For certain additional tests - to examine your general eye health and the retina, optic nerve, and blood vessels - the doctor will need to dilate your pupils using eye drops. These drops take about twenty minutes to fully open the pupils, giving the doctor a much wider view of the inside of your eyes than would be possible with constricted pupils.
After dilation, your vision might be blurred and highly sensitive to light for several hours. You won't want to walk out into bright sunlight with your eyes uncovered. If you don't have sunglasses, most eye doctors will give you disposable sunglasses to wear on the way home. Since there's no way of knowing how long it will take your eyes to return to normal, the best course is to arrange for someone to drive you home.
Ophthalmoscopic Examination
An ophthalmoscope is a specialized device through which your doctor can inspect the blood vessels and the optic nerve at the back of the eye. He or she will also examine the retina for detachment and tears, and the small areas on the retina responsible for sharp vision (the fovea) and central vision (the macula).
Slit-Lamp Examination
A slit lamp, or biomicroscope, allows the doctor to see signs of infection or disease at the front of the eye, including problems in the eyelids, cornea, conjunctiva (the thin, transparent membrane that protects the front of the eye), and iris. Using a higher-powered lens, he or she can also see to the back of the eye, detecting macular degeneration and other problems. During a slit-lamp exam, your head will be comfortably stabilized on the lamp's chin rest.
As its name suggests, the slit lamp shines slits of light into the eye. The size of the slit is adjustable, so the doctor can see very small sections of the eye at very high magnification. The lens nucleus is clearly visible, as are the lens position, the other layers of the lens, and the degree of brunescence (browning of the lens), which is responsible for some cataract patients' inability to distinguish blues and purples.
Visual Field Measurement
The simplest way to test your field of vision - how far you can see to the left and right out of the corners of your eyes - is for you to focus on the doctor's face while he or she moves a finger slowly to the side and asks you to signal when you can no longer see it. An instrument called a perimeter, which emits flashes of light, can be used to reveal blind spots. You simply stare at an image and tell the doctor when you see a flash.
Tonometry-lntraocular Pressure (lOP) Measurement
As part of a routine eye exam, your eye doctor will probably use a tonometer to screen your eyes for glaucoma. A noncontact tonorneter is generally used for screening. It expels a puff of air toward the eye and measures the resulting small, instantaneous indentation. The size of the indentation indicates the intraocular pressure (IOP) inside the eye. Other types of tonometers are placed directly on the cornea after the eye is numbed with eye drops.
Potential Acuity Testing
If you have cataracts, the doctor may perform potential acuity testing, a measure of what your vision would be like if the cataracts were removed. Potential acuity testing is especially useful in determining how much of your vision loss is due to cataracts. One way of testing potential acuity is with a pinhole acuity meter, which projects an eye chart directly onto the retina, bypassing the cataract.
Contrast Sensitivity Testing
If cataracts make it hard for you to differentiate shades of gray, you have low contrast sensitivity. Your eye doctor might measure this by using a low-contrast visual-acuity chart, or possibly a chart with different contrast levels on symbols of the same size. To find out more, you can check out How To Improve Eyesight Without Surgery.