Optic Nerve Sheath Diameter (ONSD) Measurement: Clinical Significance and Applications
Optic nerve sheath diameter (ONSD) measurement is a valuable non-invasive ultrasonographic technique for detecting increased intracranial pressure (ICP) with excellent diagnostic accuracy (sensitivity 92%, specificity 85%), making it an important bedside screening tool when invasive ICP monitoring is unavailable or contraindicated. 1
What is ONSD Measurement?
ONSD is a sonographic measurement of the optic nerve sheath, which is directly affected by changes in intracranial pressure. The optic nerve sheath is continuous with the dura mater and contains cerebrospinal fluid that is in direct communication with the intracranial subarachnoid space.
Technique:
- Use a high-frequency linear transducer (7.5-10 MHz)
- Apply gel on closed eyelid
- Measure 3mm behind the globe in the transverse plane
- Take measurements in both vertical and horizontal directions
- Perform on both eyes for comparison 1
Clinical Significance of ONSD Measurements
Normal vs. Abnormal Values:
- Normal ONSD: Generally 4.3-5.0 mm in adults 2, 3
- Abnormal/Elevated ONSD: ≥5.0-6.0 mm suggests increased ICP 1
Diagnostic Accuracy:
- Sensitivity: 92%
- Specificity: 85%
- Diagnostic odds ratio: 62 (95% CI: 33-117)
- Strong correlation with invasive ICP measurements (Pearson R = 0.68-0.80) 1, 2
Clinical Applications
1. Neurological Emergencies
- Detection of increased ICP in:
- Traumatic brain injury
- Intracranial hemorrhage
- Ischemic stroke
- Meningitis and encephalitis 4
2. Monitoring Neurocritical Patients
- Daily monitoring of neurological patients in ICU
- Early detection of malignant intracranial hypertension
- Potential marker for brain death progression 5
3. Situations Where Invasive Monitoring is Not Available
- Emergency departments
- Resource-limited settings
- Pre-hospital assessment
- Contraindications to invasive monitoring 4
4. Other Conditions
- Idiopathic intracranial hypertension
- Acute mountain sickness
- Posterior reversible encephalopathy syndrome
- Symptomatic intracranial hypotension 4
Reliability and Limitations
Reliability:
- High inter-rater reliability among trained physicians (ICC of 0.82,95% CI 0.63-0.92) 3
- Emergency physicians can accurately measure ONSD with minimal training (ICC 0.9,95% CI 0.88-0.93) 6
Limitations:
- Narrow margins between normal and pathological findings
- Requires proper technique and training
- Should not completely replace invasive ICP monitoring in all clinical scenarios 1
- May show elevated values in patients with neurological diseases even without intracranial hypertension 5
Recommendations for Clinical Practice
- Consider ONSD as a screening tool for increased ICP when invasive monitoring is unavailable or contraindicated
- Use a cut-off value of 5.05 mm (sensitivity 92%, specificity 90%) to predict elevated ICP 2
- Always correlate ONSD findings with clinical examination and neuroimaging when available
- Implement routine ONSD monitoring for neurocritical patients to detect early changes in ICP
- Ensure proper training and technique to maximize reliability of measurements
ONSD measurement represents an important advancement in non-invasive neurological monitoring that can significantly impact patient care by enabling early detection of increased ICP and guiding appropriate interventions.