Internal Auditory Canal Glioneural Hamartoma: A Rare Mass Masquerading as a Vestibular Schwannoma

Glioneural hamartomas are exceedingly rare lesions. When localized to the internal auditory canal (IAC), they can cause symptoms referrable to seventh and eighth cranial nerve compression. Here, the authors present a rare case of an IAC glioneural hamartoma. A 57-year-old male presented for evaluation of presumed intracanalicular vestibular schwannomas found on work-up of dizziness and progressive right-sided hearing loss. Surgical intervention pursued progressive symptoms and new onset headaches. The patient underwent uncomplicated retrosigmoid craniectomy for gross total resection. Histopathological evaluation revealed a glioneural hamartoma. A MEDLINE database search used the terms' cerebellopontine angle' OR 'internal auditory canal' AND 'hamartoma' OR 'heterotopia'. Clinicopathological characteristics and outcomes of the present case were compared to those in the literature. The literature review yielded nine articles describing 11 cases (eight females, three males; median age 40 years, range 11-71) of intracanalicular glioneural hamartomas. Patients most commonly presented with hearing loss and were presumed to have a diagnosis of vestibular schwannoma before histologic diagnosis. Glioneural hamartomas are rare lesions that may be found in the IAC. Although benign, they may be safely resected for cranial nerve function preservation goals with a low risk of recurrence.


Introduction
Glioneural hamartomas or heterotopia are exceedingly rare lesions that may arise in the internal auditory canal (IAC) and masquerade as a more common pathology [1]. Most masses in the IAC constitute vestibular schwannomas (VS), followed by meningiomas [2]. Similar to these more common lesions, glioneural hamartomas present commonly with symptoms of mass effect on the seventh and eighth cranial nerve (CN) complex. Preoperative imaging modalities, such as computed tomography (CT) or magnetic resonance imaging (MRI), may not help differentiate these masses from their more common counterparts. However, glioneural hamartomas are less likely to demonstrate post-contrast enhancement. Although abnormal, these are generally benign lesions that are slow growing and rarely recur after resection. Surgical intervention is often pursued goals of CN function preservation. Here, the authors present a rare case of glioneural hamartoma in a middle-aged man who was presumed to have a VS managed with surgical intervention. The clinical and pathological case characteristics are compared to those currently available in the literature to consolidate the current fund of knowledge on this rare clinical entity.

Case Presentation Clinical presentation
A 57-year-old male presented to the otolaryngology and neurosurgery clinic after evaluation of dizziness and progressive right-sided hearing loss revealed an intracanalicular mass. The patient was found to have slight asymmetric sensorineural hearing loss on the audiogram (Figure 1). His neurologic exam was otherwise unremarkable, and the MRI demonstrated a small, 0.75x0.32cm, non-enhancing mass in the IAC ( Figure 2). The mass was presumed to be a VS during preoperative work-up, and recommendations were made for radiographic surveillance versus radiation or surgery. After four months of observation, the patient returned to the clinic and elected for resection of his tumor, given his progressive symptoms and new onset headaches. The risks and benefits of all management options were discussed with the patient before pursuing a surgical intervention.

FIGURE 1: Preoperative audiogram (A) and auditory brainstem responses (B)
Audiogram recordings demonstrating moderate sensorineural hearing loss 250-8000 Hz on the right ear. The patient had normal hearing in the left ear across tested frequencies 250-800 Hz, with a mild sensorineural notch at 3000-4000 Hz. Word recognition scores were excellent when tested at a much higher than the normal conversational level in the right (96%) and normal conversational level in the left ear (96%).

FIGURE 2: Preoperative magnetic resonance imaging (MRI)
A-C: Axial T2-weighted (A) and T1-weighted fast field echo pre-contrast (B) imaging, and axial (C) and coronal (D) T1 post-contrast imaging demonstrating a T2 and T1 isointense lesion (blue circle) without contrast enhancement located anteriorly in the internal auditory canal. No evidence of significant bony remodeling or expansion of the IAC.

