A Review of Posterior Fossa Lesions

Abstract

Posterior fossa lesions can occur in patients of all ages. Patients present with subtle findings of hearing loss, tinnitus, headache, cranial neuropathies, imbalance, vertigo, nausea, and vomiting depending on the location and type of lesion. The presentation can be particularly insidious as the most common symptom is slowly progressive hearing loss. Posterior fossa lesions should be included in the differential diagnosis for patients with this presentation and appropriate audiologic testing and imaging should be performed. Management involves collaboration between multiple subspecialties and has evolved significantly over the years leading to increased patient involvement and improved outcomes.

Introduction

Lesions of the posterior fossa may be encountered by physicians of many specialties. An understanding of the clinical presentation, differential diagnosis, work–up, and treatment of these lesions will assist all physicians in the care of a patient with this medical condition. It is imperative that patients suspected of or diagnosed with a posterior fossa lesion are referred to the appropriate specialties.

The posterior fossa is bound by the clivus, petrous ridge of the temporal bones, mastoid portion of the temporal bones, and the occipital bone. It houses the cerebellum, portions of the brainstem including the pons and medulla, and the fourth ventricle.¹ Lesions in the posterior fossa can be categorized by their etiology and include vascular, infectious, traumatic, and neoplastic or by their anatomic location within the posterior fossa. They are either intra and extra–axial which describes whether they exist inside or outside of the pia mater in the brain parenchyma. Intra–axial tumors arise from the brainstem, cerebellum, or fourth ventricle.² Tumors of the posterior fossa can be seen in both adults and children with 65% of pediatric brain tumors arising in the posterior fossa.³

Many posterior fossa lesions are found is the cerebellopontine angle (CPA) which consists of cerebrospinal fluid (CSF), cranial nerves VII and VIII, the anterior inferior cerebellar artery, and arachnoid tissue. Tumors at this location represent less than 10% of all intracranial tumors in adults and less than 1% in children.⁴ The most common pathologies are acoustic neuromas and meningiomas.⁴ Lesions in this area are of particular interest to otolaryngologists as they present with otologic symptoms particularly asymmetric hearing loss and require multidisciplinary management including otolaryngology, neurosurgery, and radiation oncology.

Clinical Presentation

The presentation of a patient with a posterior fossa lesion may be different based on the type and anatomic location of the lesion. Typically, patients present with gradually progressive symptoms and signs including asymmetric hearing loss, tinnitus, vestibular dysfunction, and cranial neuropathies.^2,5 Tinnitus followed by hearing loss are the most common earliest findings. Vertigo is overall uncommon and usually mild however patients more commonly presenting with imbalance.⁶ They may also present with symptoms of increased intracranial pressure such as nausea, vomiting, papilledema, mental status changes, and ataxia.^3,6 Any patient presenting with these symptoms should be considered for work–up of an occult lesion.⁵ Asymmetric sensorineural hearing loss or sudden sensorineural hearing loss are two of the most common presentations and patients with either presentation must have a contrasted MRI to evaluate for retrocochlear pathology. Due to the increased use of MRI with contrast incidentally diagnosed posterior fossa lesions are becoming increasingly more common.⁷ Early identification of tumors, the delineation of their natural history, and the availability of MRI has led to a significant change in management with increased options for patients.⁷ Figure 1 highlights the sinister nature of these lesions as this patient’s only presenting symptom was sudden hearing loss.

Figure 1.

A young individual presented for evaluation of sudden sensorineural hearing loss and was found to have a giant acoustic neuroma. This patient had no other major additional signs or symptoms.

Work–Up

A thorough history and physical examination is the first step in evaluation. Historical symptoms including asymmetric hearing loss, asymmetric tinnitus, vestibular dysfunction, and cranial neuropathies raise suspicion and require further workup for a lesion that is more advanced.² A comprehensive physical exam includes a complete head and neck exam, cranial nerve evaluation and microscopic otoscopy.

Audiologic testing is an imperative step in the diagnostic and management algorithm.⁵ Involvement of the middle ear can result in conductive or mixed hearing loss. A complete audiologic exam includes routine air, bone audiometry, and speech discrimination. Auditory brainstem response testing is another available test which is performed in cases where hearing preservation is considered and possible.

