ESSENTIALS OF DIAGNOSIS
Asymmetric (unilateral) sensorineural hearing loss, tinnitus, and disequilibrium.
Disproportionately diminished speech-discrimination score relative to deterioration in pure-tone average.
Facial and trigeminal nerve symptoms with larger tumors.
Vestibular schwannomas (VSs) (acoustic neuromas) are nerve sheath tumors of the superior and inferior vestibular nerves (cranial nerve VIII). They arise in the medial internal auditory canal (IAC) or lateral cerebellopontine angle (CPA) and cause clinical symptoms by displacing, distorting, or compressing adjacent structures in the IAC and the CPA.
VSs are by far the most common tumors involving the CPA. VSs make up 80% of CPA tumors and 8% of all intracranial tumors. Various epidemiology studies have shown an incidence range of 10 to 20 per 1 million individuals each year. This figure correlates with 2000 to 3000 individuals diagnosed with VS each year in the United States. There is no gender bias and the age of presentation is between 40 and 60 years of age. Ninety-five percent of VS occur in a sporadic fashion. The remaining 5% of patients have neurofibromatosis type 2 (NF2) or familial VS. The age of presentation is earlier in nonsporadic VS and patients usually present in the second or third decades of life.
The CPA consists of a potential cerebrospinal fluid (CSF)-filled space in the posterior cranial fossa bounded by the temporal bone, cerebellum, and brainstem. The CPA is a roughly triangular-shaped structure in the axial plane and is filled with CSF (Figure 63–1). The superior boundary is the tentorium and the inferior boundary is the cerebellar tonsil and medullary olives. The anterior border is the posterior dural surface of petrous bone and clivus, and the posterior border is the ventral surface of the pons and cerebellum. The medial border is the cisterns of the pons and medulla and the apex is the region of the lateral recess of the fourth ventricle. The lateral opening of the fourth ventricle, the foramen of Luschka, opens into the CPA. Cranial nerves V to XI traverse the cephalic and caudal extent of the CPA. The central structures crossing the CPA to and from the IAC are the facial (CN VII) and vestibulocochlear nerves (CN VIII), respectively.
The anatomy of the CPA and its relationship to the temporal bone within the skull is shown. Inset shows the location of the cranial nerves within the IAC: the facial nerve (7) and the cochlear nerve (C) are in the anterior compartment, whereas the superior and inferior vestibular nerves (SV and IV, respectively) are in the posterior half of the IAC. 5, trigeminal nerve; 7, facial nerve; 8, cochlear nerve; IAC, internal auditory canal; CO, cochlea; GG, geniculate ganglion; ME, middle ear; EAC, external auditory canal; M, mastoid; SCC, semicircular canal; CPA, cerebellopontine angle; SS, sigmoid sinus; 4V, fourth ventricle; Cb, cerebellum; P, pons.