Relationship between the ratio of optic nerve sheath diameter to eyeball transverse diameter and morphological characteristics of dogs.

Objective: To assess the feasibility of ocular ultrasonography for measurement of the ratio of optic nerve sheath diameter (ONSD) to eyeball transverse diameter (ETD) in dogs with various morphologies and to evaluate the interobserver reliability of the ONSD/ETD ratio and its correlation with various morphological variables.

Animals: 45 healthy dogs of various breeds.

Procedures: Height, head circumference, body weight, body condition score, intraocular pressure, and blood pressure were recorded for each dog. Unsedated dogs underwent bilateral ocular ultrasonography once. A veterinarian and board-certified ophthalmologist who were unaware of subject signalment independently reviewed the ultrasonographic videos and selected 1 image for each eye on which the ETD and ONSD were measured. The ONSD/ETD ratio was calculated and compared between the 2 observers. Correlations between the ONSD/ETD ratio and various physiologic and morphological variables were assessed.

Results: 172 ONSD/ETD ratios were recorded. The ONSD/ETD ratio was calculated for at least 1 eye for 44 of the 45 (98%) dogs. Mean ± SD time required to complete the ultrasonographic examination was 90 ± 30 seconds (range, 15 seconds to 3 minutes). The mean ± SD ONSD/ETD ratio was 0.17 ± 0.01 (range, 0.15 to 0.20). The ONSD/ETD ratio did not differ significantly between the left and right eyes or the 2 observers and was not correlated with any of the variables assessed.

Conclusions And Clinical Relevance: Ocular ultrasonography was a rapid, noninvasive, and reliable method for measurement of the ONSD/ETD ratio. The ONSD/ETD ratio did not appear to be influenced by dog morphology.