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Editor,—Several lesions of the iris can clinically resemble melanoma.1 2 Failure to recognise a simulating condition could lead to misdirected therapy. The most common iris pseudomelanomas include primary iris cyst, iris naevus, essential iris atrophy, iris foreign body, peripheral anterior synechia, and iris metastasis.2
An iris foreign body can occasionally pose a diagnostic challenge, especially in the absence of a history of ocular trauma. We present a case of an iris foreign body that resembled a melanoma, in which ultrasound biomicroscopy (UBM) was instrumental in the diagnosis.
In June 1999, a 70 year old man was found on routine examination to have a brown lesion in the inferior part of his left iris. Two months later the lesion showed a possible increase in size, prompting a referral to the oncology service of the Wills Eye Hospital to rule out iris melanoma.
His visual acuity was 20/20 in both eyes and the intraocular pressures were normal. There was no melanocytosis, heterochromia, corneal pigmentation, corneal scar, endothelial defect, cells in the anterior chamber, or pupillary abnormalities. There was an irregular brown lesion in the left iris inferiorly, measuring 2 × 1 mm in diameter (Fig 1A). There were a few light brown refractile deposits on the surface of the lesion. Gonioscopy showed the lesion extending onto the trabecular meshwork without trabecular seeding but with adjacent peripheral anterior synechiae (Fig 1B). There was no intrinsic vascularity, feeder vessel, iris or angle neovascularisation, or pigment dusting. The lens was clear and the fundus in each eye was normal.
Although the lesion superficially resembled a melanoma, we felt it could be a foreign body based on the presence of focal brown deposits over the lesion, which resembled rust particles. On repeated specific questioning, the patient recalled trauma to the left eye over 50 years earlier, while hammering metal.
Ultrasound biomicroscopy3 demonstrated a highly reflective structure in the iris and anterior chamber angle, with absence of echoes posterior to the lesion, characteristic of a foreign body (Fig2A). In addition, a defect in Descemet's membrane was noted in the upper part of cornea just off the pupillary axis (Fig 2B). Subsequent slit lamp biomicroscopy with high magnification confirmed the minute defect in Descemet's membrane. Electroretinography was not performed because there was no clinical evidence of siderosis.
Several conditions can simulate iris melanoma.1 2 In a study from the Armed Forces Institute of Pathology, Ferry reported 24 pseudomelanomas among 69 eyes that were enucleated for suspected iris melanoma, a diagnostic error of 35%.1 The lesions that most commonly simulated iris melanoma in his series included anterior staphyloma, inflammatory mass, iris stromal atrophy, and corneal perforation.1 Additionally, he noted one enucleated eye with a metallic foreign body misdiagnosed as an iris melanoma. In a clinical study of 200 patients referred for an iris lesion suspected to be melanoma, Shields and associates found that 76% were pseudomelanomas.2 Iris foreign body accounted for 4.5% of pseudomelanomas in that series, and in each instance, the patient was unaware of the foreign body and could not recall ocular trauma. However, a defect in Descemet's membrane was detected on slit lamp biomicroscopy in each case.
Diagnosis of a foreign body may sometimes be difficult in the absence of a history of injury, especially when slit lamp biomicroscopy of the anterior segment does not reveal any sign of ocular trauma. Imaging modalities such as computed tomography and ultrasonography may help in such situations. However, magnetic resonance imaging is contraindicated in cases where a ferromagnetic foreign body is suspected. Standard B-scan ultrasonography has been replaced at many centres by UBM for the evaluation of anterior segment foreign bodies.3-5 We used UBM to confirm our clinical impression of an iris foreign body in this case. In addition to characterising the foreign body, UBM detected a subtle irregularity in Descemet's membrane.
It is of interest that a seemingly metallic foreign body could remain inert for 50 years without causing ocular inflammation, siderosis or secondary glaucoma, which prompted us to choose to observe this patient rather than to intervene surgically.
When the ophthalmologist is confronted with a dark iris lesion, an intraocular foreign body is a possibility, irrespective of a negative history of trauma. Slit lamp biomicroscopy coupled with gonioscopy provides excellent diagnostic clues. We believe that UBM is a sensitive imaging technique for confirmation of the diagnosis.
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