CLINICAL CASE | Difficulty Hearing
A 40-year-old woman notes that she has been having difficulty understanding what people are saying when they stand on her left side. She also finds she hears better with the phone over her right, not left, ear. On examination, when a vibrating tuning fork is held at a distance from her left or right ear, she hears better with the right ear. When the tuning fork is placed on the mastoid process, thus eliminating air conduction, the same pattern of hearing ability persists, better on the right than left side. For either side, when placed on either mastoid process, the tuning fork sounds softer than when held near the ear. She is also observed to have a mild gait instability and mild flattening of the left nasolabial fold. An MRI, with gadolinium, shows a tumor in the region of the cerebellopontine angle, encroaching into the left internal auditory canal (Figure 8–1A1, A2 shows an MRI from approximately the same level from a healthy person).
You should be able to answer the following questions based on your reading of this chapter, earlier readings, inspection of the images, and consideration of the neurological signs.
1. Under normal circumstances, would a sound be perceived as louder if the tuning fork is held in the air close to the ear or touching the mastoid process?
2. What are the key structures in the region of the cerebellopontine angle and their major functions?
3. What is the nasolabial fold?
4. How can the myriad of neurological signs—hearing impairment, flattening of the nasolabial fold, and gait instability—be explained by a single event? Key neurological signs and corresponding damaged brain structures Unilateral hearing loss on the side of the tumor
The patient has an acoustic neuroma. This is typically a Schwann cell tumor, or schwannoma, and preferentially impairs the function of the auditory division of the eighth cranial nerve. As the tumor grows, it expands the internal auditory canal, through which the nerve passes en route to the periphery (Figure 8–1B1-3). The eighth nerve peripheral auditory structures and cochlear nuclei are the only sites where injuries produce a unilateral impairment. Central auditory system lesions do not produce deafness in one ear because of the numerous opportunities for auditory information to decussate. The impairment in the patient is on the same side as the tumor hence, the impairment is ipsilateral. The eighth nerve also has a vestibular division for balance, which is discussed in Chapter 11. Flattening of the nasolabial fold
The facial nerve joins with the eighth nerve to exit through the internal auditory canal (Figure 8–1B1). As a consequence, facial nerve function can also be compromised with acoustic neuromas. The facial nerve, as we will see in Chapter 11, innervates the muscles of facial expression, unilaterally. A clear sign of weakness of these facial muscles is the flattening of the nasolabial fold. In addition ...