7.5\ Middle Cranial Fossa

Reconstruction

7.5.1\ Discussion

Reconstruction of the middle cranial fossa can be performed using titanium mesh, bone grafts, hydroxyapatite cement, free flaps (most commonly fat or myocutaneous), temporalis muscle or fascia

c

grafts, and pericranial flaps, often in combination (Figs. 7.38, 7.39, and 7.40). Titanium mesh reconstruction is particularly useful for reproducing the natural contours of the middle cranial fossa thereby providing good cosmetic results. The incidence of complications secondary to these reconstruction techniques is generally low, but includes instability of the repair, encephalomalacia, cerebrospinal fluid leaks, infection, and lesion recurrence.

Fig. 7.38  Middle cranial fossa reconstruction with titanium mesh and bone graft. The patient underwent middle cranial fossa reconstruction with mesh and bone graft for resection of TMJ pseudogout. Preoperative coronal post-­ contrast T1-weighted MRI (a) shows a large mass (*) in

the left temporomandibular joint that erodes into the middle cranial fossa. Postoperative coronal (b, c) CT images demonstrate interval resection of the tophus. There is reconstruction of the middle fossa floor with a titanium plate and bone graft

Fig. 7.39  Middle cranial fossa reconstruction with myocutaneous flap. The patient has a history of recurrent glioblastoma involving the left middle cranial fossa. Reconstruction was performed using a rectus abdominis

myocutaneous flap. Axial CT images in the soft tissue (a) and bone (b) windows demonstrate resection of a portion of the left middle cranial fossa skull base and application of a myocutaneous flap

Fig. 7.40  Middle cranial fossa reconstruction with fat and bone grafts. Coronal CT (a) image and coronal T1-weighted MRI (b) show fat graft (arrows) as well as

bone graft positioned in right the middle cranial fossa for treatment of a postoperative cerebrospinal fluid leak

7.6\ Surgical Approaches

for Vestibular Schwannoma

Resection

7.6.1\ Discussion

Several surgical approaches can be used to resect vestibular schwannomas, including the middle cranial fossa approach, the translabyrinthine approach, and the retrosigmoid approach.

The middle cranial fossa approach for cerebellopontine angle and perimesencephalic tumors consists of temporal craniotomy, extradural temporal lobe retraction, dissection of the petrous ridge dura, and drilling of the roof of the internal auditory canal, which is covered with a fascia or fat graft after the tumor is resected. The main advantages of this approach include a higher likelihood of hearing preservation and access to the fundus of the internal auditory canal for small intracanalicular tumors. Overall, this approach carries a higher risk for facial nerve injury, and retraction injury resulting in temporal lobe gliosis is found in most patients on follow-up imaging (Fig. 7.41). This approach also has limited applicability for the resection of large tumors but can be combined with the retrosigmoid approach in selected cases.

The translabyrinthine approach provides maximal exposure to the cerebellopontine angle, although it sacrifices hearing capacity. This approach is used to resect intralabyrinthine, intracochlear, and larger cerebellopontine angle tumors, as well as vestibular schwannomas in cases where hearing is poor or has been lost. The translabyrinthine approach entails complete mastoidectomy and labyrinthectomy with fat graft packing. In addition, the sigmoid sinus, tegmen tympani, and portions of the internal auditory canal can be skeletonized. The ossicles are sometimes removed, and packing material is left in the middle ear cavity in order to minimize cerebrospinal fluid leakage. Fat grafts are typically used to fill the mastoidectomy bowl, middle ear, and sometimes the internal auditory canal (Fig. 7.42). On post-contrast MRI sequences, enhancement along the periphery of the fat graft is typical and likely attributable to the presence of granulation

tissue. This enhancement usually lasts up to 1–2 years and tends to be linear and diffuse, but it can also have a “whorled” appearance. Often, the fat graft shrinks over time, losing its triangular configuration and allowing air or fluid to enter the mastoid bowl.

