Endoscopic Skull Base Surgery: the Challenge of Reaching the Brain

The head houses one of the most complex organs of the human body, the brain. This makes the anatomy of the head one of the most challenging aspects of human anatomy. The skull consists of 22 bones (some of which are unilateral while others are bilateral bones). The bones that form the head can be subdivided into two categories based on their function. The neurocranium is formed by the frontal, occipital, ethmoid, sphenoid, temporal, and parietal bones, all of which have the function of protecting the brain (Anderson et al., 2023).

Due to the vital functions of the brain, it is important for it to be enclosed in the skull as a way of safeguarding it in case of head trauma. Yet, the anatomical position of the brain, and its associated structures, introduces challenging scenarios in cases where the brain needs to be accessed for therapeutic procedures. Traditionally, medical interventions which require access to the brain are highly invasive (Iseh et al., 2011). Craniotomies require the use of a skull saw to remove parts of the bones of the head (creating a bone flap) to allow surgeons to observe the brain. Depending on what part of the brain needs to be reached, large incisions may need to be made on the face and scalp. When the problem lies in a region of the brain that is deep in the skull (located at the skull base), the surgical approach becomes particularly challenging. Overall, invasive skull surgeries prolong the recovery time of the patient post-surgery, can detrimentally impact how a person views themselves aesthetically, and can lead to severe side effects (Scholz et al., 2010).

Figure 1: Bones of the neurocranium (Knapp, 2020).

Despite the limited accessibility to the brain, healthcare professionals have derived a method to operate on parts of the brain that are stationed at the base of the skull. The method used is called endoscopic skull base surgery (ESBS). This surgery is minimally invasive and is facilitated by the use of an endoscope, an imaging tool that can transmit real-time images to the surgeons (Lee & Senior, 2008). This article will describe when this type of surgery may be needed, will explore the different surgical approaches used in ESBS, and will highlight potential advancements that can be seen in the future.

What is Endoscopic Skull Base Surgery?

ESBS is an example of a cutting-edge surgical procedure. Unlike a craniotomy, ESBS is a minimally invasive surgery. It can be performed by making a small incision or by using a transnasal approach, which eliminates the need to make any surface cuts on the face. This surgical technique also utilises an endoscope. Endoscopes are composed of a tube with a camera and a light source attached to it. The endoscope can hence be inserted into the area of interest, being used to navigate the surgical tools into the correct position and identify any nearby structures that should be avoided. In traditional skull base surgery, an operative microscope is used to give the surgeon a stereoscopic image of the brain. An endoscope is able to achieve the same while registering better images and giving doctors the opportunity to reach deeper areas. Skull base surgeries assisted by endoscopes have been found to improve the overall surgery outcomes as their use allows for the operation to be performed faster, for the patient to spend less time recovering post-surgery, and for there to be less blood loss during the surgery (Lee & Senior, 2008). Utilising a navigation system, such as the endoscope, during a surgical procedure is especially useful in oncologic cases as it can help determine if the region is tumour-free after resection (Tzelnick et al., 2023).

Figure 2: Endoscopic skull base surgery (Pacific Pituitary Disorders Center, 2023).

When is Skull Base Surgery Performed?

A patient needs to undergo skull base surgery when a cyst or a tumour has been identified in the brain or in a nearby structure, like the pituitary gland. A tumour is a growth that can occur in any tissue. It is formed by the uncontrolled division of cells. Tumours can be non-cancerous (benign) or malignant. Regardless, a growth forming in the brain can be dangerous because of the risk that it can affect surrounding structures and due to the increase in the pressure within the fixed space of the skull. The most common skull base tumour is a meningioma, a growth that occurs in one of the three membranes that surround the brain and the spinal cord called the meninges. Meningiomas are typically benign but need to be removed in order to avoid the compression of adjacent arteries or nerves (Rangel-Castilla et al., 2016).

Skull base surgery, most prominently ESBS, is also employed to remove pituitary adenomas. These are tumours that form on the pituitary gland, a gland that is attached to the hypothalamus of the brain. Due to the nature of the pituitary gland, being an endocrine organ, some adenomas are known as secreting. This means that their presence can lead to the overproduction of certain hormones, like prolactin, adrenal corticotropic hormone, and growth hormone. The disruption in normal hormonal balance can lead to conditions like Cushing disease and acromegaly, which can negatively impact a person’s quality of life. In addition, the anatomical arrangement of the pituitary gland makes it likely for pituitary adenomas to compress the optic chiasm, which can result in vision loss (Russ & Shafiq, 2020). The pituitary gland sits in a depression called the sella turcica of the sphenoid bone (Anderson et al., 2023). This makes it an area that is hard to reach if a surgical approach from the top of the head is taken. Hence, ESBS is of great significance when it comes to the removal of pituitary adenomas.

Figure 3: Pituitary adenoma (Mayo Clinic, 2019).

How is an Endoscopic Skull Base Surgery Performed?

ESBS often requires two or more surgeons to operate, using what is referred to as the four-hand technique. During the operation, one surgeon needs to handle the endoscope while the other will be using both hands to employ the surgical tools (Lee & Senior, 2008).

