An introduction to and the basics of ophthalmology (Proceedings)
Ophthalmology is a unique specialty because nearly all the ocular structures can be directly visualized. Modern equipment allows a detailed and highly magnified evaluation of the interior of the eye. Furthermore, many systemic diseases are first manifested as an ocular problem. For these reasons, a thorough eye exam can be very rewarding. This discussion will review basics of the ophthalmic exam.
As with any examination, the signalment and history are very important. Often, an accurate diagnosis can be made from this information alone. Description of the problem, duration, visual status, and degree of discomfort is all vital information. The examination should be conducted in a quiet dark room with an assistant restraining the patient. The exam should follow a systematic approach from the outside in, so that nothing is missed. I begin with the lights on by assessing the overall gross appearance, looking for any asymmetry in the globe size or position, size or shape of the pupils, or eyelid appearance. A menace response test should be performed on each eye from several directions to check different fields of view. It is important not to create an air current which may stimulate the corneal touch receptors and cause a blink reflex which can be interpreted as a positive menace. A transilluminator and a head loop are helpful for close up evaluation with the lights dimmed. A dazzle reflex is a good test of visual potential. This involves shining the light quickly into the eye and watching for a blink and/or withdrawal reflex. I begin the examination with the eyelids and steadily work toward the interior of the eye, evaluating the eyelid margins, conjunctiva, third eyelid, cornea, anterior chamber, iris, and lens. Any swelling, discoloration, opacity, etc. should be noted and determined how it compares to normal and if it is significant to the current problem. The eyelid margin should be smooth and make contact with the cornea along its entire length. The conjunctiva should not be smooth and moist with no evidence of inflammation. It is important to look under the third eyelid as this may reveal embedded foreign bodies as well as hidden structures in the ventral anterior chamber. The cornea should be clear with a shiny luster. If an opacity is seen, it should be determined if it lies on the epithelium, stroma, or endothelium as this is important for the diagnosis. A slit lamp biomicroscope is often needed to accurately localize lesions. The anterior chamber is filled with aqueous humor and should be completely clear. A cloudy appearance to the aqueous is termed aqueous flare and indicates uveitis. This is best seen with a narrow beam of light viewed from an angle. If the beam of light can be seen traversing the anterior chamber, similar to headlights in the fog, flare is present. The iris should be smooth and free of localized swellings. The lens is best evaluated after dilating the iris. Cataracts are the most common abnormality of the lens and can range from tiny insignificant lesions to mature, blinding opacities. In older patients, a cataract must be distinguished from nuclear sclerosis, which does not obstruct light and is a normal aging change.
Evaluation of the posterior segment can be performed in several ways. A direct ophthalmoscope allows a very magnified evaluation of the retina, but it can be difficult to see the entire fundus. Indirect ophthalmoscopy allows for a wider view of the retina and also allows for binocular, 3-dimentional examination with a dedicated headset. Most ophthalmologists favor indirect ophthalmoscopy, however it does take practice to become proficient with this technique. In lieu of a headset, a transilluminator and a lens can be used. The iris should be dilated for maximum visualization. I begin by centering on the optic nerve and evaluating its size, shape, and color. I then evaluate the retina systematically, beginning with the dorsal quadrants. The size and branching pattern of the retinal vessels, degree of tapetal reflectivity, degree of pigmentation, and presence of abnormal lesions (hemorrhage, detachment, mass) are noted. Most patients do not hold still for the entire exam so often a composite image must be formed from many quick glimpses.Additional diagnostic tests are often performed to gather information about specific parts of the eye. A Schirmer Tear Test is a measure of the aqueous portion of the tear film and is useful in diagnosing and monitoring for keratoconjunctivitis sicca. The test strip should be placed under the ventral eyelid to the notch and left in place for 60 seconds. A normal reading is greater or equal to 15mm. Fluorescein stain adheres to any area of the cornea devoid of epithelial cells and is used to identify a corneal ulcer. It is also useful in evaluating the tear film quality and in assessing patency of the nasolacrimal ducts. In most patients, fluorescen stain can be seen at the tip of the nose within a few minutes if the duct is patent and functional. Tonometry is the measurement of intraocular pressure and required to diagnose glaucoma. Several tonometers are currently available and in common use. Most general practices use the TonoPen™. This is an applanation tonometer and it estimates intraocular pressure by measuring the amount of force needed to flatted a small footplate at the tip of the instrument. This machine has been well tested and proved to be accurate when used correctly. It is important that the tip is perpendicular to the cornea and that very light pressure is used. If the cornea is indented when performing a measurement, then too much force is being used and a falsely high reading may result. A newer instrument is the TonoVet™ which uses a completely different technology. It is called a rebound tonometer and it estimates intraocular pressure by accelerating a plastic probe toward the cornea and measuring the speed at which it bounces off the surface. I find this device easier and quicker to use and topical anesthesia is not needed. With either instrument, proper restraint is critical to obtaining an accurate reading. Make sure the animal is as relaxed as possible and that no pressure is placed around the neck. As a general rule, it is much easier to obtain a falsely elevated reading than a falsely low one. Therefore, the lowest reading usually the most accurate.