Craniotomy fenestration refers to any neurosurgical technique in which a window or opening is deliberately created in tissue, membrane, or bone during or alongside a craniotomy. The word “fenestration” comes from the Latin for “window,” and in practice it covers a surprisingly wide range of procedures: drilling holes through a replaced skull flap, cutting open an arachnoid cyst wall, puncturing a thin brain membrane to restore fluid flow, or slitting the sheath around an optic nerve. What ties these procedures together is the core idea of making a controlled opening so that trapped fluid can move where it needs to go, or so that abnormal pressure can equalize.
Bone Flap Fenestration After Subdural Hematoma Surgery
When a surgeon removes a piece of skull (the “bone flap”) to evacuate a subdural hematoma, one option is to drill a grid of small holes through the bone flap before putting it back. These holes, typically spaced about one to two centimeters apart, turn the solid bone into something more like a colander. The goal is to let extra-axial fluid, the fluid that collects between the brain surface and the skull, drain outward through the perforated bone rather than pooling and potentially compressing the brain again.
This idea is straightforward in concept but has received relatively little formal study. A case series published in Surgical Neurology International described the technique and reported that fenestrating the bone flap before immediate replacement had the potential to reduce extra-axial fluid accumulation after surgery for subdural hematoma.1PubMed Central. Fenestration of bone flap during decompressive craniotomy for subdural hematoma The authors acknowledged, however, that the broader literature had not yet explored this approach in depth. It remains a technique some surgeons use based on anatomical reasoning and early clinical results, rather than one backed by large randomized trials.
Opening Arachnoid Cysts
Arachnoid cysts are fluid-filled sacs that develop between the brain and the arachnoid membrane. Most are congenital, and many never cause problems. When a cyst grows large enough to press on surrounding brain tissue, though, it can produce headaches, seizures, or developmental delays in children. Fenestration is one of the main surgical strategies: the surgeon opens the cyst wall to let cerebrospinal fluid (CSF) drain into the brain’s normal fluid pathways.
There are several ways to accomplish this. Open craniotomy allows direct examination of the cyst, tissue biopsy, and membrane fenestration, and it is particularly suited to multilocular cysts that have internal partitions the surgeon needs to see and open.2Journal of Cerebrovascular Sciences. Incidence and Surgical Outcomes of Intracranial Arachnoid Cysts: A Systematic Review Comparing Endoscopic Fenestration, Open Craniotomy and Cystoperitoneal Shunt Approaches Endoscopic fenestration uses a small camera and instruments inserted through a smaller skull opening, which reduces surgical trauma. A third option, cystoperitoneal shunting, routes cyst fluid to the abdominal cavity through tubing, though it introduces permanent hardware with its own set of long-term complications.
A comparative analysis of over 1,300 cases from 36 published series found no significant differences in clinical outcome among these surgical strategies. Endoscopic fenestration yielded a slight edge in quality-adjusted outcomes, though all approaches performed well.3PubMed. Pediatric intracranial arachnoid cysts: comparative effectiveness of surgical treatment options A separate study focusing on middle fossa cysts treated endoscopically found that the complication risk was not meaningfully higher than with open surgery.4PubMed. Endoscopic fenestration of middle cranial fossa arachnoid cysts: does size matter? In a 40-patient series, more than nine in ten patients had satisfactory clinical results after endoscopic cyst fenestration, and no deaths or major complications occurred, though four patients needed additional surgery and four experienced post-traumatic bleeding into the cyst after the procedure.5SpringerLink (Child’s Nervous System). Endoscopic treatment of middle fossa arachnoid cysts: a series of 40 patients treated endoscopically in two centres
Lamina Terminalis Fenestration During Aneurysm Surgery
When a brain aneurysm ruptures, blood floods the spaces around the brain. One dreaded aftereffect is hydrocephalus, a buildup of CSF that the brain cannot absorb or circulate properly because blood products have clogged the normal drainage pathways. A proportion of these patients end up needing a permanent shunt, a tube surgically routed from the brain’s ventricles to the abdomen, to manage the fluid buildup for the rest of their lives.
