Orbital blastomycosis, less commonly known as Gilchrist disease, is an orbital infection from the fungus Blastomyces dermatidis. Blastomycosis is typically acquired via inhalation of conidia (spores). Blastomycosis is a systemic pyogranulomatous infection.
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Epidemiology
Even in endemic areas, blastomycosis is uncommon with an estimated annual incidence is approximately 1 case per 100,000 individuals 1.
Clinical presentation
The majority of cases will begin and remain as pulmonary blastomycosis. However, in ~40% of cases, patients will develop extrapulmonary disease usually. Extrapulmonary manifestations usually occur via haematogenous dissemination, typically involving the skin, bone, urogenital systems, and, less commonly, the central nervous system. Specifically, orbital blastomycosis may present as eye pain, ocular gaze abnormalities, decreased visual acuity, proptosis, and/or ptosis 2,3. Patients with orbital blastomycosis may develop orbital apex syndrome, where the infection compresses structures at the orbital apex 2.
Orbital blastomycosis is difficult to differentiate from malignancy (either primary or metastatic) and therefore cases often require pathologic confirmation. Given the severity of the potential of an invasive fungal infection, timely diagnosis and treatment is necessary.
Radiographic features
CT
A relatively homogeneous, enhancing soft tissue retro-orbital mass will be seen within the affected orbit. Intraconal and/or periorbital oedema is sometimes seen. Mass effect may be seen on adjacent structures, such as the globe, optic nerve, extraocular muscles, or vasculature. Depending on the chronicity of infection, bony resorption or dehiscence may be seen on CT 3.
MRI
Similar to CT, a soft tissue retro-orbital mass will be seen:
T1: isotense to mildly hyperintense
T2/FLAIR: hypointense signal in the mass itself. Perilesional hyperintense signal is expected.
DWI: hypointense
ADC: hypointense
T1 C+: avid and relatively homogeneous contrast enhancement
Treatment and prognosis
The proper treatment depends on the patient’s immunocompetency. For instance, all immunocompromised patients with progressive pulmonary disease or extrapulmonary disease should be treated. Potential treatment agents include liposomal amphotericin B or azole antifungal agents 3,5.
Immunocompetent patients typically fare well to even disseminated blastomycosis. Patients with immunocompromised patients, whether as a result of HIV/AIDS or iatrogenic (chemotherapy, solid organ transplantation), tend to have a poor prognosis 5-7.
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