Monday, December 22, 2014

Glistenings in pseudophakic vision.

cc Rakesh Ahuja, MD. Aftercataract - Posterior capsular opacification post-cataract surgery (seen on retroillumination)

As we age, our crystalline lens becomes cloudy and we call it a cataract, maybe because the world is seen as from behind a large foaming waterfall. Already the Romans carried out cataract operations 2000 years ago, so the medical remedy is pretty much routine: the cataract is removed surgically and replaced with an intra-ocular lens (IOL). Such an IOL is generally known as a pseudophakic IOL and provides the light focusing function originally undertaken by the crystalline lens.

The ancient Greek word for lens is phakos, so phakia is the presence of the natural crystalline lens. Pseudophakia is the substitution of the natural crystalline lens with an IOL.

One problem of the pseudophakic patient is that sometimes in the weeks or months after the surgical procedure, visual discomfort due to glistenings is experienced. The glistenings are due to micro-vacuoles in the IOL, the vacuoles being part of the polymer's structure. After the IOL has been implanted, water can fill these vacuoles and water has a different refraction index (1.33) than the polymer ~1.55).

What was not known is how much the glistenings impact visual performance. One research technique was to measure the MTF. However, although MTF is related to visual acuity, it is not related to global contrast. and does not explain the visual discomfort. Alessandro Franchini, Andrea Romolo and Iacopo Franchini implemented a ray-tracing program to model and analyze the effect of the vacuoles.

They found that when a light source is in the field of view, without glistenings a clear secondary image is produced, but with glistenings light scattering introduces noise on entire visual field, reducing the global contrast.

The solution is to use hydrophobic acrylic lenses and to keep them in water before implanting them. With this, the IOL will contain 4% water instead of the usual 2%. After the lens is implanted, there will not be the current of liquids that causes the glistenings.

Citation: Alessandro Franchini, Andrea Romolo and Iacopo Franchini, Effect of glistenings on the pseudophakic patient vision, Atti della Fondazione Giorgio Ronchi, Vol. LXIX, N. 5, pp. 589–599.