Baseline Ophthalmic Parameters and High Resolution Ultrasonographic Ophthalmic Anatomy of the Atlantic Bottlenose Dolphin (Tursiops truncatus) Utilizing a Voluntary Behavioral Approach
Limited ophthalmic examination of periocular adnexal tissues and cornea has been commonplace for well-trained cetaceans. Dolphin Quest Oahu has developed a more comprehensive voluntary ophthalmic examination approach that furthers diagnostic and treatment potential.
Knowledge of normal ocular anatomy, structure, and ophthalmic measurement values are essential in diagnosing and treating ophthalmic disorders. Dolphin Quest utilized all available ophthalmic examination techniques including: slit lamp (biomicroscopy), indirect ophthalmoscopy, intraocular pressure measurement, standard ocular ultrasound exam, and high-resolution ophthalmic ultrasound examination to develop baseline "normals" for the species.
The animals were trained to remain in lateral position at the water surface with the eye exposed to air. Adnexa, cornea, anterior chamber, and iris examination were performed with a handheld slit lamp. The animals were appropriately conditioned to maintain an open eyelid for intraocular pressure (IOP) measurements by directly contacting the anesthetized corneal surface with an applanation tonometer. Posterior segment and fundus examinations have been limited in the past due to the lack of response of the cetacean iris to standard topical mydriatic agents such at tropicamide and atropine. Therefore, fundus examinations by indirect ophthalmoscopy were performed after sunset when the pupils would naturally dilate in darkness. Successful fundus photography was also performed in the same dim lighting conditions. Dolphins have a holangiotic retina with 14-16 radiating vessels from the optic disc with a 360 degree tapetal fundus. However, thorough evaluation of the posterior chamber and lens by slit lamp exam was not possible in dim lighting as the pupil would rapidly constrict in response to the intense light from the instrument.
The use of high-resolution ophthalmic ultrasound is critical in the ability to diagnose and monitor retinal disease in humans. Use of this instrument in dolphins and other marine mammals has improved assessment of intraocular structures such as the iridocorneal angle, lens, posterior chamber, vitreous, and retina. Once conditioned for tonometry, the behaviorists worked on extending the behavior for "eye open" high-resolution ophthalmic imaging. Historically, the standard for cetacean ophthalmic ultrasound has been on the closed eye. The open eye exam is important for the high-resolution ultrasound because of sound attenuation reducing signal strength. Methods for the open eye exam are to use a fluid-filled membrane over the open transducer using tear gel as lubricant and coupling medium. The covered transducer was then placed directly on the anesthetized cornea of a nearly motionless animal.
Ultrasound image documentation of normal anatomical structures is the key to recognizing abnormalities. Imaging ocular structures is made more challenging with the presence of acoustic artifacts created by unique structures such as the crystalline lens. In 10 MHz posterior segment scans the lens causes a pseudo-elevation of the retina called "Baum's Bumps". This is due to velocity and refraction effects as ultrasound waves travel back and forth through the lens, altering sound much as it does light.
|Hoku OD-10 MHz demonstrates pseudo-elevation of retina from scanning through crystalline lens.|
Vertical and horizontal scan planes reveals the anatomic relationship created by the pupil, iris, and cornea. What would appear like a shallow anterior chamber depth (indicating glaucoma, iris or ciliary body tumor or advanced cataract) in other species is normal for the Atlantic bottlenose dolphin.
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|Kolohe OS-20 MHz iridocorneal angle with thick peripheral cornea and narrow anterior chamber depth (ACD).|
|Nainoa OS-20 MHz U-shaped pupil is shown as 2 pupillary spaces in horizontal cross-section.|