Both CT and MRI can be used to image the posterior fossa, but MRI with gadolinium contrast is preferred and is the gold standard due to improved soft tissue imaging.^2,3 CT is often used as the first imaging study, especially in patients with findings that are concerning for an emergent diagnosis requiring a quick and easily available form of imaging.³ With regards to MRI, T1–weighted sequence with contrast is essential in delineating these lesions. The administration of contrast with MRI is exceptionally important in initial diagnosis as many small lesions may be missed without the administration of contrast. T2 imaging may not delineate small lesions as easily due to CSF bathing the CPA and IAC but can be used to monitor known lesions.^8,9

Table 1 summarizes the various pathologies based upon location and pathology type. Given that most pertinent and common lesions of the posterior fossa and CPA for otolaryngologists are acoustic neuromas and meningiomas these two pathologies will be presented in greater detail.

Table 1.

Table containing an differential diagnosis overview of potential lesions of the posterior fossa.¹⁰–²⁴

Posterior Fossa Lesions
Intra-axial Neoplasms	Pilocystic astrocytoma
	Medulloblastoma
	Brainstem glioma
	Hemangioblastoma
	Atypical teratoid rhabdoid tumor
	Choroid plexus papilloma
	Ependymoma and Subependymoma
	Hemangioma
	Dysplastic gangliocytoma (Lhermitte-Duclos)
	Langerhans cell histiocytosis
Extra-axial Neoplasms	Acoustic Neuroma/Vestibular Schwannoma
	Meningioma
	Cholesteatoma/Epidermoid cyst
	Arachnoid cyst
	Cholesterol granuloma
	Dermoid tumor
	Non-acoustic neuroma/schwannoma
	Teratoma
	Lipoma
	Chordoma
	Glomus Tumor/Paraganglioma
	Endolymphatic sac tumor
	Giant cell tumor
	Chondrosarcoma
	Lymphoma
	Metastasis
Non-neoplastic lesions	Trauma
	Vascular lesion
	Hemorrhagic and ischemic stroke
	Arteriovenous malformation
	Aneurysm
	Nontraumatic subdural hematoma
	Cystic lesion
	Dandy-Walker cyst
	Mega cisterna magna
	Retrocerebellar cyst
	Infectious/Inflammatory
	Abscess
	Lyme disease
	Tuberculosis
	Autoimmune dural lesions

Acoustic Neuroma/Vestibular Schwannoma

Acoustic neuromas are most common tumor of the CPA, representing over 80% of tumors at this anatomic location.² A benign tumor that arises from perineural Schwann cells of the vestibular nerve, are also called vestibular schwannomas.^2,3 They may arise from the superior or inferior vestibular nerve. Within the internal auditory canal, they have a close relationship with the facial nerve and cochlear nerve. They are typically slow growing, well circumscribed, and can cause symptoms by displacement of neurovascular structures.² The most common presentation is a slowly progressive asymmetric sensorineural hearing loss. Facial nerve palsy and paralysis are uncommon even in large lesions. Most are unilateral and non–hereditary, but a small percentage of them may be associated with Neurofibromatosis type 1 (NF1) or Neurofibromatosis type 2 (NF2).² Bilateral lesions occur with NF2 and are pathognomonic for this genetic condition.² Approximately 80% of acoustic neuromas involve the IAC and can be purely intra–canalicular (withing the IAC), purely extra–canalicular or both.^2,3,14 They are isodense or slightly hypodense on unenhanced CT and typically isointense or mildly hypointense on T1 without contrast, and avidly enhance with contrast and mildly hyperintense on T2 MRI.^2,3 The imaging of choice is MRI IAC with and without contrast. Treatment of these lesions has evolved markedly over the years.¹⁵ Treatment depends on the size, location, as well as the associated symptoms and signs. This is especially true as smaller tumors are being diagnosed more frequently with the availability and use of early imaging.¹⁵ Options for management include observation with repeat imaging, stereotactic radiotherapy (SRT), and microsurgical resection (Figure 2).

Figure 2.

Demonstrating the slow growth pattern of acoustic neuromas.

