Simultaneous repair of bilateral temporal bone meningoencephaloceles by combined mastoid-middle cranial fossa approach

Abstract

Meningoencephalocele (MEC) is a rare clinical entity caused by herniation of the dura mater and brain tissue through bony defects in the skull base. We herein report a case of bilateral MECs found after recurrent bacterial meningitis. The patient subsequently underwent simultaneous bilateral resection of the MECs and repair of the lateral skull base using a combined mastoid-middle cranial fossa approach. The bone and dura were reconstructed using free bone grafts and a temporalis fascia flap. Three years after surgery, temporal bone computed tomography showed that the reconstructed tegmen was preserved bilaterally, and there was no recurrence of meningitis or MECs. The appropriate surgical approach should be chosen based on the localization and size of the herniated tissue, and the presence of active infection or other coexisting pathology. Simultaneous bilateral resection using the combined mastoid-middle cranial fossa approach is a promising option for the treatment of bilateral multiple MECs.

Keywords:

• Meningoencephalocele
• otogenic meningitis
• skull base reconstruction

1. Introduction

Meningoencephalocele (MEC) is a rare clinical entity caused by herniation of the dura mater and brain tissue through bony defects in the skull base. MECs can be congenital, idiopathic (spontaneous), or acquired secondary to chronic inflammation, including that caused by otitis media or sinusitis, surgery, or head trauma [1]. Spontaneous MECs in the temporal bone may present with hearing loss, headache, otorrhea, and recurrent acute otitis media; in rare cases, they can present with otogenic meningitis or even a cerebral abscess [2]. To the best of our knowledge, no reports have described bilateral simultaneous surgery for bilateral temporal MECs [3–6]. We herein report a case of bilateral spontaneous MECs in the middle cranial fossa identified by recurrent bacterial meningitis and treated by simultaneous bilateral resection of the MECs and skull base reconstruction. We took advantage of the combined mastoid-middle cranial fossa approach because of the presence of multiple dehiscences in the bilateral middle cranial fossae [7]. After simultaneous bilateral resection of the MECs, we repaired the defects in the dura mater and bony wall of the middle cranial fossa using temporal fascia flaps and free bone grafts.

2. Case

A 58-year-old man had been treated twice for bacterial meningitis at 5 years and 1 year, respectively, before his first visit to our department. The patient had no disease that could affect bone metabolism or osteogenesis, and he had no history of a congenital abnormality in the craniofacial region, head trauma, or surgery. Examination revealed retraction in the posterior inferior quadrant of the left tympanic membrane, indicating recurrent suppurative otitis media and fluid collection in the left middle ear, whereas the right tympanic membrane was intact (Figure 1(A, B)). Pure-tone audiometry indicated that the left ear had a normal hearing level, while the right ear had mild conductive hearing loss with an air-bone gap of 10 dB (Figure 1(C)). A computed tomography (CT) scan showed multiple dehiscences of the bilateral middle cranial fossae and soft tissue density in the bilateral middle ear cavities (Figure 2(A–D)), which led to consultation at a tertiary center for further treatment. On magnetic resonance imaging (MRI) examination, T2-weighted imaging (T2WI) revealed a low-signal area that continued from the brain parenchyma and protruded into the middle ear, and it was surrounded by a high-signal area identical to cerebrospinal fluid (CSF) (Figure 2(E–H)). Diffusion-weighted imaging demonstrated low intensity in the corresponding area, which suggested the absence of cholesteatoma (data not shown). Based on these findings, we concluded that the repeated otogenic meningitis had been caused by bilateral MECs in the middle cranial fossa. We thus planned to perform prophylactic surgery for the bilateral MECs to prevent further meningitis.

Figure 1. Preoperative findings on examination of tympanic membranes and pure-tone audiometry. (A) The right tympanic membrane appeared to be almost intact preoperatively. (B) Preoperative examination of the left tympanic membrane demonstrated retraction in the posterior superior quadrant and fluid collection in the tympanic cavity. (C) Preoperative pure-tone audiometry showed mild conductive hearing loss in the left ear.

Figure 2. Preoperative CT and MRI revealed bilateral multiple meningoencephaloceles in the middle cranial fossa. (A,B) Preoperative CT of coronal sections showed lesions with soft tissue density (arrows) that were identified both in the mastoid and attic of the right ear. (C,D) As seen in the right ear, lesions with soft tissue density (arrows) were observed in the antrum and attic of the left ear. (E–H) MRI of coronal sections demonstrated protrusion of brain tissue continuing from the brain parenchyma and surrounded by fluid collection (arrows), suggesting encephaloceles containing cerebrospinal fluid. CT: computed tomography; MRI: magnetic resonance imaging.

