Abstract
Purpose
A spontaneous meningoencephalocele of the temporal bone may present with effusion in the middle ear, a cerebrospinal fluid leak, hearing loss, or rarely otitic meningitis. Repair of spontaneous encephaloceles in the temporal bone has been performed using transmastoid and transcranial middle fossa approaches or a combination of the two with varied results. The authors present a technical paper on the transmastoid extradural intracranial approach for the management of temporal lobe encephaloceles.
Materials/Methods
Case reports and cadaver dissections are used to provide a pictorial essay on the technique. Advantages and disadvantages compared with alternative surgical approaches are discussed.
Results
Traditional transmastoid approaches are less morbid compared with a transcranial repair as they avoid brain retraction. However, in the past, there has been a higher risk of graft failure and hearing loss due to downward graft migration and a potential need for ossicular disarticulation. For the appropriate lesion, the transmastoid extradural intracranial approach lesion offers a stable meningoencephalocele repair without the comorbidity of brain retraction.
Conclusion
The authors describe a transmastoid extradural intracranial technique via case reports and cadaver dissections for the repair of spontaneous meningoencephalocele defects larger than 2 cm. This approach provides more support to the graft compared to the conventional transmastoid repair.
1
Introduction
An encephalocele by definition is when cranial contents extend beyond the confines of the skull . The content of the herniated tissue defines its name, i.e., meningocele versus meningoencephalocele, but for ease of discussion all entities will be referred to as encephaloceles. The incidence of encephalocele (cranial and basal varieties) remains rare with estimations between 1/3,000 and 1/35,000 . While rare, the potential for neurological and infectious sequelae make it an important pathologic entity that often requires surgical intervention.
Encephaloceles are classified into cranial and basal varieties, with cranial encephaloceles being the most common form, and typically involve the occipital and frontonasal regions. Basal encephaloceles feature a dehiscence of the skull base. Temporal lobe encephaloceles are a form of basal encephalocele and are subcategorized into either lateral or midline lesions ( Fig. 1 ). Lateral temporal lobe encephaloceles are more common than midline defects. Lateral temporal lobe encephaloceles may clinically present with clear otorrhea, rhinorrhea, conductive hearing loss, chronic otitis media, middle ear effusion, seizure, meningitis, and intracranial abscess; however, they may also be asymptomatic and incidentally discovered on imaging . Midline temporal lobe encephaloceles, such as a temporosphenoidal encephalocele, can present as a mass in the sphenoid sinus or nasopharynx, with clear rhinorrhea, focal neurologic deficits, or any of the previously mentioned infectious etiologies .
Cerebrospinal fluid (CSF) leakage and otitic meningitis, however, remain the prominent complications that predispose patients with lateral temporal lobe encephaloceles to potentially life-threatening sequelae. CSF leakage occurs when the barriers retaining CSF within the subarachnoid space become disrupted. Neural tissue may prolapse through these defects, although it is often nonfunctional supportive stroma . Meningitis rates associated with CSF leakage vary widely and may differ depending on the etiology. Overall, spontaneous CSF leakage in adults carries a 14.1% risk of meningitis . Meningitis rates have been reported as low as 9% in the setting of postoperative CSF leakage or as high as 93% in a pediatric population with CSF otorrhea . CSF leakage after temporal bone fractures range from 11% to 45%, of which 7% may develop meningitis. Significant risk factors for meningitis after a temporal bone fracture include persistent leakage for longer than 7 days, concurrent infection, and fractures in the pediatric population . Regardless of the specific meningitis rate, a bony or dural defect of the temporal bone increases the risk of meningitis and serious sequelae.
Encephalocele formation through the temporal bone has multiple etiologies broadly categorized as congenital or acquired. Congenital defects are less common than acquired defects and often develop as a result of improper ossification at the petro-squamous junction or a labyrinthine malformation. The petro-squamous junction is typically ossified by 1 year of age . Gacek and colleagues expounded on the congenital defect theory, suggesting that aberrant arachnoid granulations located close to temporal bone dura have only a thin fibrous cartilage that gets eroded with CSF pressure from the subarachnoid space . Other etiologies of congenital temporal bone CSF leaks include enlargement of the petromastoid canal, cochlear aqueduct, facial canal, or Hyrtl fissure . Superior semicircular canal dehiscence is another disease entity, often with a congenital etiology, that results in a dehiscent tegmen and is commonly associated with a temporal lobe encephaloceles . Idiopathic encephaloceles are frequently reported in patients with obesity, benign intracranial hypertension, and empty sella turcica. While the etiology of encephalocele is not truly known for those patients, it has been postulated that chronic dural pulsations cause bone resorption .
