44 Neurotology Flashcards
Describe the anatomy of the cerebellopontine angle (CPA).
Describe the anatomy of the cerebellopontine angle (CPA).
The CPA is a complex three-dimensional CSF-filled space in the posterior fossa of the cranial cavity (Figure 44-1). The clivus and petrous portion of the temporal bone delineate the anterolateral boundary. The posterior boundary includes the ventral surface of the brainstem and cerebellum. Medially lies the pons and medulla of the brainstem. The superior boundary includes the cerebellum and middle cerebellar peduncle. The inferior limit is the cerebellar tonsil. The CPA is the region through which cranial nerves VII and VIII travel laterally and superiorly from the brainstem to the internal auditory canal (IAC). The trigeminal nerve is located superiorly in the CPA and travels anteriorly from the pons to Meckle’s cave. Cranial nerves IX, X, and XI are located inferiorly.
Figure text: Retrosigmoid craniotomy (left side) illustrating the surgical anatomy of the posterior fossa exposure. The venous sinuses and inner ear structures have been made visible to help clarify their relationships to intracranial structures. BS, brainstem; CC, common crus; Ch, choroid plexus; Co, cochlea; ES, endolymphatic sac; Fl, flocculus; IV, inferior vestibular nerve; JB, jugular bulb; JV, jugular vein; PA, porus acusticus; PSCC, posterior semicircular canal; SS, sigmoid sinus; SSCC, superior semicircular canal; SV, superior vestibular nerve; VA, vestibular aqueduct. Cranial nerves: 5, trigeminal; 7, facial; 8, cochleovestibular; 9, glossopharyngeal; 10, vagus; 11C, accessory (cranial portion); 11S, accessory (spinal portion).
If you are positioned at the CPA looking laterally into the IAC, what is the relationship between the facial, cochlear and vestibular nerves?
If you are positioned at the CPA looking laterally into the IAC, what is the relationship between the facial, cochlear and vestibular nerves?
The facial nerve lies anterior and superior, with the cochlear division of the eighth cranial nerve lying anterior and inferior. The superior vestibular nerve lies posterior and superior, while the inferior vestibular nerve occupies the posterior and inferior position. Bill’s bar is a vertical crest of bone separating the facial nerve and superior vestibular nerve at the lateral end of the IAC. The transverse crest separates the superior and inferior portions of the lateral IAC. The spatial relationships of the facial-vestibulocochlear nerve complex change during their transit through the IAC, as the facial nerve originates anterior to the vestibulocochlear nerve complex at the brainstem and occupies an anterosuperior position in the fundus of the IAC.
What are the vascular relationships within the CPA?
What are the vascular relationships within the CPA?
There are three major arterial branches of the vertebrobasilar system that traverse the CPA. From inferior to superior arise the posterior inferior cerebellar artery (PICA), the anterior inferior cerebellar artery (AICA), and the superior cerebellar artery (SCA). Disruption of the PICA leads to the lateral medullary syndrome (Wallenberg’s syndrome) consisting of dysphagia, dysarthria, vocal cord paralysis, vertigo, facial paralysis, ataxia, ipsilateral Horner’s syndrome, and hemisensory disturbance. The AICA follows a tortuous path with a loop lying laterally in the CPA in close association with the facial and cochleovestibular nerve roots. Disruption of the AICA leads to symptoms based on location of injury with hemiplegia and cranial nerve dysfunction from medial injury and ataxia from more lateral injury. The internal auditory artery, the blood supply to the labyrinth, is a branch of the AICA. The superior cerebellar artery is located superiorly within the CPA.
What is the differential diagnosis for a CPA mass?
What is the differential diagnosis for a CPA mass?
- Vestibular schwannoma/Acoustic neuroma (60% to 90%)
- Meningioma (3% to 7%)
- Epidermoid (2% to 4%)
- Other cranial nerve schwannomas (V, VII, IX, X, XI) (1% to 4%)
- Arachnoid cysts (1% to 2%)
- Rare: Metastasis, hemangioma, lipoma, chordoma, chondrosarcoma, dermoid, neuroepithelial tumors, endolymphatic sac tumors, ependymoma, glioma, astrocytoma, medulloblastoma, choroid plexus papilloma, aneurysm, arteriovenous malformation
What is an acoustic neuroma (AN)?
What is an acoustic neuroma (AN)?
Also known as a vestibular schwannoma, an AN is a benign tumor arising from the Schwann cells of the eighth cranial nerve. These tumors tend to arise lateral to the obersteiner-redlich zone (transition zone of central/peripheral myelin) and may affect either the superior or inferior vestibular nerve (inferior > superior). With continued tumor growth, the tumor extends medially to the CPA and may lead to marked brainstem compression. The incidence of sporadic AN is 10 per 1,000,000.
