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Atopy, proptosis, and nasal polyposis

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Q1: What is the diagnosis?

The diagnosis is allergic fungal sinusitis. The diagnosis is based on the clinical features of nasal polyposis and atopy, presence of allergic mucin, characteristic computed tomography features of soft tissue masses interspersed with hyperdense areas filling the sinuses, typical histopathological appearance of allergic mucin, and isolation of Aspergillus flavus on culture.

Q2: What is allergic mucin?

Allergic mucin is the sine qua non of allergic fungal sinusitis. It appears as a cheesy or mucinous, brown, black, or dark green clump of amorphous material interspersed with nasal polyps. It may be seen either on direct nasal examination or at endoscopy. Histological examination of allergic mucin (fig 2; see p 343) reveals eosinophilic or basophilic mucus with an infiltrate of cells which are chiefly eosinophils and many of which are necrotic. The cells are arranged in layers. Fungal hyphae, which are visualised by haematoxylin and eosin or special stains like Gomori-methenamine-silver, may be seen lying loose in the mucus. Charcot-Leyden crystals may also be seen.

Box 1: Diagnostic features of allergic fungal sinusitis

  • Recurrent nasal polyposis.

  • Presence of allergic mucin.

  • Typical histological features of allergic mucin including absence of tissue invasion by fungus.

  • Positive fungal culture; aspergillus species and dematiacious fungi usually isolated.

  • Presence of atopy as demonstrated by history, skin tests, or serology.

  • Immunocompetent patient.

Q3: What is the pathophysiology of the disease?

Allergic fungal sinusitis is believed to be the result of Gell and Coombs type I and type III hypersensitivity reaction to a fungal antigen resulting in chronic inflammation and nasal polyposis.1 This is evidenced by the fact that these patients have relatively high eosinophil counts, positive skin allergy tests, and raised total serum IgE levels. Serological evaluation by the radioallergosorbent test or enzyme linked immunosorbent assay also reveals raised IgE and IgG levels to specific fungal antigens like aspergillus or certain dematiacious fungi in a high proportion of these patients.2 Histological studies have shown that while there is fungal colonisation, there is no tissue invasion.

Discussion

Allergic fungal sinusitis is a clinical entity which was first described in 1983 by Katzenstein et al,3 drawing an analogy from allergic bronchopulmonary aspergillosis in which thick mucus plugs, similar to those seen in the sinuses of patients with allergic fungal sinusitis, are seen in the bronchi. Patients with allergic fungal sinusitis classically present with recurrent nasal polyposis, history of having had several nasal surgeries in the past and a strong history of atopy. Unlike patients with simple nasal polyposis, these patients have allergic mucin interspersed with polyps. The presence of eosinophilia, raised IgE levels and positive skin test results to fungal antigens suggest an underlying type I or IgE mediated hypersensitivity in these patients. Similarly, the presence of raised fungal specific IgG antibody levels is indicative of an underlying type III hypersensitivity.

Fungal infections of the sinuses have been classified as mycetoma, non-invasive and invasive fungal sinusitis.1 Unlike these conditions, allergic fungal sinusitis is an entity which has been classified separately because it is not a true fungal infection. It is believed to be an allergic response to a fungal antigen. Histologically, there is no tissue invasion. This is the case even in patients with clinical and radiological evidence of extensive disease. Patients with allergic fungal sinusitis are not immunodeficient. Fungal culture isolates in these patients include aspergillus species likeA flavus or A fumigatus or dematiacious fungi likeCurvularia,Alternaria, orExserophila species.4 These fungi are ubiquitous in nature with a prediliction for soil, plants, and damp areas.

Learning points

  • Allergic fungal sinusitis is a disease caused by a type I and type III hypersensitivity reaction to certain fungal antigens.

  • There is no histological evidence of tissue invasion by fungus; most authors believe that it is not an infection.

  • Allergic mucin interspersed with polyps is the characteristic clinical finding.

  • Diagnosis is based on certain definite criteria (box 1).

  • Appearance on computed tomography is fairly typical.

  • Treatment is by surgical excision with adjunctive topical steroid; the use of oral steroid and antifungal drugs is recommended by some.

Diagnostic criteria for allergic fungal sinusitis are presented in box 1. Some authors include the appearance of the computed tomogram as a diagnostic criterion.5 Soft tissue densities involving the sinuses with hyperdense areas representing allergic mucin, are the usual features. Allergic mucin shows up as hyperdense areas because of the high content of calcium and magnesium salts within. Occasionally areas of bone erosion or expansion causing extension of disease to adjacent structures like the orbit and anterior and middle cranial fossa may be present. In such cases, differentiation from malignancy is difficult and correlation with clinical and intraoperative findings is imperative.

The mainstay of treatment in allergic fungal sinusitis is surgical excision of polyps and allergic mucin and provision of aeration of the sinuses. This may be achieved by endoscopic sinus surgery or by open procedures like Caldwell-Luc or lateral rhinotomy approaches. Adjunctive steroid treatment given topically as well as orally is recommended by most authors.1 3-6 Some authors recommend antifungal drugs like itraconazole also on the basis that there may be some element of infection, despite the absence of tissue invasion.5 However, their use is controversial.

Final diagnosis

Allergic fungal sinusitis with proptosis.

References

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