CASE 16443 Published on 13.09.2019

Pneumosinus dilatans associated with fibrous dysplasia


Musculoskeletal system

Case Type

Clinical Cases


Lucas L Walgrave1, Benjamin Peersman1, Rik Verhille1, Filip M Vanhoenacker2

1 MD, 1. RZ Heilig Hart Tienen, Department of Radiology, Belgium
2 MD, PhD, 3: AZ Sint-Maarten Mechelen, Department of Radiology and Emergency, Belgium 4: Ghent University Hospital, Department of Radiology, Belgium 5: Antwerp University Hospital, Department of Radiology, Belgium


83 years, female

Area of Interest Musculoskeletal bone, Neuroradiology brain ; Imaging Technique CT, MR
Clinical History

An 83-year-old female patient was admitted to the emergency department of our hospital one hour after onset of right-sided hemiparesis and right-sided hemianopia. There was no relevant medical history.

Imaging Findings

Computed tomography (CT) of the brain revealed no intracranial haemorrhage nor imaging signs of acute ischaemia (Fig. 1). Incidentally, an enlarged left frontal sinus with unilateral osseous expansion of the adjacent frontal bone and the greater wing of the sphenoid was seen. Moreover, the affected bones were sclerotic with areas of interspersed radiolucency (Fig. 1,2).

Subsequent magnetic resonance imaging (MRI) was performed the next day to confirm the clinical suspicion of ischaemic stroke and to exclude haemorrhagic transformation after tissue Plasminogen Activator (tPA) administration. MRI revealed a subtle focus of acute ischaemia in the left lentiform nucleus, visualised as a hyperintense signal on FLAIR with diffusion restriction on diffusion weighted imaging (DWI). The osseous lesion in the left frontal bone demonstrated intermediate signal intensity on T1-weighted imaging and predominantly high signal on T2-weighted imaging. Gadolinium enhancement was inhomogeneous but without any extra-axial nor intra-axial enhancement of the brain (Fig. 3).


Pneumosinus dilatans (PSD) refers to an air-filled paranasal sinus, abnormally enlarged beyond the normal boundaries of the skull bones, and in the absence of osseous erosion, hyperostosis, or mucous membrane thickening [1]. Although the condition was first described by Meyes in 1898, the term ‘pneumosinus dilatans’ was coined by Benjamins in 1918 [2].

PSD is a rare condition with an unknown true incidence [3]. It occurs most frequently in the frontal sinuses, followed by the sphenoid, ethmoid, and maxillary sinuses [4].

The imaging characteristics for PSD are straightforward on both CT and MRI, i.e. expansion of a paranasal sinus with normal wall thickness [3]. PSD is often an incidental finding [3,5]. Unilateral frontal bossing may be present as a clinical sign [6].

The key role of imaging is to evaluate the presence of underlying conditions [3]. There is a documented association between PSD and fibrous dysplasia [7], meningiomas [3], arachnoid cysts [8], port-wine stains [9], hydrocephalus [10], but it can also be idiopathic [5]. PSD may cause spontaneous pneumocephalus [11].

The pathophysiology of PSD remains unclear, although many hypotheses have been postulated. The most cited theory comprises a one-way valve, creating a pressure gradient, thus increasing the outward pressure on the sinus wall [3,12,13]. A traction phenomenon due to an adjacent meningioma with subsequent bone remodelling has been suggested as well [3]. Whether these theories can be extrapolated to other causes of PSD such as fibrous dysplasia is still debated.

Fibrous dysplasia (FD) is a non-hereditary, benign bone disease. It is characterised by abnormal osteoblastic differentiation and maturation, leading to focal replacement of normal bone tissue by fibrous stroma and islands of immature bone [14]. FD manifests as an expansile bone lesion with smooth cortical contours. Its most common appearance on CT is that of ground-glass density, but it may be homogeneously sclerotic and even cystic [14,15]. MRI appearance is highly variable due to the variation in cellularity of FD. T1-weighted imaging may yield low to intermediate signal intensities, T2-weighted imaging may demonstrate low to high signal intensities. Similarly, gadolinium enhancement is highly variable [15].

Treatment of PSD is directed at surgically correcting the cosmetic deformity of the skull bone, and endoscopic restoration of sinus drainage [5].

In conclusion, the importance of pneumosinus dilatans is to recognise this type of sinus expansion as a clue to potential underlying pathologies.

Written informed patient consent for publication has been obtained.

Differential Diagnosis List
Pneumosinus frontalis dilatans with associated craniofacial fibrous dysplasia
Paget’s disease of bone
Skull vault haemangioma
Final Diagnosis
Pneumosinus frontalis dilatans with associated craniofacial fibrous dysplasia
Case information
ISSN: 1563-4086