Bilateral idiopathic temporal bone meningoencephaloceles - An unusual presentation.

Meningoencephaloceles (MECs) occur due to herniation of brain tissue through a bony defect in the skull base. They can be spontaneous or acquired. These are secondary to trauma, infection or neoplasia. Adult-onset spontaneous or idiopathic MECs are rare. Temporal bone MECs can present with watery discharge from the ear, conductive hearing loss or symptoms of meningitis like headache, fever, vomiting or seizures. These symptoms mimic chronic suppurative or serous otitis media. Computed tomography scan and magnetic resonance imaging differentiate between them. Awareness of this disease entity is necessary for early detection to avoid complications. We report a case of bilateral idiopathic temporal bone MECs with a rare presentation of autophony as the chief complaint. The differentiating features on computed tomography scan and magnetic resonance imaging and the surgical management are discussed.

Introduction

Meningoencephaloceles (MECs) are herniations of the brain tissue covered by meninges through openings of the skull base.[1] Awareness of this etiology is important as symptomatology and clinical findings closely mimic those of otitis media. If left untreated, it can cause meningitis. Radiology plays a pivotal role in differentiating pathologies of the temporal bone. An unusual presentation of bilateral idiopathic lateral temporal bone MECs with autophony is discussed with special emphasis on radiology and management.

Case Details

A 71-year-old woman presented with left ear autophony of 5 months' duration. It had caused considerable anxiety that warranted a psychiatric consultation. She also complained of tinnitus. She had bilateral hearing loss since 2 years and was using hearing aids. There was no history of earache, ear discharge, aural fullness, or giddiness. She denied a history of headache, fever, vomiting, head trauma, or previous ear or brain surgery.

Question 1: What is autophony and what causes it?

Answer 1: Autophony is an unusually loud or disturbing sound of the patient's voice.[2] Any condition that creates a resonant cavity, results in a sensation of enhanced perception of self-vocalization (speech hyper-resonance disorder).[3] Common conditions causing conductive hearing loss such as occlusion of the external auditory canal with wax and middle ear diseases such as serous and suppurative otitis media can cause autophony. Patulous Eustachian tube and inner ear conditions with a “third” window such as superior semi-circular canal dehiscence, wide vestibular or cochlear aqueduct, carotid artery–cochlear dehiscence, and cochlear–facial nerve dehiscence are other causes.[34]

Case details (continued):

An otolaryngology consultation was done elsewhere. The audiogram showed a right moderate-to-severe and a left moderate-to-profound mixed hearing loss with a “B”' type of curve on impedance audiometry suggestive of middle ear fluid. They had advised magnetic resonance imaging (MRI) of the brain that reported normal brain parenchyma with bilateral mastoiditis and bilateral suspicious cholesteatomas. They diagnosed her as bilateral serous otitis media (SOM), treated her medically, and subsequently advised myringotomy with grommet insertion.

Question 2: What are the causes of fluid in the middle ear behind an intact tympanic membrane?

Answer 2: The common causes are SOM, acute suppurative otitis media (ASOM), and hemotympanum following trauma. Less common causes are cerebrospinal fluid (CSF) leak or a perilymph fistula in the oval window.[4]

Case details (continued):

On presentation to our institute, a detailed history was obtained. The external ear was normal. Microscopic examination revealed bilateral intact tympanic membranes, immobile on seigalization with fluid in bilateral middle ear cavities. There was no mastoid tenderness or facial asymmetry. The nose, throat, and neck were normal. On reviewing the previous brain MRI, we suspected a pathology with fluid and soft tissue in bilateral middle ears and mastoids.

Question 3: What are the differential diagnosis and clinical differentiation of pathologies causing opacity in the middle ear cleft?

