* CN 8 is responsible for auditory and vestibular sensation. What follows is instruction on how to assess these functions:
I. Cranial Nerve 8: Simple Assessment of Auditory Function
- A simple assessment of auditory function is as follows:
- Prior to actually assessing cranial nerve 8, use an otoscope to evaluate the patient’s external auditory canals to assess for any blockages (such as ear wax or foreign bodies) and examine the patient’s eardrums.
- Stretch out your arm, away from the patient’s ear, and then bring your arm in while rubbing your fingertips together. The patient is instructed to report when he/she first hears your fingertips rustle. This is repeated on the opposite side.
- You may also test auditory function using a tuning fork. Tuning forks of 512 Hz frequently are most commonly used because 512 Hz is generally the lower frequency of speech perception. Forks with lower frequency are sometimes used, instead, because they vibrate for longer. Activate the tuning fork and hold it a distance from the patient’s ear. The patient reports how loud the tuning fork is and this test is then compare to the opposite ear.
II. Cranial Nerve 8: The Rinne and Weber Tests for Auditory Function
- The Rinne and Weber auditory tests are commonly used to differentiate between conduction hearing loss and sensorineural hearing loss. Conduction hearing loss is due to impaired sound transmission to the cochlea, simply an abnormality of the outer or middle ear, such as occlusion of the external auditory canal. Sensorineural hearing loss is due to impaired transmission within and from the cochlea, simply an inner ear abnormality or abnormality of the vestibulocochlear nerve, such as from an acoustic neuroma. The Rinne and Weber tests assess sound conduction through air (air conduction) versus sound conduction through bone (bone conduction) and are used together to localize the side of hearing loss and its type: conduction or sensorineural.
1. Rinne Test
A. To perform the Rinne test, place the stem of a 512 Hz vibrating tuning fork on the patient’s mastoid process. The patient should listen for sound from the tuning fork (ignore its vibration) and report when he/she stops hearing the sound.
B. When the patient reports that the sound stops, hold the tines of the tuning fork up to the patient’s ear. The patient should once again hear the vibrating tuning fork because air conduction is more audible than bone conduction; this is called a positive Rinne test.
2. Weber Test
A. To perform the Weber test, place the stem of a 512 Hz vibrating tuning fork in the midline vertex of the patient’s skull. The patient reports where he/she hears the sound, which should be somewhere in the center of the head, not on either side.
B. During the Weber test, conduction hearing loss in one ear will amplify the sound on that side. For instance, if the patient’s right external auditory canal is occluded, the patient will hear the sound more loudly on the right side. Alternatively, sensorineural hearing loss will lessen sound on that side.
3. Combined Interpretation of the Rinne and Weber Tests.
A. In unilateral conduction hearing loss: for the Rinne test, bone conduction is louder than air conduction (negative test) and for the Weber test, sound lateralizes to the abnormal ear.
B. In unilateral sensorineural hearing loss: for the Rinne test, air conduction is louder than bone conduction (positive test) and for the Weber test, sound lateralizes to the normal ear.
III. Cranial Nerve 8: Vestibular Function
- The vestibular division of cranial nerve 8 is evaluated during the evaluation of eye movements. Here we address the directionality of eye movements generated by the anterior and posterior semicircular canals (the vertical canals) based on whether the canal is anterior or posterior, right or left. Note that we will report the eye movement directionality from the observer’s perspective (not the subject’s). Also, to avoid confusion, although eye movements are commonly addressed in regards to the fast phase of the nystagmus they produce, here we report the slow phase of the eye movement.
- The anterior canals produce upward movement of the eyes and the posterior canals produce downward movement of the eyes.
- From the observer’s perspective, the right-side canals produce torsional movement in a clockwise direction whereas the left-side canals produce torsional movement in a counterclockwise direction.
- In benign paroxysmal positional vertigo (discussed in the CRANIAL NERVES 3, 4, 6 Special Head Position Maneuver: the Dix-Hallpike Maneuver section), calcium carbonate crystals from the utricle most often fall into the posterior canal and stimulate positional vertigo. If the right posterior canal is activated (for example), then from the observer’s perspective, the slow phase of the nystagmus is downward with clockwise rotation. The fast phase, which is how the nystagmus is actually named, is in the opposite direction: upward and counterclockwise.