Fenestration of the lamina terminalis is a maneuver aimed at preventing that outcome. The lamina terminalis is a thin membrane at the front wall of the third ventricle. During open aneurysm clipping, the surgeon already has access to this area and can make a small opening in the membrane, creating a new pathway for CSF to flow from the ventricles into the surrounding cisterns. The idea is to bypass any blockage caused by blood products and keep fluid moving.
The evidence that this works is reasonably strong. In one study, the surgeon who routinely fenestrated the lamina terminalis saw a shunting rate of only about 2% in patients whose surgical approach allowed the technique, compared with roughly 13% for other surgeons who did not perform it.6PubMed. Microsurgical fenestration of the lamina terminalis reduces the incidence of shunt-dependent hydrocephalus after aneurysmal subarachnoid hemorrhage A meta-analysis pooling data across multiple centers found that the shunt-dependent hydrocephalus rate dropped from about 15% in patients who did not receive fenestration to roughly 11% in those who did, representing about a one-third reduction in risk.7PubMed. Fenestration of Lamina Terminalis During Anterior Circulation Aneurysm Clipping on Occurrence of Shunt-Dependent Hydrocephalus After Aneurysmal Subarachnoid Hemorrhage: Meta-Analysis
A more recent study from Pakistan reinforced this picture, reporting shunt-dependent hydrocephalus in 13% of high-grade patients who received fenestration compared with 37% of high-grade patients who did not. The authors recommended routine fenestration during anterior circulation aneurysm clipping, noting that it also helps reduce brain swelling and the need for aggressive drug therapy to control intracranial pressure.8PubMed Central. Outcomes of Fenestration of Lamina Terminalis for Hydrocephalus following Clipping of Ruptured Aneurysms of Anterior Circulation This is one of the clearer examples in neurosurgery of a small additional step during an already-planned operation delivering measurable long-term benefit.
Ventricular Fenestration for Hydrocephalus
Hydrocephalus sometimes affects only one side of the brain, a condition called monoventricular hydrocephalus. This happens when a blockage prevents CSF from flowing between the brain’s lateral ventricles. The septum pellucidum, the thin partition that normally separates the two ventricles, can be fenestrated with an endoscope to restore communication and equalize pressure. This endoscopic approach has been described as the preferred treatment for monoventricular hydrocephalus, with the choice of entry point depending on the surgeon’s experience and preoperative imaging.9PubMed. Neuroendoscopic fenestration of the septum pellucidum for monoventricular hydrocephalus
The problem gets more complex with multiloculated hydrocephalus, where the ventricular system is divided into multiple separate compartments by abnormal membranes, often the result of prior infection or hemorrhage. In a review covering dozens of patients, surgeons used a variety of fenestration techniques, including open craniotomy with ventricular septal fenestration in 27 patients, endoscopic septum pellucidum fenestration in 19, and endoscopic ventricular septal fenestration in 18.10PubMed. Multiloculated hydrocephalus The choice depends on the number of compartments, their location, and whether the surgeon can reach them safely through a small endoscopic approach or needs the wider access an open craniotomy provides.
A third ventriculostomy is a related fenestration procedure in which the surgeon creates an opening in the floor of the third ventricle, allowing CSF to bypass a downstream blockage (typically in the aqueduct connecting the third and fourth ventricles) and flow directly into the basal cisterns. When the aqueduct itself is obstructed by inflammatory scarring, a stent can be placed through the narrowed passage to keep it open, which has been shown to outperform simple widening of the aqueduct alone and can serve as an alternative to shunt placement.11Journal of Neurosurgery: Pediatrics. Endoscopic aqueductal stent placement for the treatment of a trapped fourth ventricle
Fenestration for Rathke’s Cleft Cysts
Rathke’s cleft cysts are benign growths that sit near the pituitary gland at the base of the brain. They can cause headaches, vision problems, and hormonal dysfunction when they grow large enough. Surgery usually goes through the nose (a transsphenoidal approach), and the main decision is whether to simply open the cyst wall (fenestration) or try to remove it more completely (resection).
