The ophthalmic features of snakes differ considerably from those of mammals and even other reptiles; perhaps the most clinically important difference in snakes is the lack of mobile eyelids, which are replaced by the spectacle (an embryonic fusion of the eyelids).1-3 The superficial layers of the spectacle undergo periodic ecdysis with the rest of the skin.3 The spectacle forms a permanent seal over the cornea creating the epithelial-lined subspectacular space. This space is filled with an oily, tear-like secretion produced by the Harderian gland. The secretion subsequently drains from the ventral aspect of the subspectacular space, through the nasolacrimal duct, and empties into the mouth.1,2 This unique anatomic arrangement is the basis for the 3 most common ophthalmic conditions that develop in snakes: retained spectacle or dysecdysis4,5; subspectacular abscess formation secondary to ascending stomatitis, penetrating injuries, or systemic disease2,5-7; and subspectacular fluid accumulation secondary to nasolacrimal duct obstruction (also referred to as pseudobuphthalmos).4-8
For nearly 40 years, ultrasonography has been used in many species as a diagnostic tool and for anatomic determinations in various species both in vivo and in vitro.9-15 More recently, high-frequency ultrasound probes have provided increased image resolution but with decreased tissue penetration.16 Such probes are ideally suited for ophthalmic ultrasonography in which deep penetration is not required. Ultrasonography performed with ultrasound frequencies of 40 to 60 MHz provides image resolution of approximately 50 μm (similar to that achieved via low-power light microscopy) and is commonly referred to as ultrasound biomicroscopy.17-21
To the authors' knowledge, the specific anatomic dimensions of the eye and related structures have not been measured in living snakes. The purpose of the study reported here was to measure the dimensions of the eyes of clinically normal snakes by use of high-frequency ultrasound imaging and thereby establish baseline measurements for clinically important ophthalmic structures, to compare those dimensions among common species of snake, and to determine whether those features of snakes' eyes are affected by age, body length, or body weight.
UltraView 2.0 imaging system, E-Technologies Inc, Bettendorf, Iowa.
Aquasonic 100, Parker Laboratories Inc, Fairfield, NJ.
Kern TJ. Exotic animal ophthalmology. In: Gelatt KN, ed. Veterinary ophthalmology. 3rd ed. Philadelphia: Lippincott Williams & Wilkins, 1999;1279–1284.
Millichamp NJ, Jacobson ER, Dziezyc J. Conjunctivoralostomy for treatment of an occluded lacrimal duct in a blood python. J Am Vet Med Assoc 1986;189:1136–1138.
Schiffer SP, Rantanen NW, Leary GA, et al. Biometric study of the canine eye, using A-mode ultrasonography. Am J Vet Res 1982;43:826–830.
Hager DA, Dziezyc J, Millichamp NJ. Two-dimensional real-time ocular ultrasonography in the dog. Technical and normal anatomy. Vet Radiol Ultrasound 1987;28:60–65.
Dziezyc J, Hager DA, Millichamp NJ. Two-dimensional realtime ocular ultrasonography in the diagnosis of ocular lesions in dogs. J Am Anim Hosp Assoc 1987;23:501–507.
Brooks DE. Ocular imaging. In: Gelatt KN, ed. Veterinary ophthalmology. 3rd ed. Philadelphia: Lippincott Williams & Wilkins, 1999;471–473.
Hewick SA, Fairhead AC, Culy JC, et al. A comparison of 10 MHZ and 20 MHz ultrasound probes in imaging the eye and orbit. Br J Ophthalmol 2004;88:551–555.
Bentley E, Miller PE, Diehl KA. Use of high-resolution ultrasound as a diagnostic tool in veterinary ophthalmology. J Am Vet Med Assoc 2003;223:1617–1622.
Pavlin CJ, Harasiewicz K, Eng P, et al. Clinical use of ultrasound biomicroscopy. Ophthalmology 1991;98:287–295.
Pavlin CJ, Foster FS. High frequency ultrasound biomicroscopy. Imaging the eye at microscopic resolution. Ophthalmol Clin North Am 1994;7:509–522.
Pavlin C, Foster F. Ultrasound biomicroscopy: high frequency ultrasound imaging of the eye at microscopic resolution. Radiol Clin North Am 1998;36:1047–1058.
Boroffka S, van den Belt A. CT/ultrasound diagnosis-retrobulbar hematoma in a horse. Vet Radiol Ultrasound 1996;37:441–443.
Pavlin CJ, Easterbrook M, Hurwitz JJ, et al. Ultrasound biomicroscopy in the assessment of anterior scleral disease. Am J Ophthalmol 1993;116:628–635.
Heiligenhaus A, Schilling M, Lung E, et al. Ultrasound biomicroscopy in scleritis. Ophthalmology 1998;105:527–534.
Rogers M, Cartee RE, Miller W, et al. Evaluation of the extirpated equine eye using β-mode ultrasonography. Vet Radiol Ultrasound 1986;27:24.
Cottrill NB, Banks WJ, Pechman RD. Ultrasonographic and biometric evaluation of the eye and orbit of dogs. Am J Vet Res 1989;50:898–903.