Are ANs associated with inherited syndromes?
Are ANs associated with inherited syndromes?
ANs are associated with the autosomal dominant syndromes of neurofibromatosis (NF) types 1 and 2. NF1 (von Recklinghausen’s disease) is associated with intracranial and extracranial neuromas; however, fewer than 5% of patients develop AN. In contrast, NF2 is associated with bilateral AN in over 90% of patients. The presence of bilateral ANs is diagnostic of NF2. NF1 is related to a gene defect on chromosome 17, whereas NF2 may be caused by various types of NF2 gene mutations at locus 22q12.2.
What is the natural history of AN?
What is the natural history of AN?
Acoustic neuromas typically grow at a rate of 1 to 2 mm/year, but occasionally the growth rate may be as rapid as 2 cm/year or greater. Conversely, the AN may not exhibit future growth after diagnosis. As the tumor grows, extension medially into the CPA is common and encroachment on the brainstem, cerebellum, and/or adjacent cranial nerves may occur. A large CPA component may also cause obstructive hydrocephalus due to fourth ventricle compression. Although usually slow-growing, if left untreated, these tumors are lethal due to brainstem compression or intracranial hypertension.
What are the presenting signs and symptoms of an AN?
What are the presenting signs and symptoms of an AN?
Presenting signs and symptoms of AN often depend on the size of the AN, which may be small (<1.5 cm), medium (1.5 to 2.5 cm), large (2.5 to 4 cm) or giant (>4 cm). Small, intracanilicular (within the IAC) tumors present with symptoms associated with dysfunction of CN VIII such as hearing loss, tinnitus, or vertigo. As the tumor grows, hearing loss may worsen and disequilibrium may develop. With brainstem compression, facial hypesthesia ensues due to fifth cranial nerve involvement. Severe brainstem compression leads to symptoms of acute or chronic hydrocephalus such as headache, nausea/vomiting, and mental status changes. The most common symptom of AN is hearing loss (95%), while tinnitus is prominent in 56% to 63% and disequilibrium in 46% to 61%. There is substantial variability in the presenting symptoms of AN. For instance, sudden sensorineural hearing loss secondary to AN has been seen in up to 1% of patients. Facial weakness is a rare finding associated with larger tumors. Hitzelberger’s sign, numbness on the posterior aspect of the concha caused by interruption of sensory fibers of the facial nerve by a mass such as AN, was assessed historically before the era of modern radiographic diagnosis.
How is AN typically diagnosed?
How is AN typically diagnosed?
As patients typically present for evaluation of a unilateral hearing loss, audiometry is often the initial diagnostic test. Unilateral pure-tone sensorineural hearing loss with disproportionate loss of speech discrimination is suggestive of a retrocochlear lesion. Acoustic reflex testing is useful, as 88% of patients with AN have absent reflexes or abnormal reflex decay. Auditory brainstem response (ABR) testing may also be quite useful, detecting 95% of tumors. A delay in the latency of wave V of 0.2 msec or greater is considered abnormal. The false negative rate of ABR is 18% to 30%, especially for small intracanalicular lesions. In the setting of a negative ABR with strong suspicion for AN, a stacked ABR technique improves detection of small AN. Magnetic resonance imaging (MRI) with gadolinium enhancement, however, is the gold standard for diagnosing AN. ANs are isointense to brain on T1 images and mildly hyperintense on T2 with fairly avid enhancement following gadolinium contrast administration.
How are ANs treated?
How are ANs treated?
There are three main treatment modalities for AN. The decision on which modality to choose depends on many different factors. Observation with repeated imaging is an option for patients with AN, particularly elderly patients with small tumors. As the tumor enlarges there may be progression of cranial nerve dysfunction with eventual brainstem compression. Morbidity from treatment increases with increasing size of AN. Given the potential of late complications caused by tumor growth, younger patients with AN require treatment. Effective modalities include surgical excision or radiotherapy. Surgery may or may not entail a hearing preservation approach; this decision is based on the amount of residual hearing present and the location and size of the tumor. Radiation therapy may be preferentially used for those who cannot tolerate a surgical procedure or who have a limited life expectancy. Stereotactic radiosurgery is the most commonly utilized form of radiotherapy, although external beam therapy is also performed.
Is surgery or radiotherapy preferable for the treatment of AN?
Is surgery or radiotherapy preferable for the treatment of AN?
