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Ultrasound examination of optic nerve in adult cats

Evangelisti MA, Carta G, Burrai GP, Pinna Parpaglia ML, Cubeddu F, Ballocco I, et al. Repeatability of ultrasound examination of the optic nerve sheath diameter in the adult cat: comparison between healthy cats and cats suffering from presumed intracranial hypertension. J Feline Med Surg [Internet]. 2020 Jan 22 [cited 2020 Feb 5];1098612X19898006. 

Intracranial hypertension (IH) refers to an increase in the pressure within the skull (Intracranial Pressure; ICP) above the normal range. IH leads to a decrease in Cerebral Perfusion Pressure (CPP) with a consequent decrease in blood flow to the brain. IH occurs in situations of head trauma, bleeding into the skull, intracranial cancers, inflammation (ie meningitis and encephalitis) and seizures.  Detection of IH is beneficial for the treatment and prognostication of patients with intracranial disease, however traditional forms of measurement of ICP are invasive, expensive, and difficult to perform in regular practice. Optic nerve sheath diameter (ONSD) in dogs and humans has been shown to correlate with intracranial hypertension. As ultrasound is becoming more common and available in practice, therefore this is a viable technique to detect elevated ICP.

The study was designed as a prospective, single blinded, observational, cross sectional study. Two groups of cats were recruited; the first were healthy cats presented to a veterinary teaching hospital for routine neuter, and the second were cases presenting to the neurology service with clinical signs attributed to ICH and an MRI consistent with ICH. Cats with ocular trauma or other ophthalmologic disease were excluded, as were animals treated with glucocorticoids, mannitol, or hypertonic saline in the 10 days prior to analysis.

Cats were positioned in sternal recumbency and the transducer positioned on the most lateral aspect of the upper eyelid, perpendicular to the sagittal axis of the head. The probe was then rotated ~15° clockwise until the entry of the optic nerve was seen.  ONSD was measured at a position 3mm from the papilla, perpendicular to the probe and between the hyperechoic borders. Each measurement was performed three times by two people.

Fifty healthy cats and 7 cats were presumed IHC were recruited. There was good inter- and intra- measurer agreement. While weight and sex were not associated with ONSD, regression analysis showed an association with age, with increased age associated with a larger diameter.

The optic nerve sheath diameter in normal cats was a median of 1.23 +/- 0.11mm.  There was a significant difference between ONSD in cats with ICH compared to normal cats. A measurement of 1.4mm or less was consistent with a normal cat, while values >1.5mm were predictive of ICH.

There were some limitations to this study. The small sample size is a significant downside, and repeating analysis on a larger population would be beneficial. Cats did not have actual ICP measured, and so the determination of ICH was based on a combination of surrogate diagnostics.

In conclusion, measurement of ONSD in cats appears to be a well-tolerated and useful tool to screen for intracranial hypertension in cats. Its adoption in clinical practice may help to better diagnose and treat cats with intracranial disease. (MRK)

See also:

Robba C, Donnelly J, Cardim D, et al. Optic nerve sheath diameter ultrasonography at admission as a predictor of intracranial hypertension in traumatic brain injured patients: a prospective observational study. J Neurosurg. Epub ahead of print 8 March 2019. DOI: 10.3171/2018.11. JNS182077.