What muscles control the size of the pupil? Describe their innervation.

The iris sphincter muscle causes pupillary constriction and is innervated by the parasympathetic nervous system. The iris dilator muscle causes pupillary dilation and is innervated by the sympathetic nervous system. Thus, when sympathetic tone is increased, the pupil is larger, and when parasympathetic tone is increased, the pupil is smaller.

Trace the pathway of the parasympathetic innervation of the pupil.

Parasympathetic fibers begin in the Edinger-Westphal nucleus in the oculomotor nuclear complex. With cranial nerve (CN) III they exit the midbrain and travel in the subarachnoid space and cavernous sinus. They follow the inferior division of CN III into the orbit, where they synapse at the ciliary ganglion. Postganglionic fibers are then distributed to the iris sphincter and ciliary body via short ciliary nerves.

Trace the pathway of the sympathetic innervation of the pupil.

The first-order neuron begins in the posterior hypothalamus. The fibers travel caudally to terminate in the intermediolateral cell column of the spinal cord at levels C8–T1, otherwise known as the ciliospinal center of Budge. Pupillomotor fibers exit from the spinal cord and ascend with the sympathetic chain to synapse in the superior cervical ganglion, constituting the second-order neuron. The third-order neuron begins with postganglionic fibers of the superior cervical ganglion. These fibers travel with the internal carotid artery to enter the cranial vault. In the cavernous sinus the fibers leave the carotid artery to join the ophthalmic division of CN V and enter the orbit through the superior orbital fissure. The sympathetic fibers reach the ciliary body and dilator of the iris by passing through the nasociliary nerve and long posterior ciliary nerves.

Trace the pathway of the pupillary light reflex.

The pupillary light response begins with the rods and cones of the retina. Afferent pupillomotor fibers travel through the optic nerves and slightly greater than 50% decussate at the optic chiasm. They follow the optic tracts and exit before the lateral geniculate body to enter the brain stem via the brachium of the superior colliculus. Pupillomotor fibers synapse in the pretectal nuclei and then project equally to the ipsilateral and contralateral Edinger-Westphal nuclei. The pupillary fibers travel with CN III to innervate the iris sphincter and cause pupillary constriction, as described in question 2.

What is an afferent pupillary defect? How should you examine for it?

The swinging flashlight test is used to elicit a relative afferent pupillary defect (RAPD). If you shine a light into one eye of a normal subject, both pupils constrict to the same degree. If you swing the light over to the other eye, the pupil stays the same size or constricts minimally. In patients with RAPD the affected eye behaves as if it perceives a dimmer light than the normal eye; therefore, both pupils constrict to a lesser degree when the light is shone in the affected eye. Thus, if you shine the light in the right eye of a patient with left RAPD, both pupils constrict. If you swing the light to the left eye, it is perceived as dimmer and the pupils dilate. Note that this is a relative APD and signifies a difference in the pupillary response between the two eyes. However, if both eyes are equally abnormal, there may be no RAPD ( Fig. 29-1 ).

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 to 2 inches (3 to 5 cm) from each eye. Each eye is illuminated for approximately 1 second, then the light is switched quickly to the other eye. This technique allows comparison of the initial direct pupil contraction with light in each eye.

(From Kardon RH: The pupil. In Yanoff M, Duker JS [eds]: Ophthalmology, ed 2, St. Louis, Mosby, pp 1360–1369, 2004.)

A lesion in which anatomic areas may cause an afferent pupillary defect?

