MR Imaging Evolution of Endoscopic Cranial Defect Reconstructions Using Nasoseptal Flaps and Their Distinction from Neoplasm

Endoscopic Skull Base Surgery and Cranial Defect Reconstruction

The endonasal approach provides a direct and less invasive surgical corridor to primarily midline sinonasal, skull base, and intracranial neoplasms along the ventral cranial base, with equivalent or better gross total resection compared with open surgery.^{1⇓⇓⇓⇓⇓–7} Open skull base surgery, classically in the form of combined craniotomy and a transfacial or endonasal endoscope-assisted approach, is repaired with the vascularized pericranial flap routinely sutured to the edge of the dural defect. Cranial defect closure in EEA has unique challenges, including the inability to suture the reconstructive layers and the difficulty in using a pericranial flap. A variety of biologic glues and temporary supportive nasal bolsters has been used to secure the ESBR layers to the cranial defect, while awaiting granulation and incorporation of the reconstructive tissues into the surgical bed.^8⇓–10 The small dural defects have been successfully closed with nonvascularized free grafts such as seen in pituitary surgery. With the increasing size of cranial defects in EEA for resection of larger tumors and with deeper intra-arachnoid dissections and increased exposure to the cisterns, repair techniques using nonvascularized free grafts have shown unacceptable postoperative CSF leak rates.^7,8 To allow maximal resection with CSF leak rates similar to those in open surgical approaches, ESBR in the EEA routinely necessitates multilayer reconstruction using a vascularized NSF. The NSF is a pedicled vascular flap based on the posterior septal artery, a branch of the sphenopalatine artery.¹¹ This robust vascularized flap has become the mainstay in ESBR, with reported postoperative CSF leak rates in 5%–10% of cases, a range similar to that in open procedures.^7,8

Our ESBRs were performed in a multilayer stepwise fashion, beginning intracranially and advancing out to the sinonasal mucosa, using a combination of autologous fat filling the intracranial surgical bed, subdural inlay collagen matrix underneath the edges of the dural defect and/or inlay-onlay fascia lata, and the final mucosal onlay closure with single or bilateral NSFs (Fig 1). Occasionally, additional free mucosal grafts taken from the nasal septum or turbinates were required to cover the far anterior edge of the cribriform defects, which were not reachable by the pedicled NSF (Table 1).

MR Imaging Follow-Up of Endoscopic Skull Base Reconstruction

The imaging features of ESBR with the characteristic enhancing C shape and vascular pedicle configuration of the NSF and nonenhancing free grafts on immediate postoperative MR imaging have been described previously.^13,14 Kang et al¹³ reported NSFs used to reconstruct defects following resection of pituitary adenomas and found stable signal intensity but variable changes in thickness and enhancement of NSFs on follow-up MRI at 3–7 months.¹³ They described 2 flaps without enhancement in the immediate postoperative period that showed delayed enhancement on follow-up imaging and 8 flaps that enhanced in the immediate perioperative period with persistent enhancement in 7/8 flaps on follow-up imaging. Five of the initially enhancing NSFs became thinner; 2, thicker; and the average flap thickness varied from nonvisualization to 7 mm. They also noted reduction in the thickness of the multilayer reconstruction between the graft and defect. Their cohort of pituitary adenomas did not require adjuvant chemoradiation.

