Arrested pneumatisation of the skull base (sphenoid sinus)

Clinical History

40-year-old female patient underwent CT imaging following a direct head impact during a motor vehicle accident. Apart from an ongoing headache, there was no focal neurology. While the CT images of the brain demonstrated normal intracranial appearances (not fully shown), relevant skull base images showed an incidental abnormality.

Imaging Findings

Axial CT images (figures 1a-e) demonstrate an area of under-pneumatisation involving the left basi-sphenoid bone extending towards the greater wing of the sphenoid (the latter region is a recognised region of accessory pneumatisation).

The region is non-expansile, has well defined sclerotic margins, does not communicate with the aerated portions of the remaining sphenoid sinus or wider para-nasal sinus system and appears to be filled with soft tissue attenuating material on bone windowed images. Selected bone windowed images also demonstrate the presence of curvilinear calcifications within this region (arrowed on figures 2a-c). These calcifications are different from the "ring and arc" type calcifications found with chondroid series tumours. In addition, there are also fat attenuating focal areas which are best appreciated on the soft tissue windowed images (arrowed on figures 3a-c). There is no evidence of any associated disruption of skull base foramina.

Discussion

Arrested skull base pneumatisation refers to a benign developmental variant. It is one of the many recognised anatomical variations that can occur at the skull base [1-2, 5]. On imaging, it can sometimes be confused with more concerning skull base disease processes especially if the radiologist is unaware of its existence or its typical features.

At birth, the sphenoid sinus is not pneumatised at all and is filled with red marrow instead [4]. During normal development, there is often fatty change prior to pneumatisation of the sinus [3]. Fatty conversion usually commences at around 4 months of age. By 10-14 years of age, the fatty converted marrow is usually replaced by a fully pneumatised sinus lined by respiratory epithelium [6-7].

In certain individuals, the conversion of fatty marrow adjacent to the sinus into respiratory mucosa halts at some point in development and atypical fatty marrow persists into adulthood (hence termed arrested pneumatisation). Patients with arrested skull base pneumatisation are asymptomatic and the variant is usually incidentally discovered when imaging is performed for an unrelated reason.

Imaging perspective
A set of imaging criteria have been proposed for its diagnosis [2]
1. The lesion must be located at a site of normal pneumatisation or a site of recognised accessory pneumatisation
2. The lesion must be non-expansile
3. The lesion should have sclerotic, well-circumscribed margins
4. The lesion should show fatty content
5. On CT, internal curvilinear calcifications should be present - these should be morphologically different from the "ring and arc" type calcifications found with chondroid series tumours
6. Any associated skull base foramina should retain their normal appearance.
This case demonstrates all of the above features.

Take home message and teaching points
Since arrested skull base pneumatisation is a normal variant, no additional work up or treatment is required if all classical features are present on imaging.
Recognition of this variation as well as its typical features on imaging is important for a radiologist, especially in order to avoid suggesting unnecessary work up or biopsy which can potentially result in an adverse outcome.
This case highlights the typical imaging appearances of arrested pneumatisation of the skull base and emphasizes the importance in its recognition.

Differential Diagnosis List

Final Diagnosis

Arrested pneumatisation of the skull base (sphenoid sinus)

URL:	https://www.eurorad.org/case/10648
DOI:	10.1594/EURORAD/CASE.10648
ISSN:	1563-4086