Allergic fungal sinusitis

Allergic fungal sinusitis is the most common form of fungal sinusitis and is common in warm and humid climates. On imaging, it usually presents as opacification and expansion of multiple paranasal sinuses, unilaterally or bilaterally, with content that is centrally hyperdense on CT. MRI shows T2 hypointensity centrally due to the dense fungal concretions and heavy metals.

Epidemiology

Allergic fungal sinusitis accounts for 5-10% of chronic hypertrophic sinus disease going to surgery. It is seen in young immunocompetent patients (mean age range 23-42 years). In children, M:F ratio = 2:1 and in adults, M:F ratio = 1:1.4.

Associations

• asthma in 65% of cases

Clinical presentation

Patients may present with chronic headaches, nasal congestion, chronic sinusitis for years, and past sinus surgery.

Pathology

Allergic fungal sinusitis is an allergic reaction to aerosolized environmental fungi (type 1, IgE-mediated hypersensitivity reaction). Expansion, and even erosion (20%), of the sinuses is characteristic. 

Commonly implicated fungi are ¹:

• dematiaceous (pigmented) fungi: Bipolaris, Curvularia, Alternaria
• hyaline molds: Aspergillus, Fusarium

Distribution

Allergic fungal sinusitis usually involves multiple sinuses and can be unilateral or bilateral (the latter is commoner). There is a frequently a nasal component. The ethmoid sinus is the most common location, followed by the maxillary, frontal, and sphenoid sinuses. 

Radiographic features

CT

The majority of sinuses show near complete opacification. On unenhanced CT, the sinuses are typically opacified by centrally (often serpiginous ⁷) hyperdense material with a peripheral rim of hypodense mucosa. 

Approximately 40% of patients may have each of the following features ⁴:

• expansion of an involved sinus
• remodeling and thinning of the bone sinus walls
• erosion of the sinus wall

MRI

Hypointensity on T1WI and T2WI is the most common finding. 

• T1: hypointense inflamed mucosal thickness. It can have multiple T1 appearances. 
• T2
  • usually a hyperintense peripheral inflamed mucosal thickness
  • low T2 signal or signal void is due to high concentration of various metals such as iron, magnesium and manganese concentrated by fungal organisms as well as high protein and low free water content in allergic mucin
• T1 C+ (Gd): 
  • an inflamed mucosal lining has contrast enhancement 
  • no enhancement in the center or in the majority of the sinus contents (cf. neoplasms)

Treatment and prognosis

Allergic fungal sinusitis is usually treated by local excision and steroid therapy. Antifungal therapy is also attempted in some cases, but results of this treatment are equivocal. Recurrence after surgery is not uncommon; however, the inclusion of steroid therapy significantly reduces relapse.

Differential diagnosis

• sinonasal polyposis
• sinus fungal mycetoma
  • it represents a fungal colonization without hyperimmune response
  • usually involving only the maxillary sinus 
• sinonasal mucocele
  • it has the same chronic expansive features
  • no bone erosions
• non-Hodgkin lymphoma
  • sinonasal mass homogeneously hyperdense on non-contrast CT