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Objective—To assess natural variations in degree of refraction, corneal curvature, corneal astigmatism, corneal radius, and intraocular distance of healthy equine eyes.

Animals—159 horses with healthy eyes that were admitted to a veterinary teaching hospital for nonophthalmic surgeries.

Procedures—Eyes of horses were examined with a retinoscope prior to anesthesia and with a keratograph and A- and B-scan ultrasonographic biometers during surgery. In addition, manual caliper measurements of horizontal and vertical corneal radii were obtained.

Results—Mean ± SD degree of refraction in the horizontal meridian of eyes was −0.06 ± 0.68 diopters (D). Vitreous body length and horse age correlated negatively with refraction values. The horizontal corneal radius (15.96 ± 1.28 mm) was larger than the vertical corneal radius (15.02 ± 1.09 mm). Accordingly, the vertical corneal curvature (21.56 ± 1.68 D) was greater than the horizontal corneal curvature (22.89 ± 1.65 D). Axial globe length (40.52 ± 2.67 mm), anterior chamber depth (6.35 ± 0.59 mm), lens thickness (12.30 ± 0.83 mm), and vitreous body length (21.87 ± 1.85 mm) were positively correlated with body weight, height, and age. Results of keratograph and caliper measurements correlated well for horizontal corneal diameter but poorly for vertical corneal diameter. Results of A- and B-scan ultrasonography differed by ≤ 1 mm in 64% of measured eyes.

Conclusions and Clinical Relevance—Results of keratometry and ultrasonographic biometry varied widely. Additional research is needed to validate the keratograph used in our study for measurements in equine eyes.

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in American Journal of Veterinary Research


Case Description—A 6-year-old 680-kg (1,496-lb) German Warmblood gelding was evaluated because of bilateral blepharospasm and head shaking.

Clinical Findings—Moderate blepharospasm was evident bilaterally, and both eyes had hyperemic and edematous conjunctivas and lusterless corneas. For each eye, the Schirmer tear test value was only 7 mm/min. The horse's nasal mucosa was dry. Abnormal behaviors included mild repetitive vertical movement of the head, snorting, and flehmen response (classic signs of head shaking). Touching the horse's nostrils and face revealed paresthesia and dysesthesia with slight nasolabial muscle hypertrophy bilaterally. Cranial nerve examination revealed no other abnormalities. Serum thyroxine concentration was low, and results of thyrotropin-releasing hormone and thyroid-stimulating hormone stimulation tests were negative, indicating that the horse had hypothyroidism. The diagnoses included keratoconjunctivitis sicca and dry nares attributable to parasympathetic facial nerve dysfunction, head-shaking syndrome with paresthesia and dysesthesia of the face attributable to sensory trigeminal nerve disorder, and hypothyroidism. The 2 nerve dysfunctions were considered peripheral neuropathies that were most likely caused by hypothyroidism.

Treatment and Outcome—Treatment of both eyes was initiated with topical applications of cyclosporine, 0.5% sodium hyaluronate, and vitamin A ointment. Levothyroxine (20 Pg/kg [9.1 Pg/lb], PO, q 24 h) was administered. Within 3 weeks to 4 months, serum thyroxine concentration was within reference range, and clinical signs and Schirmer tear test values improved.

Clinical Relevance—Hypothyroidism should be considered as a differential diagnosis in horses with peripheral neuropathy or keratoconjunctivitis sicca. In affected horses, administration of levothyroxine may lead to resolution of neurologic signs.

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in Journal of the American Veterinary Medical Association