Animal Eye Care is proud to provide advanced diagnostic equipment and technology to ensure a complete and thorough evaluation of your pet’s ocular disease problem. Some of these diagnostic tests are performed on all patients, such as slit lamp biomicroscopy and indirect ophthalmoscopy. However most of the diagnostic tests described are performed on a case-by-case determination.
Slit Lamp Biomicroscopy and Indirect Ophthalmoscopy
Slit lamp biomicroscopy and indirect ophthalmoscopy are diagnostic tests performed on all patients at Animal Eye Care.
The slit lamp biomicroscope is a special microscope used to examine living eye tissue. It is actually a microscope combined with a slit lamp light source, and is used to evaluate the front half (“anterior segment”) of the eye, including the lens and anterior vitreous. The light can be changed from a round beam, to a tiny slit of light. The slit beam acts as a “slice ” of light that passes through ocular structures and allows the ophthalmologist to determine the size and thickness of tissues, and determine the presence and size of cataracts.
Indirect ophthalmoscopy uses special diagnostic equipment (a lighted head unit) and a condensing lens, to evaluate the back half (“posterior segment”) of the eye. It is used to examine the vitreous, and the fundus. The fundus is the view we have of the posterior layers of the eye, including the retina, optic nervehead, and (in domestic animals—NOT humans—) the tapetum. The tapetum is the “eye shine” layer—a special layer of tissue that reflects light and helps animals see better in the dark. The tapetum can be quite beautiful, in iridescent shades of green, blue, gold, orange, or yellow.
The Tonopen™ is a precision electronic tonometer (which is much easier to use than the Schiotz tonometer) to measure the intraocular pressure in domestic animals. It measures non-invasive intraocular pressure (IOP) by converting the IOP to an electrical signal. The waveform produced by each touch to the anesthetized corneal surface is analyzed and stored for a statistical comparison process. When four valid readings are obtained, the mean IOP and the standard deviation for those readings is displayed. The Tonopen™ also indicates the percent of accuracy for the reading.
Gonioscopy (“gonio–” means “angle”) is the examination of the iridocorneal drainage angle of the eyes. Aqueous humor is constantly secreted into the eye, and is constantly drained from the eyes through a sieve-like meshwork of tissue called the iridocorneal angle, or “drainage angle”. In dogs predisposed genetically to developing glaucoma, this angle is often malformed at the time of birth. Evaluation of the appearance of the angle helps determine how much at risk the eye is, for developing glaucoma. Gonioscopy is usually performed under light sedation, and a special contact lens called a goniolens is placed on the eye. The lens looks like a clear plastic mushroom. Then the ophthalmologist views the angle through the lens. The lens bends the light rays so the angle can be examined.
Doppler Indirect Blood Pressure Measurement
Particularly in geriatric cats, high blood pressure can cause intraocular bleeding and retinal disease. The retinas can actually detach suddenly, causing acute blindness. In animals with suspected high blood pressure, blood pressure measurement is performed at Animal Eye Care with a special diagnostic Doppler blood pressure unit. In a quiet room, a site on the patient’s leg or tail is shaved, and then a small cuff is wrapped around the leg or tail. A transducer is placed over the artery, the cuff is inflated, and a blood pressure reading is obtained as the cuff is allowed to slowly deflate. If the blood pressure is elevated, then it is important for the patient to be placed on the proper blood pressure lowering medications as soon as possible in an attempt to allow the retinas to reattach and vision to improve, and to reduce stress on the kidneys and heart. High blood pressure can also cause headaches in animals. How do we know this? Because people with high blood pressure also can experience headaches, and once the blood pressure is controlled in animals, the animal usually acts perkier and obviously feels better.
Like other nervous tissue of the body, the retina generates electrical currents. The electroretinogram (ERG) is a measurement of electrical function that helps determine how well the rods and cones of the retina are functioning. ERG testing is sometimes needed to help determine whether or not animals with cataracts have operable eyes. If the lens is white and opaque and the health of the retina is unknown, then cataract surgery carries a higher risk for failure. It is important to know if the retina is healthy prior to performing cataract surgery, and ERG testing assists in determining retinal health.
Other indications for ERG testing are to diagnose and differentiate blindness caused by retinal disease, such as Sudden Acquired Retinal Degeneration syndrome (SARDs), from brain or optic nerve-induced blindness.
Electroretinography is a painless procedure that is performed under sedation or brief general anesthesia. In a darkened room, special electrodes are attached to the skin, and a special contact lens is placed on the eyes. Bright strobe lights are flashed into the eyes, and a waveform response then shows up on a computer screen. The height of the waveform is measured, and this value correlates to how well the retina is functioning.
Animal Eye Care performs electroretinography using a sophisticated electrodiagnositic unit and computer software. If ERG testing is recommended for your pet, we will discuss the procedure and schedule ERG testing as an outpatient procedure (admit the patient to our hospital in the morning, and discharge the patient in the afternoon).