Ocular tonometry

Tonometry is the procedure eye care professionals perform to determine the intraocular pressure (IOP), the fluid pressure inside the eye. It is an important test in the evaluation of patients at risk from glaucoma. Most tonometers are calibrated to measure pressure in millimeters of mercury (mmHg), with the normal eye pressure range between . In applanation tonometry the intraocular pressure (IOP) is inferred from the force required to flatten (applanate) a constant area of the cornea, for the Imbert-Fick law. The Maklakoff tonometer was an early example of this method, while the Goldmann tonometer is the most widely used version in current practice. Because the probe makes contact with the cornea, a topical anesthetic, such as proxymetacaine, is introduced on to the surface of the eye in the form of an eye drop. Goldmann tonometry is considered to be the gold standard IOP test and is the most widely accepted method. A special disinfected prism is mounted on the tonometer head and then placed against the cornea. The examiner then uses a cobalt blue filter to view two green semicircles. The force applied to the tonometer head is then adjusted using a dial connected to a variable tension spring until the inner edges of the green semicircles in the viewfinder meet. When the area of a circle with diameter has been flattened, the opposing forces of corneal rigidity and the tear film are roughly approximate and cancel each other out allowing the pressure in the eye to be determined from the force applied. Like all non-invasive methods, it is inherently imprecise and may need to be adjusted. The Perkins tonometer is a type of portable applanation tonometer, which may be useful in children, anesthetised patients who need to lie flat, or patients unable to co-operate with a sitting slit lamp examination, that yields clinical results comparable to the Goldmann. Dynamic contour tonometry (DCT) uses the principle of contour matching instead of applanation. The tip contains a hollow the same shape as the cornea with a miniature pressure sensor in its centre.