Image from RK Jackler Skull Base Surgery Atlas.

Meningioma

Meningiomas represent another common tumor in the posterior fossa and are the second most common lesions. They are typically benign, but can be locally aggressive and can invade bone through haversian canals or cause adjacent bone to become hyperostotic.² On unenhanced CT 50% are hyperdense and 50% isointense on T1 and T2 MRI.³ They may also have calcifications and a homogenous enhancement with iodine.² On T2 they may be hyperintense and often have a dural tail which may help differentiate them from other CPA tumors.³ They also have a broad base and the majority extend into the middle fossa.² Interestingly patients with meningiomas will often maintain their hearing even with particularly large lesions.

A grading system developed by the World Health Organization (WHO) is widely used and based off of histopathologic appearance.² The grading system ranges from Grade I to III with Grade I representing benign lesions that make up 90% of these tumors (Figure 3). Often clinically small meningiomas are indistinguishable from acoustic neuromas and pathologic diagnosis is made during microsurgical resection. Similar to acoustic neuromas management options include observation, SRT and microsurgical resection.

Figure 3.

Illustration demonstrating the propensity of meningiomas to develop along foramina in the skull base as well as venous sinuses.

Image from RK Jackler Skull Base Surgery Atlas.

Management

Management starts with ensuring that the patient received a multidisciplinary approach. Depending on the type of lesion, these patients are often referred to neurosurgery and may also require referral to interventional neuroradiology, medical oncology, and radiation oncology. Regardless of the natural history of the particular lesion, it is important to initiate a treatment plan. Patients are provided with information regarding their diagnosis, natural history, and their options for treatment. Additionally, they are involved in the decision–making process in lesion management.

The treatment options that exist for posterior fossa masses include observation, microsurgery, and stereotactic radiotherapy.¹⁵ Treatment decision is based on the type, size, and location of the lesion, whether there are related symptoms or signs, and to a large degree patient’s preference. Treatment decisions should be made with the goal of tumor control with maximal patient benefit without creating morbidity.¹⁵ Observation or “watchful waiting” requires a commitment to repeat imaging to monitor for growth. This approach is often utilized in small lesions and lesions in patients who are older or with poor health. The main risks of observation include lesion growth and loss of hearing. Lesions are initially imaged with repeat MRI with contrast with audiogram at six months and if stable are imaged annually with audiologic testing. Indications to pursue alternative treatment include growth of the lesion and changes in hearing which can be a relative indication.

Surgical Management

Microsurgical resection is indicated for large lesions, lesions in young patients, growing lesions, and patients who have failed SRT. There are several microsurgical approaches which are indicated based upon the size of the lesion, location, and patients hearing.² For giant lesions these approaches may also be combined. The goal is for gross total resection of a lesion with preservation of vital structures and minimizing any morbidity. This modality is the only curative treatment and patients can benefit from tumor control, management of mass effect, and pathologic diagnosis. Possible complications include CSF leak, meningitis, facial nerve injury, loss of hearing, changes in facial sensation, and injury to the anteroinferior cerebellar artery.² All cases utilize facial nerve monitoring and hearing preservation cases utilize electrophysiologic monitoring of the cochlear nerve. Figure 4 demonstrates the various exposures and approaches to the posterior fossa. Approaches for the majority of CPA lesions include translabyrinthine, retrosigmoid, and middle cranial fossa approaches.

Figure 4.

Demonstration of various exposure from several different posterior fossa approaches.

Image from RK Jackler Skull Base Surgery Atlas.

Translabyrinthine Approach

The translabyrinthine approach is indicated for a variety of size tumors at the CPA. It provides wide exposure of the internal auditory canal into the CPA allowing for full identification of the facial nerve. Additionally, it has the benefit of having little to no brain retraction and intradural drilling. However, it does result in complete hearing loss and exposure can be limited by the pneumatization of the mastoid.² It is recommended for patients without serviceable hearing and in cases were hearing preservation is unlikely (Figure 5). Reconstruction is performed by placing an abdominal fat graft at the conclusion of the case.

Figure 5.

Demonstration of exposure via the translabyrinthine approach. This approach offers wide exposure of the IAC and CPA but sacrifices any residual hearing.

