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High-yield strategies for cranial nerve testing

Which tests deliver the quickest results?

May 2008

Published in the May 2008 issue of Today’s Hospitalist

This is the second in a two-part series on conducting a neurological exam. An elderly patient comes into the emergency room complaining of vertigo. You want to rule out several possible causes, from a stroke in the brainstem or cerebellum to a peripheral lesion such as labrynthitis. What test or procedure do you use?

While there are many testing possibilities, neurologist S. Andrew Josephson, MD, urged hospitalists to stick with two tried-and-true options: the coordination and the cranial nerve exams.

“If somebody has vertigo with a cranial nerve abnormality, it is a brainstem stroke, period,” he explained. “If somebody has vertigo with an asymmetric cerebellar exam, it’s a cerebellar stroke, period. If both exams are normal, a peripheral localization is most likely.”

“A visual field cut can identify a parietal or temporal lobe lesion that would have otherwise been missed.” 

andrewjosephson~ S. Andrew Josephson, MD, UCSF

When it comes to cranial nerve testing, neurologists have a huge arsenal of tests from which to choose. (For tips on testing coordination, see “High-yield strategies for a neurological exam in the April 2008 issue.) But at the University of California, San Francisco (UCSF) annual meeting on managing hospitalized patients, Dr. Josephson suggested that hospitalists focus on the following tests, all of which are relatively easy to conduct and yield good results.

Visual field testing
Suppose you see a patient who presents with blindness in the left eye. You probably know that visual field testing can help determine whether a cerebral lesion is the cause of that blindness. But administering the full version of the test, which involves asking patients to count numbers while they look at all eight quadrants, is difficult and time-consuming.

Dr. Josephson, who is assistant professor of neurology at UCSF, suggested another approach: Sit or stand at the same level as the patient with your hands placed an equal distance from each other. Ask patients to look straight ahead at your nose with each eye and point out which hand is wiggling, testing all fields of vision. If any are abnormal, a lesion must be present in the optic pathways. A binocular lesion (involving both eyes), suggests a problem behind the optic chiasm in the brain itself, rather than in the optic nerve.

The comatose patient
If a patient is comatose, Dr. Josephson said, you’ll need to modify your cranial nerve testing. But there are some shortcuts that work well.

The pupillary exam may be the most important initial test, he noted, reminding hospitalists that there are only two reasons a person may be in a coma. Either the patient has a dysfunction of his bilateral cerebral hemispheres, or the patient has a problem with the reticular activating system in the brainstem.

As a result, Dr. Josephson said, you need a screening tool that will identify patients whose brainstem is working. Because all cranial nerves (II-XII, excepting XI) originate in the brainstem, the brainstem is the root of the problem if any of the cranial nerves are abnormal.

Looking at the patient’s pupils, therefore, is particularly helpful. “If pupils are non-reactive or asymmetric,” he said, “it’s a brainstem problem. If they are normal, it’s more likely the hemispheres. And if the pupils are uneven, either there is a problem with cranial nerve III in the brainstem or there is such a big mass lesion in the hemispheres that herniation is occurring, pressing on cranial nerve III in the brainstem.”

Make sure you’re aware if patients received a drug like an Atrovent nebulizer that can cause unilateral pupillary dilation. Another caveat is to make sure that there is no light stimulus that could interfere with the exam.

“If you think that somebody’s pupils are 5 mm and fixed, which is a sign of dysfunction,” Dr. Josephson said, “make sure that everything is dark and you are using a really bright light.”

Doll’s eyes
A simple test for cranial nerves III and VI is the oculocephalic maneuver, otherwise known as “doll’s eyes.” If the patient is in a coma, move his head to one side. If the eyes gradually drift back toward the midline, that’s a normal response; if the eyes remain deviated out towards the side of the head, this brainstem reflex is absent, suggesting dysfunction of the midbrain and pons. Touching the cornea and eliciting a blink tests cranial nerves V and VII. Gagging, coughing and watching for spontaneous breathing tests cranial nerves IX and X.

For patients who are comatose, delirious or confused, or simply uncooperative, visual fields (cranial nerve II) can be tested by looking to see if the patient “blinks to threat.” As patients look straight ahead, move your finger, not your whole hand, at their eye from each quadrant, Dr. Josephson recommended. The goal is to test “if they blink each time it looks like you are about to poke them in the eye.”

“It’s very helpful in confused patients,” he said. “A lot of times, a visual field cut can identify a parietal or temporal lobe lesion that would have otherwise been missed.” He said he records the results of this simple maneuver as “does or doesn’t blink to threat on one side.”

It’s important to perform such cranial nerve exams because treatable basilar artery thrombosis, a cause of stroke in younger people, is all too often missed. “Anybody who is in a coma who has any cranial nerve lesion has a basilar artery thrombosis until proven otherwise,” Dr. Josephson said.

“The one thing you want to ask yourself on the coma exam is, ‘Is there anything wrong with the cranial nerves?’ And if the answer is ‘yes,’ patients get imaged until you exclude that they have a brainstem lesion.”

For patients with emboli to the basilar, he said, mechanical embolectomy can be successful even 12 to 16 hours after the event.

Deborah Gesensway is a freelance writer reporting on U.S. health care from Toronto, Canada.