Answer 3: Opacity can be due to inflammatory conditions such as ASOM, chronic suppurative otitis media (CSOM), SOM, cholesterol granuloma, tuberculosis, and neoplasia like paraganglioma, facial nerve tumor, and malignancy. Fluid and soft tissue are seen concomitantly in CSOM with cholesteatoma and rare conditions such as MECs with CSF leaks. Clinically, the temporal bone MECs present with conductive hearing loss, which may be due to CSF or herniated cranial contents filling the middle ear cavity.[1] They can also present with otorrhea and at times with recurrent meningitis or pulsatile tinnitus.[15] Other less common symptoms are facial nerve weakness and seizures.[6] These symptoms mimic suppurative otitis media. However, the otorrhoea is purulent in CSOM but clear and watery in a MEC with CSF leak. Our patient was diagnosed elsewhere as SOM and a bilateral myringotomy with grommet insertion was planned. Patients with MECs are often diagnosed initially as CSOM or SOM and receive a full range of medical treatment for otitis media, including myringotomy and grommet insertion, resulting in disastrous consequences of persistent watery discharge from the ear and meningitis.[6] In fact, Kutz et al.[7] reported that the most common presentation was otorrhea through a patent tympanostomy tube with an incidence of 68%.

We advised a dedicated MRI of the temporal bone. This revealed disease in the epitympanum and mastoid bilaterally, appearing isointense to the brain on T1W and iso-to-hyperintense on T2W images with no restricted diffusion. CSF fluid intensity of the left middle ear and bilateral mastoid air cells was observed. The soft tissue appeared contiguous with the brain parenchyma on high-resolution T2W coronal images, suggesting bilateral MECs. High-resolution computed tomography (HRCT) scan of the temporal bones in the corresponding coronal plane revealed focal bony defects along the posterior tegmen tympani extending into the tegmen mastoideum at the expected site of encephalocele, larger on the left side with opacification of the left tympanic cavity and bilateral mastoid air cells [Figure 1].

Figure 1

High-resolution T2W coronal images reveal non-dependent soft tissue in the epitympanum (solid white arrows), contiguous with adjacent temporal lobes, (a) right side, (b) left side, suggestive of encephaloceles with CSF in the left middle ear and mastoid air cells. HRCT images reveal defects in the tegmen mastoideum (broken white arrows) (c) right side (d) left side corresponding to the site of brain herniation.

Question 4: How do MECs differ radiologically from CSOM with cholesteatoma?

Answer 4: In both conditions, an HRCT study will show defects of the tegmen tympani and tegmen mastoideum along with opacification of the middle ear cleft. However, in CSOM, the mastoid air cells will be sclerosed, whereas, in MECs, they will be well preserved. Since a CT scan cannot differentiate these etiologies, a dedicated MRI study is necessary.[8] MRI differentiates these two conditions by characterizing the nature of the soft tissue. Additionally, cholesteatomas can be accurately diagnosed on MRI by the presence of restricted diffusion on the non-echo planar diffusion-weighted sequences.

Based on the age, history, clinical findings, audiometry, temporal bone MRI, and HRCT, a diagnosis of bilateral idiopathic temporal bone MECs was made.

Question 5: What is the etiopathogenesis and classification of MECs?

Answer 5: The exact mechanism of MEC formation is elusive. It is assumed that in areas of pre-existing bony defects, the chronic effect of normal intracranial pressure and pulsations cause gradual thinning and eventually loss of dural integrity with resulting CSF leaks and MECs.[9] Arachnoid villi are occasionally ectopically located against the base of the cranial fossa surfaces instead of the usual intrasinus locations. Decades of CSF pulsations may lead to cortical erosion and perforation of the dura mater, giving rise to MECs and CSF leaks.[10] The CSF enters the air-filled spaces of the skull base, including the mastoid and middle ear spaces.[7] Increased intracranial pressure may also cause meningoceles in a subset of patients.[11] Spontaneous meningoceles or MECs account for only 8.6% of all MECs.[12] The incidence is more common in females. The most common site for spontaneous MECs is the tegmen tympani.[1] They can also occur in the tegmen mastoideum. The classification is described in Table 1.

