Middle Fossa Encephaloceles Treated via the Transmastoid Approach

Introduction: Middle fossa, or less commonly, posterior fossa, encephaloceles are rare lesions resulting from herniation of the inferior temporal lobe through bony defects in the tegmen tympani or mastoideum. Clinical manifestations include meningitis, epilepsy, cerebrospinal fluid(CSF) leak, and conductive hearing loss. Etiology is variable, with known causes including trauma, iatrogenic injury, long-standing infection, and idiopathic intracranial hypertension(IIH). A significant number of cases are idiopathic. Surgical goals include obliteration of the intracranial-extracranial communication, resection of nonfunctioning parenchyma, and dural and bony dehiscence repair. The middle fossa craniotomy (MCF), transmastoid (TM), and combined approaches have been described. The minimally invasive TM route provides excellent exposure of the pathology and allows for ample working room to repair the defect. The approach avoids the morbidity of craniotomy and temporal lobe retraction, and allows direct evaluation of middle ear structures. Skull base reconstruction can be facilitated by TM placement of an autologous mastoid cortical bone graft. We describe short-term follow-up in patients treated via TM repair at our facility.

Methods: We retrospectively reviewed patients with symptomatic encephaloceles treated via the TM approach. All procedures were performed by our multidisciplinary neurosurgery and neurotology team. In all cases, a lumbar drain was placed for temporary CSF diversion. Otology harvested graft materials and performed the TM exposure. The neurosurgeon then performed the resection of the encephalocele and dural defect and bony dehiscence repair. Reconstruction was accomplished with dural substitute and fascia, hydroxyapatite cement, and either a mastoid cortical bone plate or a porous polyethylene implant.

Results: A total of eight encephaloceles in six patients were treated (Table 1). Defect etiologies included spontaneous(50%), iatrogenic(12.5%), and secondary to chronic infection(25%) or osteoradionecrosis(12.5%). One case was iatrogenic following prior tympanomastoidectomy at an outside facility. One patient presented with bacterial meningitis and all with conductive hearing loss. Defects were most often within the tegmen mastoideum (62.5%). On short-term follow up (average 2.25 months) no patients experienced postoperative CSF leak and all experienced preservation of hearing and facial nerve function as well as significant symptom improvement.

Conclusions: Middle fossa encephaloceles have multiple etiologies and can present with a wide range of clinical manifestations. Surgical repair is required to eliminate the communication between the intracranial and extracranial compartments. The TM approach effectively exposes the pathology and allows for a durable repair while avoiding the morbidity of a MCF craniotomy. Our short-term results support the more widespread use of this approach for the treatment of these lesions.