Ocular Motility, Gaze Palsies & Pupillary Disorders

Extraocular Muscles and Cranial Nerve Palsies

Who moves what. The lateral rectus (CN VI) abducts, and the superior oblique (CN IV) depresses the adducted eye and intorts it. CN III drives the medial, superior, and inferior recti, the inferior oblique, and the levator palpebrae, and it carries Edinger-Westphal parasympathetic fibers through the ciliary ganglion to the pupillary sphincter and the ciliary muscle (accommodation). Shorthand: LR6, SO4, everything else 3.

CN III palsy. Unopposed lateral rectus and superior oblique pull the eye down and out; levator weakness causes ptosis; loss of parasympathetic fibers leaves a fixed, dilated pupil and lost accommodation. Diplopia is both horizontal and vertical.

The pupil separates compression from ischemia. Pupillomotor fibers run on the surface of CN III. Compression by a posterior communicating artery aneurysm or uncal herniation dilates the pupil early, whereas diabetic or hypertensive microvascular infarction damages the nerve's core and spares the pupil. A pupil-involving CN III palsy is a potential surgical emergency and needs urgent vascular imaging.

CN IV palsy. Superior oblique weakness leaves the eye elevated (hypertropia) and extorted. Vertical diplopia worsens looking down and in, as when reading or descending stairs, and patients tilt the head away from the affected side to compensate. Head trauma is the most common acquired cause. CN IV is the only cranial nerve to exit the dorsal brainstem, and it decussates, so a trochlear nucleus lesion weakens the opposite superior oblique.

CN VI palsy. Lateral rectus weakness turns the eye in (esotropia), with horizontal diplopia worse looking toward the lesion and at distance. CN VI's long course up the clivus and over the petrous apex is stretched by raised ICP (idiopathic intracranial hypertension, mass), making it a false-localizing sign. It also lies free in the cavernous sinus beside the internal carotid, whereas III, IV, V1, and V2 run in the lateral wall, so CN VI is often hit first by cavernous sinus lesions.

Horizontal Gaze and Its Lesions

The gaze circuit. A leftward saccade starts in the right frontal eye field, which drives the left paramedian pontine reticular formation (PPRF), the horizontal gaze center. The PPRF activates the left abducens nucleus: its motor neurons abduct the left eye, and its interneurons cross and ascend in the right medial longitudinal fasciculus (MLF) to the right CN III medial rectus subnucleus, adducting the right eye. Vertical gaze, by contrast, is organized in the midbrain.

Cortex looks toward the lesion; pons looks away. A destructive FEF lesion, such as an MCA stroke, lets the intact FEF push the eyes toward the lesion and away from the hemiparesis; an FEF seizure drives them away from the focus. A PPRF or abducens nucleus lesion causes an ipsilateral horizontal gaze palsy, so the eyes rest looking away from the lesion, toward the hemiparesis.

Internuclear ophthalmoplegia (INO). With an MLF lesion, the eye on the side of the lesion fails to adduct on lateral gaze while the opposite, abducting eye shows nystagmus. Convergence is preserved because vergence input to CN III bypasses the MLF. Name the INO for the eye that cannot adduct; it sits on the same side as the damaged MLF. Bilateral INO in a young adult suggests multiple sclerosis, and unilateral INO in an older adult suggests a pontine infarct.

One-and-a-half syndrome. A lesion of the PPRF or abducens nucleus plus the adjacent ipsilateral MLF gives a gaze palsy toward the lesion (the "one") plus an INO on looking away (the "half"). The only horizontal movement left is abduction of the opposite eye, with nystagmus. Causes are MS and pontine stroke.

The Sympathetic Pathway and Horner Syndrome

Three neurons. The oculosympathetic pathway descends from the hypothalamus to the ciliospinal center (C8–T2), exits with the T1 root over the lung apex to the superior cervical ganglion, and then climbs the internal carotid to the dilator pupillae and Müller (superior tarsal) muscle. Facial sweat fibers leave with the external carotid, so a lesion along the internal carotid causes little or no facial anhidrosis.

Horner syndrome. Mild ptosis (Müller muscle), miosis with anisocoria greatest in dim light (the small pupil cannot dilate), and ipsilateral facial anhidrosis.

Pupillary Reflexes and Abnormal Pupils

The light reflex. Light travels through the optic nerve and tract to the pretectal nuclei, which project to both Edinger-Westphal nuclei; CN III and the ciliary ganglion then constrict the sphincter. Either eye therefore normally drives equal direct and consensual constriction.

Relative afferent pupillary defect (Marcus Gunn pupil). With an optic nerve lesion (optic neuritis, severe retinal disease), swinging the flashlight to the affected eye makes both pupils dilate, because that eye sends a weaker signal. Resting pupils are equal; efferent lesions cause anisocoria instead.

Argyll Robertson pupils. Bilateral small, irregular pupils that constrict with accommodation but not to light (light-near dissociation), classically from tertiary neurosyphilis (tabes dorsalis) with a dorsal midbrain (pretectal) lesion; diabetes can mimic it.

Adie tonic pupil. Postganglionic parasympathetic (ciliary ganglion) denervation, often in young women after a viral illness: a large pupil with poor light reaction, slow tonic constriction to near, and slow redilation. Denervation supersensitivity makes it constrict to dilute (0.1%) pilocarpine. Holmes-Adie syndrome is a tonic pupil plus absent knee and ankle jerks.

Pharmacologic mydriasis. Anticholinergic exposure (scopolamine patch, ipratropium nebulizer, atropine, jimsonweed) or a sympathomimetic dilates the pupil with full eye movements and no ptosis. After an anticholinergic, 1% pilocarpine fails to constrict it because the muscarinic receptors are blocked; a sympathomimetic pupil usually still constricts, and a CN III palsy pupil constricts briskly.