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Cisternostomy is neurosurgical procedure which utilises skull base and microvascular techniques to access and open the basal subarachnoid cisterns in severe head trauma.

In this practice, the basal cisterns are opened to atmospheric pressure and cerebrospinal fluid (CSF) is drained from them to cause a “back-shift” of CSF from the brain to cisterns through the Virchow Robin Spaces (VRS), thereby reducing intraparenchymal pressure that occurs in trauma due to “CSF shift edema” The procedure has been found to reduce morbidity and mortality in subdural hematoma (SDH) cases as well.

Cisternostomy also avoids the risks associated with the removal of the bone flap in decompressive craniectomy (DC) or decompressive hemicraniectomy (DHC) and the need for cranioplasty.

The procedure accesses the basal cisterns through the axial and sagittal unlocking of the frontal and temporal lobes by drilling off the sphenoid ridge and dissection of the orbito-meningeal band. The dura is opened at the base and after the opticocarotid cistern is opened and the blood is washed out, the carotico-oculomotor cistern is opened. Then the membrane of Liliequist is also opened through one of the cisterns, leading to the posterior fossa cisterns. A feeding tube is inserted into the prepontine cistern and kept for five days to drain CSF.

The entire procedure takes about ten to twenty minutes to complete post learning curve. Cisternostomy is increasingly being regarded as a more efficient and effective replacement to practices such as decompressive craniotomy. Its potential application is being regarded highly due to its immediate effect in controlling intracranial pressure (ICP) and its role in the improvement of brain oxygenation and metabolism.

Pathophysiology
The basis of this procedure “CSF shift edema”. To understand this, one must understand that the CSF communicates from the cisterns to the interstitial fluid (ISF) via Virchow Robin spaces and this mechanism is important for the cleaning and cooling functions of the brain.

Severe head trauma results in subarachnoid hemorrhage. This results in blood entering the cisterns at a very high pressure and this results in increased pressure within the cisterns.

The increase in pressure within the cisterns provokes a shift of CSF into the brain through the Virchow Robin spaces resulting in “CSF shift edema”. The opening of the cisternal spaces to atmospheric pressure favours the resolution of the CSF shift edema by reversing the gradient of CSF flow. The positive effect of the cisternal drainage in relieving brain swelling is presumed to be due to the reversal of CSF shift edema. The effect is further enhanced if the CSF is drained through a cisternal drain in the postoperative period as well.

The interaction of the CSF through the VRS with the interstitial spaces in the brain plays a vital role in cisternostomy. Research done with fluorescent molecular weights in mice indicated that the ventricular CSF only affects the immediate periventricular region, entering the parenchyma minimally while the subarachnoid CSF supplied via the basal cisterns enter it rapidly through perivascular spaces.

Additionally, these observations have led to the formulation of the cleaning and cooling theory which posits that the CSF has temperature control function as well as waste clearance roles such as the clearance of tau proteins, lactate and free radicals within the brain.

Procedure
The surgical procedure is performed with the patient in the supine position. The head is rotated and extended about 10-15° to the contralateral side, until the malar eminence reaches the uppermost part. A frontotemporal craniotomy is performed. The sphenoid ridge resection starts in the lateral-medial direction. The sphenoid ridge is drilled off with a drill burr until the orbito-meningeal arteries are encountered. The drilling is continued until a flattened bone surface is obtained.

At this point, the skull drilling stops and the dura is carefully detached from the bone.

The superior orbital fissure and the orbito-meningeal band are usually seen at this point (Fig. 3A to be included). The meningo orbital band, which marks the lateral edge of the superior orbital fissure is dissected medially to avoid the risk of oculomotor nerve injury (Fig. 3B). The division of the band releases the dura propria from the inner layer of the cavernous sinus dura and unlocks the brain in the axial plane. After the meningo orbital band is cut, the dura covering the temporal lobe can be separated from the inner layer of the cavernous sinus.

This maneuver allows one to get to the base very close to the cisterns. The basal dura is opened in a linear fashion near the orbital roof. This maneuver allows easy (and sometimes rather difficult) access to the interoptic, optico-carotid and the lateral carotid cisterns that can now be opened for draining. Cerebrospinal outflow immediately provides brain relaxation which enables further reaching the membrane of Liliequist.

The membrane of Liliequist can be approached through the optico-carotid window or the lateral carotid window. The membrane, made up of two layers, can be opened by a sharp dissection. During Liliequist's membrane opening, particular attention is advisable to avoid injury to the posterior communicating artery and the ipsilateral P1 segment along with small perforators. After the membrane is widely opened, the basilar quad consisting of the basilar artery, both P1 segments, the superior cerebellar arteries and the third nerve of both sides is visualized.

A number 8 feeding tube is inserted to drain the CSF from the cisterns. A rapid decrease in brain tension and a regain of pulsatility in the brain matter is observed after this stage is completed.

The dura is closed with just approximation and the drain is left in place for at least five days for continued CSF drainage that helps prevent the cascade of secondary damage, by efficiently regaining the CSF flow. Bipolar coagulation is almost never used and hemostasis is done with the help of oxidized cellulose and continuous irrigation.

As a result of traumatic brain injuries, the cisterns are usually filled with blood. Irrigation provides blood and clots removal, and stops any venous bleeding avoiding the use of the bipolar coagulation. Once the cisterns are opened, a feeding tube, 2mm in diameter, is positioned into the prepontine cistern which helps to irrigate out the blood for up to five days.

Procedural changes can also occur in case of a surface hematoma with significant mass effect. In this case, instead of performing a lobectomy, a minimal cortical incision is made and anesthesiologists are asked to assist with the evacuation of the clot by a Valsalva maneuver.

