From the Research
Optic nerve sheath diameter (ONSD) measurement is a valuable non-invasive tool for assessing increased intracranial pressure (ICP) in clinical settings, with a cutoff value of >5.85 mm predicting ICP >20 mmHg with a sensitivity of 92.3% and specificity of 85.7%. The normal ONSD measurement is typically less than 5.0-5.8 mm in adults, with measurements above 5.8-6.0 mm strongly suggesting elevated ICP 1. This technique can be performed quickly using bedside ultrasound (with a high-frequency linear transducer at 7.5-10 MHz) or through CT/MRI imaging. When performing ultrasound measurement, the probe should be placed gently on the closed eyelid with gel, and measurements taken 3 mm behind the globe in the transverse plane. Multiple measurements (typically 2-3 per eye) should be averaged for accuracy.
Key Points to Consider
- ONSD assessment is particularly useful in emergency settings where invasive ICP monitoring is unavailable or contraindicated, such as in trauma patients, those with suspected idiopathic intracranial hypertension, or when evaluating for cerebral edema.
- The technique works because the optic nerve sheath is continuous with the dura mater surrounding the brain, allowing cerebrospinal fluid pressure changes to be transmitted to the perioptic subarachnoid space, causing measurable distension of the optic nerve sheath.
- While ONSD measurement cannot replace invasive ICP monitoring for continuous assessment, it serves as an excellent screening tool with good sensitivity and specificity for detecting elevated ICP.
- A study published in 2022 found a strong positive correlation between direct ICP and ONSD after intubation with a correlation coefficient of 0.969 (P <0.001) 1.
- Another study published in 2019 found that an ONSD ≥6.0 mm + Marshall score ≥3 on initial CT head demonstrated a 92.5% sensitivity, 92.6% specificity, and 96.1% positive predictive value for developing an ICP ≥20 mmHg during hospitalization 2.
Clinical Application
- ONSD measurement can be used as an initial screening test when physicians suspect a patient has elevated ICP 3.
- The measurement of ONSD by ultrasonography is an important and reliable tool in the assessment of normalization of ICP post CSF diversion procedure 1.
- ONSD measurement is a non-invasive method for the estimation of increased intracranial pressure in patients with severe traumatic brain injury using optic nerve sheath diameter measured on computed tomography head 2.