A lesion anywhere in the afferent pupillary pathway may cause an RAPD—that is, retina, optic nerve, optic chiasm, optic tract, or along the course of pupillary fibers from the optic tract to the pretectal nuclei. Pupillary fibers leave the optic tract prior to the lateral geniculate body. Therefore, any lesion from the lateral geniculate body posteriorly does not cause an RAPD. A retinal lesion causes an RAPD only if it is rather large. An optic nerve lesion causes an RAPD in the ipsilateral eye. A lesion in the optic chiasm may cause an RAPD if fibers from one optic nerve are affected more than those from the other. An optic tract lesion causes an RAPD in the eye with the most visual field loss. Typically, in patients with a mass lesion of the optic tract, an RAPD is produced in the ipsilateral eye because of ipsilateral optic nerve compression, but an ischemic lesion causes an RAPD in the contralateral eye because slightly more fibers cross than remain uncrossed. A lesion in the brain stem in the area of the pretectal nuclei may cause an RAPD without visual defects if the pupillomotor fibers are affected between the optic tract and the pretectal nuclei.

What is anisocoria? How should one examine a patient with anisocoria?

Anisocoria is a difference in the size between the two pupils. In anyone who has anisocoria, the pupil size should be measured in both bright and dim light. If the anisocoria is greater in bright light, the larger pupil is abnormal and constricts poorly, which is usually caused by a defect in parasympathetic innervation. If the anisocoria is greater in dim light, the smaller pupil is abnormal because it dilates poorly, usually because of a defect in pupillary sympathetic innervation. If the difference in the size of the two pupils remains the same in bright and dim light, the anisocoria is probably physiologic and not pathologic.

What is the differential diagnosis of a unilateral dilated, poorly reactive pupil?

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  Third-nerve palsy

  
  
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  Pharmacologic paralysis (an anticholinergic medication such as atropine)

  
  
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  Adie’s tonic pupil

  
  
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  Iris damage (e.g., sphincter tears secondary to trauma or posterior synechiae)

What are the clinical findings in a third-nerve palsy?

CN III innervates the superior, medial, and inferior recti and inferior oblique and levator palpebrae muscles. Therefore, in a complete CN III palsy, ptosis is complete and the eye is in the down-and-out position; it does not move up, down, or medially. The parasympathetic nerves that innervate the pupillary sphincter travel with CN III; therefore, if those fibers are affected, the pupil will be dilated and nonreactive.

What are some possible causes of third-nerve palsy?

In adults the most common causes are microvascular ischemia in the nerve, aneurysm (usually of the posterior communicating artery), trauma, and neoplasm. In children, aneurysm is rare, and consideration must be given to ophthalmoplegic migraine although injury, infection, and tumor are more common.

What is the significance of pupil involvement or pupil sparing in third-nerve palsy?

Pupil involvement in third-nerve palsy suggests a compressive lesion such as aneurysm or tumor. Pupil sparing is suggestive of microvascular ischemia. The parasympathetic fibers are on the outer portion of CN III and are more susceptible to external compression and less susceptible to ischemia, which is usually axial in the nerve.

What is the appropriate workup for an isolated third-nerve palsy with pupillary sparing?

In patients in the vasculopathic age group, the most likely cause is microvascular ischemia. The classic teaching is that patients may simply be followed with the expectation that the ocular misalignment will improve. However, the recommendation is evolving that all third-nerve palsy patients should have imaging with either magnetic resonance imaging (MRI)/magnetic resonance angiography (MRA) or computed tomography (CT)/computed tomography angiography (CTA). Certainly a medical workup for hypertension or diabetes is appropriate. If there is no improvement in 3 to 6 months, neuroimaging should be performed. Patients too young for the vasculopathic age group should all have an MRI and MRA scan. If the scan is negative, other hematologic investigations and lumbar puncture should be considered.

What is the appropriate workup for an isolated third-nerve palsy with pupillary involvement?

The first step is to perform an emergent MRI and MRA or CT and CTA scan. This important MRA or CTA must be done before catheter arteriography. If the scan is negative, a catheter arteriogram must be performed to rule out an aneurysm in institutions in which MRA or CTA may not confidently exclude small ones. If the scan is negative in children younger than age 10, an arteriogram is not necessary because the likelihood of an aneurysm is very low in young children.