Rodents: ophthalmology

image of Rodents: ophthalmology
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The anatomy and physiology of the eyeball (bulbus oculi) is fairly uniform amongst rodent species, but they do show some interesting structural differences regarding the bony orbital anatomy. Three elements constitute the zygomatic arch in rodents: (i) the zygomatic bone; (ii) the zygomatic process of the maxillary bone; and (iii) the zygomatic process of the temporal bone. The presence of a prominent and well developed zygomatic process of the maxillary bone is what differentiates the zygomatic arch of rodents from that of dogs and cats. The chapter covers Basic ocular anatomical features of rodents; Physical restraint and ophthalmic examination; Ophthalmological diseases such as disorders of the eyelids and periocular infections, conjunctivitis and other conjunctival diseases, corneal diseases and diseases of the lens.

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15.1 Dorsal view of skulls of four different rodent species (without the mandible): chinchilla; albino Wistar rat; golden hamster; gerbil. Note the convex shape of the zygomatic arch in (a) and (b), and its concavity in (c) and (d), as well as its thin aspect in (b) and (c); the proportionally larger interorbital breadth in (a) and (d), and the round shaped orbit in (a). Bar: 10 mm. (Courtesy of Marcello Machado.)
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15.2 Selected normal features of rodent eyes. Note the large eyes of this rat with heavily pigmented iris. Note the slit pupil of the chinchilla eye. Albino guinea pig: note the semi-transparent appearance of the iris. It is even hard to tell the pupillary margins. The overall red appearance of the eye is due to the lack of pigment and predominant reddish fundic reflection. Note the reddish appearance of the fundus of this albino rat. Additionally, note the holangiotic retina with arterioles and venules that radiate from the optic disc like spokes on a bicycle wheel.
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15.3 Protocol for ophthalmic examination of rodents. (Modified from .)
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15.4 Ophthalmic examination in rodent species. This hamster was restrained by the scruff of the neck and transitory exophthalmos occurred (arrow). Use of the conventional 6 mmwide Schirmer tear test (STT) strip in a guinea pig. Normal production can be low in this species (about 1–2 mm). (Courtesy of Giuseppe Visigalli.)
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15.5 Representative cases of ophthalmic conditions caused by tooth-root infections. Periocular abscess with blepharitis and hemifacial swelling causing marked facial asymmetry (arrowed) due to a cheek tooth-root infection in the right eye of this hamster. Exposure keratitis (arrowed), chemosis and exophthalmos due to a retrobulbar abscess in a chinchilla, secondary to a tooth-root infection (an apical abscess).
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15.6 Representative cases of conjunctivitis in rodents. Note that the resulting exudate made the eyelids of the left eye stick together. In this case, the conjunctivitis was associated with severe respiratory disease. This bilateral blepharoconjunctivitis case was associated with a skin infection caused by spp. (Courtesy of Carlos Alexandre Pessoa.)
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15.8 Guinea pig with ‘pea eye syndrome’. Note the nodule that protrudes from the inferior conjunctival sac (arrowed). (Courtesy of Daniel H Johnson.)
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15.9 This guinea pig was diagnosed with a corneal dermoid. In this case a patch of normal skin tissue is abnormally located in the cornea (arrowed). Surgical excision is the treatment for the condition. (Courtesy of Daniel H Johnson.)
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15.10 Corneal ulcers and stromal abscesses in rodents. There is a very discrete superficial corneal epithelial defect in this hamster. The epithelial defect created a corneal opacity (arrowed). A deeper ulcer with more evident margins (arrowed) and mild stromal bacterial invasion. The ulcer was stained with fluorescein dye. Note the conjunctivitis, thick mucopurulent discharge and severe white purulent stromal plaque (arrowed) in this chinchilla suffering from a stromal abscess. Note the intense stromal keratitis and vascularization (arrowed) partially obscuring the stromal abscess in this gerbil.
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15.12 Hamster demonstrating luxation of the lens from its normal position into the anterior chamber (arrowed). (Courtesy of Giuseppe Visigalli.)
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15.13 Cataracts in rodents. Mature cataracts in a guinea pig. (Courtesy of Marc H Kramer.) Mature cataracts in an agouti with diabetes mellitus.
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15.14 Chinese chipmunk (), 5-year-old male, presenting with exophthalmos in the left eye. The patient was anaesthetized and a CT scan analysis of its head was performed. A tumour at the base of its brain also invading the orbit was diagnosed. In this CT scan section it is possible to appreciate the forward dislocation of the left eyeball (exophthalmos). (Courtesy of Yasutsugu Miwa.)
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