Freckle in the wrong place

Juan Ding, OD, PhD

Choroidal nevus is a collection of pigmented cells in the choroid, a thin layer of tissue just underneath retina. In layman’s term we often refer nevus to a ‘freckle’. Traditionally choroidal nevus is considered a benign condition. True, only about 1 in 8000 of nevi will turn into a melanoma. So the vast majority of choroidal nevus is benign. But does that mean that they don’t cause a problem? The answer is it depends. For some unfortunate people, it does cost their vision.

I recently had the pleasure of meeting a gentleman in his 60s who had vision loss in his right eye for 20+ years. The cause? A choroidal nevus that happened to be in the worst location, under the fovea. This is where our central vision is, so as a result of a nevus growing there his visual acuity was 20/400. He used to see a renowned ocular oncologist for many years and it remained stable for years, meaning it’s not turning into a melanoma. However, due to the unfortunate location of this freckle, his right eye cannot see details at all.

So given that this is chronic and stable, he was in the clinic actually complaining about something else. He has developed double vision and had to pull over while driving because of seeing two roads. Important to note, even though the nevus took away his central vision, his peripheral vision was quite normal and he relied on peripheral vision of both eyes to drive. I measured and found that he had a mild esophoric deviation, this means that his eyes are converging a little bit too much towards each other. But normally this is a small misalignment and most people compensate for this well. In his situation though, because his right eye does not see too well, it is not able to send a clear signal to the brain which then in turn is not able to feed back to both eyes to fuse the images into single vision. 

Solution to this double vision issue? A small amount of prism made it go away.

So how does this freckle that has such a big impact on his vision look? Figure 1 below is a color photo of this freckle, which was in the center of the eye.

Figure 1. Fundus image of a choroidal nevus at fovea. The arrow points to a black blob which is the nevus.

Figure 2 below shows the cross section showing how thick this freckle is.

Figure 2. Cross section view of the fovea with nevus under the retina.

So are choroidal nevi good or bad? It appears that they may not be as benign as we often believe. A retrospective medical review looked at more than 3000 eyes with stable choroidal nevi and found that of those that had nevi under the fovea, 26% developed vision loss over 15 years [1]. And it is not even that rare to have nevi under fovea, about 6% of choroidal nevus patients in this review series had their nevi right under the fovea [1]. So what is the mechanism for reduced vision if the nevi are stable? It appears that even though the size and thickness of the nevi may not change over time, they may cause edema of the retina, detachment of the RPE and thinning of the photoreceptors. If these changes occur at the fovea, then you get reduced central vision; if in the periphery as in most cases, there may be a peripheral visual field defect. It is not rare to have visual field defects from choroidal nevi; in fact, it is quite common to have field defects, ranging from 38% to 85% depending on the population analyzed [2, 3]. 

In my patient’s case, you can see that it is not even a large or thick nevus. However, it is right in the fovea and there is a thin layer of fluid between RPE and the retina, and the foveal region looks significantly thinned out (Figure 2).

It is easy to overlook choroidal nevus, because it is so commonly seen in an eye doctor’s office, and it rarely converts to melanoma. However, it is important to realize that even though it is not a cancer, it is a tumor all the same. It is not just an inert growth under the retina, but rather can often slowly but actively cause changes to the retina, affecting its function. If it happens to be under the fovea, significant vision loss can happen to patients. 

References

[1] Shields CL, Furuta M, Mashayekhi A, et al. Visual Acuity in 3422 Consecutive Eyes With Choroidal Nevus. Arch Ophthalmol. 2007;125(11):1501–1507. doi:10.1001/archopht.125.11.1501

[2] Tamler  EMaumenee  AE A clinical study of choroidal nevi.  AMA Arch Ophthalmol 1959;62 (2) 196- 202

[3] Flindall  RJDrance  SM Visual field studies of benign choroidal melanomata.  Arch Ophthalmol 1969;81 (1) 41- 44

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