THE PUPIL

The size of the normal pupil varies at different ages, from person to person, and with different emotional states, levels of alertness, degrees of accommodation, and ambient room light. The normal pupillary diameter is about 3-4 mm, smaller in infancy, and tending to be larger in childhood and again progressively smaller with advancing age. Pupillary size relates to varying interactions between the sympathetically innervated iris dilator, with supranuclear control from the frontal (alertness) and occipital lobes (accommodation). The pupil also normally responds to respirations (ie, hippus). Twenty to 40 percent of normal patients have a slight difference in pupil size (physiologic anisocoria), usually of about 0.5 mm. Mydriatic and cycloplegic drugs work more effectively on blue eyes than on brown eyes.

Neuroanatomy of the Pupillary Pathways

Evaluation of the pupillary reactions is important in localizing lesions involving the optic pathways. The examiner should be familiar with the neuroanatomy of the pathway for reaction of the pupil to light and the miosis associated with accommodation (  Figure 14-30).

A. Light Reflex:

The pathway for the light reflex is entirely subcortical. The afferent pupillary fibers are included within the optic nerve and visual pathways until they exit the optic tract just prior to the lateral geniculate nucleus. They enter the midbrain through the brachium of the superior colliculus and synapse in the pretectal nucleus. Each pretectal nucleus decussates neurons dorsal to the cerebral aqueduct to the ipsilateral and contralateral Edinger-Westphal nucleus via the posterior commissure and the periaqueductal gray matter. A synapse then occurs in the Edinger-Westphal nucleus of the oculomotor nerve. The efferent pathway is via the third nerve to the ciliary ganglion in the lateral orbit. The postganglionic fibers go via the short ciliary nerves to innervate the sphincter muscle of the iris

B. The Near Reflex:

When the eyes look at a near object, three reactions occur-accommodation, convergence, and constriction of the pupil-bringing a sharp image into focus on corresponding retinal points. There is convincing evidence that the final common pathway is mediated through the oculomotor nerve with a synapse in the ciliary ganglion. The afferent pathway enters the midbrain ventral to the Edinger-Westphal nucleus and sends fibers to both sides of the cortex. Although the three components are closely associated, the near reflex cannot be considered a pure reflex, since each component can be neutralized while leaving the other two intact-ie, by prism (neutralizing convergence), by lenses (neutralizing accommodation), and by weak mydriatic drugs (neutralizing miosis). It can occur even in a blind person who is instructed to look at his nose. Bilateral overaction of the near reflex is accommodative spasm. Bilateral accommodative paresis occurs in botulism poisoning and in the Fisher variant of Guillain-Barré syndrome.

ARGYLL ROBERTSON PUPIL

A typical Argyll Robertson pupil is strongly suggestive of central nervous system syphilis associated with tabes dorsalis or general paresis. The pupil is less than 3 mm in diameter (miotic) and does not respond to light stimulation but does accommodate; this finding is nearly always bilateral. The pupils are commonly irregular, eccentric, and dilate poorly with mydriatics as a consequence of concomitant iris atrophy. Less commonly, the sign is incomplete (slow response to light) or unilateral or associated with tonic pupils (mimicking Adie's syndrome). Some degree of Argyll Robertson pupil is present in over 50% of patients with central nervous system syphilis. A wide variety of other central nervous system diseases infrequently cause incomplete Argyll Robertson pupil. These include diabetes, chronic alcoholism, encephalitis, multiple sclerosis, central nervous system degenerative disease, and tumors of the midbrain. The periaqueductal gray matter of the midbrain is the usual site of the lesion and thus affects the light reflex. The near reflex pathway is more ventral and thus is spared.

TONIC PUPIL

Tonic pupil occurs because of an abnormal pupillary constrictor mechanism in which all or a segment of the sphincter muscle contracts slowly (tonically) to near stimulation and relaxes even more slowly, but either response is better than the light response. It is usually associated with loss of deep tendon reflexes (Adie's syndrome). It results from damage to the ciliary ganglion, which carries 30 nerves destined for the ciliary body to one destined for the iris sphincter. Thus, accommodation is more apt to be preserved by a ciliary body lesion and is also-as a consequence of preferential innervation-more likely to reinnervate after an injury. This can produce segmental pupillary innervation. A weak (0.1%) solution of pilocarpine instilled into the conjunctival sac causes a tonic pupil to constrict as a result of denervation hypersensitivity; normal pupils are not affected. Some preganglionic oculomotor nerve lesions have, however, been shown to have denervation hypersensitivity probably related to a direct iris pathway that does not synapse at the ciliary ganglion. Bilateral tonic pupils should raise a question of autonomic neuropathy.

