Anterior segment optical coherence tomography and ultrasound biomicroscopy in the diagnosis of subconjunctival mycosis mimicking nodular scleritis

Background

Subconjunctival mycosis (fungal ball) is a rare presentation and can be easily misdiagnosed as nodular episcleritis or scleritis. Sporothrix schenckii, a dimorphic fungal pathogen, has been identified as the most common aetiological agent.1–4 Previously published case reports and series have shown that these lesions respond well to antifungal agents, such as topical fluconazole, oral itraconazole or potassium iodide.1–4 In this case report, we report the utility of ocular imaging techniques in diagnosing subconjunctival mycosis and eliminating other differential diagnoses.

Case presentation

A woman in her 20s presented with reports of a painless, progressive mass lesion over the conjunctival surface of her left eye. The problem was observed over a month prior to the visit. She was diagnosed with nodular scleritis in another hospital. She was treated with topical and oral prednisolone in tapering doses for 1 month. There was no history of penetrating trauma or foreign body entry. On examination, her vision was 20/20 in both eyes. A solid, non-mobile, congested nodular lesion was observed over the inferotemporal bulbar conjunctiva (figure 1A). The lesion showed internal transilluminance on an optical section of the slit-lamp biomicroscope (figure 1B). Our main differential was conjunctival cysticercosis, and we decided to investigate further.

Investigations

She underwent a spectral domain anterior segment optical coherence tomography (AS-OCT) (Optovue, Fremont, California) scan which revealed a smooth nodular elevated lesion with thickened zones of hyperreflectivity (figure 1C,D) caused by the compaction of the subconjunctival tissue, suggesting a cyst wall. The posterior extent of the lesion could not be delineated because of the resolution limits of the machine. Ultrasound biomicroscopy (UBM) (Aviso S, Quantel Medical, Paris, France V.5.0.0) confirmed that the lesion was confined to the subconjunctival space. It also showed the uniform hyperechoic nature of the mass without extending to the anterior sclera. No hyperreflective lesion corresponding to a scolex was observed (figure 1E,F).

Differential diagnosis

The patient was initially diagnosed with nodular scleritis. Clinically, the lesion appeared raised with congestion and tenderness similar to nodular scleritis. However, the lack of severe pain at onset and no improvement despite treatment with steroids for adequate duration ruled it out. Nodular episcleritis is a similar, painless differential. However, it also responds to steroids. We considered a diagnosis of conjunctival cysticercosis because of the cystic appearance and the presence of transillumination under a slit-lamp biomicroscope. However, ultrasonography confirmed the absence of the pathognomonic hyperechoic scolex within the lesion.5 Another differential diagnosis was a benign conjunctival stromal tumour. It appears similar to a subconjunctival solid mass; however, it lacks the transillumination effect.6 Finally, melanoma and amelanotic melanoma might also mimic this presentation; however, the lack of feeder vessels and the presence of transillumination rule them out. Moreover, AS-OCT would have shown its characteristic features, such as hyper-reflective subepithelial lesion with normal thickness and the appearance of a basal epithelium or an epithelial cleavage, indicating atypical melanocytes.7–9

Treatment

The patient underwent an excision biopsy of the cyst under local anaesthesia. A peritomy was performed adjacent to the lesion to excise the cyst in toto. Pus was observed on the cyst surface, oozing out of the cystic cavity. The cyst walls were necrotic and had to be excised piecemeal. The bed was gently debrided, and an amniotic membrane graft (AMG) was placed over the bare scleral area and secured with fibrin glue. The patient was started on topical levofloxacin 1.5% six times a day and oral ciprofloxacin 500 mg two times per day for a week postoperatively.

The pus was sent for microscopy (smears seen with grams and potassium hydro-oxide calcofluor white stains), and the cyst wall fragments were sent for histopathology. The pus was inoculated in blood agar, chocolate agar, potato dextrose agar and thioglycolate broth. The microbiology of the samples was negative on the smear and culture. Histopathology of the excised tissue showed plaque of fungal filaments with necrosed fibrocollagenous tissue (figure 2A). The Gomori methenamine silver stain confirmed thin septate fungal filaments with few cut ends of fungal hyphae (figure 2B,C). This established the diagnosis of subconjunctival mycosis. We started the patient on topical natamycin 5% 2 hourly (stepped down after 21 days) and oral ketoconazole 200 mg two times per day. Both medications were stopped after 36 days.

Outcome and follow-up

One week postoperatively, mild congestion was noted over the bulbar conjunctiva in the area of the cyst excision and AMG placement. The antifungals were continued for 5 weeks. Four months postoperatively, the condition and the congestion over the area of excision resolved with mild conjunctival fibrosis and the integration of the AMG (figure 2D–F). No recurrences were noted.