Optic Nerve Sheath Diameter

Optic Nerve Sheath Diameter (ONSD)

Optic Nerve Sheath Diameter (ONSD) measurement by bedside ultrasound is a rapid, non-invasive, repeatable method for estimating intracranial pressure (ICP). Although ONSD does not directly measure ICP, enlargement of the optic nerve sheath strongly correlates with raised ICP because the optic nerve sheath is anatomically continuous with the intracranial subarachnoid space.ONSD should complement—not replace—clinical assessment, neuroimaging, and definitive neuromonitoring in patients with suspected intracranial hypertension.

Anatomy

Optic Nerve

The optic nerve (cranial nerve II)

• Approximately 50 mm long
• Extension of the central nervous system
• Covered by the three meningeal layers:
• Dura mater
• Arachnoid mater
• Pia mater

Unlike peripheral nerves, the optic nerve is actually white matter of the brain.

Optic Nerve Sheath

The sheath consists of

• Outer dura
• Middle arachnoid
• Inner pia

Between the arachnoid and pia lies the subarachnoid space, which contains

• Cerebrospinal fluid (CSF)
• Trabeculae
• Blood vessels

This space is continuous with the intracranial CSF compartment.

Equipment

High-frequency linear probe

Preferred frequency

• 7.5–15 MHz
• 10–18 MHz ideal

Machine preset

• Small parts
• Ophthalmic preset (preferred)
• Low mechanical index
• Low thermal index

Safety

Ultrasound energy may theoretically injure the retina.

Recommendations:

• Mechanical Index (MI) < 0.23
• Thermal Index (TI) as low as possible
• Use ophthalmic preset if available
• Keep examination as short as practical (typically <1 minute per eye)

Avoid prolonged Doppler insonation of the eye.

Measurement Technique

• Patient position —Supine,Head neutral,Eyes closed,Relaxed eyelids.If patient is Awake instructed him  maintain a forward gaze with eyes closed.No pressure should be applied to the eyeball.
• Identify Retina and Move exactly 3 mm posterior to Draw a perpendicular line
• Measure outer dura to outer dura This is the ONSD.
• Bilateral Measurement
• Take Three measurements per eye (if feasible) and Average the values,Average of both eyes improves reliability.

Why Measure 3 mm Behind the Globe?

The sheath is

• Most distensible
• Least affected by surrounding orbital tissues
• Most reproducible

within the first 3 mm behind the retina.

Hence all guidelines recommend measurement exactly 3 mm posterior to the globe.

Normal Values

Adults

• Normal: <5.0 mm
• Borderline: 5.0–6.0 mm
• Suggestive of raised ICP: ≥6.0 mm
• Markedly enlarged: >6.5 mm

Because published thresholds vary by population and ultrasound technique, many ICUs use a cutoff between 5.5 and 6.0 mm to suggest elevated ICP rather than relying on a single universal value.

Children

Age-specific normal values are lower than in adults.

Approximate reference ranges:

Relationship Between ONSD and ICP

Approximate correlation:

This relationship is approximate and influenced by individual anatomy and measurement technique.

Diagnostic Accuracy

Across meta-analyses, ocular ultrasound ONSD demonstrates approximately:

• Sensitivity: 85–95%
• Specificity: 80–90%
• Area under ROC curve: ~0.90

Performance depends on operator experience, chosen cutoff, and the reference standard (invasive ICP monitoring vs. CT findings).

How to Use ONSD

Measure ONSD:

• At baseline when raised ICP is suspected.
• After interventions expected to change ICP.
• When the patient’s neurological status changes.
• To assess trends rather than isolated values.

Contraindications

Absolute

• Suspected globe rupture
• Open eye injury

Relative

• Recent ocular surgery
• Severe orbital trauma
• Ocular infection
• Corneal ulcer
• Chemical burns

Limitations

• Operator dependent
• Inter-observer variability
• Lack of universal cutoff
• Cannot provide an exact ICP value
• May remain enlarged for some time after ICP normalizes
• Influenced by chronic optic nerve sheath remodeling

Situations Where ONSD May Be Misleading

False positives

• Optic neuritis
• Optic nerve tumors
• Long-standing papilledema
• Idiopathic intracranial hypertension with chronically enlarged sheath
• Orbital pathology

False negatives

• Very early ICP elevation
• Technical error
• Poor acoustic window
• Individual anatomical variation

Common Pitfalls

• Applying pressure on the globe
• Measuring at the wrong depth
• Measuring inner-to-inner instead of outer-to-outer
• Off-axis imaging
• Excessive gain obscuring margins
• Measuring only one eye
• Ignoring ocular pathology
• Interpreting ONSD in isolation without the neurological examination and neuroimaging

Clinical Interpretation

ONSD should be viewed as a screening and monitoring tool, not a standalone diagnostic test.

• Normal ONSD (<5 mm in most adults): Raised ICP is less likely but not excluded.
• Borderline ONSD (5.0–6.0mm): Interpret alongside symptoms, neurological examination, and imaging.
• Enlarged ONSD (≥6.0 mm): Supports the possibility of elevated ICP and should prompt further evaluation and, when appropriate, definitive neuroimaging or invasive monitoring.

REFERENCES

1. Lau, T.; Ahn, J.S.; Manji, R.; Kim, D.J. A Narrative Review of Point of Care Ultrasound Assessment of the Optic Nerve in Emergency Medicine. Life 2023, 13, 531. https://doi.org/10.3390/life13020531
2. Richards E, Munakomi S, Mathew D. Optic Nerve Sheath Ultrasound. [Updated 2026 Jan 31]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2026 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK554479/