The pupil

Both pupils are equally round and approximately 3–4 mm in diameter. Anisocoria refers to a difference in pupil size, and 4% of normal people may have as much as 1 mm of difference. Miosis is a constricted pupil, and mydriasis is a dilated pupil. Pupil size is determined by a dilator muscle controlled by the sympathetic nerve and a constrictor muscle that has cholinergic innervation via CN III (Fig. 3.42).

Fig. 3.42 Radial fibers of dilator muscle and ring of constrictor muscle near pupillary margin. Courtesy of Pfizer Pharmaceuticals.

Less common causes of optic neuropathy include drug or tobacco or alcohol toxicity, folic acid or vitamin B12 deficiency, tumors, or thyroid disease. Infections such as mumps, measles, and influenza rarely may be associated.

Sympathetic nerve

The iris dilator muscle and the Müller’s muscle that elevates the lid are both stimulated by the sympathetic nerve that begins in the hypothalamus (Fig. 3.43) and descends down the spinal column. At C8–T2 it synapses and then exits and passes over the apex of the lung. It ascends in the neck until it synapses, and follows the carotid artery into the skull and orbit. It dilates the pupil in response to the “fight or flight” stimulus. Damage to this nerve causes Horner’s syndrome (Fig. 3.44): miosis, ptosis, and decreased sweating (anhidrosis).

Fig. 3.44 Right Horner’s syndrome. Associated pain on the same side is highly suggestive of a dissection of the wall of the carotid artery and should be referred immediately to the emergency room for vascular imaging. If caught early, anticoagulation may prevent a stroke. (see Figs 3.45 and 3.46)

Fig 3.45 Carotid artery dissection: Magnetic resonance angiography (MRA) of right internal carotid (≠) shows decreased blood flow. Left internal carotid artery is normal (≠≠).

Fig. 3.46 MRA shows right wall hematoma seen on the black blood image (≠) and is compatible with a right carotid dissection. Left carotid is normal (≠≠).

Pupillary light reflex (Fig. 3.47)

Light shining on the retina stimulates the optic nerve and then the optic chiasm and optic tract. Here, it exits from the visual pathway to stimulate the Edinger–Westphal nucleus in the midbrain. The pupillary fibers leave the nucleus and travel with CN III until it synapses at the ciliary ganglion in the orbit. It innervates the iris sphincter muscle. Light shining in one eye causes that pupil and the pupil of the other eye to simultaneously constrict. The latter is referred to as the consensual light reflex. Both pupils also constrict when the eye accommodates from distance to near. This normal state may be noted as PERRLA—pupils equally round and reactive to light and accommodation. An MRI of the brain, neck, and upper chest should be considered when Horner’s syndrome occurs in children to rule out neuroblastomas when there are no other obvious causes such as birth trauma.

Fig. 3.47 Pupillary light reflex.

Y G O L O M L A H T H P O - O R U E N 40

Causes of irregular pupils

Adie’s pupil (tonic pupil)

This is a dilated pupil with a reduced direct and consensual light reflex. It reacts slowly to accommodation, and eventually becomes smaller and stays smaller than the other eye, hence the name tonic pupil. It is due to a benign defect in the ciliary ganglion (Fig. 3.47). Resulting denervation hypersensitivity causes the tonic pupil to constrict intensely compared with the other eye in response to one drop of weak pilocarpine 1/10%.

Visual field testing

The field of vision of each eye extends to 170° in the horizontal and 130° in the vertical meridian. Routine testing of vision with a Snellen chart recorded as 20/20 only means that the central few degrees corresponding to the macula are normal.

1 Amsler grid. This hand-held black crosshatched card tests the central 20° of the visual field. Waviness of lines is called metamorphopsia, and is characteristic of a wrinkled retina from macular disease (Fig. 3.48 and Appendix 2).

2 A tangent screen is a sheet of black felt (Fig. 3.49). It measures the central 60° of field. The patient is seated 1 or 2 meters from the screen, with one eye occluded. The examiner moves a small white ball centrally until the patient first sees it. Areas blind to this small object are tested with progressively larger objects.

