Anatomy of Acoustic Neuroma

Elissa H. Kawamoto, B.S.
Graduate Student, Department of Speech, Language and Hearing Science, University of Arizona

In order to develop strong diagnostic and clinical skills, one must have a deep understanding of the anatomy and physiology of the entire system related to ones field of study. Considerable attention must be directed towards understanding the region where vestibular schwannomas grow. This is because if a vestibular schwannoma is missed, the growth will expand and begin to impact surrounding cranial nerves, cerebellar arteries, and parts of the brainstem, and/or the cerebellum. Vestibular schwannomas are best known as acoustic neuromas however, the proper terminology is vestibular schwannoma because it is an over production of schwann cells on the vestibular nerve. Since most refer to this lesion as an acoustic neuroma, I will use this term throughout this paper. Obviously, there is much to be said about this topic. However, by reading this paper, one will be informed in a few pages of some basic aspects of acoustic neuromas. This paper will overview the key anatomy related to the acoustic neuroma including the internal auditory meatus, brainstem relationships, vascular supply to the acoustic neuroma and neighboring nerves, symptomatology, and diagnostic tools.

Where does this all start?

All of the peripheral hearing structures are housed and protected by the temporal bone (Musiek and Baran, 2007).  The tympanic portion of the temporal bone is the most lateral aspect of the skull. Moving medially from this point of reference, one will observe the tympanic membrane, middle ear muscles, cochlea, and semicircular canals, among many other important anatomical structures. Nerves from the cochlea and semicircular canals project medially towards the brainstem. In order for the nerve fibers to proceed from the hearing organs to the brainstem, they travel through a small canal called the internal auditory meatus (IAM). The IAM is within the petrous portion of the temporal bone and is approximately an inch in length (Portmann, Sterkers, Charachon, and Chouard, 1975). According to findings from Papangelou (1972), the average vertical diameter of the IAM is 4-6 mm with a horizontal diameter of 4-5 mm. Rhoton and Tedeschi (2008) describe the four portions within the IAM as the facial nerve running through the anterior-superior portion, the auditory nerve running through the anterior-inferior portion, the superior vestibular nerve running through the posterior-superior portion, and the inferior vestibular nerve running through the posterior-inferior portion. The nerve fibers course through the IAM and exit at the porus acousticus. According to Gruskin, Carberry, and Chandrasekhar (1997) in most cases, acoustic neuromas will originate from the vestibular branch within the IAM. Due to the narrow space within the IAM, acoustic neuromas will first compress the auditory nerve (AN), and occasionally, erode the temporal bone. However, in a majority of cases, the acoustic neuroma will grow medially and extend out into a recess called the cerebellopontine angle (CPA). The CPA is the angle between the cerebellum and pons (Musiek and Baran, 2007). It may also be described as the space between the porus acousticus and the brainstem. The acoustic neuroma will often grow into the CPA because it will better accommodate the growth in comparison to the small and bone restricted IAM.

What nerves are involved?

As stated before, acoustic neuromas often originate from the vestibular nerve within the IAM. The vestibular and auditory nerves compromise the VIII^th cranial nerve. The VIII^th cranial nerve is approximately 17-20 mm in length (Bebin, 1979). The AN is tonotopically organized so that the high frequencies are represented on the periphery, and the low frequencies are represented in the core of the nerve trunk (Musiek and Baran, 2007). The AN is typically the first nerve to become compressed by the lesion. Even though acoustic neuromas originate on the vestibular nerve, in most cases, auditory symptoms are first to surface and are also detectable by audiologists (Angeli and Jackson, 1997).  Since the tumor presses on the outer-most portion of the auditory nerve, the high frequencies that are represented on the periphery, will be affected. This damage will result in a high-frequency sensorineural hearing loss (Tucci, 1997). Additional to the VIII^th cranial nerve, there is the VII^th cranial nerve running through the IAM towards the brainstem. The VII^th cranial nerve consists of the nervus intermedius and the facial nerve. The nervus intermedius and facial nerve run as separate branches prior to exiting the porus acousticus and join together once they exit into the CPA (Bebin, 1979). Facial sensation primarily involves the nervus intermedius whereas facial movement involves the facial nerve. Once all of these nerves have reached the CPA, they enter the brainstem at the cochlear nucleus in the pons.

