An additional challenge for head and neck radiologists: anatomic variants posing a surgical risk – a pictorial review
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Anatomic variants in the head and neck are quite numerous and occur frequently: a minority of them increase the risk of complications during surgical procedures and may be visualized on cross-sectional images. As some of these complications are potentially fatal, awareness (and accurate reporting) of such variants is a basic responsibility of radiologists, particularly when surgery in the pertinent anatomic area is under consideration.
KeywordsHead and neck Anatomic variants CT MRI Surgery
Cone beam CT
Magnetic resonance imaging
Anatomic variants in head and neck are numerous and frequently encountered
Some anatomic variants, if not known, can trigger serious complications during surgery
Radiologists should inform surgeons about the presence of relevant anatomic variants
Anatomic variants in the head and neck are quite numerous and occur frequently, particularly in the sinonasal region. Some of them, mainly in the paranasal region, are known to predispose to pathology, whereas most bear little (if any) clinical significance. Only a minority of variants increase the risk of complications and iatrogenic damage during surgical procedures. As some of these complications are fatal, awareness and accurate reporting of such variants is a basic responsibility of radiologists. In this pictorial review, anatomic variants posing a surgical risk will be classified under four main categories: abnormal bone pneumatization, bone dehiscence and asymmetry, anomalous vessel course, and anomalous nerve course.
Abnormal bone pneumatization
In the paranasal area, the Onodi cell is probably the most alarming variant. This cell is the extension of an ethmoid cell above and/or lateral to the sphenoid sinus; hence, it is also referred to as a sphenoethmoid cell. Given its location, it may have an intimate relationship with the optic nerve canal.
In fact, a more restrictive definition of the Onodi cell includes an optic nerve canal protrusion or dehiscence. During endoscopic sinus surgery, the transgression of the walls of an overlooked Onodi cell may result in irreversible optic nerve injury and/or profuse hemorrhage. The prevalence of this anomaly is variable: Shin et al.  found an incidence of ~ 30% with a good correlation between multidetector CT (MDCT) and intraoperative findings. However, literature data on its prevalence are inconsistent, with incidence ranging between 10.9%  and 65% .
Another point of concern for optic nerve injury during surgery is the anterior clinoid process pneumatization. In this case, the risk of iatrogenic damage is related to the thickness of the bony walls of the process and to the degree of pneumatization surrounding the nerve. The prevalence of this variant is low (6–13%) .
Both clinoid pneumatization and Haller’s cell are readily detected on coronal MDCT scans or on cone beam CT (CBCT) scans.
The anatomy of the frontal sinus is largely conditioned by the degree of pneumatization of surrounding air cells (mostly agger nasi cells) and by the cranial attachment of the uncinate process . Some configurations (i.e., supraorbital ethmoid air cell and deep olfactory fossa) significantly influence the complexity of the procedure and may increase the relative risk of complications.
Bone dehiscence and asymmetry
Anomalous vessel course
Both of these variant vessels may pose a surgical threat, principally when tracheostomy or thyroid/parathyroid surgery is planned; identification on cross-sectional imaging requires careful assessment of vascular structures in the peritracheal soft tissues.
The incidence of this variant is linearly related to patient age; thus, it is possibly explained by increased tortuosity and atherosclerotic changes or by hypertension. Severe complications may be generated by retropharyngeal carotid artery injury, even during routine surgery such as tonsillectomy or peritonsillar abscess drainage. Interestingly, a change in position (from and to retropharyngeal) on MDCT examinations acquired at different time points has been described in 6.3% of cases .
Anomalous nerve course
In the sinonasal area, anomalous nerve course is often the consequence of abnormal pneumatization and/or dehiscence of the bony walls of their canals. As a result, cranial nerves in the maxillofacial area and temporal bone may be seen coursing within air cavities. The maxillary and vidian nerve may protrude into the sphenoid sinus when the pterygoid root is pneumatized thus creating a lateral recess. Less commonly, the infraorbital nerve may protrude into or hang in the maxillary sinus.
In the lower neck, relevant nerve course variants may be detected only during surgery, such as the extralaryngeal ramification of the recurrent nerve and the numerous variations of the course of the spinal accessory nerve .
Some anatomic variants in the supra- and infrahyoid neck may increase surgical risk and, if overlooked, have the potential to generate serious complications. Often such variants produce subtle findings on cross-sectional images. It is the responsibility of the radiologist reporting a head and neck scan to carefully scrutinize the regional anatomy in search of variants and to report on them, particularly when surgery is under consideration.
DF, DL, IZ, and MR contributed to the study concept and design. MB and GP contributed to the editing and reviewing of the manuscript. All authors read and approved the final manuscript.
The authors state that this article did not receive any funding.
Ethics approval and consent to participate
Patients consent was waived according to the Declaration of Helsinki.
Consent for publication
Not applicable to our study.
The authors declare that they have no competing interests.
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