Evaluation of transpalpebral ultrasonographic measurement of optic nerve sheath diameter for indirect assessment of intracranial pressure in anesthetized and standing healthy adult horses.

Objective: To determine whether an association exists between direct intracranial pressure (ICP) measurement and ultrasonographic measurement of optic nerve sheath diameter (ONSD) in anesthetized and standing horses.

Design: Cross-sectional study performed on a convenience sample of healthy adult horses.

Setting: University teaching hospital.

Animals: Eight adult horses donated to the University. Enrolled horses were free of abnormalities on physical examination, CBC, neurological evaluation, and ophthalmological examination.

Measurements And Main Results: Horses were anesthetized in lateral recumbency for placement of an ICP transducer. Three head positions (neutral, elevated, and lowered) were used to alter ICP. ICP and ONSD in 2 directions (D1 and D2) were recorded at 5 and 10 minutes after position change to elevated and lowered. ICP and ONSD measurements were repeated in standing sedated horses 24-36 hours after recovery from anesthesia. Linear regressions were performed with ICP as the dependent variable and ONSD as the independent variable by head position and times. Linear regressions were also performed for change from neutral under anesthesia, with ONSD as the independent variable and ICP as the dependent variable, by head position and times. Significance was set at P < 0.05. There was a moderate association between ICP and ONSD in horses with head lowered at 5 and 10 minutes (R values = 63%-78%) and weak association in head elevated at 10 minutes (R values = 56%-63%). There was a weak association between change from neutral ICP and change from neutral ONSD in the elevated anesthetized position at 10 minutes for summed D1 + D2 (R  = 33%).

Conclusions: Consistent associations between direct ICP and ONSD in anesthetized or standing horses were not observed. This inconsistency limits the clinically utility of transpalpebral ultrasonographic ONSD measurement for ICP monitoring in horses.