Transnasal Approach

Incisions can be avoided when performing ESBS if a transnasal approach is taken. There are two openings in the nose, known as nares. An endoscope can be navigated through the nasal openings, passing through the nasal cavity near the nose septum and the sphenoid sinus (air cavity found in the sphenoid bone), to reach the pituitary gland. This approach has been successfully used in the resection of pituitary adenomas (Li et al., 2020). Once the surgical removal of a tumour is completed, bleeding, damage to the nasal septum, and potential leakage of cerebrospinal fluid need to be managed. Cerebrospinal fluid leakage after surgery has been found to be a serious complication of endonasal surgical approaches (Ishii et al., 2014).

Figure 4: Four hand surgery approach (Lee & Senior, 2008).

Transorbital Approach

Transorbital neuroendoscopic surgery (TONES) is an approach to ESBS that is not yet widely implemented. A transorbital route allows regions in the anterior skull base to be reached which would not be considered operable if using other surgical corridors (like the transnasal approach) (Feller et al., 2023). For the surgery to be performed, an incision is usually made in the crease of the upper eyelid. This is considered a cosmetically favourable incision site, commonly seen in blepharoplasty. The surgeon would then need to work their way through the orbit, dissecting orbital muscles and reaching the bones that make up the orbit. Bone drilling is then further needed to allow sufficient space for the resection of tumours (Vural et al., 2021). TONES is a technique that has been used to successfully remove meningiomas. However, it is an extremely challenging procedure that requires an interdisciplinary team of surgeons who have expert knowledge of the orbital and cranial cavity (Somma et al., 2022). A literature review done in 2021 described the outcome of TONES in 69 patients, indicating that pre-operative symptoms, like vision problems and neurological problems, had been improved in all cases by the surgery. Like with the transnasal approach, cerebrospinal fluid leaks post-surgery a significant complication (Houlihan et al., 2021). Therefore, it is important that more clinical studies are done to improve the transorbital approach, making it more accessible and safer.

Future advances

Skull base surgery is dependent on the expert knowledge, skill, and decision-making of the surgeon. It is a complex procedure that requires a large team of healthcare professionals. A potential advancement in the operating theatre is the use of surgical robotics. Medical robots have been integrated into the healthcare setting in the form of robotic arms that can be controlled by the surgeon, even from a distant site. However, robots have also been designed to be autonomous, performing programmed actions, and semi-automatic, where they can respond to cues from the surgeon but perform tasks independently (Kimia Kazemzadeh et al., 2023). A team at the University of Washington introduced the concept of an Artificially Intelligent Neurosurgical Robotic Assistant. Instead of having another surgeon or surgical assistant perform tasks such as navigating an endoscope and applying suction to certain regions, the team is attempting to design a robotic assistant that would be responsive to environmental stimuli, like the voice commands and the actions of the surgeon. This could be an impactful change in how ESBS is performed as the robotic assistant could also be programmed to identify parts where the tumour has been left unresected, devising an action plan for the surgeon. Therefore, robotics in the field of neurosurgery has significant potential for improving the efficacy of ESBS (Sekhar et al., 2020).

Figure 5: Robotic surgical assistant (Sekhar et al., 2020).

A limitation of ESBS is the requirement for surgeons to rely on information projected onto a monitor (from the endoscope) to perform the surgical intervention. Researchers are aiming to integrate virtual reality and augmented reality into the operating theatre to tackle this issue. Zeiger and colleagues displayed an example of how augmented reality can aid in performing endoscopic endonasal skull base surgery. Personalised three-dimensional models were created using images recorded by CT and MRI technology of the patient’s brain, and were used by surgeons to pre-plan their surgical approach. During the operation, these three-dimensional models were displayed alongside, with the potential to be overlaid, the transmission presented by the endoscope. This technique was utilised in 134 cases of tumour resections. Surgeons reported that the use of augmented reality in this way had guided them to be more cautious of vital vessels and nerves during the surgery and had aided in their dissections to reach the tumour. In addition, designing personalised three-dimensional models of a patient’s neuroanatomy can similarly aid patients. Mian and colleagues demonstrated that presenting patients with 3D printed models of their skull base, which presented the location of their tumour, improved patient understanding of the operation that they were to undergo. Furthermore, being exposed to such a model alleviated pre-operative anxiety in most of the patients (Mian et al., 2022). Overall, the use of technology to facilitate neurosurgery is something that needs to be further researched as it shows promise to advance the way skull base surgery is conducted.

Conclusion

Skull base surgery is a procedure commonly performed to reach parts of the brain to resect brain tumours, such as meningiomas and pituitary adenomas. Endoscopic skull base surgery (ESBS) is an approach that is minimally invasive and guided by real-time imaging. There are different surgical corridors that can be utilised during an ESBS, with the transnasal and transorbital approaches being explored the most in literature. The use of robotics, augmented reality and other technologies can drive the field of neurosurgery forward. Yet, more clinical studies are still required to make ESBS a safer procedure, effectively minimising the risk of complications, like cerebrospinal fluid leak.

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