Uses and Outcomes
Cisternostomy had been found to be useful in cases that demand a need for decreased brain swelling and intracranial pressure. It is currently regarded by many practitioners as being a modern and much better alternative to decompressive hemicraniectomy in traumatic brain injury cases but its application, and the procedure itself, is not traditionally uncommon in cases of aneurysm and hematoma.

The procedure is applicable to the following conditions:

Intraoperative malignant brain edema Edema in intracerebral hemorrhage Ruptured aneurysm Arteriovenous malformation (AVM) Tumors Subarachnoid bleed CSF shift edema

Various studies have observed the following outcomes post cisternostomy:

Normalization of intracranial pressure (ICP) Improved cerebral microdialysis glucose Decrease in lactate/pyruvate ratio Improvement of edema and brainstem compression

Its use in clinical studies have showed a lower mortality rate of 13.8% in comparison with other methods such as decompressive hemicraniectomy. This is also followed by lesser number of days required to be spent in ventilator as well as a better Glasgow Outcome Scale(GOS) score of 3.9 points in comparison to 2.8 following DHC.

Clinical Research and Studies
In a personal series, cisternostomy was tried out in over 1000 cases and was found to decrease intraoperative brain swelling, mortality and morbidity. Although the procedure was initially performed in conjunction with decompressive hemicraniectomy, the latter was completely replaced by cisternostomy alone as the surgeon’s experience increased and better results were obtained exclusively through its practice. After cisternostomy, the brain was observed to be lax, and bone flaps were repositioned without further complications.

In another study done by Dr. Iype Cherian, a total of 285 patients underwent decompressive hemicraniectomy, 272 underwent DHC with cisternostomy and 476 underwent cisternostomy only. Among the cisternostomy group, there were 52 mild head injury cases, 206 moderate head injury cases and 218 severe head injury patients. For severe head injuries, the mortality for cisternostomy was 13.8%, for DHC was 34.8% and for DHC in conjunction with cisternostomy was 26.4%. Mean GOS at 6 weeks was 2.8 for DHC-treated patients, 3.7 for DHC with cisternostomy and 3.9 for cisternostomy alone respectively in severe head injured patients. The mean days on the ventilator for the three groups were 2.4, 3.2 and 6.3 respectively for cisternostomy only, DHC with cisternostomy and DHC only. Cisternostomy was also carried out in the mild and moderate head injury group as a prophylactic measure in patients with acute subdural hematoma associated with mass effect and midline shift.

Another observation was made with a 50-year old man admitted to the emergency unit after a severe TBI and a Glasgow Coma Scale(GCS) of 6. CT scans revealed multiple bilateral brain contusion and a small unilateral acute subdural hematoma. Condition of the patient worsened with increased edema around the parenchymal contusions which led to a predominantly unilateral mass effect. A surge in positive results was observed on conducting cisternostomy. The GCS after extubation was 14/15 and the patient was successfully transferred to neurorehabilitation.

While DHC only brings the ICP to atmospheric pressure, it fails to deal with the intra-brain pressure, which causes severe brain swelling and herniation despite the ICP being reduced. DC results in higher rates of vegetative state, lower severe disability and upper severe disability as shown by the RESCUEicp trial of 2016. Furthermore, the morbidity of these procedures lead to axonal stretching and consequent increase in edema. DC/DHC procedures also fail to address the paravascular systems of the brain that is shown to have more interaction with the parenchyma than its ventricular counterpart. . The DECRA (Decompressive Craniectomy in Patients with Severe Traumatic Brain Injury), the largest randomized trial in diffuse TBI, also failed to show the effectiveness of decompressive craniectomy in adults with traumatic brain injury. Due to these reasons, it has been argued that cisternostomy could replace DC/DHC as a far more effective neurosurgical treatment for severe TBI in addition to other applications.

Limitations
The biggest drawback associated with cisternostomy is the steep learning curve and expertise required to tackle complex microsurgery. The technical difficulties of performing cisternostomy with a tight edematous brain requires a great amount of training to be able to successfully carry it out and is the main reason as to why DHC, a technique that is over 110 years old and requires much less precision, is more prevalent. The need for operating with high magnification operating microscopes is also a limiting factor.

There has also been speculation regarding the feasibility of subfrontal retraction in the setting of severe trauma. Although not applicable if ischemic changes have set in, a basal approach and the use of the “2-minute window” surgeons get after removing the subdural hematoma from the base helps to get into the interoptic cisterns.

The possibility that the pediatric brain may not respond predictably and a sliver of basifrontal lobe may need to be removed in subpial fashion at times to reach the cisterns. Furthermore, there is also a predisposition for the formation of pseudomeningocele as the dura is left open in order to facilitate CSF drainage from the cisterns to the subgaleal space, where it could be absorbed.

The procedure has been observed not to work when associated ischemia occurs, when blood pressure is low and with malignant brain infarcts. It is also inapplicable to patients with low GCS.

Development
Cisternostomy was as a neurosurgical procedure was developed by Dr. Iype Cherian around 2009 during his practice in India when accidentally it appeared to him that opening the membrane of Liliequist in a traumatic brain played a role in rapidly decreasing the ICP and resulted in better prognosis. The initial years of research consisted of modifying surgical procedures with a combination of decompression coupled with cisternal drainage.

There is a trend of both acceptance and reluctance with regard to the adoption of cisternostomy as a vital protocol for the management of brain trauma. However, evidently positive prognosis in some centers in India, Nepal, China and parts of Europe have created waves in the field of neurotraumatology. An on-going neurotrauma study, Global Neurotrauma Outcomes Study funded by the NIHR regards cisternostomy as one of the techniques vital to the reduction of ICP in emergency TBI. There are ongoing studies and training of young neurosurgeons to skillfully perform cisternostomy that are expected to establish this procedure and perhaps revolutionize the current management of traumatic brain injury.