1st Cranial nerve
Smell, which is controlled by the 1st (olfactory) cranial nerve, is normally assessed only after a head injury or when lesions of the anterior fossa (e.g., meningioma) are suspected or when patients report irrelevant smell or taste.
While one nostril is closed, the patient is asked to identify smell (such as soap, coffee, or cloves) delivered to each nostril. When malingering is suspected, irritants such as alcohol, ammonia, and other irritants are used to test the nociceptive receptors of the 5th (trigeminal) cranial nerve.
2nd Cranial nerve
Visual performance is examined for the second (optic) cranial nerve using a Snellen chart for distance vision or a portable chart for near vision; each eye is assessed separately, with the other eye covered.
Standard pseudoisochromatic Ishihara or Hardy-Rand-Ritter plates with numbers or figures placed in a field of specially colored dots are used to evaluate color perception.
Direct confrontation is used to examine visual fields in all four visual quadrants. The pupillary responses are assessed both directly and indirectly. In addition, a funduscopic examination is performed.
3rd, 4th, and 6th Cranial nerves
Eyes are examined for symmetry of movement, globe position, asymmetry or drooping of the eyelids (ptosis), and twitches or flutters of the globes or lids for the 3rd (oculomotor), 4th (trochlear), and 6th (abducens) cranial nerves. The patient is asked to follow a moving target (e.g., examiner's finger, penlight) across all four quadrants (including across the midline) and toward the tip of the nose; this test can reveal nystagmus and ocular muscle palsies. End-lateral gaze nystagmus with a fine amplitude is normal.
In a dimly light space, anisocoria (Anisocoria is unequal pupil size) in pupillary size should be noted. The symmetry and briskness of the pupillary light response are assessed.
5th Cranial nerve
The 3 sensory divisions (ophthalmic, maxillary, and mandibular) of the 5th (trigeminal) nerve are assessed using a pinprick to test facial sensibility and a wisp of cotton against the lower or lateral cornea to assess the corneal reflex. The angle of the jaw should be evaluated if face sensation is gone; sparing of this area (innervated by spinal root C2) suggests a trigeminal deficit. Depressed or absent corneal sensation, which is frequent in contact lens wearers, should be separated from a weak blink due to face weakness (e.g., 7th cranial nerve paralysis). Even if the blink rate is reduced, a patient with facial weakness feels the cotton wisp on both sides normally.
The masseter muscles are palpated as the patient clenches his or her teeth, and the patient is asked to open his or her mouth against opposition. The jaw deviates to that side if a pterygoid muscle is weak.
7th Cranial nerve
Hemifacial weakness is used to assess the 7th (facial) cranial nerve. The nasolabial fold is depressed and the palpebral fissure is widened on the weakened side of the face during spontaneous conversation, especially when the patient smiles or, if obtunded, grimaces at a noxious stimulus; on the weakened side, the nasolabial fold is depressed and the palpebral fissure is widened. The etiology of 7th nerve weakness is central rather than peripheral if the patient only displays lower facial weakness (i.e., furrowing of the forehead and eye closure are retained).
Sweet, sour, salty, and bitter solutions placed with a cotton swab on one side of the tongue, then the other, can be used to assess taste in the anterior two thirds of the tongue.
A vibrating tuning fork held adjacent to the ear can identify hyperacusis ( a disorder in loudness perception)f, which indicates stapedius muscle weakness.
8th Cranial nerve
Because the 8th cranial nerve (vestibulocochlear, acoustic, auditory) conveys both auditory and vestibular input, a thorough examination is required.
Hearing evaluations
Tests of vestibular function
Hearing is first checked in each ear by whispering something into one ear while occluding the other. Formal audio-logic testing should be performed on any suspected hearing loss to validate findings and distinguish conductive hearing loss from sensorineural hearing loss. To separate the two, the Weber and Rinne tests can be performed at the bedside, although they are difficult to do efficiently outside of specialized facilities.
The presence of nystagmus can be used to assess vestibular function. The existence of nystagmus and its characteristics (such as direction, duration, and triggers) can help detect vestibular diseases and distinguish central from peripheral vertigo. Vestibular nystagmus is made up of two parts:
Vestibular input causes a sluggish component.
A quick-acting, reversible component that induces movement in the opposite direction (called beating).
9th and 10th Cranial nerves
Typically, the 9th (glossopharyngeal) and 10th (vagus) cranial nerves are assessed simultaneously. It is recorded whether the patient's palate raises symmetrically when he or she says "ah." The uvula is lifted away from the paretic side if one side is paretic. A tongue blade can be used to touch one side of the posterior pharynx, then the other, to see if the gag reflex is symmetrical; bilateral absence of the gag response is frequent among healthy persons and is unlikely to be relevant.
Suctioning the endotracheal tube generally causes coughing in an intubated, unconscious patient.
The vocal chords are examined if hoarseness is detected. Isolated hoarseness (with normal gag and palate elevation) should initiate a search for lesions compressing the recurrent laryngeal nerve (e.g., mediastinal lymphoma, aortic aneurysm).
11th Cranial nerve
The muscles supplied by the 11th (spinal accessory) cranial nerve are tested:
The patient is asked to turn the head against resistance provided by the examiner's hand as the examiner palpates the active muscle for the sternocleidomastoid (opposite the turned head).
The patient is instructed to raise their shoulders against the examiner's resistance for the upper trapezius.
12th Cranial nerve
By asking the patient to extend their tongue and evaluating it for atrophy, fasciculations, and weakness, the 12th (hypoglossal) cranial nerve is assessed (deviation is toward the side of a lesion).
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