eye disorders
The Eye Examination
Tonometry
With tonometry, the pressure within the eye can be measured. Normal pressure within the eye is 8 to 21 millimeters of mercury (mm Hg). Pressure in the eye is measured to detect certain types of and monitor their treatment. The noncontact air-puff tonometer can be used to screen for elevated pressure in the eye. This device is not highly accurate, but it is useful in identifying people who may need further testing. A small puff of air is blown against the cornea, which causes the person to blink but is not uncomfortable. The puff of air flattens the cornea, and the device measures the time (in thousandths of a second) it takes to do so. It takes less time for the puff of air to flatten the cornea in an eye with normal pressure than it does an eye in which pressure is elevated. Applanation tonometry is a more accurate method. The applanation tonometer is usually attached to a slit lamp. After numbing the eye with drops, the doctor observes the eye through a slit lamp while the applanation tonometer is gently moved until it rests on the cornea. The amount of pressure it takes to flatten the cornea is related to the pressure within the eye. Portable, handheld applanation instruments are also used for tonometry. Eye drops that numb the eye are given, then the instrument is gently placed on the cornea and a pressure reading is obtained. Portable applanation tonometers can be used in the emergency department or a doctor's office to quickly detect increased pressure in the eye. A handheld rebound tonometer can be used to measure the pressure within the eye without the use of eyedrops to numb the eye. It is useful in children and is widely used in emergency departments by nonophthalmologists. 