Preoperative CT Checklist

Like an airline pilot before preparing for take-off, you must go through a systematic check of the CT scan before surgery so as to avoid the surgical equivalent of a crash. Particular problems occur in the absence of the middle turbinate or where there is a hypoplastic maxillary sinus, a sphenoethmoidal air cell, or an asymmetrical skull base. It is vital to systematically check the CT scan. You must do this or you will be more likely to miss a danger area or a relevant variation in the anatomy (Fig. 8.9a-d).

A systematic, five-step approach to examining CT scans will ensure that no problem is overlooked (Mason et al., 1998b).

Step 1. When placing the scans on the viewing box, orientate the scan sequence from anterior to posterior and ensure that the sides are marked and placed as though as you are looking at the patient. Follow the cuts anterior to posterior; follow the septum, note any deviation, and look for the size and extent of the eth-moidal bulla, which is a relatively consistent landmark.

Step 2. Examine the lamina papyracea, uncinate process, and middle turbinate. See whether the lamina papyracea is dehiscent. It is important to define the site and insertion of the uncinate process and its proximity to the lamina papyracea so that a middle meatal antrostomy can be undertaken safely (Fig. 8.10a, b). Delineate the insertion of the uncinate process as it extends upward from the superior aspect of the inferior turbinate and establish whether

Uncinate Process
Fig. 8.9 a A polyp arising from the lamella lateralis. b Asym- hiscent optic nerve (+) and carotid artery (*) in the sphenoid metrical skull base. c, d CT (c) and endoscopic (d) views of de- sinus, left side.
Inferior Nasal Concha Hypertrophy
Fig. 8.10a, b Localize the uncinate process (arrow) from its free margin posteriorly and follow it anteriorly and upward.
Nasal Turbinate Concha

Fig. 8.11 a The uncinate process inserting into the skull base (arrow) on the right. b The uncinate process inserting into the middle turbinate (arrow) on the right. c The uncinate process inserting into the lateral nasal wall (arrow) on the right and the middle turbinate on the left.

Fig. 8.11 a The uncinate process inserting into the skull base (arrow) on the right. b The uncinate process inserting into the middle turbinate (arrow) on the right. c The uncinate process inserting into the lateral nasal wall (arrow) on the right and the middle turbinate on the left.

it inserts onto the lamina papyracea, skull base, or middle turbinate (Fig. 8.11a-c). In particular, determine the proximity of the uncinate process to the orbit and define the degree of aeration of any agger nasi air cells (Fig. 8.12). Ensure that the maxillary antrum is not hypoplastic because hypoplasia increases the risk of entering the orbit. Look for an infraorbital cell that may be entered during a middle meatal an-trostomy and may be mistaken for the antrum itself (Fig. 8.13a, b). Examine whether there is a middle turbinate, as its absence is one of the main factors associated with complications involving the orbit and the skull base (Fig. 8.14). Define whether there is a concha bullosa or whether the middle turbinate is paradoxical.

Step 3. Examine the area of the frontal recess. The frontal recess lies anterior and superior to the ethmoid bulla. The infundibulum expands superiorly to form the frontal recess, but the way it does so varies. Ante b a c

Recessus FrontalisAerated Inferior Turbinate
Fig. 8.13 a Hypoplasia of the maxillary sinuses. b A left infraorbital cell (arrow), whose roof looks thin.

rior ethmoid air cells can interrupt its path, and the degree of aeration of the agger nasi air cells will define how narrow the frontal recess is. If an agger nasi cell is well pneumatized and it extends into the frontal sinus, it is called a bulla frontalis. Sizable agger nasi cells may reduce access to the frontal sinus to leave a slit between the middle turbinate and the medial surface of the agger nasi cells. A key aspect of frontal recess surgery is to define the insertion of the uncinate process as this may also "guard" anterior access to the frontal recess by forming a web if it attaches to the skull base or middle turbinate (Fig. 8.15a-d).

Step 4. Determine the height of the skull base. Identify the relationship between the height of the cribriform plate and the fovea ethmoidalis (the roof of the ethmoid sinuses). Sometimes their relationship is flat, but sometimes the cribriform plate can be at a much lower level (Fig. 8.16a-d). A well pneumatized supraorbital cell will cause the anterior ethmoid

Ethmoid Bulla
Fig. 8.15 Serial CT scans in the same patient. a Bilateral bulla frontalis. b Bulla frontalis and the start of agger nasi cells below.

artery to traverse the ethmoid in a separate channel below the ethmoid roof (Fig. 8.17 a-c). Damage to the anterior ethmoid artery may cause retraction into the orbit with subsequent immediate formation of an orbital hematoma and exophthalmos.

Supraorbital Cells
c The attachment of the uncinate process. d Bilateral suprabul-lar recesses and supraorbital cells.

The medial aspect of the ethmoid roof joins the cribriform plate via a thin vertical strut, called the lateral lamella. This is particularly thin bone that can easily be traversed. The fovea ethmoidalis is asymmetrical in approximately 7 % of individuals, and it is important to

Low Lying Cribriform Plate Sinuses
Fig. 8.16a-d Variations of the position of the cribriform plate that depend on the degree of pneumatization of the skull base.

look for this (Fig. 8.18a, b). The height of the posterior ethmoid cells varies, and it can be estimated by examining the distance between the roof of the maxillary sinus in the posterior coronal cuts and the height of the posterior skull base. There are rarely more than two or three cells on either side in the posterior ethmoid sinuses. Approximately 18% of patients have an Onodi (sphenoethmoid) air cell, which increases the risk to the optic nerve. An Onodi cell is a posterior ethmoid cell that is lateral to the sphenoid sinus (Fig.8.19 a-c).

Step 5. Examine the sphenoid sinus. The perpendicular plate of the vomer always attaches to the midline and is a useful landmark. The sphenoid septum is frequently asymmetrical and may attach laterally to the prominence created by the internal carotid artery or the optic canal (Fig. 8.20). It is important to note whether the carotid artery is medially placed or dehiscent. The optic nerve indents the lateral wall of the sphenoid in up to 20% of individuals (Fig. 8.21 a-d). Axial views are not done routinely in many departments because of the extra irradiation and time that it

Middle Turbinate Pneumatized
Fig. 8.18a, b These two CT scans show how the degree of pneumatization between the left and right side affects the exposure of the anterior ethmoid artery. The presence of a
Anterior Ethmoid Cells Scan

Fig. 8.17a, b Sequential CT scans showing the anterior ethmoid artery leaving the orbit across the ethmoid sinuses to the septum. c Endoscopic view of the anterior ethmoid artery, which travels anteriorly as it goes medially after leaving the orbit.

Anterior Ethmoidal Artery

supraorbital cell (*) makes it more likely that the anterior ethmoid artery is dehiscent (arrow).

Planning and Staging the Procedure 141

Planning and Staging the Procedure 141

Preoperative ChecklistEthmoidal Sinus Axial

takes if a nonspiral CT scanner is used. Axial views are easily obtained from spiral CT data, and they are particularly desirable in cases involving malignancy or sphenoid pathology where disease involves the orbit. Check for any bony dehiscence (Fig. 8.22 a-d).

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Responses

  • yusef
    What is nasal turbinates?
    7 years ago
  • ferdinand
    What Is A Nasal Turbinate?
    7 years ago
  • lia
    How to reduce turbinates?
    6 years ago
  • HARIS
    How does air cells occurs in the middle turbinate?
    3 years ago

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