What is tonometry?
What is the intraocular pressure?
Tonometry is very useful to doctors for measurement of the pressure in the eye, or the intraocular pressure (IOP). An elevated IOP can be dangerous because people with varying degrees of IOP elevation may develop damage to the optic nerve. The optic nerve collects all of the visual information from the retina of the eye and transmits that information to the brain, where the signals are interpreted as vision. When changes occur in the optic nerve leading to decreased peripheral vision and loss of the nerve tissues, a diagnosis of glaucoma can be made.
Glaucoma is a fairly common condition. Many people with glaucoma have not been checked and therefore do not know that they have it. Thus, glaucoma screening efforts as well as regular eye examinations are essential to detect glaucoma at the earliest possible stages.
Glaucoma is usually, but not always, associated with elevated pressure in the eye Actually, glaucoma is now considered a disease of the optic nerve, or optic neuropathy. Generally speaking, the vision loss in glaucoma usually occurs in both eyes and is thus termed bilateral. As in many other disease states, the vision loss may not be symmetric, that is, one eye may be worse than the other. Vision loss due to glaucoma often begins with a subtle decrease in peripheral vision. If the glaucoma is not diagnosed and treated, it may progress to loss of central vision and blindness.
Vision loss in the chronic open-angle form of glaucoma generally occurs gradually over many years, while the vision loss of acute angle closure glaucoma may occur within a matter of days if not immediately treated. Since patients with open-angle glaucoma rarely notice their gradual peripheral visual field loss, they may not visit an eye doctor until advanced changes have occurred. Unfortunately, the visual field loss in glaucoma represents permanent damage to the optic nerve and is therefore irreversible. For this reason, glaucoma is often called the sneak thief of sight.
Who is at risk for glaucoma?
Glaucoma occurs when the normal fluid in the front of the eye chamber, the aqueous humor, is blocked from leaving the eye during the normal aqueous turnover process. This blockage can occur for a number of reasons, the most common of which is simply poor outflow due to chronic open-angle glaucoma. A number of other problems can also impede the ability of the outflow channels to function properly, thus creating elevated IOP. Glaucoma is rarely if ever a disease of excessive aqueous humor production. An eye doctor, your ophthalmologist or optometrist, can usually detect those individuals who are at risk for glaucoma. These patients are called "glaucoma suspects" and must be monitored closely for subtle changes in their visual field or in their optic nerve.
There are many risk factors for glaucoma, including:
- family history of glaucoma,
- anatomical variations including narrow filtering angles or anatomical damage to the filtering angles,
- elevated intraocular pressure (IOP),
- diabetes mellitus,
- previous ocular trauma, injury, or surgery,
- use of steroid pills, eyedrops, patches, injections, or nasal sprays,
- excessive pigment in the front or anterior segment of the eye,
- many congenital disorders of the eye such as Peter's anomaly or Sturge-Weber syndrome,
- many degenerative disorders of the eye such as pseudoexfoliation syndrome,
- extremely advanced cataract,
- African American heritage,
- proven visual field loss or anatomical changes in the optic nerves,
- inflammatory disorders of the eye such as iritis, uveitis, or pars planitis,
- many infectious diseases of the eye such as Herpes simplex, toxoplasmosis, Fuch's uveitis syndrome, or shingles (Herpes zoster),
- an extremely thin cornea,
- and excessively high myopia, generally greater than 6 diopters.
The eye doctor also can diagnose patients who already have glaucoma by observing their nerve for damage or detecting visual field loss with a peripheral visual field test. Patients with glaucoma must be monitored closely for the remainder of their lifetime in order to adequately treat the IOP and assess treatment effectiveness. It is extremely cost effective to avoid visual disability due to glaucoma, enabling the at-risk patient or the patient with glaucoma to have an active lifestyle with functional central and peripheral vision.
How is tonometry done?
Tonometry is generally performed upon an anesthetized ocular surface. Anesthesia is generally rendered with a single drop of topical anesthetic, such as proparicaine (Alcaine) or tetracaine (Pontocaine). The tonometer device lightly touches the surface of the eye, ever so slightly indenting the cornea. The resistance to indentation is measured by a precisely calibrated pressure sensing device, the tonometer. Several types of tonometers are available for this test, the most common being the applanation tonometer:
- Goldman applanation tonometer: the "gold standard" instrument attached to the slit lamp biomicroscope used in all eye doctors' offices. It requires a cobalt blue light source and a small droplet of fluorescein on the ocular surface. A tiny pressure sensor attached to a spring-loaded arm is gently placed against the tear film, and the doctor or technician reads the pressure through the microscope under the blue light.
- Tono-Pen handheld electronic contact tonometer: This widely used, portable, handheld device runs on hearing aid batteries and calibrates digitally with the push of a button. It requires a disposable sterile cover for each patient. The sterile device tip is gently placed against the tear film by the doctor or technician, and the pressure reading appears on the digital readout simultaneous to a faintly audible beep.
- pneumotonometer contact device: The device is operated similarly to the handheld tonopen tonometer, but due to its larger size, it's not readily portable. It requires a continuous gas supply and separate gauge container with analog readout attached to a long tube and pressure probe. This is an older technology and has largely been replaced by the handheld tonopen tonometer.