Treatment for a given AN depends on many factors, including the tumor size, location, age of the patient, residual hearing, vestibular symptoms, and preference of the patient. Understanding the risks and benefits of each modality is critical for patient counseling. Rates of cranial nerve palsies are low with each modality and are influenced by the extent of surgery (gross total resection vs. near-total or sub-total resection) and radiation dose (less with marginal doses of 13 Gy) along with surgeon and treatment center experience. The rate of recurrence of surgically excised AN ranges from 1% to 20% depending on extent of resection. The rates of regrowth after radiosurgery have been reported at 5% to 15%. Radiation offers the advantage of earlier return to normal activity. Variation in protocols for radiation therapy and the paucity of long-term follow-up, however, make it difficult to interpret the data for patient counseling on long-term results.
What are the results of AN treated with radiotherapy?
What are the results of AN treated with radiotherapy?
There are different protocols utilized to treat AN including stereotactic radiosurgery (SRS), where radiation is delivered in a single fraction, or stereotactic radiotherapy (SRT), where radiation is delivered in fractions. Two common systems for delivering radiation in treatment for AN are the Gamma Knife system, a cobalt-60 system, and the CyberKnife system, a linear-accelerator based system. Current radiation doses are on the order of 1200 to 1300 rad, which results in a decreased incidence of cranial nerve palsies compared to higher doses used previously. Reports of progression-free survival and rates of tumor control are comparable between protocols and between systems. Rates of 10-year progression-free survival for AN range from 92% to 98% with overall rates of tumor control 93% to 99%. Complications associated with radiation include trigeminal neuropathies (0% to 27% for SRS), facial nerve palsy/paresis (0% to 23%), and hearing loss which appears to worsen over long-term follow-up. Radiation-induced carcinogenesis is a rare potential late complication of radiation for acoustic neuromas.
What are the primary surgical approaches for ANs and their indications?
What are the primary surgical approaches for ANs and their indications?
Translabryinthine:
- ANs larger than 2.0 cm (hearing preservation unlikely)
- Small ANs without serviceable hearing
- Approach for facial nerve decompression and vestibular neurectomy in the presence of poor hearing
- Exposure for other CPA tumors with poor hearing
Retrosigmoid:
- ANs with servicable hearing and minimal involvement of the IAC (not extending to the lateral portion of the IAC)
- Meningiomas with limited IAC involvement
Middle fossa:
- Small intracanalicular ANs with <1.0cm CPA extension
- Superiorly based meningiomas of the IAC/petrous bone
What are additional lateral skull base approaches to the CPA and their primary indication?
What are additional lateral skull base approaches to the CPA and their primary indication?
- Retrolabyrinthine: Posterior craniotomy between the sigmoid sinus and otic capsule providing limited exposure of the posterior fossa used for vestibular neurectomy.
- Transcochlear: Anterosigmoid posterior fossa craniotomy providing an enhanced view of the anterior aspect of the CPA extending the translabyrinthine craniotomy. Meningiomas medial to the IAC, chordomas, chondrosarcomas, as well as residual/recurrent AN may be approached in this manner.
- Infratemporal fossa: Approach used to access tumors of the temporal bone extending inferior to the jugular foramen near the petrous internal carotid artery (ICA) or tumors of the deep lobe of the parotid gland with temporal bone involvement.
What is a meningioma?
What is a meningioma?
Meningiomas are benign tumors thought to arise from arachnoidal cap cells associated with the arachnoid villi in the CPA. Meningiomas account for 20% of intracranial neoplasms and are the second most common lesion of the CPA. These lesions are generally sporadic. Like AN, they may be seen with greater incidence in NF2. Although these lesions are benign, 5% may become malignant. Presentation of a meningioma depends on location and involvement of nearby structures. Meningiomas in the CPA present similarly to ANs with hearing loss, tinnitus, and imbalance. Classic imaging characteristics include enhancement on a postcontrast MRI with a characteristic dural tail and hyperostosis observed on CT. For growing or symptomatic meningiomas, surgical resection is warranted. SRS or SRT for small lesions (<3 cm) or for postsurgical residual disease may provide tumor growth control.
How are paragangliomas associated with the temporal bone staged?.
How are paragangliomas associated with the temporal bone staged?
The Fisch staging system is often used to classify temporal bone paragangliomas based on their extension into adjacent structures and their intracranial extension (Table 44-2).
How is squamous cell carcinoma of the EAC staged and treated?
What are the outcomes?
How is squamous cell carcinoma of the EAC staged and treated? What are the outcomes?
The University of Pittsburgh staging system for EAC tumors is the most widely accepted system (Table 44-3). Two-year survival rates by stage are as follows: T1, 100%; T2, 80%; T3, 50%; and T4, 15%.
