T2‐weighted Magnetic Resonance Imaging Measurements of Optic Nerve Sheath Diameter in Dogs With and Without Presumed Intracranial Hypertension

Abstract

Intracranial hypertension is a cause of cerebral ischemia and neurologic deficits in dogs. Goals of this retrospective study were to test interobserver agreement for MRI measurements of optic nerve sheath diameter and associations between optic nerve sheath diameter, signalment data, and presumed intracranial hypertension status in a cohort of dogs. A veterinary radiologist interpreted scans of 100 dogs and dogs were assigned to groups based on presence or absence of at least two MRI characteristics of presumed intracranial hypertension. Two observers who were unaware of group status independently measured optic nerve diameter from transverse T2-weighted sequences. Mean optic nerve sheath diameter for all dogs was 3 mm (1-4 mm). The mean difference between observers was 0.3 mm (limits of agreement, -0.4 and 1.0 mm). There was no correlation between optic nerve sheath diameter and age for either observer (r = -0.06 to 0.00) but a moderate positive correlation was observed between optic nerve sheath diameter and body weight for both observers (r = 0.70-0.76). The 22 dogs with presumed intracranial hypertension weighed less than the 78 dogs without (P = 0.02) and were more often female (P = 0.04). Dogs with presumed intracranial hypertension had a larger ratio of optic nerve sheath diameter to body weight for each observer-side pair (P = 0.01-0.04) than dogs without. Findings indicated that the ratio of MRI optic nerve sheath diameter relative to body weight may be a repeatable predictor of intracranial hypertension in dogs.

CT dimensions, volumes and densities of normal canine eyes

“…Although the findings of the present study are similar to those previously reported, from 1.2 to 2 mm, the authors agree that MRI would give a better resolution of the optic nerve sheath due to the higher contrast resolution for soft tissues and its ability to define fluid in the nerve sheath. Recently, the width of the optic the nerve sheath has been measured in 100 normal dogs on MRI, showing a mean diameter of 3 mm, slightly larger than in the present study (Scriviani and others 2013). The authors could not identify any change with age or sex, but a positive correlation was present with the bodyweight (Lee and others 2003).…”

Transcranial Doppler Ultrasound Examination in Dogs with Suspected Intracranial Hypertension Caused by Neurologic Diseases

“…In addition to TCD, various noninvasive methods for assessment of intracranial pressure have been applied to dogs . The combination of MRI findings is useful and reliable for the diagnosis of intracranial hypertension in dogs .…”

“…In addition to TCD, various noninvasive methods for assessment of intracranial pressure have been applied to dogs . The combination of MRI findings is useful and reliable for the diagnosis of intracranial hypertension in dogs . In a recent study, the combination of these MRI findings yielded a sensitivity of 72% and a specificity of 96% for the diagnosis of intracranial hypertension in dogs .…”

“…Increased intracranial pressure causes a shift of intracranial structures such as brain herniation and compression of the CSF space, which are observed on MRI . Therefore, we classified the dogs by the MRI findings of suspected intracranial hypertension . Secondly, the study population was restricted to dogs with clinical conditions for which general anesthesia was acceptable, and dogs with severe intracranial hypertension that precluded isoflurane general anesthesia were excluded from the study.…”

“…The MRI findings indicative of intracranial hypertension were recorded according to the following criteria: (1) effacement of the cerebral sulci, (2) brain herniation (foramen magnum, transtentorial, subfalcine), (3) compression of CSF space (third ventricle, fourth ventricle), and (4) displacement of the lamina quadrigemina . Dogs were suspected of having intracranial hypertension if any ≥2 of the above‐mentioned MRI findings were identified …”