Paranasal sinus mucocoele

Paranasal sinus mucocoeles represent complete opacification of one or more paranasal sinuses by mucous, often associated with bony expansion.

Clinical presentation

Clinical presentation depends on two factors:

1. location and direction of expansion
2. presence of infection

Mucocoeles distort local anatomy and exert pressure on adjacent structures as they enlarge. Examples include:

• frontal mucocoele extending into the front of the orbit, presenting as a mass
• posterior ethmoidal mucocoele compressing the orbital apex
• sphenoidal mucocoele extending posteriorly impinging upon the pituitary and brainstem
• maxillary mucocoele may elevate the orbital floor and result in proptosis 

If a mucocoele becomes infected presentation is similar to acute sinusitis, with potential extension of infection into adjacent spaces:

• intracranial - subdural empyema, meningitis, cerebral abscess
• orbit - subperiosteal abscess
• subcutaneous - e.g. Pott's puffy tumour

Pathology

Mucoceles most likely occur as a result of obstruction of the ostium of a sinus with resultant accumulation of mucus and eventual expansion of the sinus. Some authors (the minority) believe that they represent a mucous retention cyst which gradually enlarges, eventually filling the whole sinus ². Chronic non-invasive fungal sinusitis has also been associated with the formation of mucocoeles ^2-3. From the point of view of radiologists which of these hypotheses is correct is immaterial. 

The content of the mucocele is variable, and this impacts on the imaging appearances (see below). Most simple mucocoeles are formed by clear thick mucous. If the mucocoele has been infected (pyocoele) then the content is similar to pus. 

Location

The frontal sinus is particularly prone to developing a mucocoeles, and up to two thirds of all mucocoeles occur there. The ethmoidal sinuses are the next most common, whereas maxillary and sphenoidal sinuses are infrequently involved ². 

Associations

• cystic fibrosis - particularly if occurring in children ⁴

Radiographic features

Mucocoeles are best imaged with a combination of CT (to assess bony changes) and MRI (to assess any extension into the orbit or intracranial compartment).

Plain film

Plain films do not have a significant role to play in the diagnosis of mucocoeles. If obtained they demonstrate opacification and expansion of the affected sinus.

CT

The affected sinus is completely opacified, and the margins expanded and usually thinned. Areas of complete bony resorption may be present resulting in bony defect and extension of the 'mass' into adjacent tissues ^2-3. Peripheral calcification is sometimes seen ². Following administration of contrast only peripheral enhancement (if any) is seen.

The content of the sinus is variable, depending on the degree of hydration, ranging from near water density, to hyperdense as secretion become increasingly thick and dehydrated ³.

MRI

MRI signal intensity is very variable and depends on the proportions of water, mucus and protein ^1-3.

• T1 
  • water rich content - low signal (most common)
  • protein rich content - high signal
• T2 
  • water rich content - high signal (most common)
  • protein rich content - low signal
• T1 C+ (Gd) - enhancement if present, only occurs at the periphery
• DWI - variable

It should be noted that colonisation with fungus can lead to very low signal on both T1 and T2 weighted sequences, mimicking a normal aerated sinus ³.

Treatment and prognosis

In general drainage and marsupialisation of affected sinus is sufficient. If large bony defects are present, reconstructive surgery may be necessary (e.g. orbital floor). If drainage remains impaired, recurrence may occur.

Differential diagnosis

There is often little if any differential, provided contrast has been administered. Differential includes:

• paranasal sinus tumours e.g inverted papilloma or sinonasal carcinoma
  • most of these enhance
• mucus retention cyst
  • does not completely fill the sinus (by definition)
  • no bony expansion
• antrochoanal polyp
  • does not completely fill the sinus
  • focally protrudes through the osteomeatal complex
• acute sinusitis
  • no bony expansion