CLINICAL CASE | Acoustic Neuroma
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.
Figure 8–1A1 is an MRI, with gadolinium, showing a lesion in the left internal auditory canal, which is indicative of an acoustic neuroma. Figure 8–1 A2 shows an MRI from approximately the same level from a healthy person.
Based on your reading of this chapter, answer the following questions.
1. Explain why this patient has the following three signs: unilateral hearing loss, gait instability, and mild flattening of the left nasolabial fold.
2. What is the significance of preservation of the left hearing impairment whether the tuning fork is held at a distance from her ear or when it contacts the mastoid process?
Key neurological signs and corresponding damaged brain structures Unilateral hearing loss An acoustic neuroma—typically a Schwann cell tumor, or schwannoma—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 lesions 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.
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 skin fold that extends from the nose to the lateral edge of the mouth; sometimes this is termed a smile line. In addition to the muscles of the lower face, eighth nerve damage also can weaken other facial muscles. Eighth nerve functions are discussed in Chapter 11.
Gait instability This can be produced either by compromised function of the vestibular division of the eighth nerve or by compression of the pons and cerebellum by the expanding tumor. This patient does not report vertigo, a sign of vestibular dysfunction. ...