Image from RK Jackler Skull Base Surgery Atlas.

Retrosigmoid Approach

The retrosigmoid or suboccipital approach provides an opportunity for hearing preservation and it provides a wider panoramic exposure to the posterior fossa. The wide exposure of this approach makes it very versatile. Tumor extension into the IAC is an important consideration as violating the inner ear significantly decreases chances of hearing preservation. One particular consideration of this approach is that it requires cerebellar retraction, which could create edema narrowing the surgical corridor.² This could lead to long term cerebellar encephalomalacia. Pending the placement of the sigmoid, cerebellum and patient habitus this can result in a tight surgical corridor. In cases without hearing preservation the option of translabyrinthine versus retrosigmoid largely comes down to surgeon preference (Figure 6).

Figure 6.

Exposure derived from a retrosigmoid approach. This is a possible hearing preservation approach though tumors that extend laterally into the IAC can be limiting. Additionally, the retrosigmoid approach provides a more panoramic view of the posterior fossa.

Image from RK Jackler Skull Base Surgery Atlas.

Middle Fossa Approach

The middle cranial fossa approach is a subtemporal extradural approach which is indicated for small intracanalicular tumors with usable hearing. An extended approach is possible though larger tumors are poor candidates for this approach.² The limitation of this approach is that tumors that extend to or that are in the distal IAC and the CPA can have limited exposure. This approach requires retraction of the temporal lobe for exposure which has inherent risks including seizure, stroke, and aphasia particularly in elderly patients. Additionally, depending on the nerve or origin of the tumor there is a short term slightly higher risk of initial facial paresis with this approach primarily within the first year of surgery (Figure 7).²⁵

Figure 7.

The middle cranial fossa approach is a hearing preservation approach for small intracanalicular tumors with limited medial extension.

Image from RK Jackler Skull Base Surgery Atlas.

Stereotactic Radiotherapy

Stereotactic radiotherapy also known as stereotactic radiosurgery is an additional treatment option for patients. Examples of this include Gamma Knife, CyberKnife, and proton beam radiation. This treatment modality is an option for patients with growing tumors that cannot undergo microsurgical resection, do not wish to proceed with surgery, or are not too large. The goal of SRT is not to eliminate the tumor but cease growth of the lesion.²⁶ Ionizing radiation is delivered via a cobalt source or linear accelerator (LINAC) in an exceptionally precise manner to the intracranial lesion.²⁶ This limits exposure to normal healthy tissue and maximizes dose delivery to the tumor for therapeutic effect. This is accomplished via localization and targeting of the lesion. Treatment planning utilizes data from CT and MRI to localize the tumor plan treatment delivery.^{26, 27,28} Patients are typically treated with 12 Gy single dose or 18 Gy fractionated treatments. Control rates for growing tumors is 80–85% vs. 90–95% for all lesions following SRT. Hearing preservation rates are approximately 20% for useful hearing at five years and is an important consultation topic. A negative of this treatment modality is that diagnosis and treatment is based off of imaging and a pathologic specimen is not obtained. A very rare but important consideration is that there have been approximately 20 cases of malignant transformation following radiation which is imperative to discuss with patients.²⁹ As the tumor is not removed it is recommended that patient undergo annual imaging to monitor for any changes.

Conclusions

Posterior fossa lesions can occur in patients of all ages with particularly innocuous presenting symptoms. When a patient presents with hearing loss especially asymmetric or sudden loss, cranial nerve dysfunction, or symptoms that may represent obstructive hydrocephalus it is imperative that the treating physician consider a posterior fossa lesion in the differential diagnosis. Work–up should always include a thorough history and physical exam, imaging, audiologic testing, and referral to an appropriate skull base center. Once the diagnosis has been made, a management plan can be formulated. Depending on the location, size, and pathology of the lesion different treatment options exist. These include observation, stereotactic radiotherapy, and microsurgical resection which is in stark contrast compared to historical considerations were nearly all tumors underwent microsurgical resection. Treatment options are managed in a multidisciplinary fashion and in conjunction with patient’s goals of therapy. This has resulted in improved outcomes and quality of life as well as patient satisfaction.