Lateral aspect of the brainstem

As the auditory nerve travels medially through the CPA, and enter the brainstem at the root entry point between the dorsal and ventral cochlear nuclei. According to Musiek and Baran (2007), this entry point is at a sulcus located between the pons and medulla, called the pontomedullary junction. Once the VII^th and VIII^th cranial nerves enter the pons, they reach the cochlear nucleus (CN). The CN is the first structure of the central auditory nervous system and is responsible for relaying auditory input from the periphery to the brain. Due to its location, as acoustic neuromas grow into the CPA, they may continue to grow medially, eventually compromising the CN and sometimes if large enough, the fourth ventricle on the affected side. The neural structures that are most commonly compressed by the acoustic neuroma are the pons, medulla, fourth ventricle, and cerebellum (Stangerup, Caye-Thomasen, and Thomsen, (2006). The cerebellum has many functions therefore, if compromised, there will be serious affects such as ataxia.

Vascular supply

One cannot discuss the effects of an acoustic neuroma without mentioning the vascular supply. It seems little attention is paid towards learning about vascular supply and as audiologists, one must be able to recognize that with acoustic neuromas, there is a great chance that the blood supply to the cochlea will be compromised. It should be noted that the two main vascular systems that supply the CANS are the vertebral-basilar system and the internal carotid system (Musiek and Baran, 2007). The one that is of primary concern in relation to this pathology is the vertebral-basilar system. The two vertebral arteries converge to form the basilar artery that gives rise to the anterior-inferior cerebellar artery (AICA). According to Bebin (1979), AICA courses on the ventral surface of the pons and is one of the largest branches of the basilar artery to reach the cerebellum. AICA also supplies the CN and its branches course through the IAM. Once the tumor grows out into the CPA, it may push the lateral branch of AICA upward. An important branch called the internal auditory artery (IAA) often originates from AICA (Musiek and Baran, 2007). The IAA is important because it supplies blood to the nerves within the IAM, the bony structures, and the cochlea and vestibular systems in the periphery. There is probability this artery may be compromised and when blood supply is interrupted, cochlear and VII^th nerve damage may result.

Symptomatology

The symptoms of an acoustic neuroma are directly related to the anatomy. One must think critically about the anatomy to understand how a lesion in this small space will impact an individual. According to Bebin (1979), there are three main ways acoustic neuromas can create intracranial changes. One way is if the tumor continues to grow inside the IAM, it will erode the temporal bone. Additionally, there can be changes related to compression of cranial nerves, brainstem, cerebellum, and relevant arteries. Lastly, this tumor, if large, can result in hydrocephalus or papilledema. It has been stated previously that in most cases, acoustic neuromas originate on the vestibular nerve. Some may find it surprising that vestibular symptoms are often minimal or absent in most individuals with an acoustic neuroma. However, in a few cases, some report having a slight sensation of unsteadiness and instability in gait. So rather than reports of vestibular disturbances, the first symptom patients report is a hearing loss or difficulty understanding speech (Angeli and Jackson, 1997). Audiologic test results reported by Johnson (1977) demonstrated that in 500 cases of acoustic neuromas, there was a unilateral sensorineural hearing loss present in 95% of those patients. Angeli and Jackson (1997) report that another common symptom is unilateral tinnitus in the affected ear. Additionally, if the facial nerve is elongated or compressed, there may be symptoms such as spasms, twitching, or facial weakness. If the nervus intermedius is irritated, there may be some facial numbness or a tingling sensation present. The facial symptoms are present in the same side that the tumor is. More often than not, this lesion is unilateral leaving only 5% of acoustic neuromas bilateral (Gruskin, Carberry, and Chandrasekhar, 1997).