Table 1

Classification of meningoencephaloceles (MECs)

Based on the location in the skull base
Anterior cranial fossa (frontal lobe MECs)	Posterior wall of the frontal sinusCribriform plate, fovea ethmoidalis
Middle cranial fossa (temporal lobe MECs)	Midline: roof, lateral wall of the sphenoid sinusLateral: tegmen tympani or tegmen mastoideum, petrous apex
Posterior cranial fossa	Perisinus or retro sigmoid mastoid air cells
Based on etiopathogenesis
Spontaneous	Congenital (childhood-onset)Idiopathic (adult-onset)
Acquired	Post-traumatic: following blunt or penetrating trauma after head injuryIatrogenic: post-surgical
Neoplastic	Post radiation

Since the symptoms were more severe in the left ear, surgical excision of the MEC with the repair of the skull base defect via a trans mastoid approach was planned. The patient was given a guarded prognosis as regards improvement in autophony and tinnitus.

Question 6: What are the surgical approaches and their selection criteria for the repair of temporal bone MECs?

Answer 6: Once a CSF fistula has been confirmed, surgical repair is recommended to decrease the incidence of meningitis.[7] Temporal bone MECs can be managed by a trans mastoid, transcranial, or combined approach. The choice of surgical approach depends on the size of the defect and volume of the herniated brain. Small MECs can be repaired by a trans mastoid approach and the defect can be closed in layers.[13] In our case, the MEC measured 7.1 mm anteroposteriorly and 3.8 mm transversely, making it amenable to a trans mastoid repair. This approach has added advantages of avoiding a craniotomy, reducing morbidity, and hospital stay, and thus hastening the recovery.

The mastoid cortex was exposed through a post aural incision. A temporalis fascia graft was harvested. Following a cortical mastoidectomy, a lobulated yellowish mass covered by a thin membrane, probably the arachnoid, was seen occupying the mastoid cavity, prolapsing into it through a tegmen defect [Figure 2]. Active CSF leak was observed. The defect was exposed along all its margins. A tissue biopsy was sent for histopathology. The mass was reduced with bipolar cautery and made flush with the tegmen bone so that subsequent repair material would stay in contact with the bone making the repair robust. The defect was closed by an overlay graft of the tragal cartilage. A layer of dural regenerative matrix followed by temporalis fascia was placed over it [Figure 3]. Fibrin glue was applied followed by gel foam. The wound was closed in layers. The postoperative period was uneventful with no soakage of the mastoid dressing. The histopathology report was suggestive of degenerative glial tissue. Her postoperative HRCT scan done after 6 months showed an intact repair with the restoration of middle ear aeration [Figure 4]. Her complaints of autophony were resolved.

Figure 2

(a) Intraoperative image showing cortical mastoidectomy with a yellowish soft tissue mass of the meningoencephalocele covered with arachnoid (black arrow). (b) Following cauterization and excision (white asterisk).

Figure 3

(a) Bony defect in the tegmen mastoideum covered by tragal cartilage (solid black arrow) and (b) subsequently by temporalis fascia (broken black arrow).

Figure 4

Postoperative high-resolution coronal CT image reveals the tegmen mastoideum repair in place (solid white arrow) with a well-aerated middle ear cavity (white asterisk).

Conclusion

Autophony was the presenting complaint in our patient with temporal bone MECs. Awareness of this etiology and a high degree of clinical suspicion are needed for an early diagnosis. HRCT scan and dedicated MRI of the temporal bone with/without contrast are complementary and mandatory for the diagnosis and planning of the surgical approach which is determined by the size of the defect. Surgical reduction by a trans mastoid approach and repair of the defect by layered reconstruction is the preferred option for the management of small MECs and must be undertaken to prevent life-threatening complications. Long-term follow-up is recommended to rule out recurrences.

Declaration of patient consent

The authors certify that appropriate patient consent was obtained.

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Conflicts of interest

There are no conflicts of interest.