Surgical intervention
The patient underwent an elective right retrosigmoid craniectomy to resect his IAC mass. Intraoperative neuromonitoring with somatosensory evoked potentials was employed along with electromyography of CN VII. Auditory brainstem responses were monitored throughout the procedure. The CN VII/VIII complex was isolated before drilling the porus acusticus. Direct nerve stimulation demonstrated intact CN VII/VIII with evidence of the tumor located anterosuperiorly to the facial and cochlear nerve ( Figure 3). The tumor was firm and appeared grossly similar to its surrounding tissues. Using careful microdissection, the tumor was able to be removed en bloc. Due to anatomic constraints, the lateral aspect of the IAC was not able to be thoroughly examined. Still, the size and appearance of the specimen were consistent with a gross total resection. Dura and superficial tissues were closed in standard fashion. There were no intraoperative complications, and the patient was extubated without issue.

FIGURE 3: Intraoperative microscope photo (annotated inset)
A metal retractor lies medially over the cerebellum at the image's inferior aspect. At the same time, the porus acusticus has been drilled to expose the contents of the internal auditory canal (IAC, seen at the superior aspect of the image). Viewed posteriorly through a retrosigmoid approach, the dura lining the IAC (yellow asterisk) has been sectioned and reflected superiorly and inferiorly to expose the seventh and eighth nerve complex (green shading). A portion of the tumor (blue oval) is located ventrally and superiorly in the IAC.

Histology
Histopathological evaluation revealed a glioneural hamartoma (or heterotopia) ( Figure 4). Histologic sections demonstrated a portion of glioneuronal tissue consistent with hamartomatous brain tissue. Small fragments of nerve were included. Neurofilament stain highlighted large neuronal cell bodies, some in clusters and intervening axons.

Postoperative course
The patient tolerated the procedure well despite mild postoperative nausea and persistent dizziness. He had normal facial function (House-Brackmann 1). He was discharged home on postoperative day four with recommendations for home therapy services. Postoperative imaging was consistent with gross total resection of mass without recurrence. Audiology evaluation demonstrated slight worsening in hearing loss on the right side, amenable to hearing aid placement. He was last seen in the clinic 2 years after surgery with normal facial nerve function and no evidence of tumor recurrence.

Author
Although rare, they may present with symptoms similar to their more common counterparts of auditory and vestibular dysfunction. This was how the majority of patients in the review of the literature, as well as in the present case, had presented. Unlike the neoplasms in this location, hamartomas are generally isodense with brain parenchyma. Interestingly, in our review of the literature, a slight majority of cases (n = 5/8) reported enhancement of the lesion [3,7,8], and 1 case reported minimal enhancement [9]. In the present case, no enhancement of the IAC lesion was noted. No cases, including ours, reported systemic involvement, and all lesions appeared in isolation.
Management of these lesions should be tailored to each patient. Asymptomatic or incidentally identified small lesions may be monitored, given the benign nature of glioneural hamartomas. Conversely, atypical or symptomatic lesions may warrant surgical resection to diagnose and preserve CN function. The surgical approach should be up to the discretion and comfort level of the surgeons as well as dependent on the patient's anatomy. In all cases with reported post-surgical outcomes, including ours, facial nerve and auditory nerve function were preserved [4,[6][7][8], even though one case reported a transient postoperative facial nerve palsy that improved after 1 year [3]. No cases in the literature (n = 7/7) reported lesion recurrence on follow-up, as expected with this diagnosis [3][4][5][6][7][8][9].

Conclusions
Glioneural hamartomas are rare non-neoplastic lesions found in the IAC, causing symptoms related to mass effect on the CN VII/VIII complex, but can be safely managed with preservation of CN function and low risk of recurrence. These cases are important to note as they are not common lesions but can cause significant CN deficits secondary to their location. Additionally, they may be easier to recognize once they are already symptomatic, given their slow-growing nature and the lack of consistent enhancement on imaging.

Additional Information Disclosures
Human subjects: Consent was obtained or waived by all participants in this study. Conflicts of interest: In compliance with the ICMJE uniform disclosure form, all authors declare the following: Payment/services info: All authors have declared that no financial support was received from any organization for the submitted work. Financial relationships: All authors have declared that they have no financial relationships at present or within the previous three years with any organizations that might have an interest in the submitted work. Other relationships: All authors have declared that there are no other relationships or activities that could appear to have influenced the submitted work.