- The airpuff noncontact tonometer, which generally requires no anesthetic drop, is widely used in doctor's offices, clinics, and screening facilities. It is very safe due to the "no touch" technology, but it often produces falsely elevated readings, particularly in patients who squeeze their muscles upon anticipation of the air puff. The patient simply sits then places their chin in a rest while looking straight ahead, while the operator activates the air puff mechanism while aligning each eye individually.
- Tactile finger applanation over the closed eyelid by a skilled eye doctor is an age-old traditional method utilized by the experienced practitioner.
- Intraocular sensors for experimental or intraoperative use during surgery are utilized in research and technology development companies. Hopefully, these will become universally available for long-term use and patient self-readout.
- Patient self-testing devices are in their infancy. The ProVision device marketed by Bausch & Lomb allows the patient to gently press a calibrated spring-loaded piston against the closed eyelid over the upper, outer quadrant of the eyeball while gazing downward. The true IOP is reached when the pressure applied onto the globe produces faint lights in the eye, or phosphenes, readily perceived by the patient and thereby recorded at home.
After the eye has been numbed by the technician or doctor with anesthetic eyedrops, the tonometer's sensor is placed against the surface of the eye. The firmer the tone of the surface of the eye, the higher the pressure reading. The doctor can record the pressure reading, and this can be used to diagnose or monitor the treatment of glaucoma. Also, the pressure reading is lower than otherwise expected if the cornea is thin. Thus, patients with a thin cornea may obtain a falsely low IOP reading when the actual pressure is high. These patients must be watched more closely for glaucoma.
What is tonometry used for?
Tonometry is an essential component of the global examination in all eye check-ups and specifically for a glaucoma examination. A complete glaucoma examination has a number of important components.
These tests together give a complete picture of the patient's ocular health and therefore the risks for developing or controlling a given disease such as cataracts, macular degeneration, or glaucoma. The following tests make up a glaucoma examination:
- medical and ocular history
- visual acuity testing
- refraction to measure the glasses or lens correction
- tonometry testing for IOP
- Slit lamp biomicroscope examination is the "stethoscope" of the eye doctor. This high-quality optical device allows the doctor to view the eyes in stereo under high magnification while the patient places their chin on a rest and their forehead against a bar to maintain consistent positioning. Numerous details can be discerned by the trained observer, rendering possible a specific diagnosis, including the etiology (or cause) of an individual patient's glaucoma.
- gonioscopy to examine the aqueous humor outflow channels
- peripheral visual field testing
- Optic nerve head photography is taken by high-quality cameras and recorded, usually digitally. This allows precise comparison from year to year, permitting analysis of subtle anatomical changes over time.
- Optic nerve head topography, including imaging techniques like Optical Coherence Tomography (OCT), create precise digital images of the nerve thickness around the optic nerve to within 1/1000 of a millimeter. These detailed images of the retina document any changes in a precise objective way. Subtle loss of nerve thickness indicates the progression of glaucoma.
Once the examinations have been completed, a decision is made as to whether or not glaucoma treatment should be given, maintained at its current level, or modified. If the patient is doing well, then glaucoma treatment may be withheld or cut back. If the patient is doing poorly, then more treatment may be recommended. There are several approaches to glaucoma treatment, any or all of which may be appropriate for a given patient. These include but are not limited to the following:
- topical eyedrop medications: beta blockers, prostaglandins, alpha agonists, aqueous suppressants, and miotics
- oral ocular hypotensive or pressure-lowering medications: most commonly acetazolamide (Diamox) or methazolamide (MZM).
- laser treatment: SLT (selective laser trabeculoplasty) or ALT (argon laser trabeculoplasty) for open-angle glaucoma and PI (peripheral iridotomy) for closed-angle glaucoma
- filtering procedure: Traditional trabeculectomy surgery permits drainage of the aqueous humor in the front of the eye to a bubble or bleb on the conjunctiva, or clear surface over the white part of the eye. This surgery has been performed for centuries and has been highly perfected under the outstanding control of the operating-room microscope by skilled surgical hands.
- shunting procedure: The eye doctor places a plastic tube and emptying chamber into the eye in the operating room. Commonly used devices include the Molteno shunt, the Express shunt, the Baerveldt valve, and the Ahmed valve. This procedure accomplishes the same effect as the trabeculectomy but generally lasts longer because the plastic tube avoids closure by normal progressive tissue scarring.
- outflow enhancement procedures: These include the viscocanalostomy or the canaloplasty operations that allow the drainage of aqueous humor without a conjunctival bleb or plastic drainage device. Instead, the natural drainage pathways in the anterior or front of the eye are opened surgically, allowing more facile egress of aqueous into the channels of the ocular surface and orbit, or eye socket.
- Newer oral medication strategies to preserve or enhance optic nerve head function: These include antioxidants such as ginkgo biloba or prescription neuro-protectants such as memantine (Namenda).
The bottom line for any patient, and particularly the patient who might be at risk for glaucoma, is to have regular visits to your eye doctor. Patients over the age of 50, or patients at risk of glaucoma over the age of 40, should have regular annual eye examinations.
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"Open-angle glaucoma: Epidemiology, clinical presentation, and diagnosis." UpToDate.com