which could involve the optic tract, LGN, optic radiations, or occipital cortex. A superior quadrantanopia (sometimes called "pie in the sky") localizes to the temporal lobe (Meyer loop), while an inferior quadrantanopia ("pie on the floor") localizes to the parietal lobe (Baum loop). A homonymous hemianopia with macular sparing is characteristic of an occipital cortex lesion, because the macular representation has dual blood supply from both the posterior cerebral artery and the middle cerebral artery. Bilateral occipital infarcts can produce cortical blindness with preserved pupillary responses; when the patient is unaware of their blindness, this is called Anton syndrome.

Confrontation Visual Field Testing

At the bedside, each eye is tested independently with the other covered. The examiner presents finger counts or finger wiggling in each quadrant. A red target can be used to detect subtle desaturation from optic nerve lesions. Kinetic perimetry at the bedside involves bringing a target from the periphery inward to map the boundaries of the visual field.

Pupillary Examination

Anatomy of the Pupillary Light Reflex

The afferent limb of the pupillary light reflex travels from the retina through the optic nerve and optic tract to the pretectal nucleus in the midbrain. The efferent limb projects from the pretectal nucleus to the bilateral Edinger-Westphal nuclei, then through CN III parasympathetic fibers to the ciliary ganglion, and finally via the short ciliary nerves to the sphincter pupillae muscle. The sympathetic pathway follows a three-neuron chain: first-order neurons from the hypothalamus descend to the ciliospinal center of Budge at C8-T2, second-order neurons ascend to the superior cervical ganglion, and third-order neurons travel along the internal carotid artery through the cavernous sinus to reach the eye via the long ciliary nerves, innervating the dilator pupillae muscle.

Relative Afferent Pupillary Defect (RAPD)

The relative afferent pupillary defect, also known as the Marcus Gunn pupil, is detected with the swinging flashlight test. When the light is swung from the normal eye to the affected eye, the affected pupil paradoxically dilates instead of constricting. An RAPD indicates asymmetric optic nerve disease such as optic neuritis, ischemic optic neuropathy, or compressive optic neuropathy. A large retinal lesion can produce a mild RAPD. Importantly, cataracts and refractive errors do not cause an RAPD, and an RAPD cannot be produced by an efferent (CN III) lesion.

Horner Syndrome

Horner syndrome presents with the triad of miosis (small pupil), ptosis (1-2 mm), and anhidrosis (the distribution of which depends on the lesion level). Dilation lag in dim light is a characteristic feature. First-order (central) Horner syndrome results from hypothalamic or brainstem lesions such as stroke or demyelination. Second-order (preganglionic) causes include apical lung tumors (Pancoast tumor), thoracic surgery, and neck masses. Third-order (postganglionic) causes include carotid dissection (which is urgent), cavernous sinus lesions, and cluster headache. Pharmacologic testing can help localize the lesion: cocaine drops fail to dilate the Horner pupil; apraclonidine reverses the anisocoria by dilating the Horner pupil through upregulated alpha-1 receptors; and hydroxyamphetamine differentiates third-order from first- or second-order lesions. An acute painful Horner syndrome must be treated as carotid dissection until proven otherwise.

Anisocoria Approach

When anisocoria is greater in bright light, the larger pupil is the abnormal one, suggesting CN III palsy, pharmacologic mydriasis, or Adie tonic pupil. When anisocoria is greater in the dark, the smaller pupil is abnormal, pointing to Horner syndrome or pharmacologic miosis. Physiologic anisocoria produces less than 1 mm of difference and does not change with lighting conditions.

Adie Tonic Pupil

Adie tonic pupil is a dilated pupil with a sluggish or absent light response but a preserved tonic near response. It results from damage to the ciliary ganglion (postganglionic parasympathetic). The hallmark is light-near dissociation: the pupil responds poorly to light but constricts slowly with prolonged near effort. Denervation supersensitivity causes the pupil to constrict with dilute pilocarpine (0.1%), which would not affect a normal pupil. This is a benign condition, usually unilateral, most common in young women, and may be associated with absent deep tendon reflexes (Holmes-Adie syndrome).

Ocular Motor System

CN III (Oculomotor) Palsy

The oculomotor nerve innervates the medial rectus, superior rectus, inferior rectus, inferior oblique, and levator palpebrae superioris. It also carries parasympathetic fibers to the pupillary sphincter and ciliary body. A complete CN III palsy positions the eye "down and out" with complete ptosis and a dilated, fixed pupil.

The critical distinction is between pupil-involving and pupil-sparing CN III palsies. A pupil-involving CN III palsy is a posterior communicating artery aneurysm until proven otherwise and represents a surgical emergency. The parasympathetic fibers run on the surface of the nerve, so external compression from an aneurysm affects them first. A pupil-sparing CN III palsy is more likely due to microvascular ischemia from diabetes or hypertension, which affects the center of the nerve while sparing the superficial parasympathetic fibers. Aberrant regeneration -- where eyelid elevation occurs with eye depression, for instance -- is seen with chronic compression (meningioma, aneurysm) but never with microvascular palsies. Its presence should prompt imaging for a structural cause even if the palsy was initially pupil-sparing.

CN IV (Trochlear) Palsy

The trochlear nerve innervates the superior oblique, which intorts and depresses the eye in adduction. CN IV palsy is the most common cause of isolated vertical diplopia. The head tilt test (Bielschowsky test) reveals worsening hypertropia with head tilt toward the affected side, and patients often present with a compensatory contralateral head tilt. Congenital CN IV palsy is common and may decompensate later in life. Trauma is the most common acquired cause due to the nerve's long intracranial course and vulnerability at the tentorium.

