Arrested pneumatisation of the skull base

Arrested pneumatisation of skull base is an anatomical variant that most commonly occurs in association with the sphenoid sinus. It is known that the sphenoid bones undergo early fatty marrow conversion antecedent to normal pneumatisation. However, for unclear reasons, some individuals experience failure of pneumatisation before respiratory mucosa has fully extended into sites of early fatty marrow conversion. These individuals are then left with persistent atypical fatty marrow adjacent to the sinus that persist into adulthood. If unrecognised, imaging features may at times create diagnostic difficulty in interpretation of skull base CT and MR scans.

Clinical presentation

Since it is a developmental abnormality it usually manifests as an incidental finding. No specific clinical symptoms are attributed to this entity. 

Location / distribution

It usually occurs in regions of normal or accessory sphenoid sinus pneumatisation including the basisphenoid bone, pterygoid processes and clivus. Contiguous involvement across multiple sphenoid sub sites is common.

Pathophysiology

The normal process of pneumatisation of the skull base and paranasal sinuses start at the age of 4 months and develops through to young adulthood. Red bone marrow is being replaced by fatty marrow prior to pneumatisation of the paranasal sinuses, including the sphenoid bone. The precise mechanisms that remain largely unclear. This bone marrow conversion precedes the invasion of epithelial cells to form the respiratory mucosa. When one of the steps described above is halted, no or reduced pneumatisation of the sinus will occur.

Radiographic features

Imaging consists of CT and MR studies involving the skull base. The non-expansile nature of the lesion is best evaluated at the inferior orbital fissure and vidian cannal, which are not displaced nor disrupted.

CT

Characteristic features on CT are the presence of 

• a non-expansile lesion with
• internal curvilinear calcifications and 
• sclerotic margins 

MRI

Hallmark of MR imaging is 

• presence of internal fat and microcystic components
• absence of any mass effect 

• T1 -
  • hyper intense fatty component
  • hypo intense microcystic and calcified components
  • sclerotic border is hypo intense
  • no enhancement of the lesion after Gd administration
• T2 - microcystic components are hyper intense on T2

Arrested pneumatisation can be diagnosed when a lesion fulfills the following criteria: 

1. the lesion must be located at a site of normal pneumatisation or of recognized accessory pneumatisation. 
2. the lesion must be non-expansile with sclerotic, well-circumscribed margins. 
3. the lesion should show fatty content. 
4. on CT, internal curvilinear calcifications should be present 
5. any associated skull base foramina should retain a normal appearance

Differential diagnosis

General differential considerations include

• fibrous dysplasia
• ossifying fibroma
• chondrosarcoma
• osteomyelitis (around skull base)
• chordoma 
• bone metastasis(es)

In contrast with arrested pneumatisation, all of these conditions lack the presence of internal fat, or usually show signs of mass effect on the surrounding structures.