A propensity-matched analysis of 242 patients found that fenestration and resection produced similar rates of improvement in visual and hormonal symptoms. Where they diverged was telling: patients who underwent more aggressive resection had lower rates of headache resolution (around 34 to 43%) compared with those who received fenestration (59%). The resection group also had higher rates of new growth hormone deficiency, roughly three to five times the rate seen after fenestration.12PubMed. Resection versus fenestration for Rathke’s cleft cysts: a propensity score-matched analysis of long-term outcomes In other words, the less aggressive approach achieved equal or better symptom relief while causing less collateral damage to the pituitary gland. This finding has reinforced a trend toward conservative fenestration for many Rathke’s cleft cysts, reserving more extensive removal for recurrent or atypical cases.
Optic Nerve Sheath Fenestration
Optic nerve sheath fenestration (ONSF) is a different procedure from anything done inside the skull, but it shares the underlying logic: make a window to relieve pressure. In idiopathic intracranial hypertension (sometimes called pseudotumor cerebri), elevated pressure inside the skull can be transmitted to the sheaths surrounding the optic nerves, threatening vision. ONSF involves making small slits or windows in the optic nerve sheath behind the eye, allowing trapped CSF to drain and protecting the nerve.
An updated meta-analysis and systematic review found that ONSF improved visual acuity in about 41% of patients and visual fields in roughly 76% of cases. Even more strikingly, optic disc swelling improved in 97% of patients. The study also noted that a transconjunctival approach, accessing the nerve sheath through the tissue lining the eye, had higher success rates for visual field improvement than other surgical routes.13SpringerOpen / Acta Neurochirurgica. Effectiveness of optic nerve sheath fenestration in preserving vision in idiopathic intracranial hypertension: an updated meta-analysis and systematic review ONSF does not address the underlying elevated intracranial pressure throughout the brain; it is a targeted rescue procedure for the optic nerve when vision loss is progressing faster than systemic treatments can control.
Brain Relaxation Through Cistern and Membrane Opening
Fenestration and related opening techniques also serve a less obvious purpose during surgery: brain relaxation. When surgeons open the skull for any reason, a tense, swollen brain can be difficult and dangerous to work around. Techniques that release CSF from the brain’s internal cisterns or membranes can cause the brain to settle and soften, giving the surgeon room to operate without excessive retraction.
During anterior circulation aneurysm clipping through a smaller approach called the mini-spheno-supraorbital craniotomy, surgeons reported that opening the lamina terminalis and the basal cisterns provided adequate brain relaxation to allow safe dissection and vessel control.14PubMed Central. The Mini-Spheno-Supraorbital Craniotomy for Treatment of Ruptured Anterior Circulation Aneurysms Basal cisternostomy, a technique in which the surgeon opens the CSF-containing spaces at the base of the brain, has been proposed as both a standalone procedure and as an addition to decompressive surgery for traumatic brain injury.15PubMed. Cisternostomy as an Adjuvant or Standalone Approach for Management of Traumatic Brain Injury: A Systematic Review and Network Meta-Analysis
When basal cisternostomy was added to standard decompressive hemicraniectomy for moderate to severe traumatic brain injury, a meta-analysis found that patients spent about three fewer days in intensive care, had significantly less brain herniation through the skull opening, were far less likely to need high-dose medication to manage brain swelling, and had lower odds of dying before discharge.16PubMed Central. Basal cisternostomy as an adjunct to decompressive hemicraniectomy in moderate to severe traumatic brain injury: a systematic review and meta-analysis These results are promising, though the technique is still gaining adoption and longer-term outcome data are accumulating.
