Skull Base Anatomy

Kevin C. Welch, MD

Introduction
The term “skull base” refers to the bottom of the skull, or the plate of bone upon which the brain sits. The skull base is an important area since it separates the brain from the remainder of the anatomic structures in the head, including the sinuses, eyes and ears. The skull base contains many openings. Nerves and blood vessels travel through these openings. The skull base is important to rhinologists (nasal and sinus surgeons) since infections and tumors in the sinuses can directly or indirectly impact the function of the nerves coming out of the brain as well as the brain itself.

Overview
The skull base is made up of several different bones that are fused (joined) together. When these bones fuse together, the skull base looks much like a bowl with holes in the bottom (See Figure 1). The large hole in the skull base (red circle) is where the spinal cord leaves the brain and travels through the skull.  
  
Figure 1: The "skull base" as seen from the top of an individual (the brain is removed). Notice the complexity of the shape and notice the many holes and crevices in the bone. Nerves and blood vessels pass through these holes.
 
When flipped over, the skull base can be viewed from the bottom side (See Figure 2). Although the view is very different, the holes and crevices can be seen again. What is different is that other important structures can be seen as well. These important structures include the upper jaw and palate (red arrow) as well as openings in the back of the nose (red arrowhead). The large opening in the skull base (red circle) is where the spinal cord comes out of the skull.
 
Figure 2: The skull base viewed from the bottom. Notice all the holes and crevices that are similar to those found on the top side of the skull base (Figure 1).
 
When we view the front of the skull (See Figure 3), which contains the skull base, we see a picture that is much more familiar to us since it is the framework of the human face. The eye sockets are present (red arrow) as is the nasal cavity (red arrowhead).
 

Figure 3: The front of the skull base -- a more familiar view to us all. Rhinologists (sinus surgeons) operate many times through the nasal cavity (red arrowhead) to access areas the sinuses and the skull base.
 
Importance of the skull base
The skull base is important as previously mentioned since many different types of sinus tumors can occur in this area. Additionally, fluid from the brain (cerebrospinal fluid or CSF) can leak out of the brain into the sinuses in this area as well. 

Since the sinuses are located just below the skull base, rhinologists (sinus and skull base surgeons) are well-equipped and knowledgeable about diseases that occur in the skull base. For patients with pituitary gland tumors, the pituitary gland sits in the middle of the skull base and is directly above and behind the sphenoid sinus. This is another area that rhinologists are able to access for surgery in a minimally invasive manner (See Figure 4).

 
Figure 4: This image is taken during a surgery on a patient's pituitary gland tumor. The surgery was done entirely through the nose without incisions on the face.

surgery of the skull base
The skull base, specifically tumors or defects involving the skull base, can be reached during surgery through a number of different "approaches" (i.e., ways of getting there). The skull base can be reached by creating incisions on the skin. These incisions can be on the face, on the scalp, behind the ear or on the neck under the jaw bone. As unsettling as this sounds, it is often necessary to do this in order to reach the skull base and treat the problem that is present. However, in many situations (and the number of situations continues to grow), the skull base can be reached with an endoscope (a small rigid telescope). Endoscopic surgery is often the preferred method of getting to the skull base since there are no external incisions involved and it is considered a "minimally invasive" way of performing surgery. Depending on the circumstances, endoscopic surgery of the skull base provides as good or better results and may involve fewer risks and faster recovery than open approaches. This is highly variable, of course, depending on the problem that is present. Figure 5 shows the CT scans as well as a photograph during the removal of a sinus cancer that was performed entirely through the nostrils.

Figure 5: An intraoperative picture during the removal of a sinus cancer that involved the skull base. The green crosshairs on the CAT scan images show that tumor has been removed from the skull base.

Is endoscopic skull base surgery right for me?
The decision to perform endoscopic skull base surgery depends upon many factors. Most importantly, the surgeon or surgeons must decide if the problem can be treated completely by performing endoscopic surgery. If the problem cannot be treated with the use of an endoscope, an "open" approach with incisions may need to be performed. If you have a sinus tumor, cancer, brain fluid leak, pituitary tumor or other condition that involves the skull base, you should discuss with your surgeon what is best for you.
 
 Revised 02/17/2015
©American Rhinologic Society