Diagnostic criteria of Allergic fungal sinusitis

Introduction:

Diagnosis of allergic fungal sinusitis is made from a combination of clinical, radiological, microbiologic and histopathologica information. Hence the diagnosis of allergic fungal sinusitis cannot be made with certainty till surgery is over.

Diagnostic criteria include:

1. Presence of allergic mucin: Allergic mucin is thick tenacious and dark colored. It is very difficult to remove this secretion with a suction. When suction is applied it resembles an elastic strand of tissue. In rare cases this allergic mucin may assume the shape of fungal ball. Microscopically allergic mucin contains onion skin laminations of necrotic and degranulating eosinophils on mucin background. Occasionally charcot layden crystals can also be seen. Fungal hyphae may be present but are scarce. Fungal stains are necessary to demonstrate these hyphae. These hyphae don't invade tissue. If evidence of tissue invasion is present then diagnosis of allergic fungal sinusitis should not be entertained.
2. Presence of nasal polypi with evidence of eosinophilic inflammatory infiltrate.
3. Evidence of Type I hypersensitivity
4. Imaging: CT scan paranasal sinuses reveal multiple opacified sinuses with central hyperattenuation, mucocele formation, erosion of lamina papyracea / skull base, pushing borders. Bone erosion in patients with nasal polyposis should raise suspicion of allergic fungal sinusitis

Role of MRI in the diagnosis of AFRS:

This is indicated when there are intracranial and orbital complications due to allergic fungal sinusitis. MRI reveals central low signal on T1 and T2 weighted images. These areas correspond to allergic mucin associated with peripheral high signal intensity due to the presence of oedematous mucosa.