Endoscopic Versus Open Craniotomy Approaches
A recurring theme across nearly all fenestration contexts is the shift from open craniotomy toward endoscopic methods. Over the past couple of decades, advances in camera technology, flexible instruments, and neuronavigation have made it possible to perform many fenestration procedures through much smaller openings. For multiloculated hydrocephalus, endoscopy has become more common than open craniotomy as the technology has matured.17Journal of Korean Neurosurgical Society. Multiloculated Hydrocephalus: Open Craniotomy or Endoscopy?
Open craniotomy still holds advantages in specific situations. Complex multilocular arachnoid cysts, cases where tissue biopsy is needed, and scenarios where the anatomy is distorted or the cyst is in a difficult location may all favor direct surgical access. The decision is not one-size-fits-all, and experienced neurosurgeons often make the call based on preoperative imaging, the patient’s anatomy, and the specific pathology being treated. The general trend, though, is toward less invasive approaches whenever they can achieve the same result.
Endoscopic Fenestration of Chronic Subdural Hematoma Membranes
Chronic subdural hematomas sometimes develop internal membranes that divide the blood collection into compartments, and an associated pocket of clear fluid called a subdural hygroma can form beneath the inner membrane. Standard burr-hole drainage may not adequately address this layered problem. Endoscopic fenestration of the inner membrane allows the surgeon to open up the partition, aspirate the trapped hygroma, and collapse the membrane, relieving the pressure gradient that was keeping the fluid compartments separated.18PubMed Central. Endoscopic Treatment of Chronic Subdural Hematoma Combined with Inner Subdural Hygroma This is a relatively targeted application of fenestration, but it illustrates how the same basic principle, making a window in a membrane to equalize pressure, gets adapted to different clinical problems.
Checking Whether a Fenestration Stays Open
Creating a fenestration is one thing; knowing whether it remains patent (open and functional) over time is another. This is especially relevant for third ventriculostomies, where the opening in the floor of the third ventricle can scar shut. Surgeons use a specialized type of MRI called cine phase-contrast imaging to watch CSF flow through the fenestration in real time. The technique has been compared against direct surgical re-exploration and found to be a reliable way to assess whether fluid is still moving through the opening.19PubMed Central. Third ventriculostomy patency: comparison of findings at cine phase-contrast MR imaging and at direct exploration
Quantitative versions of this imaging can measure the stroke volume of CSF pulsing through the fenestration with each heartbeat. Higher stroke volumes appear to predict better clinical outcomes, and the imaging has shown that a well-functioning ventriculostomy maintains its CSF flow over follow-up periods of months to years.20American Journal of Neuroradiology. Functional Analysis of Third Ventriculostomy Patency by Quantification of CSF Stroke Volume by Using Cine Phase-Contrast MR Imaging For patients and families, this means that post-surgical monitoring can often be done noninvasively, without needing to go back into the operating room to check whether the opening is still working.
Training the Next Generation of Surgeons
Fenestration procedures, particularly the endoscopic ones, demand a specific set of skills: working through a narrow corridor, navigating with a camera rather than direct vision, and manipulating delicate structures in tight spaces. Training for these techniques has been evolving alongside the procedures themselves. Realistic physical simulators built from 3D-printed skulls and brain models, some incorporating special-effects techniques to mimic the feel and appearance of real tissue, have been developed specifically for practicing procedures like endoscopic third ventriculostomy.21Journal of Neurosurgery: Pediatrics. Creation of a novel simulator for minimally invasive neurosurgery: fusion of 3D printing and special effects
Virtual reality is also entering the picture. A craniotomy skills curriculum piloted with 40 neurosurgical residents and young attendings in a lower-resource healthcare setting tested whether VR training before hands-on practice on a 3D-printed model improved performance compared with going straight to the physical model. Half the group trained in VR first; the other half did not.22Neurosurgery. Virtual Reality and 3D-Printed Model-Based Craniotomy Skills Curriculum for Under-Resourced Healthcare Systems These tools are especially valuable in settings where access to cadaver labs or high-volume mentored caseloads is limited, helping to bridge a training gap that has historically existed between well-resourced and under-resourced neurosurgical programs.