Diagnostic Tools

The purpose of this section is to simply overview the diagnostic tools for it has been written about extensively in many journal articles. What will be mentioned will be limited to what is relevant within the scope of this paper. How does one detect acoustic neuromas early on? Pure tone thresholds will not always detect acoustic neuromas. About 5% of people with acoustic tumors have normal audiograms (Musiek, Kibbe-Michal, Geurkink, Josey, and Glasscock III, 1986). The most definitive tool to detect an acoustic tumor is MRI however, there is a risk of over referral especially in some circumstances in present day practices. According to Tucci (1997), the most sensitive diagnostic tool for audiologists to use to screen for acoustic tumors is the auditory brainstem response (ABR). The ABR is abnormal in 90% of auditory neuroma cases. It is marked by increased latencies or in some cases, absent ABR waves (Musiek, Josey, and Glasscock III, 1986). Although pure tone thresholds cannot always allude to an acoustic neuroma, it is important to remember that often one of the first symptoms is difficulty hearing or understanding speech (Angeli and Jackson, 1997). If a patient presents a unilateral high-frequency sensorineural hearing loss, one must always assume it is an acoustic neuroma until proven otherwise.

References

1. Angeli S. I., & Jackson C. A. (1997). Neurotological Evaluation. In W. House, C. Luetje, & K. Doyle (Eds.), Acoustic Tumors Diagnosis and Management (2nd ed., pp, 85-91). San Diego, CA: Singular Publishing Groups, Inc.
2. Bebin J. (1979). Pathophysiology of Acoustic Tumors. In W. House & C. Luetje (Eds.), Acoustic Tumors (Vol. I, pp. 45-83). Baltimore, MD: University Park Press.
3. Gruskin P., Carberry J. N., & Chandrasekhar S. S. (1997). Pathology of Acoustic Tumors. In W. House, C. Luetje, & K. Doyle (Eds.), Acoustic Tumors Diagnosis and Management (2nd ed., pp, 27-83). San Diego, CA: Singular Publishing Groups, Inc.
4. Johnson E. W. (1977). Auditory Test Results in 500 Cases of Acoustic Neuroma. Arch Otolaryngol, 103, 152-158.
5. Musiek F. E., & Baran J. A. (2016). The Auditory System. San Diego, CA: Plural Publishing, Inc.
6. Musiek F. E., Josey A. F., & Glasscock III M. E. (1986). Auditory Brain Stem Response – Interwave Measurements in Acoustic Neuromas. Ear and Hearing, 7, 100-105.
7. Musiek F. E., Kibbe-Michel K., Geurkink N. A., Josey A. F., & Glasscock III M. E. (1986). ABR results in patients with posterior fossa tumors and normal pure-tone hearing. Otolaryngology – Head and Neck Surgery, 94, 568-573.
8. Papangelou L. (1972). Study of the Human Internal Auditory Canal. The Laryngoscope, 82/4, 617-624.
9. Portmann M., Sterkers J. M., Charachon R., & Chouard C. H. (1975). The Internal Auditory Meatus. Edinburgh and New York: Churchill Livingstone.
10. Rhoton A. L. Jr, & Tedeschi H. (2008). Microsurgical Anatomy of Acoustic Neuroma. Neurosurgery Clinics of North America, 19, 145-174. doi: 10.1016/j.nec.2008.02.005
11. Stangerup S. E., Caye-Thomasen P., Tos M., & Thomsen J. (2006). The Natural History of Vestibular Schwannoma. Otology & Neurotology, 27, 547-552.
12. Tucci D. L. (1997). Audiologic Testing. In W. House, C. Luetje, & K. Doyle (Eds.), Acoustic Tumors Diagnosis and Management (2nd ed., pp, 93-104). San Diego, CA: Singular Publishing Groups, Inc.

Elissa Kawamoto is a graduate student at the University of Arizona. She is from Phoenix, Arizona and received her Bachelor of Science in Speech, Language, and Hearing Sciences from the University of Arizona. Throughout her graduate studies, Elissa has been working on various projects with Dr. Frank Musiek, Dr. David Velenovsky, and Dr. Linda Norrix. She is involved in Dr. Frank Musiek’s Neuroaudiology Lab and Dr. David Velenovsky’s Reflectance Lab.