Uveal Pseudomelanoma From Hypermature Cataract
Hypermature cataracts are very mature crystalline lenses in which cortical hyperosmolarity results in an influx of fluid. The degree of opacity frequently prohibits direct fundus evaluation, requiring ultrasound to visualize intraocular contents. Rarely, hypermature cataracts can present a posterior dome-shaped artifact on ultrasound which may be mistaken for uveal melanoma but in fact presents a pseudomelanoma. This is an important consideration as accurate identification and timely treatment of uveal melanoma is crucial.
The human lens is located posterior to the iris and is contained within the lens capsule, which is supported by suspension cables called zonules that fixate the lens to the ciliary body. This complex is responsible for accommodative changes to the lens . The lens, along with the cornea, is responsible for focusing light on the retina. If light is focused in front of the retina (i.e. the focusing power is too strong), the eye is myopic; conversely, if light is focused behind the retina (i.e. the focusing power is too weak), the eye is hyperopic. The lens is composed of epithelial cells which have replicated, elongated, and lost their organelles to become lens fibers. Lens fibers are constantly produced throughout life, with older lens fibers forming the central nucleus of the lens and newer lens fibers forming the peripheral cortex. The lack of organelles is crucial for lens clarity but results in susceptibility to oxidative damage.
Cataracts are the result of changes to the intraocular lens with progressive opacification resulting in impaired visual acuity and contrast sensitivity. Cataracts may arise from congenital, age-related, metabolic, inflammatory, and traumatic etiologies . The three main types of cataracts include cortical, wherein the peripheral cortex becomes opacified; nuclear sclerotic, wherein the central nucleus of the cataract is affected; and posterior subcapsular, wherein the affected region lies between the central posterior lens capsule and posterior cortex. Generally, cataract presentation and symptoms slowly worsen. With time, a cataract may become mature when it becomes completely opacified. A hypermature cataract occurs when the cortex of a mature cataract reaches a hyperosmotic state resulting in a net influx of fluid, causing in a tense lens capsule . Surgery remains the only treatment for cataracts. These dense cataracts pose diagnostic and surgical challenges to the ophthalmologist. The degree of opacity limits posterior visualization and ultrasound imaging is frequently required.
Ocular ultrasound is a non-invasive imaging modality frequently used to visualize intraocular contents. It involves the production of high frequency acoustic waves from a transducer which travel through a medium and are reflected back to the probe. The signal is transmitted to a receiver and subsequently an amplifier with resulting image generation. There are two types of ocular ultrasound: A-scan and B-scan.
In A-scan, short for amplitude scan, sound waves are generated at a frequency of 8 MHz. Upon interaction with a tissue interface, an echo returns to the transducer with subsequent wave or spike generation. The height of the spike illustrates the intensity of the return echo. It is affected by several factors, including the difference in density between tissue interfaces. A-scan measurement details or descriptors include length, regularity, reflectivity, and sound attenuation . Thus, A-scan can characterize the internal structure of the eye in one-dimensional format .
In B-scan, short for brightness scan, sound waves are commonly generated at a frequency of 10-20 MHz in a linear fashion . Similar to A-scan, an echo is generated on interaction with tissue and returns to the transducer with subsequent production of a two-dimensional image . The intensity of the echo is dependent on the density of the of the tissue and is termed hyperechoic if more intense or brighter than baseline and hypoechoic if less intense or darker than baseline. There are 5 views routinely used: 4 transverse views and 1 longitudinal macular view. B-scan provides anatomic visualization and characterization of lesion’s motility and relationship with surrounding tissue.
The uveal tract consists of the pigmented vascular layers of the globe including the iris, ciliary body, and choroid. Uveal melanoma is a malignant tumor arising from melanocytes within this pigmented layer and is the most common primary intraocular malignancy of adulthood. The incidence of new uveal melanoma is approximately 5.1 per million with approximately 1500 new cases diagnosed each year in the United States. Approximately 85% of uveal melanomas arise in the posterior choroid (defined as posterior to the equator); the anterior choroid (anterior to the equator), ciliary body, and iris compromise the remaining 15% . Accurate diagnosis of uveal melanoma is of the utmost importance as prompt treatment is crucial given the high risk of metastasis and subsequent mortality. Ocular ultrasound plays a crucial role in the diagnosis and prognostication of uveal melanoma.
On A-scan, uveal melanomas generally present with low to medium internal reflectivity, regular internal structure, high sound attenuation, and visible vascular pulsations. On B-scan, uveal melanoma findings include a dome-shaped or mushroom-shaped mass, choroidal excavation, and associated subretinal fluid with or without retinal detachment .
Many pathologies can simulate uveal melanoma. In a 2005 review of 12,000 cases referred for evaluation of presumed uveal melanoma, Shields et al identified that 1,739 patients actually had pseudomelanoma. The most common masquerading pathologies included choroidal nevus (851), peripheral exudative hemorrhagic chorioretinopathy (139), congenital hypertrophy of retinal pigmented epithelium (108), hemorrhagic detachment of the retina or retinal pigmented epithelium (86), circumscribed choroidal hemangioma (79), and age related macular degeneration (76) .
Cataract Induced Pseudomelanoma
Cataract is an uncommon etiology of uveal pseudomelanoma, with Shields et al identifying just 10 cases in their 2005 study. A subsequent 2013 study by Shields et al described 20 cases of hypermature cataracts simulating uveal melanoma . In contrast to other etiologies of pseudomelanoma wherein a chorioretinal lesion was confused with melanoma, those caused by cataracts were due to ultrasound findings as the cataract prohibited direct visualization of the suspected lesion.
Two of these 20 cases in Shields et al’s 2013 study involved luxated lenses, which appeared as elliptical or dome shaped masses on the retinal surface. In these patients, it was found that the lens shifted with patient positioning. In the 18 subjects with anatomically positioned lenses, it was determined that oblique positioning of the ultrasound probe in evaluating the far peripheral retina may result in a dome shaped mass concerning for melanoma. In actuality, this represents the equatorial lens sitting adjacent to the ciliary body . This is supported by identifying this pseudo-mass in all 4 quadrants. Additional ultrasound findings consistent with a cataract induced pseudomelanoma include uniform elliptical or dome shape and consistent hyperechoic cortical rim surrounding a hypoechoic nucleus with absent vascular pulsations. Following cataract surgery, there were no lesions were identified in these patients.
Hypermature cataracts frequently prevent direct visualization of the retina, necessitating use of ultrasound to assess intraocular anatomy. Oblique positioning of the ultrasound probe may result in equatorial lens artifact concerning for uveal melanoma.
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