Mar 29, 2019
Welcome back to our exploration the eye. We have now reached the deepest limits of the anterior chamber, the forward-most space of the eye. When we move into the eye's deeper spaces, the clear liquid of the anterior chamber is replaced with gel and all of the moving, focusing, familiar parts of the eye give way to the amazingly complex neural networks where the actual work of vision takes place. The anterior chamber is functionally the "cam-era" portion of the eye, while the "film" is further back. We’ll get there.
Lots of things can happen in the anterior chamber. The aqueous humor is produced by se-cretory cells of the ciliary body, which is the unlabeled thickening that supports the suspen-sory ligaments and makes up the base of the iris. The ciliary body is muscular and provides the pull that stretches the lens and allows focus for near-field vision.
Aqueous fluid is produced on the surface of the ciliary body, then runs over the lens, pass-es through the pupil and finally passes through the filtration angle where the clear cornea meets the white sclera of the globe. There is a critical balance between fluid production and fluid loss which maintains the intra-ocular pressure within the range necessary to keep all the sensitive structures inside the eye healthy.
But the ciliary tissue is highly vascular. Micro-organisms can find their way into the anteri-or chamber and infect the eye. Sometimes, antigen-antibody complexes leak into the aque-ous fluid, triggering inflammation within the anterior chamber. As we age, the lens dries, shrinks, and leaks protein. Because lens protein is never normally released, the immune system recognizes it as "foreign." All of these conditions cause anterior uveitis (inflamma-tion of organs within the anterior chamber) and lead to fogging of the cornea, pain, con-striction of the pupil, and redness.
Worst of all, protein complexes can clog the critical filtration angle, leading to an increase in intra-ocular pressure called glaucoma. If the pressure inside the eye gets too high, the sen-sitive neurons of the retina are killed. Glaucoma is an extreme emergency that often pre-sents with a suddenly red, painful eye. Specialized equipment is required to measure and monitor the intra-ocular pressure and treatment needs to be initiated immediately because , permanent blindness can occur in as little as twelve hours. Many veterinarians immediately refer suspected glaucoma patients to a specialist in ophthalmology, who has the equipment and specialized medicines needed to stabilize the patient before irreversible damage occurs.
Even when anterior uveitis is less severe, inflammation of structures within the anterior chamber can cause bleeding or adhesions between the iris and the lens or the back side of the cornea. Even if the rest of the eye remains functional, derangements of the camera struc-tures of the anterior chamber can severely interfere with vision. These kinds of adhesions are called senechia and most commonly result from direct injuries - wounds or blows - to the eye.
One problem is that an acutely red, painful eye can be the result of glaucoma - with high intra-ocular pressure, or also from anterior uveitis - often with low intra-ocular pressure. One clue is the size of the pupil: When an eye is in pain, spasms of the ciliary body can re-sult in unequal pupillary size, with constriction of the pupil in the painful eye. Glaucoma-tous eyes are also in pain but may show dilation of the pupil because of poor retinal func-tion. Either way, these eyes are in real trouble and need to be attended to immediately. Any delay risks permanent loss of vision.
Veterinarians tend to react strongly to all eye problems, largely because the consequences of failing to provide necessary care can be so terribly and irreversibly severe. In the face of uncertainty, it is far, far better to over react (i.e.: to immediately send a questionable patient to the ophthalmologist for evaluation) than to find oneself saying, "Gee, if I had responded more aggressively yesterday, this patient would not be blind today." This is an approach I urge all pet owners to adopt: Any eye problem should be considered a potential emergency because when an eye case goes bad, it goes very, very bad. If we’re going to make a mis-take, let’s make the mistake that leaves our pet with its vision.
Tune in next month for a new graphic and a fascinating peek into the posterior chamber!
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