3Hemisphere perimeters (Fig. 3.50) test the entire 170° of horizontal field and 130° of vertical field. Automated perimeters are expensive but save examiners’ time and gives record of field. Static perimeters project increasingly intense stimuli at one location until it is first seen.

4Confrontation testing is used when instruments aren’t available. The patient is seated opposite the examiner. The patient closes his or her right eye; the examiner closes his or her own left eye, and each fixates on the other’s open eye. The examiner moves an

Fig. 3.48 Amsler grid. Distortion in macular degeneration.

Fig. 3.49 Tangent screen central 60°. It is useful when patients cannot perform the more difficult automated perimetry.

Fig. 3.50 Automated hemisphere perimeter tests central and peripheral fields.

object in from the periphery and it should be seen simultaneously by both individuals. This technique compares patient’s and examiner’s field.

Scotomas due to ocular and optic nerve disease

A scotoma is loss of part of the field. Relative scotomas are areas of visual field blind to small objects, but able to perceive larger stimuli. Absolute scotomas are totally blind areas.

The normal blind spot is an absolute scotoma located 15° temporal to central fixation, which corresponds to the normal absence of rods and cones on the optic disk. It is plotted first (Fig. 3.51). If the blind spot cannot be located, the test is considered unreliable.

Central scotomas (Fig. 3.52) occur in macular degeneration. Central and paracentral scotomas (Fig. 3.53) are most characteristic of optic nerve disorders.

Unilateral altitudinal scotomas are defects above or below the horizontal meridian and are caused by an occlusion of a superior or

Fig. 3.51 Normal blind spot.

Fig. 3.52 Central scotoma.

Fig. 3.53 Paracentral scotoma.

inferior retinal artery or vein and retinal detachments (Fig. 3.54).

Scotomas due to brain lesions

Field defects help to localize the site of brain lesions. Light focused on the temporal retina passes through the optic nerve and stimulates the occipital cortex on the same side, whereas fibers carrying impulses from the nasal retina cross over in the optic chiasm and stimulate the brain on the opposite side (Fig. 3.55). Therefore, defects at or posterior to the chiasm cause loss of vision in both eyes and respect the vertical meridian. If the defects are equal and on the same side, they are called homonymous (1, 4, 5). If they are unequal on the same side, they are termed incongruous (3). If they are on opposite sides in each eye, they are referred to as bitemporal or binasal (2).

Fig. 3.54 Altitudinal scotoma.

Fig. 3.55 Lesions of the visual pathway.

Color vision

Color vision depends on the ability to see three primary colors: red, green, and blue. Partial defects are inherited in 7% of males and 0.5% of females and are detected using Ishihara or American optical pseudoisochromatic plates. Loss of color vision could limit one’s ability to become an electrician, airline pilot, or other profession requiring color discrimination. Acquired color defects may be due to retinal or visual pathway disease, the most common of which is optic neuritis. In acquired cases, test each eye separately and look for differences.

Circulatory disturbances

affecting vision (Refer to Fig. 3.4)

The blood supply to the brain originates from the two carotid arteries in the anterolateral neck and the two vertebral arteries passing through the cervical vertebrae. Transient loss of vision in those persons younger than age 50 is often due to a migrainous spasm of a cerebral artery. This causes scintillating scotomas (Fig. 3.57), which are brief flashes of light and/or zigzag lines that may precede a headache. It may progress to a homonymous hemianopsia lasting for 15–20 minutes.

If neurologic symptoms persist, they should go to the emergency room.

In older persons, the transient blurring is more often due to arteriosclerosis and is referred to as a transient ischemic attack (TIA). These visual obscurations—also called

Fig. 3.56 CT scan of pituitary adenoma pressing on the optic chiasm which lies anterior and superior to it. Courtesy of Sandip Basak, MD.

ministrokes—are caused by cholesterol, fibrin, or calcific emboli liberated from plaques, most often in the carotid artery. Symptoms occur as these emboli pass through the eye or visual cortex of the brain and usually last less than a half hour but could last up to 24 hours. If it lasts longer, it could turn into a permanent obstruction called a stroke.

