Optic Nerve Sheath Diameter and ICP: ONSD Ultrasound Technique, Values and Limits

Why the optic nerve sheath reflects ICP

The optic nerve is surrounded by a sheath containing cerebrospinal fluid that communicates with the subarachnoid space. The retrobulbar portion of the sheath is distensible. When intracranial pressure rises, fluid is pushed into the sheath and its diameter increases, usually within seconds to minutes, and it tends to narrow again as pressure falls.

That makes ONSD ultrasound attractive in the emergency department and ICU: it is fast, repeatable, needs no catheter and can be done in a patient who is not yet suitable for invasive monitoring. It does not give continuous pressure or a true waveform. Anyone searching for a non invasive ICP waveform should know that ONSD offers a snapshot and not a trace. See normal ICP values and waveforms for the invasive waveform.

ONSD ultrasound technique

Ocular ultrasound for ICP uses a high-frequency linear probe placed gently on the closed upper eyelid with a generous layer of gel and no pressure on the globe. The operator finds the optic nerve as a hypoechoic band behind the globe and measures the sheath diameter, usually 3 mm behind the retina, perpendicular to the nerve axis. Measurements are normally taken in both eyes and in two planes, and averaged.

Keep the scan short and use the lowest acoustic output that gives a usable image, in line with the as-low-as-reasonably-achievable principle for ocular safety. Training matters: manual caliper placement is the main source of inter-operator variability.

Optic nerve sheath diameter normal value and cut-offs

Published studies report a range of cut-offs in adults, roughly from 4.8 to 6.3 mm, depending on the reference standard, the population and the measuring plane. Many protocols treat a diameter above about 5 mm as suggestive of raised ICP, and values in children are lower and age-dependent. The optic nerve sheath diameter normal value therefore cannot be a single figure and should be taken from your local protocol or the cited study.

ONSD is better used to decide whether ICP is likely to be raised than to calculate an exact pressure in mmHg. Interpret it with the clinical picture and imaging, and confirm with invasive monitoring where treatment decisions depend on a number. For the comparison of approaches see invasive vs non-invasive ICP monitoring.

Limits of ocular ultrasound for ICP

Measurements can be affected by optic nerve pathology, orbital trauma, previous surgery, and cases where the sheath no longer distends normally. A delay between a pressure change and the sheath response is also possible, so the finding should not be used alone in unstable patients.

These limits explain why research groups combine ONSD with other markers. They also explain why automated tools that standardise the measurement are of interest to hospitals that perform many scans.

Training and quality assurance help here: periodic comparison of measurements between operators, and review of stored images, makes results more consistent across a department.

• Operator dependence of caliper placement
• Variable cut-offs between studies and populations
• Optic nerve or orbital pathology can confound readings
• Gives a snapshot, not continuous pressure

AI-assisted analysis with FAST-ICP

FAST-ICP automatically analyses transorbital ultrasound images and measures the ONSD as well as a Deformability Index, guides probe positioning in real time and typically completes an examination in under a minute without equipment calibration. It runs on the Nisonic P-100 system and is designed to be installed on other scanners too. At present it is labelled for research use only.

If your department is considering ONSD research, contact Beckfinity to discuss the system and the manufacturer's research programme.