Temporal behaviour and clinical implications of ONSD in neurocritical patients: beyond static measurements

This thesis investigates the temporal behaviour of the optic nerve sheath diameter (ONSD) as a non-invasive neuromonitoring tool in patients with acute brain injury admitted to the Neuro-Intensive Care Unit. ONSD, measurable at the bedside via transorbital ultrasonography, has been established as a surrogate marker of intracranial pressure (ICP) due to the anatomical continuity between the optic nerve sheath and the intracranial subarachnoid space. However, its dynamic properties — particularly its recoil behaviour following resolution of intracranial hypertension — remain poorly understood. This prospective observational study, conducted at the Azienda Ospedaliero-Universitaria Pisana, enrolled adult neurocritical patients with conditions including traumatic brain injury, subarachnoid haemorrhage, and intracerebral haemorrhage. Serial ONSD measurements were performed to characterise the time course of sheath distension and recovery, and a novel concept of "ONSD burden" — the cumulative area of ONSD elevation above a defined threshold over time — was developed and analysed in relation to patient outcomes. Key findings indicate that the recoil capacity of the optic nerve sheath is highly variable and influenced by individual anatomical and histological factors, including tissue elasticity. Additional investigations explored the effect of gaze deviation and bed inclination on ONSD measurements, ONSD asymmetry in unilateral brain lesions, methodological considerations regarding measurement distance, and ONSD behaviour during cerebral circulatory arrest. The results support the use of ONSD as a screening tool for initial ICP elevation, while underscoring its limitations in longitudinal monitoring and advocating for its integration within a comprehensive neuro-POCUS approach.