Department of Animal Medicine and Surgery, Veterinary Teaching Hospital, University of Murcia, Spain
Diseases of the eyes of reptiles are frequently seen in veterinary practice. The causes of these lesions are often identical to these responsible for inducing ophthalmic disease in mammals and birds. Although the approach to treatment is similar to that in mammals, anatomic differences in the reptile eye cause conditions specials to reptiles. Because the ciliary muscles of reptiles are composed of striated muscle rather than smooth muscle fibers, conventional mydriatic agents (parasympatholytics) such as tropicamide and atropine are ineffective in producing pupillary dilation. The second difference is that in those reptiles lacking movable eyelids, the corneas are covered by a clear protective epidermally derived structure called spectacle. This structure appears impervious to topical medications, thus making treatment of the globe difficult. The spectacle is periodically replaced along the rest of the skin during molting cycles (ecdysis).
Ocular malformations (especially microphthalmos) occur with some frequency in captive bred reptiles possible a consequence of inbreeding, or environmental conditions. Other congenital abnormalities include cyclopia, synophthalmia, and anophthalmos. These frequently occur with skeletal abnormalities.
Blepharitis, usually of bacterial or fungal etiology occurs commonly in reptiles, occasionally extending to involve and destroy the globe. Swelling of the eyelids and conjunctiva are seen in lizards and chelonians. Conjunctivitis may be associated with these infections. Orbital abscesses are commonly encountered in some groups of lizards (especially chameleons). Most of these infections are associated with Gram- bacterial isolates and are very difficult to manage. The spectacle of snakes and some lizards presents peculiar problems unique to these animals. The spectacle may fail to be shed at ecdysis. This mostly occurs in snakes kept in excessively dry conditions or which are dehydrated or nutritionally deficient. Also is caused by a lack of suitable scabrous substrates upon which a snake can rub its chin and rostrum to initiate the molting process. Occasionally the problem is exacerbated by ectoparasites (mites and ticks) feeding at the peripheral margin of the spectacle. Usually correcting the animal's hydration and environmental moisture just prior to shedding corrects the abnormality at the nest shed.
Blockage of the nasolacrimal duct (which cannot occur in chelonians because it is absent) results in a backup of tear secretion in reptiles with a spectacle. Most notably this occurs in snakes and geckos. The nasolacrimal duct can also be congenitally absent or become blocked by pressure from adjacent tissue (granulomas or neoplasia) or by fibrosis (burn injuries to the roof of the mouth). The fluid content may initially be clear but later become turbid and flocculent. Often culture of the fluid reveals bacteria such Aeromonas spp and Pseudomonas spp. The presence of protozoa in these infections of the corneospectacular space has been demonstrated repeatedly although their role in the disease is uncertain. Although some of these blockages and infections will clear spontaneously, in many cases the infection is unrelenting and progresses to panophthalmitis or extends into the periocular tissue spaces. Affected animals should be investigated for evidence of systemic infections.
Corneal disease (ulcers, lipid dystrophies) occurs occasionally. Corneal ulceration may be associated with foreign bodies or trauma as in other species. Keratitis in tortoises (Testudo spp.) is seen as a white corneal mass. Such keratitis is contagious and should be considered a herd problem.
Uveitis is rarely diagnosed in reptiles although it does occur associated with systemic infectious disease.
Cataracts in tortoises (Testudo spp.) have been associated with freezing episodes. The chelonian lenses are extremely soft and almost fluid like in consistency. It is hypothesized that because of this they are particularly prone to damage from low temperatures. In some cases these changes are reversible, although it may take up to 18 months for the lens to clear. Also cataracts often occur in older reptiles (etiology unknown).
Vitamin A deficiency occurs with frequency in chelonians kept in captivity. Although this is often reported in young, rapidly growing aquatic species fed meat diets deficient in vitamin A (skeletal muscle meat) and dried insects, it also occurs in terrestrial species (box-turtles) inappropriately kept in captivity. This nutritional disease causes squamous metaplasia of the orbital glands and their ducts. Epithelial desquamation blocks the ducts, and the glands increase in size, resulting in orbital and eyelid edema and secondary conjunctivitis and blepharitis. Occasionally there is secondary bacterial infection.
Neoplasms (fibropapillomas, fibrosarcomas, fibromas, papillomas) occur around the eyelids with some frequency--the occurrence of epizootics in chelonia and lizards have suggested that there is probably an underlying infectious (viral) etiology. Surgical debulking is rarely effective in curing these diseases.
The principal groups of birds that veterinary ophthalmologists examine in their consultations include cage birds, sport, zoo and wildlife birds. The sensory organ of the vision in birds presents a high specialization for adjustment to living conditions. Their visual sharpness is 2 to 8 times higher that of mammals. Their visual fields are up to 360°, the range of stereopsis is 0° to 70°, the maximum spatial frequency (skill to distinguish a certain movement in simple images) is over 160 images / second (10-15 in humans) and a minimal detection of movements over 15°/hour (movements that are performed in a very slow way).
The perception of ultraviolet light is a common skill in the diurnal birds due to a special sensibility to the ultraviolet beams of the rods of the retina, playing a very important role in their communication, camouflage, orientation and others. Knowledge of anatomical and physiological particularities of eyes of these species will help in the interpretation of the ocular exploration and in the emission of appropriate diagnoses. Some of the most important differences that can be outlined, with regard to the eyes of the mammals, include the small ocular size of some species and different morphologies of the eyeball depending of the species, open orbit, voluntary contraction of the pupil (striated sphincter muscle of the iris), ossicles in the sclerotic, avascular retina and presence of the pecten in vitreous chamber (vascular structure that nourishes the retina).
The ophthalmological examination in birds is similar to those of mammals with some particularities derived from the anatomical and physiological differences. Includes a physical ocular examination and complementary techniques, such as tonometry, ophthalmoscopy, electroretinography and ultrasonography, among others, in order to identify the ocular affections and to evaluate the severity.
Schirmer's test: It is carried out fundamentally in birds of great size. In a study carried out in 255 birds of 42 species, values have been obtained for Schirmer's test in Psitaciformes of 3.2-7.5 mm/min without topic anesthesia and 1.7-4.5 mm/min by topic anesthesia; in Falconiformes 4.1-14.4 mm/min without anesthesia and 2-4.2 mm/min with topic anesthesia; in Accipitriformes 10.7-11.5 mm/min without anesthesia and 3.6-5.9 mm/min with topic anesthesia.
Tonometry: The devices used to perform the tonometry tests are Tonopén® (applanation), Tonovet ® (rebound) and tonometer of Schiotz (indentation). In birds, the values of intraocular pressure described are the following: In turkeys, 25 mm Hg (applanation); in birds of prey, 11-16 mm Hg and in psittacines 20-25 mm Hg (applanation).
Direct and indirect ophthalmoscopy: The mydriasis necessary for the ophthalmoscopy in birds can be obtained by means of general anesthesia with ketamine or by topical or intracameral tubocurarine (d-tubocurarine chloride®, Sigma Chemical CO) (20 mg/ml). A study in raptors proved the efficiency of three curariform agents.
The most frequent ocular diseases reported include malformations (palpebral agenesia, microphthalmia, cataracts), primary or secondary inflammatory diseases of eyelids and conjunctiva (poxvirus, chlamydias), traumatisms (ocular hemorrhages, uveitis, cataracts, chorioretinitis), Retinal diseases that include congenital anomalies, degeneration, inflammation and detachment, neoplasms and nutritional disorders (vitamin A deficiency).
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