HORNER'S SYNDROME

Horner's syndrome is caused by a lesion of the sympathetic pathway, either (1) in its central portion, which extends from the posterior hypothalamus through the brainstem to the upper spinal cord (C8-T2); or (2) in its preganglionic portion, which exits the spinal cord and synapses in the superior cervical (stellate) ganglion; or (3) in its postganglionic portion, from the superior cervical ganglion via the carotid plexus and the ophthalmic division of the trigeminal nerve, by which it enters the orbit. The sympathetic fibers then follow the nasociliary branch of the ophthalmic division of the trigeminal nerve and the long ciliary nerves to the iris and innervate Müller's muscle and the iris dilator. Iris dilator muscle paresis causes miosis, which is more evident in dim light. Melanocyte maturation in the iris of a neonate depends upon sympathetic innervation; thus, less pigmented (bluer) irides occur if a congenital sympathetic lesion is present. Unilateral miosis, ptosis, and absence of sweating on the ipsilateral face and neck make up the complete syndrome. Postganglionic fibers to the face for sweating and vasoconstriction follow the external carotid. Causes of Horner's syndrome include cervical vertebral fractures, tabes dorsalis, syringomyelia, cervical cord tumor, cervical rib, Lyme disease, apical bronchogenic carcinoma, aneurysm of the carotid or subclavian artery, brachial plexus injuries, and injuries to or dissection of the carotid artery high in the neck. Pharmacologic testing with topical cocaine in the conjunctival sac can differentiate Horner's syndrome from physiologic anisocoria, and hydroxyamphetamine can further localize the process to the postganglionic neuron, thus assisting in defining the cause of the syndrome.

Raeder's paratrigeminal syndrome is Horner's syndrome associated with unilateral headache or facial pain in the distribution of the trigeminal nerve. If associated with a sixth, third, fourth, or second cranial nerve palsy, complete neurologic evaluation for basilar skull tumor is required. Without these additional cranial nerves, Raeder's syndrome is a benign condition perhaps related to cluster headache.

AFFERENT PUPILLARY DEFECT

Optic nerve fibers from the right eye decussate at the chiasm to enter the left tract as well as continuing into the right tract, and the same is true on the left side. The pupillary light pathways enter the midbrain through the brachium of the superior colliculus to synapse in the pretectal nucleus; here, they decussate also, as each pretectal nucleus connects to the ipsilateral and contralateral Edinger-Westphal nucleus. For this reason, light shone into the right eye produces an immediate direct response in the right and an immediate indirect consensual response in the left eye (Figure 14-31). The intensity of this response in each eye is proportionate to the light-carrying ability of the directly stimulated optic nerve.

One of the most important assessments to make for the patient complaining of decreased vision is whether it is due to a local ocular problem, eg, cataract, or to a more serious optic nerve problem. Even dense cataracts do not change the light afferent pathways to the brain; hence, a comparison is possible. If an optic nerve lesion is present, the direct light response in the involved eye is less intense than the consensual response (in the involved eye) evoked when the normal eye is stimulated. This phenomenon is called a relative afferent pupillary defect (RAPD) (Figure 14-32). It will be positive also if there is a large retinal lesion. Causes of unilateral decreased vision without an afferent pupillary defect include refractive error, cloudy media (cataract), amblyopia, hysteria or malingering, a macular lesion, and chiasmatic problems. It is anatomically possible for a relative afferent defect with normal visual function to occur if the brachium of the superior colliculus is damaged by a thalamic hemorrhage.

Amaurotic pupillary defect is the term applied to an eye that does not even see light owing to severe unilateral retinal or optic nerve disease. Obviously, a blind eye would not have a direct light response, nor could it induce a consensual response in the normal eye. However, a light shown directly into the normal eye would induce a direct response there and a consensual response in the blind eye (Figure 14-33).

10.1036/1535-8860.ch14

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