You Can’t Hide From the B-scan

It is fairly common to encounter patients with dense cataracts that do not allow the doctor a very good view of the posterior structures of the eye. These structures include the optic nerve and retina. In our office we use the ScanMate Ultrasound B-unit from DGH to better evaluate the retina and optic nerve when we cannot directly view these structures with standard exam techniques.

Retinal detachments are easily picked up with the B-scan. This instrument is invaluable when the doctor cannot get a good peripheral view of the retina due to cataracts.

Even when the view of posterior eye structures is unimpeded, B-scan can come in handy. Buried drusen in the optic nerve can cause visual field loss. A B-scan can confirm that what appears to be buried drusen really is, and not a more serious optic nerve problem.

The B-scan experience is very comfortable for the patient. We obtain data through a closed eye, so no “goopy stuff” needs to be applied to the open eye when using the probe to obtain an image.

Not - So - Peripheral Vision

At my office, we do a visual field screening test with almost every standard eye examination. This is sometimes incorrectly referred to as a “peripheral vision test”. My Humphrey-Zeiss automated visual fields tester can measure the visual field out to the far periphery, but during the screening mode it only tests the central 30° of the visual field. The vast majority of the neurological disorders we are looking for will be picked up in this 30° central test.

Two main disorders we are looking for are brain tumors and strokes. Both of these disorders can cause a visual field loss because they disrupt the “wiring” in the visual pathway in the brain. The visual cortex, or “seeing” part of the brain, is located at the very back of the cranium. The “wires” in the visual pathway that connect the retina in the eyes to the visual cortex travel through the parietal and temporal lobes of the brain. The “wires” responsible for the left half of our visual field travel in the right side of the brain, and those for the right half of the visual field travel in the left side of the brain. Therefore, any disruption of the wires (from, say, a stroke or a tumor) on the left side of the brain can cause a visual field loss on the right. Problems on the right side of the brain will manifest in the left half of the visual field. When visual fields are screened, careful attention is paid looking for field loss present in both eyes symmetrically on either the left or right half of the field.

It's Good to Be Thick (In Some Places):

Eye pressure is nearly always measured during a comprehensive eye examination. It is an important measurement because high fluid pressure in the eye can cause the gradual death of the optic nerve. This disease process often leads to visual field loss and sometimes central acuity loss, and is known as glaucoma. Other risk factors exist for glaucoma other than high eye pressure. One important risk factor is central corneal thickness.

When eye pressure is measured, the cornea is flattened (applanated) from its normal convex shape to a flat (Plano) state. This flattening is achieved with a puff of air (non-contact tonometry) or with a probe (Goldmann tonometry). The amount of force required to flatten (applanat) the cornea is measured and converted to a fluid pressure in millimeters of mercury. Typical normal eye pressure runs from 8 to 21 millimeters of mercury. When the cornea is flattened, the calculations used to convert the force required to flatten it to fluid pressure, assume that the cornea is “average” thickness. Average is about 550 microns thick centrally.

I was taught in optometry school 30 years ago that almost all corneas are average thickness. We now know that this is not true. Since thin corneas are a contraindication for some refractive surgeries (i.e. LASIK), many practioners have ultrasound–A units that measure central corneal thickness quickly and easily. In my office, use of my ultrasound–A unit (pachymeter) has shown a wide variance among patient’s corneal thickness. It is not unusual to see thick corners (580 microns and higher) or thin corneas (520 microns and lower). Since it is easier to flatten a thin cornea, most experts’ feel that a patients true eye pressure is actually higher than that measured if the patient has a thin cornea. Patients with thick corneas probably have lower pressure than that measured.

Thin corneas are an independent risk factor for glaucoma. Irrespective of a patient’s true pressure, the theory goes that patients with thin corneas probably have other structures in the eye that are thinner than normal, which may be more easily damaged by fluid pressure. I always tell patients it’s good to be thick in some places. The cornea is definitely one of those places.