There is not a well-recognized surgical indication for MFAC. Symptomatic MFACs are the absolute indication. Most of MFACs are incidentally identified and the patients usually have no obvious symptom or clinical sign, even the cysts are quite large.  It has been shown that AC larger than 5 cm was associated with cyst rupture or hemorrhage. For these MFACs, some researchers believe that they should be operated on for their mass effect, While others argue that surgery may hurt rather than help asymptomatic patients in their cognitive function, especially for young children. Recently, more evidences have shown that MFACs have a negative effect on general cognitive ability and that this impairment can be improved by surgery.[34, 19, 1] But the remarkable complications reported in recent years’ studies hinder the pursuit of these functional improvements. The surgical techniques of MFAC fenestration have been demonstrated in numerous studies. Many postoperative short-term complications are directly attributed to inappropriate manipulations during surgery. Based on the experience in this relatively large series, we share some techniques which, in our opinion, are important in preventing these complications.
SH is one of the most common complications following AC fenestration. As the subdural space was opened during surgery, a certain amount of SH showed in the postoperative head CT is reasonable. Many researchers did not recognize this as a complication. As reported, it occurred in 4–10% of patients who underwent MFAC marsupialization, and nearly 60% of patients require secondary surgery.[38, 24] In this study, SH was regarded as a complication only if it required re-operation.
It was proposed that the wide excision of the cyst’s membrane and increased production of fluid from the cyst wall contributing to the complication. We agree with the theory that there could be increased production of fluid from the cyst wall after surgery. There are evidences that the component of fluid inside the ACs are different from CSF. The fluid transport across the cyst wall may already reach a constant level. Once abrupted by trauma or surgery, fluid production from the cyst wall could change in the imbalance condition. ACs having rich microvilli in the luminal epithelium may be more likely to develop SH after surgery.
To avoid this complication, marsupialization is now considered obsolete. But still, we find high re-operative SH rates in some recently published fenestration studies. We think there are two key points in preventing postoperative SH: 1) the larger the fenestration size the better: a small fenestration can create the turbulent CSF flow. It cannot effectively drainage the cystic fluid. Furthermore, a significant narrow fenestration will act as a slit-valve, aggravating the accumulation of extra-axial CSF; 2. The smaller the opening of the outer membrane the better: If there is a larger surgical opening of the outer wall than the fenestration into the cisterns, the fluid will readily accumulate in the subdural space because there is lower flow resistance. Our experience is that one should carefully dissect the dura before entering the cyst. This can not only keep the entrance as small as possible, but also secure the vessels on the cyst (Fig. 4). We advise keeping the other walls of the cyst (besides the entrance and fenestration site) intact during manipulation.
Another important non-surgical risk factor for SH is low age. Children less than 2 years old are more vulnerable to SH because of immature CSF drainage pathways. Choi et.al. showed that 50% of infants who underwent MFAC fenestration required reoperation because of SH. Similarly, Dong et.al. reported a 75% reoperation rate in children < 2 years old. The logistic analysis in the present study also indicated that a lower age could be a risk factor for developing SH. As a result, we agree that infants and young children should not be subjected to surgery unless surgery is essential.
Cranial nerve and vessel protection
The direct reason for immediate postoperative cranial nerve palsy and hemorrhage is accidental injury by the surgeon. We have emphasized the necessity for a large fenestration above. However, a larger fenestration means there are more risks we need to bear. Here we describe the details of fenestration in three anatomic spaces.
The most accessible space for fenestration is the tentorial-oculomotor space. There is sparse vascularity and the space is usually large (Fig. 3A). The important structure is the underlying basilar artery and its branches. Using microscopy, one can elevate the membrane with suction or micro-forceps to keep it away from the underlying vessel, and open the membrane with micro-scissors. The risk of causing cranial palsy in this space is low because the surgeon usually does not need to retract the oculomotor nerve for more room. For Galassi II MFAC, the temporal lobe may block the surgical view for this space. One can slightly push the temporal lobe away using a small retractor. Do not cut the tentorial edge, as it will not only give rise to annoying hemorrhage, but also destroy the integrity of the cyst.
The second accessible space is the oculomotor-ICA space. The structures worth noticed are the branches of the communicating segment of ICA, namely the posterior communicating artery and anterior choroidal artery. Due to the longstanding compression of the cyst, these arteries are usually elongated and variant. They can be very close to the oculomotor nerve. Sometimes this space may be narrow. As the arteries are unmovable, the oculomotor nerve is the only structure that can be manipulated. This is the situation one could easily paralyze the nerve by accidentally poking it. To push the nerve posteriorly, we like to use suction rather than micro-forceps, because the water accumulated in this area frequently blurs the view. One should ensure that the suction has been turned down. Micro-scissors should only cut parallelly to the ICA and oculomotor nerve.