The retrosigmoid approach for cerebellopontine angle tumors consists of creating a bone flap and performing a dural incision over the ipsilateral cerebellar hemisphere, posterior to the sigmoid sinus and inferior to the transverse sinus. The mastoid air cells are commonly entered, and bone wax is applied along the edges in order to prevent leakage of cerebrospinal fluid. The cerebellar hemisphere is retracted medially, and the medial portion of the posterior internal auditory canal wall is resected, once the intracranial portion of the tumor is resected. However, an internal labyrinthectomy is often necessary in order to access the fundus of the internal auditory canal. Fat graft is also sometimes inserted into the cerebellopontine angle region if air cells are encountered in the wall of the resected medial internal auditory canal. Occasionally, prominence of the cerebrospinal fluid lateral to a flattened cerebellar hemisphere results from retraction and often gradually dissipates over time (Fig. 7.43).

Although there is no consensus for when to obtain baseline postoperative imaging, it is generally recommended that this is performed between 6 months and 1 year. Patients with known subtotal resection, nodular or mass-like enhancement in the internal acoustic canal, or a history of neurofibromatosis type II undergo serial imaging thereafter. Residual tumor is deliberatively left in some cases, particularly in the lateral internal auditory canal, which is difficult to access via a retrosigmoid approach, in order to minimize the risk of facial nerve and vascular injury. Fat-suppressed post-contrast T1-weighted MRI sequences are particularly useful for the evaluation of tumor (Fig. 7.44).

Various surgical complications can be encountered on postoperative imaging. For example, fat grafts can undergo necrosis, which may appear as cystic change within and adjacent to the residual fat graft (Fig. 7.45). Rarely, aseptic lipoid meningitis can result from ­fragmentation and dis-

persal of the fat graft in the subarachnoid space (Fig. 7.46). Other complications may include leakage of cerebrospinal fluid into the mastoid air cells and middle ear (Fig. 7.47), particularly in patients with overpneumatized air cells that are transgressed by the surgical approach, pseudomeningocele from leakage of cerebrospinal fluid into the overlying scalp (Fig. 7.48), her-

niation of the cerebellum into the surgical cavity (Fig. 7.49), endolymphatic sac fenestration with loss of T2 signal (Fig. 7.50), infectious of inflammatory labyrinthitis (Fig. 7.51), labyrinthitis ossificans (Fig. 7.52), wound infection (Fig. 7.53), territorial infarction (Fig. 7.54), and venous sinus thrombosis (Fig. 7.55).

Fig. 7.41  Middle cranial fossa approach. Axial FLAIR (a) and coronal post-contrast T1-weighted (b) MR images demonstrate encephalomalacia and volume loss in the right inferior temporal lobe (arrows) ipsilateral to the

middle cranial fossa approach for cerebellopontine angle schwannoma resection. Sequelae of translabyrinthine resection are also noted on the left side without associated brain parenchymal injury

Fig. 7.42  Translabyrinthine approach with fat graft reconstruction. Axial CT (a) and T1-weighted MRI (b) show obliteration of the internal auditory canal and mastoid bowl with fat graft. The axial T2-weighted MRI (c) shows absence of the right vestibule and semicircular

canals, but the right cochlea remains intact. Granulation tissue enhancement. Axial contrast-enhanced fat-­saturated T1-weighted MRI (d) shows linear enhancement along the periphery of the fat graft (arrow) and along the overlying incision plane, which likely represents granulation tissue

Fig. 7.46  Fat graft aseptic lipoid meningitis. Axial (a, b) T1-weighted MR images demonstrate numerous high T1 signal foci scattered in the subarachnoid spaces including

the suprasellar cistern, which represent fragments of the fat graft used during translabyrinthine resection

Fig. 7.48  Pseudomeningocele. Axial T2-weighted (a) and coronal (b) T1-weighted MR images show the large subgaleal fluid collection (*) that has cerebrospinal fluid

signal characteristics and extends far superiorly within the subgaleal space

Fig. 7.50  Postoperative endolymphatic sac fluid signal loss. Preoperative axial T2-weighted MRI (a) shows a large left vestibular schwannoma with mass effect on the pons and middle cerebellar peduncle, which are otherwise intact. There is normal signal within the bilateral inner ear

structures. Postoperative axial T2-weighted MRI (b) shows interval resection of the mass. There is diminished signal within the left cochlea, labyrinth, and semicircular canals (encircled)

Fig. 7.53  Wound abscess. Axial T2-weighted (a), axial (b) and coronal (c) post-contrast T1-weighted, and ADC map (d) show a rim-enhancing fluid collection with debris

and an area of restricted diffusion in the right translabyrinthine resection site (arrows)