The olfactory nerve conveys the sense of smell to appropriate centers of the cerebral cortex via olfactory receptors located in the superior nasal septum. Smell is properly tested through the use of nonirritating volatile oils or liquids, for example, oil of wintergreen, oil of cloves, eucalyptus, oil of cinnamon, vanilla, or anise. One nostril is occluded while the patient inhales vigorously with the stimulant held close to the open nostril and with the patient’s eyes closed. The fact that the patient can sense an odor is more important than being able to identify the particular stimulant. Even some bedside substances can be useful for rough, qualitative testing of smell, such as coffee grounds, lemon oil, or flavored toothpaste. Obviously a conscious patient is necessary for testing of the olfactory nerve. Some irritative substances may actually stimulate the trigeminal nerve and confuse the examiner, even though they are attractive to use in semiconscious or obtunded patients. Examples of the substances to be avoided are chloroform, menthol, camphor, ammonia, strong acetic acid, alcohol, and formaldehyde.

The optic nerve conveys retinal fibers to the lateral geniculate bodies of the thalamus through the optic chiasm. From the lateral geniculate bodies, the optic radiation fibers travel to the primary visual cortex to begin the process of constructing vision based upon the visual stimulation of the retinal fibers. For the scope of this text, only the basic bedside clinical tests of vision are of concern. These fundamentally consist of confrontation testing and visual acuity testing. Visual acuity is tested in the conscious, cooperative patient by means of one of the easily available reading cards, such as the Snellen, Jaeger, or Rosen-baum reading cards. Confrontation testing consists most commonly of having a patient stare at a fixed point in the center of his or her vision (such as the examiner’s nose, if the examiner is in front of the patient) and counting fingers of the examiner’s hand, held in front of
the patient. Ideally, each eye is tested independently by covering the contralateral eye with an opaque object. The examiner should hold up fingers of his or her own hand in each of four quadrants of the patient’s scope of vision for each eye. It is important to remember that for the purpose of counting fingers, the fingers should be held up steadily and not moved or wiggled. Moving the fingers while asking the patient to count them will actually test the sense of visual movement and not object perception. In semiconscious, uncooperative or poorly communicative patients, visual confrontation testing may be performed at the bedside by flicking fingers close to each eye but without touching the eye. A blinking response indicates visual field perception. There are two caveats. First, the flicking should not be so vigorous as to induce air movement strong enough to stimulate the eyelashes or cornea, which would induce a blinking reflex. Second, this method can only grossly test visual perception without giving any definitive sense of quadrantanopsias. It can usually be done only in the field of the nasal retina.

The optic nerve, because it conveys light perception, is also involved in the pupillary light reflex (consensual pupillary constriction in response to light stimulation), but because of the involvement of the third cranial nerve in this reflex arc, this is treated in the next section.

The oculomotor nerve subserves four of the six oculomotor muscles; the superior rectus, medial rectus, inferior rectus, and inferior oblique muscles. It also innervates the levator palpebrae superioris, which contributes to the elevation of the upper eyelid. There is a parasympathetic component to the innervation of the oculomotor nerve, which innervates the sphincter pupillae muscle and ciliary muscles. These latter muscles control pupillary size and are intimately involved in the pupillary light reflex and in accommodation to near vision. The trochlear nerve innervates the superior oblique muscle. The superior oblique muscle depresses the eye when it is adducted and has some rotatory effect. The abducens nerve innervates the lateral rectusmuscle, which abducts the eye.

In a conscious patient testing of the oculomotor, trochlear, and abducens nerves consists basically of having the patient track a moving target, such as the examiner’s finger. Each eye may be tested
independently. In poorly cooperative patients the examiner may observe spontaneous eye movements and use gross stimulation, such as moving from one side of a patient’s bed to the other while calling or clapping, to observe for tracking eye movements. Some comatose patients may have certain kinds of roving eye movements, which can give information about which extraocular muscles are intact. Testing trochlear nerve function independently is difficult at the bedside, most especially in an intensive care setting. The most common clinical presentation of acute isolated trochlear nerve palsy in an ambulatory patient is a head tilt—the patient presents with his or her head tilted forward and to the contralateral side of the affected muscle. Increasing the head tilt in the direction of the palsied side may induce vertical diplopia, which is part of the testing maneuver for trochlear muscle weakness (Bielschowsky test). Obviously, this is somewhat difficult in many intensive care unit (ICU) patients. Examination of eye movements must include observation of nystagmus and other eye-movement abnormalities such as forced gaze in a particular direction or opsoclonus (irregular oscillations of the eyes in both vertical and horizontal directions), as these may help to localize neurologic lesions.

For patients who cannot cooperate (for example, comatose patients or those on sedation), the performance of oculocephalic maneuvers can elicit eye movements. The oculocephalic reflex depends upon an intact vestibular system for its reliability. In a patient with an intact oculocephalic reflex, turning the patient’s head in a given horizontal direction will drive the eyes to the opposite side. This tests the ipsilateral medial rectus (third cranial nerve) and the contralateral lateral rectus (sixth cranial nerve). Vertical eye movements can be tested with a vertical motion of the head (with the head held in the midline position). Isolation of a particular extraocular muscle abnormality is difficult, though, because of the combination of extraocular muscle function in the production of vertical eye movements. In addition to eye movements, other clinical features will indicate oculomotor dysfunction. The presence of ptosis may suggest a lesion in the oculomotor nerve.

Finally, testing of the pupillary light reflex is an absolutely essential component of bedside cranial nerve testing. Light is conveyed through the optic nerves to the midbrain and into the Edinger-Westphal nucleus. Efferent motor and parasympathetic fibers to the pupil are conveyed by the oculomotor nerve. There are multiple crossing points along the pathway, so the pupillary light reflex is bilateral in nature. It is tested with the swinging flashlight test, in which a bright light is directed into each eye individually. The examiner must observe for
both the direct response and the consensual response. The pupil in a normal eye constricts in response to light shown directly into it; this is the direct response. The contralateral pupil should simultaneously (and equally) constrict if it is also normal, which represents the consensual response. An abnormality in one of the responses suggests a lesion somewhere along the pathway, which can involve both the optic nerve, the oculomotor nerve, or midbrain connections. In a conscious, cooperative patient, the accommodation reflex can also be tested. In the normal accommodation reflex, if a patient looks to a far object and then shifts his or her gaze to a near object, such as a hand held in front of the patient’s face, the pupils constrict. The efferent limb of the reflex is conveyed by the oculomotor nerve.