Allergic Fungal Rhinosinusitis
Allergic fungal sinusitis (or allergic fungal rhinosinusitis; AFRS) is a term introduced by Robson in 1989 to describe a constellation of unusual findings in a unique group of patients suffering from chronic sinusitis. Although AFRS represents the most common type of fungal sinus infection, our understanding of this disease is still limited.
AFRS is initiated when an atopic individual is exposed to inhaled fungi. It is estimated that an active man will inhale approximately 5.7 x 107 spores of various species within a 23 hour period. The fungi deposits within a sinus cavity, and an escalating immunologic reaction, Gell and Coombs types I (and possibly III), takes place to the non-invading organism. Mucosal edema, stasis of secretions, and an inflammatory exudate all combine to obstruct the sinus ostia. The process may then expand to involve adjacent sinuses and may produce sinus expansion and bony erosion. Secondary bacterial infection may also occur, simulating an acute exacerbation of underlying chronic sinus disease.
AFRS must be distinguished from other forms of fungal rhinosinusitis. It is helpful to consider fungal rhinosinusitis along an immunologic spectrum with reference to the host, irrespective of the offending fungi. The immunocompromised patient may develop invasive fungal rhinosinusitis, which is often rapidly fatal. The immunocompetent host may also develop invasive rhinosinusitis, though the course is generally much more chronic. Mycetomas, or fungus balls, occur in immunologically competent hosts and are almost never invasive. Saprophytic fungal aggregates are often seen endoscopically growing on dried crusts of mucus in areas of mucociliary transport disruption; these crusts with their saprophytic fungal growths seem to have no clinical importance. At the opposite end of the spectrum from the immunocompromised is the immunologically �hypercompetent,� the atopic patient. It is in this host that AFRS manifests.
Symptoms of AFRS
Patients with may be similar to other chronic rhinosinusitis patients. The patient typically has a history of nasal polyposis, and may have had prior sinonasal surgery. The patient may have documented atopic disease. Proptosis is common in children with AFRS. Seventy-five percent of patients who are ultimately diagnosed as having AFRS describe expelling darkly-colored rubbery nasal casts. This is composed of the tenacious allergic mucin in which the hyphal forms can be found.
Bent and Kuhn developed the following diagnostic criteria for: 1) type I hypersensitivity, 2) nasal polyps, 3) characteristic CT scan findings, 4) positive fungal stain or culture, and 5) allergic mucin with fungal elements and no tissue invasion. Marple utilizes the same diagnostic criteria, though he excludes 4) as a negative culture may be due to laboratory inexperience, and a positive culture may represent a saprophytic growth of fungi. In actuality, the atopic patient who lacks polyps, a characteristic CT and/or positive fungal cultures, but has the characteristic histopathology of allergic mucin with hyphal elements is still diagnosed as having AFRS.
The incidence of polyposis in AFRS is almost 100%. Nasal polyposis is a nonspecific indicator of chronic nasal inflammation, and patients undergoing FESS for polypoid rhinosiusitis will be expected to be afflicted with AFRS in 5 to 10% of such cases. Preoperative steroids reduce nasal polyposis and facilitate identification of surgical landmarks, but may obfuscate the diagnosis of AFRS. Graham and Ballas described such a case in which preoperative high dose steroids resolved the eosinophilic mucin. The pathologist saw only a amtte of hyphae and diagnosed a mycetoma. Only when the steroids were removed postoperatively then the AFRS recurred and the diagnosis was confirmed by characteristic histopathology.
Imaging in AFRS
AFRS CT scan findings are characteristic. Central areas of hyperattenuation within the sinus cavity on CT scan correspond to areas of hypointensity on T1-weighted MR images, and signal void on T2-weighted MR images. These central ares represent the proteinaceous allergic mucin. The central high attenuation seen on CT scan may take on various patterns including a star-filled sky, ground glass, or serpiginous pattern. Boney loss is common as the expanding inflammatory lesion pushes and thins surrounding bone. The disease may progress from the frontal sinus into the orbit below, or posteriorly through the posterior table to involve the anterior skull base. Disease may spread laterally from the ethmoid region to erode the lamina papyracea. Although exposed, the soft tissue barriers of dura and periorbita are not invaded by the fungus. Almost half of AFRS patients have only unilateral disease, although involvement of the nose and contiguous sinuses is common.
Pathology in AFRS
The mucus evacuated from AFRS patients is indistinguishable from the mucoid impactions of patients with ABPA. The mucus is thick and tenacious; its color ranges from tan to green, brown or black.
As mentioned, species of the dematiaceous family are most commonly the cause of AFRS. Examination of the allergic mucin with a hematoxylin and eosin stain reveals eosinophils, Charcot-Leyden crystals and, possibly, fungal hyphae within a background of eosinophilic or basophilic mucinous material. The Charcot-Leyden crystals are composed of lysophospholipase, and range in size from 2 to 60 micrometers. The crystals are depicted especially well with a Brown-Brenn stain. Gomori methenamine-silver stain is typically utilized to visualize the fungal elements within the allergic mucin. Fontana-Masson stain may actually allow the microscopist to more easily distinguish the dematiaceous fungi from other septate fungi, as it stains the melanin which are characteristic of the former group and also provides a more clear background.