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Hearing and Vestibular Sense (CN VIII)

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Hearing. Can the patient hear fingers rubbed together or words whispered just outside of the auditory canal and identify which ear hears the sound? A tuning fork can be used to distinguish neural from mechanical conductive hearing problems. In the Rinne test the sound heard when holding a vibrating tuning fork just outside each ear (air conduction), is compared to the sound heard when placing the tuning fork handle on each mastoid process (bone conduction). Normal individuals will hear the tone better by air conduction. In conductive hearing loss, bone conduction is greater than air conduction, because bone conduction bypasses problems in the external or middle ear. In sensorineural hearing loss, air conduction is greater than bone conduction in both ears (as in normal hearing), however, hearing is decreased in the affected ear. In the Weber test, the tuning fork is placed on the vertex of the skull in the midline, and the patient is asked to report the side where the tone sounds louder. Normally, the tone sounds equal on both sides. In sensorineural hearing loss, the tone is louder on the normal side. In conductive hearing loss, the tone is louder on the affected side. You can verify that the tone is louder on the side of conductive hearing loss on yourself by closing each ear alternately while humming.

42. Hearing

Sequence

Vestibular Sense. Vestibular sense is generally not specifically tested, except for in the following important situations:

Patients with vertigo. Barany or Hall-Pike positional testing can help distinuish peripheral from central causes of vertigo. The patient sits on the bed or examining table, and the examiner supports the patient's head as the patient lays back with one ear down, and with the head extending over the edge of the table. This maneuver does not need to be done especially briskly. The patient is asked to keep their eyes open and report any sensations of vertigo, while the examiner looks for nystagmus . This change of position causes maximal stimulation of the posterior semicircular canal of the ear that is down, and of the anterior semicircular canal of the ear that is up. The maneuver is also done with the other ear down. With peripheral lesions, there is usually a delay of a few seconds before the onset of nystagmus and vertigo. The nystagmus is horizontal or rotatory and does not change directions. Nystagmus and vertigo then fade away within about a minute. If the same maneuver is repeated, there is often adaptation, so that the nystagmus and vertigo are briefer and less intense each time. In contrast, with central lesions, the nystagmus and vertigo may begin immediately, and there tends to be no adaptation. Horizontal or rotatory nystagmus can also be seen with central lesions. However, vertical nystagmus, nystagmus that changes directions, or prominent nystagmus in the absence of vertigo are seen only in central, and not in peripheral lesions.

43. Positional Vertigo Test

Sequence

Patients with limitations of horizontal or vertical gaze. Testing the vestibulo-ocular reflex can help localize the lesion (see Neuroanatomy through Clinical Cases, Chapter 13). As we mentioned when discussing Extraocular Muscles the vestibulo-ocular reflex can be tested in two ways. The first is using the oculocephalic maneuver, in which the eyes are held open and the head is turned rapidly either from side to side or up and down. The second is using caloric testing, in which cold or warm water is instilled into one ear, producing asymmetric stimulation of the semicircular canals.

Patients in coma. The vestibulo-ocular reflex is often the only way to test eye movements in these patients. Further details of these tests and their significance are provided in Neuroanatomy Through Clinical Cases Chapters 3, 12 and 13.

What is Being Tested?

Hearing loss can be caused by lesions in the acoustic and mechanical elements of the ear, the neural elements of the cochlea, or the acoustic nerve (CN VIII). After the hearing pathways enter the brainstem, they cross over at multiple levels and ascend bilaterally to the thalamus and auditory cortex. Therefore, clinically significant unilateral hearing loss is invariably caused by peripheral neural or mechanical lesions. Abnormalities in vestibular testing can be associated with lesions in the vestibular apparatus of the inner ear, the vestibular portion of CN VIII, the vestibular nuclei in the brainstem, the cerebellum, or pathways in the brainstem (such as the medial longitudinal fasciculus) that connect the vestibular and oculomotor systems. See Neuroanatomy through Clinical Cases, Chapters 12 and 14 for greater details.

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