Our long-term follow-up (range, 3–41 months; average, 16.4 months) showed some differences from the published results of Kang et al.¹³ None of our enhancing NSFs lost their immediate postoperative solid enhancement, and all demonstrated mild reduction in thickness. Our average flap thickness of 4.5 mm was comparable. None of our cases, including a similar cohort with a transellar approach without adjuvant treatment, had increases in flap thickness as seen in 2 patients in the study of Kang et al (an increase of 1 and 2 mm in flap thickness). This discrepancy may be explained by sampling error. During the immediate postoperative period, the NSF stood out as the only enhancing tissue among the nonenhancing free grafts and postoperative sinus contents. Therefore, its unique enhancing C-shaped configuration was easily delineated. Delayed enhancement of the free grafts, retraction and reduction in the thickness of the reconstructive layers, and integration of the enhancing NSF into the reconstruction site resulted in its less distinct configuration on follow-up MR imaging (Fig 3). The nonenhancing NSFs on immediate postoperative MR imaging probably compromised the vascular pedicles. Theoretically, they served as free mucosal grafts at the reconstructive site. Therefore, these nonenhancing NSFs behaved in a manner similar to those in the free mucosal grafts on follow-up; both appeared as thin enhancing granulation-mucosalization, radiographically and clinically.

The reduction in thickness of free grafts in our cohort varied. The fascia lata remained thicker than DuraGen, and its noticeable reduction in thickness probably reflects resolution of immediate postoperative swelling with increasing T2-hypointensity (Fig 4). Enhancement on follow-up reflects the incorporation of the fascia lata into the granulation tissue at the cranial defect. In reconstructions using only inlay subdural collagen matrix as the dural graft, the nonvisible to ≤2-mm nonenhancing collagen matrix became indistinguishable from the enhancing granulation at the defect (Fig 5).

In addition, the stabilization of the mature imaging features occurred within 2–6 months following surgery, with little or no further change on subsequent imaging. Seventy-four percent of our follow-up MRIs were obtained within the first year after surgery, because the initial follow-up intervals were generally closer, with a resulting higher number of studies, especially those for oncologic surveillance. Close to equal proportions of the imaging studies were performed in patients after they received adjuvant chemo- and/or radiation treatment (45%) and in patients who did not or have not received adjuvant treatment (55%). Notably, the adjuvant radiation or chemoradiation and the sinonasal inflammatory changes had no appreciable effect on the normal imaging evolution of ESBRs in our oncologic patients (Fig 4).

Imaging studies evaluating postoperative endoscopic pituitary surgery demonstrated the involution of surgical materials and identification of residual-recurrent adenoma by location, characteristic signal intensity, and nodular enhancing patterns that were identical to those of the corresponding preoperative adenoma.^15,16 Our evaluation of parasellar neoplasms following EEAs is concordant with those of MR imaging studies of transellar hypophysectomy. Our study also showed that the full-thickness enhancement of T2-isointense NSFs, which is different from the thin peripheral enhancement of the adjacent T2-hyperintense sinonasal mucosa, may mimic residual or recurrent tumor in patients with sinonasal neoplasms if the radiologist is not familiar with the normal appearance and evolution of these reconstructions (Fig 4). To avoid this pitfall, proper identification of the normal NSF on imaging and evaluation of the reconstructive margins for the presence of nodular tissue with signal characteristics and enhancement similar to that of preoperative neoplasm and different from that of the reconstructive layers are paramount (Fig 5).

Limitations and Implications for Patient Care

Limitations of our study include its retrospective nature and small sample size limited to an adult population with predominantly midline neoplasms. Even though the scanner variability makes our findings applicable to most practices, its effects on our results are uncertain, and that uncertainty may explain some of the minor differences in our results compared with those of Kang et al.¹³ Our reported imaging features of free grafts were limited to collagen matrix dural grafts and the fascia lata, and different materials used at other institutions may appear different. However, these free tissue grafts are most commonly used at many centers.^8⇓–10 The vascularized NSF, the workhorse of ESBR in EEA, is always present irrespective of the type of free grafts and reconstruction techniques.

With the ongoing evolution of surgical navigation, endoscopic instrumentation, and the growing success of EEA and ESBR in achieving gross total resection and reliable closure similar to that in the open approach, EEA is becoming a surgical standard for carefully selected cases in the minimally invasive surgical arena.¹ As these procedures are increasing across the country, neuroradiologists will increasingly face the challenge of postoperative surveillance following EEA and ESBR, which potentially have unique patterns of neoplastic residua and recurrence compared with those of traditional open surgery.