An inverted U-shaped skin flap was designed (Figure 3(A)), and the temporalis fascia was elevated from the temporalis muscle to create a fascial flap. The temporalis muscle with periosteum was reflected anteriorly to widely open the surface of the squamous portion of the temporal bone. When the dura mater of the middle cranial fossa was elevated following the craniotomy, dehiscence of the temporal bone was visualized. Both areas that formed two independent MECs were separated from the dura mater, and the lesions were pushed down to the middle ear side (Figure 3(B)). Following a cortical mastoidectomy performed by an intact canal wall technique, we found MECs measuring up to approximately 8 mm in the mastoid cavity (Figure 3(C)) and 6 mm in the attic. The MECs were carefully dissected from the adhesion to the malleus. The defects in the dura mater were covered with a temporal fascia flap laid in the epidural space of the middle cranial fossa in addition to an absorbable adhesive material composed of polyglycolic acid (Durawave, Gunze Co., Ltd., Osaka, Japan) to prevent CSF leakage. The bone defects were rigidly repaired using free cortical bone grafts harvested from the temporal bone (Figure 3(D)). Absence of CSF leakage was double-checked by the Valsalva maneuver. A similar surgical procedure was performed on the left side, and two independent MECs in the attic and antrum, measuring 7 and 1 mm, respectively, were identified and removed. The dura mater and bony wall in the middle cranial fossa were then reconstructed (Figure 3(E)). Figure 3(F) shows a schematic that demonstrates how the skull base and dural defects were reconstructed. No intraoperative complications were encountered during either the middle cranial fossa or transmastoid approach. The operation time for the bilateral surgical procedure was 10 h 20 min.

Figure 3. Intraoperative findings and schematic of operative procedures. (A) Schematic drawing of skin incision. (B,C) Bone defects and meningoencephaloceles were identified in the middle cranial fossa (arrow in B) and in the mastoid cavity of the right ear (arrows in C). (D) A free bone graft was placed in the bone defect of the middle cranial fossa. E. A meningoencephalocele was also identified in the mastoid cavity of the left ear. (F) Schematic drawing of reconstruction with an extradural flap of the temporalis fascia and free bone grafts for repair of the dura mater and bony wall of the middle cranial fossa.

The pathological diagnosis was consistent with MECs containing fibrous connective tissue with calcification as well as the choroid and brain parenchyma. In some areas, infiltration of inflammatory cells, predominantly lymphocytes, was also observed.

Three years after surgery, temporal bone CT showed that the reconstructed tegmen was preserved bilaterally, and there was no recurrence of meningitis or MECs. The aeration of the mastoid cavity was good, and no signs of middle ear effusion were present (Figure 4(A–D)).

Figure 4. Postoperative CT demonstrated no recurrence of the meningoencephaloceles or cerebrospinal fluid leakage. (A,B) Postoperative CT of coronal sections showed good aeration in the attic, tympanic cavity, and mastoid of the right ear 3 years after the operation. (C,D) No protrusion from the cranium to the left middle ear cavity was identified. CT: computed tomography.

3. Discussion

Anatomical skull base defects are not uncommon; approximately 15% to 34% of autopsies of normal individuals reportedly reveal temporal bone defects or dural exposure [2,3]. The most common sites for spontaneous MECs in the temporal bone are the tegmen tympani and its posterior extension, the tegmen mastoideum. Multiple bone wall defects are present in less than 1% of cases, and one study showed that among 262 patients with MECs, only 13 (4.9%) patients had bilateral MECs [1,6]. The exact mechanism of MEC formation remains unclear. and several mechanisms have been postulated for the thinning of the tegmen tympani. The chronic effects of normal intracranial pressure and pulsations, focal inflammation, or the aging process are assumed to cause gradual thinning and eventually loss of dural integrity with resulting CSF leakage and MEC formation [6,8,9]. According to previous reports, spontaneous MECs are uncommon. In particular, a review of 262 MECs showed that 125 (47.7%) were due to otitis media, 61 (23.3%) were congenital or spontaneous, 55 (20.9%) were postoperative, 21 (8.0%) were traumatic, and 1 (0.6%) each was due to a subdural abscess or irradiation [1]. Sanna et al. [10] also reported 122 MECs (133 sides), of which 61 (46%) were iatrogenic, 33 (25%) were spontaneous, 29 (22%) were due to otitis media, and 10 (8%) were traumatic. Interestingly, structural defects in the middle fossa tegmen have been associated with the development of both MECs and superior semicircular canal dehiscence (SSCD) [4]. The prevalence of SSCD is 30 times higher in patients with than without spontaneous CSF leakage, and patients with SSCD also have a high rate of canal defect complications (up to 76%). Two different diseases causing temporal bone skull base defects often overlap, suggesting shared underlying mechanisms between SSCD and MEC [11].

