The EyeWhen Shadows Float Before Your Eyes
My introduction to floaters came on a sunny September afternoon in the orchard. I was high on a ladder picking Jonagold apples when I felt a pop in my left eye, followed by blurriness. I thought that maybe a branch had slapped across my glasses, dirtying the lens, and I went inside to clean it. But the blurriness - a sort of floating haze - persisted overnight.
After hearing the symptoms, my ophthalmologist, Dr. William Kirber, diagnosed a posterior vitreous detachment, one of several causes of floaters, sensations that many people describe as specks, bugs or cobwebs floating in their fields of vision. In 85 percent of cases, the floaters caused by posterior detachment are mere annoyances, but when they occur suddenly, immediate medical examination is essential.
The most common type of floater is caused by aging. Sitting directly between the lens and the retina is a cavity known as the vitreous or vitreous humor. Its outer boundary is defined by a crust like the skin of Jell-O. The function of the vitreous, which makes up four-fifths of the volume of the eye, is to give it its shape, to be a shock absorber in younger eyes and, some argue, to nourish the inner eye.
The vitreous cavity is filled with a clear, sticky gel that is 99 percent water. The balance consists of strands of a protein called collagen and acidic molecules. With aging, the molecules break down, releasing water that gathers in pockets. At the same time, strands of collagen, normally translucent, bunch into larger opaque fibrils or wispy sheaths.
It is these clumps of protein and water, of varying shape, meandering unpredictably within the vitreous cavity, that are the source of most floaters. As light passes through the vitreous, floaters cast annoying shadows on the retina, the eye's movie screen. Most adults experience this type of floater at some time.
Much less common is a second type, floaters caused by specks or globs of blood, the result of bleeding brought on by rips in the retina or as a complication of diabetes.
In each case, the brain almost always gets used to the floaters or they migrate out of view. The third kind of floater, however, caused by a posterior vitreous detachment, is far more likely to impair vision.
In posterior vitreous detachment, as the fluid from the degenerating vitreous gel moves between the skin of the vitreous and the retina, the vitreous separates from the retina, usually near the optic nerve. As the vitreous tears from the retina, a fragment of tissue remains attached to the receding vitreous.
Because the tissue is tethered to the vitreous, this fragment, known as Weiss's ring, sometimes remains in the central field of vision. Under the ophthalmologist's slit lamp, Weiss's ring resembles a blob of oil floating on water under a lattice of blood vessels. How disabling this vitreous detachment is depends on how far the vitreous peels from the retina.
Posterior vitreous detachments occur in fewer than 10 percent of people under 50, but in 60 percent of people over 70. In 10 to 15 percent of detachments, as the vitreous separates, it tears the retina, requiring immediate treatment, either with a laser or through surgery.
The question is, When are floaters alarming and when are they benign?
"Almost everyone has floaters," Dr. Jim Garrity, a Mayo Clinic ophthalmologist, stresses in a periodic lecture he offers for general practitioners. "It is the sudden onset of the tiny ones accompanied by flashes of light that you have to be concerned about."
Many ophthalmologists underrate floaters because they cannot be visualized precisely and because the potential complications from removing the vitreous, the only sure remedy, heavily outweigh the benefits, some experts say.