ABSTRACT

Ocular tonometry is a class of noninvasive measurement that allows an ophthalmologist (and some optometrists) to estimate the internal hydraulic pressure in the eyeballs, called the

intra-ocular pressure

(IOP). The IOP is the result of the constant production (and outflow) of the liquid

aqueous humor

(AH). The AH is found around the lens and in the anterior chamber of the eye. It serves as a nutrient solution for the lens, the iris and the inside of the cornea, and its pressure helps to maintain the proper shape of the eyeball. Aqueous humor (AH) is continuously formed in a normal eye at a rate of about = 2 mm

/min. by the cells of the

ciliary process,

a tissue lying behind the lens having an exposed area of about 6 cm

(Guyton, 1991). AH is formed by the active metabolic “pumping” of Na

ions from inside ciliary process cells to the perilenticular region inside of the eyeball. There is also active transport of ascorbate and certain amino acids into the eyeball. Osmotic pressure and diffusion down concentration gradients cause water, chloride, glucose and bicarbonate ions to follow the ions pumped into the eyeball. AH contains mainly low-molecularweight substances, including Na

, K

, HCO

, citrate, ascorbate, urea, glucose, etc. Clearly, in the steady state, the AH must exit the eye at the same volume flow rate

that it enters. Outflow of AH is through the

Canal of Schlemm

into the episcleral veins, and thence into the main venous circulation, etc. The eyeball is slightly elastic, with most of its compliance coming from the thin clear cornea. Normal intraocular pressure (IOP) is about 16 mm Hg. If there is an increase in the outflow resistance, the normal IOP rises, and if the IOP exceeds its normal high range (about 30 mm Hg), the condition known as

glaucoma

exists. In extreme situations, the IOP can exceed 60-80 mm Hg. Such acute glaucoma sharply reduces normal arterial blood flow to the retina, causing poor oxygenation and impaired nutrition of retinal neurons and glial cells. If prolonged, glaucoma can lead to the death of retinal neurons, including the loss of retinal ganglion cells, the axons of which compose the optic nerve. Such neuron loss is irreversible, causing loss of visual field, acuity, and even total blindness. Thus, it is medically important as part of every routine eye examination to measure the IOP, especially in older patients, who are more susceptible to glaucoma.