Five percent of TIAs go on to develop a stroke within a month. So even if the symptoms have already cleared by the time the patient reaches your office, they should be cautioned about the risks of a permanent stroke and advised to see their primary care physician within a short time. If the TIA is still occurring after your examination, they should be sent directly to the emergency room since it could progress to a stroke. Eighty percent of strokes are ischemic and twenty percent are hemorrhagic. In the ER, they can be thoroughly evaluated to see if they meet the stringent guidelines to receive tissue plasminogen activator (tPA). There is a three hour therapeutic window from the onset of ischemic symptoms to administer this thrombolytic drug which could increase the chance of recovery from a stroke by 30–50% (Fig. 3.58). A CT scan is usually performed first in the emergency room to be sure it is ischemic and not hemorrhagic. Only then can tPA be safely administered.

Tests for decreased circulation

A non-invasive duplex ultrasonography could show carotid stenosis and decreased blood flow. If positive, a CT angiogram may be ordered. Invasive arterial catheter angiography is infrequently used since there is a 1% chance of procedure-related stroke (Fig. 3.59), but it is still the gold standard. Carotid endarterectomy is performed in symptomatic (high risk) patients with 50% narrowing or in asymptomatic patients with 70% stenosis (see Appendix 2).

The right and left cavernous sinuses in the brain drain the superior and inferior

Fig. 3.58 MRI of right occipital infarct. Courtesy of Rand Kirtland, MD.

Fig. 3.59 Arteriogram of internal carotid artery narrowing.

Fig. 3.61 Carotid cavernous fistula.

Carotid-cavernous fistulas usually result from trauma to an aneurysm of the carotid artery in the cavernous sinus (Figs 3.60 and 3.61). It connects high-pressure arterial to lowpressure venous circulation causing a pulsating exophthalmos with a bruit over the eye, and tortuous —”corkscrew” — conjunctival vessels. Diagnosis is confirmed with carotid arteriography that shows an enlarged superior ophthalmic vein draining in a retrograde way towards the orbit, instead of towards the cavernous sinus.

It must be distinguished from a cavernous sinus thrombosis which causes a non-pulsatile exophthalmos. The latter is often due to infection carried to the sinus via the superior and inferior ophthalmic veins. An MRI is useful to show widening of the cavernous sinus. A carotid-cavernous fistula and a cavernous sinus thrombosis are two causes of exophthalmos that can mimic orbital cellulitis (see Fig. 3.62). What both conditions have in common with orbital cellulitis are conjunctival vascular engorgement and chemosis (see Fig. 3.63); lids that are often swollen shut; and possible involvement of cranial nerves III–VI and the sympathetic nerve. Orbital cellulitis is usually unilateral; cavernous sinus thrombosis is commonly bilateral; and carotid-cavernous fistula is unilateral unless there are large connections between the right and left sinuses.

SOV

IC

Fig. 3.62 Carotid cavernous fistula causing bilateral VI CN palsies. Contrast injection into the femoral artery showing an enlarged superior ophthalmic vein (SOV) with retrograde flow and internal carotid a. (IC) within cavernous sinus).

SOV IC

Fig. 3.63 Cerebral angiography through a femoral artery injection and passage of detachable platinum embolization coil through the superior ophthalmic vein located by a cutdown incision near the skinfold of the upper lid. Note the successful clouding of the obliterated cavernous sinus [up arrow] due to thrombosis and a narrowed SOV and uninterrupted blood flow through the distal internal carotid artery (IC) as it exits the sinus. Courtesy of Stavropoula I. Tjumakaris, MD and Robert Rosenwaisen, MD. Thomas Jefferson University Hospital, Cerebrovascular Neurological Surgery Dept.