Anatomic abnormalities in the paranasal sinuses and skull base are not uncommon. Awareness of these abnormalities may be of assistance in pre-operative planning. This content presents a template-driven approach to the analysis of computed tomography scans in preparation for endoscopic sinus surgery.
With the increased availability of thin-cut computed tomography (CT) scans; and the ability to view anatomic variations in the coronal, axial, and sagittal planes, detailed knowledge of a patient’s anatomy is available before surgery. This article presents a template-driven, methodical approach to CT evaluation of anatomic “danger zones” as well as other areas pertinent to planning for surgery on the paranasal sinuses.
Preoperative checklist overview
When reviewing a patient’s CT before surgery the author uses a methodical, step-by-step approach that involves 3 complete passes through all the sinuses. Anatomy is evaluated first by looking at each sinus: frontal, maxillary, ethmoid, and sphenoid, in all 3 CT views:
- 1.
Coronal
- 2.
Axial
- 3.
Sagittal.
This procedure is followed by carefully scrolling through each CT view (coronal, axial, sagittal), and looking at every sinus on each view. Finally, the skull base is carefully evaluated for any defect. Typically, the author scrolls from anterior to posterior (coronal), from superior to inferior (axial), and from lateral to medial (sagittal). The direction of scrolling is not as important as the need to perform this evaluation in the same manner each time. A reference “checklist” is shown in Box 1 . A methodical approach allows the surgeon to become increasingly familiar with the appearance of “normal” anatomy, and consequently more adept at recognizing anatomic variations.
Organized by sinus
Frontal
Follow frontal sinus (FS) outflow tract
Superior uncinate process attachment
Frontal cells
Anterior and posterior table dehiscence
Orbital roof dehiscence
Ethmo-frontal angle
Maxillary
Uncinate process atelectasis/Silent sinus
Nasolacrimal duct
Haller/Infraorbital ethmoid cell
Infraorbital nerve location
Dehiscent maxillary sinus roof
Ethmoid
Anterior ethmoid artery
Medial skull base—Keros levels
Skull base asymmetry
Low/sloping skull base
Ethmoid cell character—large cells versus tightly packed cells
Skull base thickness; presence of polyps
Lamina papyracea
Sphenoid
Location of sphenoid ostium
Onodi cells
Bone over carotid
Location of optic nerve
Is there a skull base defect in ethmoid (E), frontal (F), sphenoid (S) sinuses?
Organized by CT view
Axial (superior to inferior)
F Follow FS outflow tract
Anterior + posterior table dehiscence
E Ethmoid cell character—large cells versus tightly packed cells
Lamina papyracea
S Location of sphenoid ostium
Bone over carotid
Location of optic nerve
Coronal (anterior to posterior)
F Follow FS outflow tract
Superior uncinate process attachment
Frontal cells
Anterior and posterior table dehiscence
Orbital roof dehiscence
M Uncinate process atelectasis/Silent sinus
Nasolacrimal duct
Haller/Infraorbital ethmoid cell
Infraorbital nerve location
Dehiscent maxillary sinus roof
E Anterior ethmoid artery
Medial skull base—Keros levels
Skull base asymmetry
Low/sloping skull base
Ethmoid cell character—large cells versus tightly packed cells
Skull base thickness; presence of polyps
Lamina papyracea
S Onodi cells
Location of Optic nerve
Sagittal
F Follow FS outflow tract
Frontal cells
Anterior and posterior table dehiscence
Ethmo-frontal angle
S Location of sphenoid ostium
Frontal sinus
Precise knowledge of the anatomy of the frontal sinus outflow tract is imperative when manipulation of this area is to be performed. The axial CT scan is evaluated, scrolling from superior to inferior ( Fig. 1 ). Typically, as the inferior aspects of the sinus are approached, the agger nasi and/or frontal cells can be appreciated on the CT defining the often labyrinthine drainage pathway. Most commonly, these cells push the drainage pathway medially, although variations occur frequently.