CN VI (Abducens) Palsy

The abducens nerve innervates the lateral rectus, which abducts the eye. CN VI palsy produces horizontal diplopia that is worse at distance and worsens looking toward the affected side. The nerve's long intracranial course makes it vulnerable to raised intracranial pressure, where it serves as a false localizing sign not specific to any focal lesion. Other causes include microvascular disease (diabetes, hypertension), pontine lesions (stroke, demyelination), cavernous sinus pathology, and Gradenigo syndrome (petrous apicitis).

Internuclear Ophthalmoplegia (INO)

An INO results from a lesion of the medial longitudinal fasciculus and produces an ipsilateral adduction deficit with contralateral abducting nystagmus. It is named for the side with the adduction deficit. In a young patient, bilateral INO is multiple sclerosis until proven otherwise. In an older patient, a unilateral INO suggests brainstem ischemia or stroke. Wall-eyed bilateral INO (WEBINO) refers to bilateral INO with exotropia.

Supranuclear Gaze Disorders

Horizontal gaze palsies have different localizations depending on the level of the lesion. A frontal eye field lesion causes the eyes to deviate toward the lesion in acute stroke, while a pontine gaze center (PPRF) lesion causes the eyes to deviate away from the lesion. Vertical gaze palsies localize to the dorsal midbrain, where Parinaud syndrome produces upgaze palsy, convergence-retraction nystagmus, light-near dissociation, and lid retraction (Collier sign); this syndrome is caused by pineal tumors, dorsal midbrain strokes, or hydrocephalus. Progressive supranuclear palsy produces a vertical supranuclear gaze palsy (especially downgaze) along with postural instability and axial rigidity.

<image>A detailed anatomical diagram of the afferent visual pathway from retina to occipital cortex, showing the optic nerve, optic chiasm (with nasal fiber crossing), optic tracts, lateral geniculate nucleus, and optic radiations (Meyer loop in temporal lobe, Baum loop in parietal lobe). Alongside the pathway, labeled visual field defect patterns are shown at each level of potential lesion: monocular scotoma (optic nerve), bitemporal hemianopia (chiasm), homonymous hemianopia (optic tract), superior quadrantanopia (temporal lobe), inferior quadrantanopia (parietal lobe), and homonymous hemianopia with macular sparing (occipital cortex). Each visual field defect is shown as a pair of circular field charts with the affected region shaded.</image>

<image>A clinical flowchart for the evaluation of anisocoria. The diagram begins with measuring pupil size in light and dark conditions. If anisocoria is greater in light, the larger pupil is abnormal, branching to CN III palsy (with sub-branches for pupil-involving vs pupil-sparing, and compressive vs microvascular etiologies), pharmacologic mydriasis, and Adie tonic pupil (with dilute pilocarpine test). If anisocoria is greater in dark, the smaller pupil is abnormal, branching to Horner syndrome (with pharmacologic testing steps: cocaine/apraclonidine, then hydroxyamphetamine to localize first/second vs third order) and pharmacologic miosis. If anisocoria is equal in light and dark, physiologic anisocoria is indicated.</image>

<image>An illustrated summary of the three ocular motor cranial nerve palsies. For CN III: eye position diagram showing "down and out" with ptosis and mydriasis, labeled anatomy of the oculomotor nerve with parasympathetic fibers on the surface, and comparison of pupil-involving (aneurysm, compression) vs pupil-sparing (microvascular) etiologies. For CN IV: eye position diagram showing hypertropia in primary gaze worsening with contralateral gaze and ipsilateral head tilt (Bielschowsky test), with illustration of superior oblique action. For CN VI: eye position diagram showing esotropia with abduction deficit, with illustration of lateral rectus action and a note about false localizing sign with raised ICP.</image>

<image>A diagram illustrating internuclear ophthalmoplegia (INO). The figure shows the medial longitudinal fasciculus (MLF) connecting the CN VI nucleus in the pons to the contralateral CN III subnucleus (medial rectus) in the midbrain. A lesion is marked in the left MLF. The resulting left INO is demonstrated with eye position diagrams showing: normal rightward gaze, but on leftward gaze, the left eye fails to adduct while the right eye shows abducting nystagmus. A table below compares causes: bilateral INO in young patients (MS) vs unilateral INO in older patients (brainstem stroke).</image>

Clinical Pearls

A relative afferent pupillary defect is the single most important sign of optic nerve dysfunction and is not caused by media opacities such as cataracts or by refractive error. A pupil-involving CN III palsy is a posterior communicating artery aneurysm until proven otherwise, and emergent CTA, MRA, or conventional angiography is mandatory. An acute painful Horner syndrome must prompt urgent vascular imaging to rule out carotid artery dissection. Bilateral INO in a young patient is nearly pathognomonic for multiple sclerosis. In acute stroke, the eyes "look toward the lesion" with frontal eye field damage but "look away from the lesion" with pontine gaze center damage. Aberrant regeneration following a CN III palsy indicates a compressive rather than microvascular cause and warrants imaging even if the palsy was initially pupil-sparing. When evaluating visual field defects, macular sparing localizes to the occipital cortex, and congruity of the field deficit increases the more posterior the lesion.

References

• Liu GT, Volpe NJ, Galetta SL. Neuro-Ophthalmology: Diagnosis and Management. 3rd ed. Elsevier; 2019.
• Prasad S, Galetta SL. Approach to the patient with acute monocular visual loss. Neurol Clin Pract. 2012;2(1):14-23.
• Biousse V, Newman NJ. Neuro-Ophthalmology Illustrated. 3rd ed. Thieme; 2020.
• Brazis PW, Masdeu JC, Biller J. Localization in Clinical Neurology. 8th ed. Wolters Kluwer; 2022.

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