Acquired nontraumatic etiologies of temporal bone encephaloceles include spontaneous perforation, chronic middle ear disease with or without cholesteatoma, and neoplasms. Traumatic etiologies are the most common acquired source and include temporal bone trauma, post-radiotherapy dehiscence, and iatrogenic causes . Reported rates of acquired temporal encephaloceles include mastoid surgery (21–46%), chronic middle ear disease (7–22%), trauma (8–13%), and spontaneous perforation (25–80%) . Furthermore, modern lateral skull base surgery, such as for acoustic neuroma removal, has a well-documented CSF leakage rate between 3.8–10.6%, regardless of whether a translabrynthine, retrosigmoid, or infratemporal fossa approach is used .
Current surgical management of temporal encephaloceles with or without CSF otorrhea includes three different approaches: classic transmastoid extracranial, middle fossa extradural, or the newly described transmastoid extradural intracranial (TMEDIC) approach ( Fig. 2 ). No consensus exists for which method is preferred, and all repairs must be tailored to the patient’s specific defect. The classic transmastoid extracranial approach provides a less secure encephalocele repair because the graft attached to the undersurface of the tegmen is prone to detachment, thus re-exposing the temporal bone defect. The middle fossa extradural approach carries with it potential for higher surgical and perioperative morbidity on account of the craniotomy, temporal lobe retraction, need for a lumbar drain, and often a 3–5-day hospital admission. The authors demonstrate a minimally invasive TMEDIC method of temporal encephalocele repair with stepwise images from cadaveric prosections as well as intraoperative photos. Semaan and colleagues first reported this particular transmastoid approach in 2011, and similar approaches have since been described .
2
Surgical technique: cadaver demonstration
A description of this surgical technique was previously published by the senior neurotologists (MTS and CAM) and is further illustrated in this article using formalinized silicon-injected cadaver dissections. A postauricular incision is made elevating skin flaps in the plane of the superficial temporalis fascia ( Fig. 3 ). The pericranium is incised staying parallel to the linea temporalis and then curving inferiorly toward the mastoid tip ( Fig. 4 ). The temporalis fascia and pericranium are elevated off of the mastoid cortex ( Fig. 5 ). A wide mastoidectomy is performed. The tegmen mastoideum and sigmoid sinus are skeletonized. The posterior wall of the external auditory canal is left intact. The antrum, incus, semicircular canals, and facial nerve are further dissected for illustrative purposes and may or may not need to be identified intraoperatively ( Fig. 6 ). While skeletonizing the tegmen mastoideum and sigmoid sinus, care must be taken to anticipate the encephalocele location as suggested by preoperative radiographic imaging. Bone adjacent to the tegmen defect is often quite thin and, if unstable, should be removed with a diamond burr ( Fig. 7 ). If needed to isolate the encephalocele, the dissection can be continued in an anteromedial direction via removal of the zygomatic root and opening of the antrum to expose the tegmen tympani. Opening the facial recess with a #2 diamond burr or disarticulating the incus obtains further exposure of the lateral epitympanum. Descriptions of autologous incus repair or prosthetic ossicular chain reconstruction are beyond the scope of this article.
If the encephalocele has a stalk, it is cauterized with bipolar cautery and then amputated at its base ( Fig. 8 ). If the herniated tissue is sessile, bipolar cautery is used to coagulate the tissue. Tissue that does not regress intracranially is sharply amputated, leaving a retracted encephalocele remnant ( Fig. 9 ).
With the defect well demarcated, the wound and mastoid cavity are thoroughly irrigated with antibiotic solution. The defect size is measured. Through the postauricular incision, conchal cartilage and temporalis fascia are harvested ( Fig. 10 ) and prepared for later use.
Using blunt instrumentation, such as a Freer or #6 Rhoton elevator, the dura is elevated off the skull base circumferentially around the defect, via the defect in the tegmental bone ( Fig. 11 ). This maneuver creates an extradural–intracranial (epidural) pocket in which the shaped conchal cartilage autograft can be placed. If the pocket is formed correctly, the cartilage fits in a locking manner between the dura and bony floor of the middle cranial fossa ( Fig. 12 ). The weight of the overlying temporal lobe adds to the cartilage graft’s stability ( Fig. 13 ). Temporalis fascia is then draped over the tegmental defect on the extracranial side ( Fig. 14 ). The mastoid cavity is filled with tissue sealant after a free muscle graft is placed in the antrum, which limits diffusion of the sealant into middle ear space and/or epitympanum ( Fig. 15 ). Closure is performed in a multilayer fashion followed by application of a mastoid dressing.
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