Awareness of these cells, and the resultant drainage pathway, is reinforced with evaluation of the coronal plane, scrolling from anterior to posterior ( Fig. 2 ). In most cases, these same frontal-area cells can be identified, thereby increasing the surgeon’s 3-dimensional understanding of the drainage pathway location. Also of importance on the coronal views is the posteromedial location of the drainage opening within the frontal sinus. Therefore, when the sinus is opened, enlargement is typically performed in an anterior and lateral direction. The anatomy of this drainage pathway may also be appreciated on sagittal films, typically viewed from lateral to medial ( Fig. 3 ). Scrolling in the medial direction provides a view of the unobstructed pathway into the frontal sinus.
The varying sites of superior uncinate process attachment are well known. It is similarly well known that the site of attachment affects the frontal sinus drainage pathway and, consequently, the entrance into the frontal sinus during endoscopic surgery. Recent studies have delineated that the superior attachment of the uncinate is fluid at location, and may attach at locations between the traditional 3 sites: lamina, skull base, middle turbinate. This site of attachment may be appreciated most easily on coronal images ( Fig. 4 ).
Frontal cells can alter and obfuscate the drainage pathway of the frontal sinus. Awareness of the impact of frontal cells on sinus drainage can be appreciated on CT scan. Typically, these cells are best appreciated on sagittal view, although coronal and axial films should also be reviewed because they allow for optimal understanding of the presence of these cells, and how the cells specifically alter the drainage of the frontal sinus. Type I frontal cells are single cells superior to the agger nasi cell that do not extend into the frontal sinus ( Fig. 5 ). Type II frontal cells are 2 or more cells superior to the agger nasi cell that do not extend into the frontal sinus. Type III cells extend into the sinus proper ( Fig. 6 ), whereas Type IV cells have been defined as either entirely located within the frontal sinus, or as having more than 50% of their volume in the frontal sinus. As with frontal cells, supraorbital and suprabullar cells can also be appreciated on CT, although their impact on the drainage pathway is usually less severe.
During examination of the frontal sinus, the area is also carefully inspected for anterior and posterior table dehiscence. Dehiscence or malformation of the posterior table of the frontal sinus is easily appreciated on axial and sagittal films preoperatively ( Fig. 7 ). While it may not always be necessary to repair a dehiscence, it is imperative that the surgeon be aware of any dehiscence prior to surgery because dehiscence may be associated with the presence of a meningo-encephalocele, or a cerebrospinal fluid (CSF) leak. Failure to appreciate the exposure of the cranial contents from bony dehiscence may lead to an incomplete initial surgical plan, resulting in the need for a secondary procedure.
Anterior table dehiscence or malformation can also be easily recognized on axial and sagittal CT images. As with the posterior table, failure to appreciate an abnormality in the anterior table may lead to an incomplete primary surgical intervention and the later need for an unplanned secondary intervention. In many cases, appreciation of abnormalities prior to surgery may lead to a single, all-encompassing surgical plan that addresses the issues brought up by these abnormalities ( Fig. 8 ).
After checking the anterior and posterior table, the orbital roof is evaluated for dehiscence. The frontal sinus often pneumatizes over and “rests” on the orbital roof ( Fig. 9 ). Part of the orbital roof is, in fact, the floor of the frontal sinus. This intimate relationship may have surgical implications in the case of frontal sinus lesions. Mucoceles, acute infections, tumors, and other lesions may erode the orbital roof, increasing the vulnerability of the orbital contents. These changes can often be appreciated on preoperative CT, particularly in the axial and coronal planes ( Fig. 10 ).
Finally, the entryway into the frontal sinus is evaluated by looking for the ethmo-frontal angle. As the ethmoid roof is followed anteriorly, it slopes upward along the posterior table of the frontal sinus. The ethmo-frontal angle ( Fig. 11 ) is best appreciated on sagittal CT, and has been noted to range from 135° to 171°. Moreover, this angle has asymmetry within any particular patient. Although knowledge of this angle has little impact on surgeons performing “frontal recess exploration,” it may be of use to those instrumenting up within the frontal sinus proper.
Frontal sinus
Precise knowledge of the anatomy of the frontal sinus outflow tract is imperative when manipulation of this area is to be performed. The axial CT scan is evaluated, scrolling from superior to inferior ( Fig. 1 ). Typically, as the inferior aspects of the sinus are approached, the agger nasi and/or frontal cells can be appreciated on the CT defining the often labyrinthine drainage pathway. Most commonly, these cells push the drainage pathway medially, although variations occur frequently.
