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.
