Use of Rebound Tonometry as a Diagnostic Tool to Diagnose Glaucoma in the Captive California Sea Lion
One of the most common medical problems seen in the California Sea Lion (Zalophus californianus) is ocular disease. The prevalence of ocular lesions in captive California Sea lions is estimated to be very high in comparison to other marine mammals.2 Eye disease can range from a pinpoint corneal opacity to loss of vision due to cataracts, lens luxation, and secondary glaucoma. Glaucoma is a disease that has not been evaluated extensively in the sea lion. Observing clinical signs and measuring intraocular pressures (IOP) is critical for early diagnosis. IOP is a measurement that provides us with information regarding the balance between aqueous humor production and outflow. The objective of my future Masters project is to measure IOP in clinically normal California Sea Lions (Zalophus californianus) with and without ocular pathology.
A tonometer is a non-invasive instrument that records the IOP by gently touching the cornea. After trying various tonometers, the Tono-Vet® (Lumic International, Baltimore, MD) was selected to be used in this study. This new rebound tonometer was introduced in 2004 with a design that calibrates the instrument for various species of animals. The Tono-Vet® does not use applanation of the corneal surface, but rather a new rebound method to estimate IOP. An electrical magnetic tonometer probe comes into contact with and rebounds from the corneal surface to estimate an IOP. The speed of this instrument provides a safer advantage for the person taking the measurements. The probe touches the cornea so quickly that a topical anesthetic is not necessary. In order to record an accurate IOP, six measurements are taken and averaged resulting with the mean value.4 A complete ophthalmic examination has been or will be performed by a veterinary ophthalmologist. This includes slit lamp biomicroscopy, indirect ophthalmoscopy, as well as fluorescein staining. Three measurements will be taken on each eye with the Tono-Vet® on a weekly basis for a series of eight consecutive months. Preliminary data has been collected on thirteen sea lions in this study. Six of the sea lions have ocular pathology ranging from corneal edema, ulcers, cataract (varying degrees), and secondary glaucoma. Four of them have had cataract surgery with and without axial corneal scarring and three of them have no ocular pathology. The future objective of this study is to establish a normal baseline range for IOP values in captive sea lions with no ocular pathology. This range will provide a comparative measurement when evaluating a diseased eye. By measuring the IOP regularly in young adult sea lions, veterinarians will be able to determine when IOPs begin to change so that medical management can be started prior to loss of vision. The goal is to have ten sea lions with no ocular pathology participate in this study. There is a request for the help and authorization of all marine facilities that would like to include their sea lions in this study.
The author would like to give a special thank you to Mississippi State University College of Veterinary Medicine for the instrumentation and initial funding of this Masters study and the Institute for Marine Mammal Studies of Gulfport for providing the startup venue and guidance for the project. Additional thanks to Lumic International, Dan Scott and Associates, and Reichert Ophthalmic Instruments for also helping provide the tonometers. A note of appreciation is given to the members of my Masters committee: Dr. Skip Jack, Dr. Lora Ballweber, Dr. Moby Solangi, Dr. William Miller, Dr. Carmen Colitz, and Dr. Maya Magdalena Rodriguez for their support and encouragement throughout this study. A special thanks to the trainers (Tim Hoffland, Marci Romagnoli, and Shannon Huyser) and Dr. Connie Chevis for having an open mind to train this behavior for the first time. This research would not be possible without the help and skills of all of the staff, doctors, trainers, and animals of the following places: Miami Seaquarium, Miami, FL, Dolphins Plus, Key Largo, FL, Theater of the Sea, Islamorada, FL, Atlantis Marine World, Riverhead, NY, SeaWorld, Orlando, FL, Wildlife Conservation Society, Bronx, NY, and Marine Life, Gulfport, MS.
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