The ICA-optic space is the most challenging one for fenestration. As the ICA was pushed anteriorly and medially by the cyst, this space is usually very narrow. Although one can access the inner membrane of the cyst, the Liliequist membrane underneath is hard to reach. We suggest that one should only open this space if at least 5 mm fenestration can be done. As we have discussed above, making an incomplete fenestration (a one-way valve) is not beneficial or even harmful for the patients.
A total of 70.6% (12/17) of postoperative hemorrhage/hematomas are subdural hematomas. Chronic or traumatic subdural hematomas were excluded from the systemic review. Acute subdural hematoma following MFAC fenestration could result from the bleeding of the cyst wall. Subdural hemorrhage is not unusual during the manipulation of the cyst’s wall. It particularly occurs in between the inner wall of the cyst and the tentorium cerebelli where the vasculatures are rich. In this situation, one should avoid using coagulation inside the cyst, as it increases the risk of neurovascular injury. Irrigation will be more appropriated. Significantly, do not use cold water irrigation after fenestration. It could contract basilar or carotid arteries, which induce rapid hemodynamic changes (e.g., tachycardia, hypertension) in pediatric patients.
Infection and CSF leak
The reported PCNSI rate in MFAC surgery is comparable to the general neurosurgical procedure. CSF leak is one of the risk factors for PCNSI. The simple way to prevent both complications is water-tight closure of the dura. As we cut the dura circularly and perform dura tenting around the opening, dural substitutes are demanded. We do not use the native dura because it tends to shrink after coagulation or dry in the air. In this situation, the native dura is of high tension and is easy to be torn during suturing. CSF may leak from the suture’s pinhole in the dura as well. As such, we are fond of using a large, loose temporal fascia for duraplasty (Fig. 2C). One can also use fibrin glue to prevent leakage from the pinhole. We did not use those supporting materials simply for cost-saving. To evacuate the air, filling the cranial cavity with warm water is required before the complete closure.
Unlike the endoscopic technique, we use a larger incision and bone flap, because we need to harvest temporal fascia and perform water-tight duraplasty. Usually, we only spend about 20 minutes on MFAC fenestration. The most time-consuming part is the closure of the dura and skin. One can use the running suture technique to save time. The surgery duration of our series is comparable to other studies. No prophylactic antibiotic was applied postoperatively. By using this technique, we have one PCNSI (2.6%) and no CSF leak in our series.
The microscopic and endoscopic technique
Traditionally, Fenestration was performed under the microscope. The first endoscopic AC fenestration was performed in the 1990s. Although the technique was primarily used in suprasellar AC, it had been rapidly spread in all kinds of AC. While endoscopic fenestration in suprasellar AC has stood the test of time, the question of which tool is more suitable in MFAC is still under debated. A meta-analysis published by Chen et.al showed that “endoscopic fenestration may be the best initial procedure in MFAC”. But we found that some data in this meta-analysis was not consistent with the result we found (for example, the complication rate in Levy et. al  study was over 10%, while Chen et.al recorded it as 0%). Therefore, we performed another systemic review for microscopic and endoscopic fenestration up to date.
In our review, we found no significant difference in complication rate between the microscopic and endoscopic techniques. Interestingly, it seems that institutes using both two techniques bear a higher complication rate (Supplemental table 2). Hitherto, there were two comparative studies for these two techniques, which showed that the complication rates were lower in the microscopic group.[4, 39]
We are fond of microscopic technique because: 1) Two-hands manipulation under the microscope can deal with the narrow gaps during fenestration; 2) In endoscopic technique, the illumination is non-uniform and highly directional. This makes it possibly overlook some small bleedings in the surgical field, especially in the extremely large cyst. Amelot et.al. had reported that the endoscopic group has more risk of developing SH.
There are several limitations in our study: 1) we only investigated the short-term complications because the follow-up period was short; 2) we did not study patients’ clinical outcomes (changes in the size of MFAC, symptoms and signs); 3) the sample size was relatively small.
This study focuses on introducing surgical techniques that can decrease the complication rate. To further reinforce our result, we are conducting a comparative study for MFAC fenestration in the next few years, with a longer follow-up period and larger series.