Various imaging modalities are used for the diagnosis of MEC. CT is often used to reveal the dehiscence of the skull base and the protrusion of a MEC to the middle ear space [12]. Although CT requires a relatively shorter time for scanning, it shows less sensitivity in differentiating an occupying lesion in the middle ear from MEC and other lesions, such as fluid collection, granulomatous inflammation, cholesteatoma, cholesterol granuloma, and other middle ear tumors. In contrast, MRI is generally effective for qualitative assessment, identifying brain tissue and MEC-associated fluid collection showing equal intensity to the brain parenchyma and CSF, respectively, in several different sequences. Differential diagnosis between MEC and cholesteatoma often becomes essential because of accompanying bone defects in the middle cranial fossa. Typically, cholesteatoma shows low intensity on T1WI and high intensity on T2WI, whereas cholesterol granuloma shows high intensity on both T1WI and T2WI. Diffusion-weighted imaging also helps in the differential diagnosis, demonstrating restricted diffusion coefficients as hyperintense signals in cases of cholesteatoma [12,13]. Thus, MRI is the main imaging technique for assessing complications of middle ear pathologies leading to CSF fistulas, meningitis, or intracranial abscess formation.

The surgical approaches to MECs are chosen according to the size, location, and number of skull base defects and MECs. The transmastoid approach is generally used for a single, small dehiscence localized in the tegmen mastoideum or tegmen antri. If the bone defects or MECs are larger and/or multiple, as in the present case, the middle cranial fossa approach is preferable because it facilitates easier repair of bone or dural defects [14]. About half of patients with MECs have multiple lesions, limiting the indications for the transmastoid approach; this sometimes leads to a combination of both the transmastoid and middle fossa approaches [15]. Although there is no clear consensus regarding the indication for the combined approach, one of the advantages of the combined approach is that the lesion associated with otitis media in the middle ear can be simultaneously removed together with the MECs.

There is ongoing debate regarding the treatment plans for bilateral MECs. Although bilateral simultaneous surgery requires a longer operation time, it can reduce the number of hospitalizations, thus decreasing the socioeconomic burden on the patient. One of the possible disadvantages of simultaneous bilateral surgery for bilateral MECs is the risk of complications such as meningitis or CSF leakage; therefore, reconstruction of the dura mater and/or bone in the lateral skull base should be secured. The materials often used for the repair of defects in the bone and dura are autologous grafts such as temporalis fascia, muscle, cartilage, and bone grafts; use of artificial materials such as hydroxyapatite and titanium mesh can also be considered. Different materials can be used to repair the bone and dural layer. Simple repair with only a single layer of temporalis fascia is associated with a high rate of recurrence of MECs or CSF leakage. Better results have been described with use of multilayer repair using a temporalis fascia in association with a muscle flap, cartilage graft, or bone graft [10]. Use of a multilayer reconstruction technique can also reportedly achieve a high closure rate comparable to that in previously published reports [12]. In the present case, we successfully treated and reconstructed the dural and bone defects after resection of MECs using both autologous materials, including bone grafts and a temporalis fascia flap, in addition to artificial materials to seal the CSF leakage.

4. Conclusions

We have herein reported a case of bilateral MECs found after recurrent bacterial meningitis and treated by simultaneous bilateral resection of the MECs and reconstruction of the lateral skull base utilizing a combined transmastoid and middle cranial fossa approach. The bone and dura were repaired using free bone grafts and a temporalis fascia flap. The appropriate surgical approach should be chosen based on the localization and size of the herniated lesions, the preoperative auditory function, and the presence of active infection or other coexisting pathology. Simultaneous bilateral resection using the combined approach is a promising option for the treatment of bilateral multiple MECs.

Ethical approval

The patients have provided written informed consent forms to the publication of clinical information about them, and the identities of the patients have been protected. This study was conducted in accordance with the ethical standards in the Declaration of Helsinki of 1975 and its amendments or comparable ethical standards.

Informed consent

Authors confirm that consent was obtained from the patients for this study.

Acknowledgment

The authors thank Angela Morben, DVM, ELS, from Edanz (https://jp.edanz.com/ac), for editing a draft of this manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

Authors declare no financial supports.